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Ž .Current Opinion in Colloid & Interface Science 7 2002 21�41

Particles as surfactants�similarities and differences

Bernard P. Binks�

Surfactant & Colloid Group, Department of Chemistry, Uni�ersity of Hull, Hull HU6 7RX, UK

Abstract

Colloidal particles act in many ways like surfactant molecules, particularly if adsorbed to a fluid�fluid interface. Just as theŽ .water or oil-liking tendency of a surfactant is quantified in terms of the hydrophile�lipophile balance HLB number, so can

that of a spherical particle be described in terms of its wettability via contact angle. Important differences exist, however,between the two types of surface-active material, due in part to the fact that particles are strongly held at interfaces. Thisreview attempts to correlate the behaviour observed in systems containing either particles or surfactant molecules in the areasof adsorption to interfaces, partitioning between phases and solid-stabilised emulsions and foams. � 2002 Elsevier ScienceLtd. All rights reserved.

Keywords: Particle wettability; Particulate monolayers; Contact angles; Partitioning of particles; Solid-stabilised emulsions and foams

1. Introduction

It is well known that low molar mass surfactantsand surface-active polymers are used to aid dispersionof powdered materials in a liquid, can form a varietyof aggregated structures in aqueous or non-aqueous

Ž .media including microemulsions in their mixturesand are commonly employed as emulsifiers in thepreparation of emulsions and as stabilisers in theproduction of foams. Much less well appreciated is

Ž .that solid particles nano- or micro- can function insimilar ways to surfactants but certain differences inbehaviour are inevitable, e.g. individual particles donot assemble to give aggregates in the same way thatsurfactant molecules form micelles and hence solubil-isation phenomena are absent in the particle case.Partly as a result of the current activity in nanoparti-

� �cle technology for producing new materials 1,2 , therehas been a resurgence of interest recently in the field

� Tel.: �44-1482-465450 fax: �44-1482-466410.Ž .E-mail address: [email protected] B.P. Binks .

Ž .of particles at interfaces both planar and curved .This review aims to highlight the similarities anddifferences in the behaviour of surfactants and parti-cles and includes their adsorption to interfaces, theirpartitioning between immiscible liquids and their abil-ity to stabilise emulsions and foams. Since this arearepresents a new perspective, references to existingolder literature are included which serve as relevantbackground and maintain continuity.

For surfactants present in oil�water mixtures, thesystem HLB is the most important variable in de-

Žtermining whether aggregated surfactant micelles or.microemulsion droplets resides in either water, oil or

a third phase. It has been shown that the packingparameter of the surfactant in situ at the oil-waterinterface determines the tendency of the surfactantmonolayer to curve towards water or oil or remain

� �effectively planar 3 . This in turn is set by the geome-try of the surfactant molecules, hydrated by water onone side of the monolayer and solvated by oil on the

Žother. Thus, for hydrophilic surfactants ionics in theabsence of salt or non-ionics with a high degree of

1359-0294�02�$ - see front matter � 2002 Elsevier Science Ltd. All rights reserved.Ž .PII: S 1 3 5 9 - 0 2 9 4 0 2 0 0 0 0 8 - 0

( )B.P. Binks � Current Opinion in Colloid & Interface Science 7 2002 21�4122

Ž .Fig. 1. Upper Position of a small spherical particle at a planarŽfluid�water interface for a contact angle measured through the

. Ž . Ž .aqueous phase less than 90� left , equal to 90� centre andŽ . Ž .greater than 90� right . Lower Corresponding probable position-

ing of particles at a curved fluid�water interface. For ��90�,Ž .solid-stabilised aqueous foams or o�w emulsions may form left .

For ��90�, solid-stabilised aerosols or w�o emulsions may formŽ .right .

.ethoxylation , the area per head group is larger thanthat of the chain and the monolayers curve around oil

Ž .resulting in oil-in-water o�w micro- and macroemul-Žsions. For more lipophilic surfactants ionics in the

presence of sufficient salt or nonionics with low de-.grees of ethoxylation , the area per chain exceeds that

of the head group and water becomes the dispersedŽ .phase in water-in-oil w�o micro- and macroemul-

sions. For conditions in which the head group andchain areas are similar, monolayers have a net curva-ture of zero and new aggregates form including lamel-lar phases and bicontinuous microemulsions. The se-quence of transitions for multiphase microemulsion

Ž .systems Winsor I�Winsor III�Winsor II and thecorresponding emulsion inversion can be achieved bychanges in salt concentration, oil type or temperatureand by addition of cosurfactant, and is reasonably well

� �understood 4 .In the case of spherical particles which adsorb to

Ž .interfaces water�air or water�oil , the relevantparameter is thought to be the contact angle � whichthe particle makes with the interface. For hydrophilicparticles, e.g. metal oxides, � measured into the aque-ous phase is normally�90� and a larger fraction ofthe particle surface resides in water than in the non-polar phase. For hydrophobic particles, e.g. suitablytreated silica, � is generally greater than 90� and theparticle resides more in air or oil than in water. Byanalogy with surfactant molecules, the monolayerswill curve such that the larger area of the particlesurface remains on the external side, giving rise to airor oil-in-water when ��90� and water-in-air or oil

Ž .when ��90� Fig. 1 . The former include aqueousfoams and o�w emulsions, respectively, whilst thelatter include aerosols and w�o emulsions.

2. Adsorption to interfaces

There are now many methods to synthesise small,monodisperse particles of different shape and surfacecoating. If the coating, e.g. alkylsilane or fluorocar-bon, is homogeneous over the particle surface fol-lowing, say, reaction in the vapour phase, such parti-cles are surface-active but, unlike surfactants, are notamphiphilic. If, however, the coating can be restrictedto a particular area of the surface only, heteroge-neous or ‘Janus’ particles result which are both sur-face-active and amphiphilic, i.e. the particle has aspecific area which is oil-liking and a specific areawhich is water-liking. It is important to understandwhy hydrophobic or hydrophilic particles are strongly

Ž . Ž .attached to fluid � �fluid � interfaces. Consider aŽ .spherical particle s of radius r which is initially

totally in phase � and is subsequently adsorbed to the�� interface. The interfaces s� , s� and �� haveinterfacial tensions � associated with them. Ignoring

Ž .the line tension � acting at the three-phase ��scontact line, adsorption of the particle at the interfaceresults in an area of the s� interface being lost butbeing replaced by an equal area of the s� interface.More importantly however, an area of the planar ��

Ž .interface normally of high tension is also lost due tothe presence of the particle, and this area depends on�. The energy of attachment of a particle to afluid�fluid interface is related not only to the contactangle but also to the tension of the interface � .��

ŽAssuming the particle is small enough typically less.than a few microns in diameter so that the effect of

gravity is negligible, the energy E required to removethe particle from the interface is given by

22 Ž . Ž .E� r � 1�cos� 1��

in which the sign inside the bracket is negative forremoval into the water phase, and positive for re-

Ž .moval into the air or oil phase. Inspection of Eq. 1�8 Žfor given r�10 m characteristic of fumed silica,

. �1 Ž .later and � �0.036 N m toluene�water shows��

that the particle is most strongly held in the interfacefor ��90� with E being 2750 kT. Either side of 90�,E falls rapidly such that for � between 0 and 20� orbetween 160 and 180� this energy is relatively smallŽ .�10 kT . It will be shown later that this extremevariation of E with wettability has a major influenceon the ability of particles of different hydrophobicityto stabilise emulsions. For angles �90� the particle ismore easily removed into water than into air or oil,i.e. the particle is relatively hydrophilic and for ��90�the reverse is true, i.e. the particle is relatively hy-drophobic.

( )B.P. Binks � Current Opinion in Colloid & Interface Science 7 2002 21�41 23

Fig. 2. Variation of the energy required to detach a single sphericalparticle exhibiting a contact angle of 90� from a planar oil�water

Ž �1 .interface of interfacial tension 50 mN m with particle radius atŽ298 K. Note the low energies for radii�0.5 nm similar to surfac-

.tant molecules .

One consequence of the very high energy of attach-ment of particles to interfaces, relative to the thermalenergy kT, is that particles once at interfaces can bethought of as effectively irreversibly adsorbed. This isin sharp contrast to surfactant molecules which ad-sorb and desorb on a relatively fast timescale. Since Edepends on the square of the particle radius, it de-creases markedly for smaller and smaller particles.Fig. 2 shows the variation in the detachment energyfor spherical particles of different radius for ��90�

�1 Žand � �50 mN m typical of an alkane�water��

. Žinterface . Clearly, very small particles �0.5 nm in.radius of the size comparable to most surfactant

Ž .molecules are easily detached several kT and andmay not be too effective as stabilisers.

2.1. Liquid��apour surface

� �It is surprising to note that Okubo 5 appears to bethe only author reporting the air�water surface ten-sions of deionised colloidal suspensions. His interestarose from work on colloidal crystal formation inaqueous dispersions of charged particles, which is dueto the electrostatic repulsion between particles withexpanded electrical double layers. For sphericalpolystyrene particles, the lowering of surface tensionfrom the value of the pure solvent depends on the

Ž .particle volume fraction and on particle size. AtpŽ .low particle concentration �0.005 , for which thep

suspensions are turbid and milky and termed gas- orliquid-like, the tension is not significantly reduced. At

Ž .higher concentrations �0.005 , for which the sus-ppensions form crystal-like structures including bril-

liant iridescent colours arising from Bragg diffraction,the tension falls continuously with concentration. Us-ing 12 types of latex particles of different size andcharge density, it is concluded that surface activity isonly high for surfaces between air and crystal-like

Ž .structured suspensions. At a constant high 0.02 ,pthe surface tension passes through a sharp minimumfor particle diameters of approximately 100 nm, thelowering being as much as 20 mN m�1. By contrast,the surface tension of aqueous suspensions of spheri-cal silica particles ranging in diameter from 6 to 184nm either remained constant or increased slightlywith concentration. Most of the suspensions wereeither gas or liquid-like however. For those that dis-played crystal-like structures in bulk, the tension diddecrease but only by as much as 2 mN m�1.

Colloidal crystal formation occurs when the effec-Žtive diameter of the particles equal to twice the

.Debye length plus particle diameter is equal to orgreater than the mean inter-particle distance. Theinter-particle distance in the two-dimensional crystalat the surface is likely to be less than that in bulk dueto the thinner double layers at a surface. The differ-ence in surface activity between polystyrene and silicasuspensions is attributed to the surface nature of theparticles, i.e. hydrophobic for polystyrene and polarand hydrophilic for silica, implying that the contact

Ž .angle through water is higher in the former case.Unfortunately, the excess concentration of particlesat the air�water surface could not be calculated usingthe Gibbs equation since the variation of the meanactivity of the spheres with sphere concentration,which is very sensitive to the degree of dissociation,was unknown.

Clearly, adsorption of particles to interfaces is ex-pected to be slow compared with surfactant moleculesand the above study represents a special case. It ismore common to investigate spread or insoluble par-ticle layers at interfaces using a Langmuir troughequipped with a microscope. Typically, the surface

Ž . Ž .pressure � - surface area A isotherms for particlemonolayers are determined and the aggregate mor-phology and fractal dimensions have been reported.The studies include, inter alia, the use of micron sized

� � � � �polystyrene 6 ,7 , or glass 8 particles and nanometre� � � �sized silica 9 , magnetic maghemite 10 and silver

� � �11 particles. In general, the forces responsible forthe interactions between particles in bulk are also

� �operative in particulate monolayers. Pieranski 12noted, however, that for charged particles at an inter-face between water and a medium of low dielectric

Ž .constant air or oil , electrostatic repulsion is en-hanced over that operating in the bulk aqueous phaseparticularly at large particle separations. The reasonis that the interfacial particle has an asymmetricdistribution of counterions resulting in a dipole mo-


ment normal to the water surface. Repulsive interac-tions between the dipoles due to neighbouring parti-cles occurs through the phase of low dielectric con-stant, whereas repulsion through the aqueous phase isscreened by free ions in solution. It has been showntheoretically that the interaction energy betweencharged particles near an air�water surface decaysinversely with the cube of the separation between thedipoles and is inversely proportional to the electrolyte

� �concentration 13 .The air�water surface has proved the most popular

choice of interface. Horvolgyi et al. have investigated´ ¨Žmonolayers of silanised glass spheres 75 �m in di-

. � �ameter using both a Langmuir 14 and a Wilhelmy� �8 film balance. The advantages of using relativelylarge particles are that their behaviour on the watersurface can be readily visualised and in situ contactangle determinations can be conveniently performed.By changing the extent of silylation the hydrophobic-ity of the particles can be altered easily. Using acombination of combustion gas analysis, infra-redspectroscopy, krypton adsorption and contact angle

� �measurements, Fuji et al. 15 showed that for glassŽ .beads coated with trimethylsilyl TMS groups the

air�water contact angle dependence on TMS surfacecoverage was S-shaped, changing markedly at approxi-mately 50% monolayer coverage. From the monolayer

Ž .studies using particles which were weakly ��55� ,Ž . Ž .moderately � � 72� and highly � � 90� hy-

drophobic, hexagonal packing of the particles was� �observed in all cases at high compression 14 . How-

ever, the degree of particle attachment to the surfacevaried with hydrophobicity with the most hydrophilicparticles being pushed irreversibly into the waterphase, and the most hydrophobic particles collapsingto form a whitish multilayer. Particles of intermediatehydrophobicity were most stable and further compres-sion resulted in creasing of the monolayer withoutparticle expulsion. On the assumption that the col-lapse pressure � can be equated with the energycrequired to remove particles, in unit area, out of themonolayers into the more ‘wetting’ phase, the visualobservations were consistent with the influence ofcontact angle on this energy. In a similar study, how-

� �ever 8 , there was little agreement between the con-Ž .tact angles determined from the isotherms via � c

and those measured microscopically for a series ofincreasingly hydrophobic glass spheres. The conclu-sion is that the proposed method for the determina-tion of � from collapse pressures is suspect and this isreinforced later.

In addition to the appearance of ordered arrays ofmicron-sized charged particles, the spontaneous for-mation of mesostructures or loosely bound internallyordered aggregates of particles at air�water surfaceshas been reported. In a study utilising polystyrene

Ž .latex particles 0.95 and 2.84 �m in diameter , Ghezzi� � �et al. 16 show a whole array of patterns formed in

monolayers subjected to various kinds of externalinfluence, including ionisation of the air phase, apply-ing an electric current and exposing the layer to UVlight. Depending on the perturbation applied, thelattice-like structure formed fractal aggregates, clus-ters, striations and loops. It is argued that the chargeof the particles is neutralised for a number of reasonsand that such particles are forced into the secondaryminimum of the potential energy-particle separationcurve, of approximately kT in depth. This would helpexplain the observation of the metastability ofmonolayers when mesostructures can be present im-mediately after spreading. An interesting study aimedat investigating Langmuir monolayers of magnetic

� �nanoparticles was reported by Lefebure et al. 10 .Ž .The particles were maghemite grains �-Fe O ren-2 3

dered partially hydrophobic using lauric acid, andwere monodisperse of three diameters: 7.5, 11 and15.5 nm. For the smallest particles, the isothermdisplays a zero pressure plateau followed by a rapid

Žincrease in pressure when A�A �1 where A is theo o.area per particle in an ideal close-packed lattice . For

the larger particles, the monolayers are less compress-ible and the isotherms are shifted to higher areas.Electron microscopy of deposited monolayers re-vealed that dense, circular, compact domains formfrom the smallest particles, whereas the largest parti-cles are organised in long chains and form digitatedaggregates. In this system, the attractive interactionsbetween particles are van der Waals and anisotropicmagnetic dipolar ones, both of which increase withparticle size. It is at first sight surprising then toobserve that a film made of small particles appearsmore compressible than one made of larger particles,since the latter constitute a more attractive systemand so should be more compressible. The apparentcontradiction is resolved if one considers that theisotherms reflect the behaviour of particle domainsand not of isolated particles. It turns out that theratio of the magnitude of the magnetic and van derWaals interactions is �1 for particles larger than 12nm in diameter, and when dipole�dipole interactionsare strong enough, chained and rather compressibleaggregates with aligned dipoles are predicted to form,as opposed to more compact aggregates when isotropicinteractions are dominant. The effect of an externalmagnetic field on the structure within the particlemonolayer would be an interesting extension.

The behaviour of non-spherical particles at a water� � �surface has been investigated by Brown et al. 17 .

Using the technique of photolithography, particlesincluding gold diamonds and rectangles, Nichrome�gold crosses and disks and silicon dioxide diamondswere prepared. By evaporating two different materi-


Ž .als, curved particles disks were synthesised such thatone material is on one face and the other material onthe opposite face. Such particles at an interface intro-duce a new form of capillary force that arises fromdistortions in the surface of water induced by thechemical or physical anisotropy of the particles. Forsuch disks of 5 �m diameter, the particles aggregatedto give small clusters which ‘coalesced’ with eachother upon surface compression. Eighteen percent ofthe particles had their Nichrome surface upward whilst82% had their gold surface upward. This is not sur-

Žprising given that a gold surface is hydrophobic ��. Ž .90� and Nichrome is hydrophilic ��5� . Long chains

containing over 20 particles were also observed inwhich the particles were connected along their axis ofcurvature. Such anisotropic forces in two dimensionshave not been studied previously on a colloidal lengthscale and extend the field of control of particles at

� �interfaces. In contrast, Nakahama et al. 18 havedeveloped an alternative novel way of preparing parti-cles possessing surface regions of differing wettabili-ties by first forming a particle monolayer on waterand then spreading a reactive polymer material on thesurface during particle transfer to a substrate. Thepart of the particle attached to the solid remainsuncoated whilst that exposed to the polymer duringtransfer becomes coated. In a theoretical study, Binks

� �and Fletcher 19 considered the difference in theadsorption of a spherical particle to an interfacebetween particles possessing a homogeneous surfaceof uniform wettability and so-called Janus particles.For Janus particles, calculations are presented whichshow how the particle amphiphilicity, tuned by varia-tion of either the relative surface areas or the differ-ent wettabilities of the two surface regions on theparticles, influences the strength of particle attach-ment. Increasing the amphiphilicity of the particlesproduces a maximum of a 3-fold increase in surfaceactivity compared with homogeneously coated parti-cles for average contact angles around 90�. In addi-tion, it was shown that, unlike particles of homoge-neous surface coating, Janus particles remain stronglysurface active for average contact angles approachingeither 0 or 180�.

The structure and evolution of monolayers ofmonodisperse silica particles of 2 �m in diameter atvarious liquid�vapour interfaces has been reported by

� �Hansen et al. 20,21 . In the first study, the particleswere alkoxylated using octanol or octadecanol andtheir behaviour at the toluene�air surface was visu-alised using video-enhanced microscopy and imageanalysis. The choice of the non-aqueous liquidminimises the influence of electrostatic phenomena,and the alkyl coating permits the short range particleinteractions to be varied in two ways; firstly by varyingthe range of the steric repulsion by means of the

thickness of the coating, and secondly by controllingthe magnitude of the van der Waals interactionthrough changes in the contact angle. Immediatelyafter spreading, the particles formed a 2-D film withsubstantial long range order with a tendency towardshexagonal packing. With time the particles begin todiffuse over larger distances and clusters start toform. These grow in size until eventually a largeparticle network is formed. The octyl system displayed

Ž .a diffusion limited cluster aggregation DLCA -likestructure of fractal dimension D �1.45, whereas thefoctadecyl system realised a more dense structure withD �1.55. This difference is consistent with the mag-fnitude of the attractive energy at contact of �30 kTfor the thinner coating and �15 kT for the thicker

� �one 20 . The temporal evolution of the cluster massdistribution displayed a transition between a regimeóf slower DLCA to a faster convection limited cluster

� �aggregation. In a subsequent study 21 , the silicaparticles were reacted with either octyl- or octadecyl-trichlorosilane and investigated at the surfaces ofeither benzene, toluene, 1,4-dioxane, 2-meth-oxyethanol, 1,2-dichloroethane or water. As a resultof the different contact angles for the different liq-uids, it was found that the extent of the hexagonally

Ž .ordered particle domains or packing density de-creased exponentially with an increase in the attrac-tive energy between the particles at contact, drivenmainly by an increase in the contact angle measuredinto the liquid.

2.2. Liquid�liquid interface

Monolayers of charged polystyrene latex particlesŽ .2.6 �m in diameter have also been formed atalkane�water interfaces using a modified trough de-

� � �sign 6 ,7 , and provide new features. When an alkaneis layered onto a particle layer spread initially at theair�water surface, the original disordered monolayerrapidly becomes a highly ordered hexagonal array atthe oil�water interface. The monolayer retains a highdegree of order even in the presence of 0.1 M NaCl inthe aqueous phase, which is sufficient to cause con-siderable particle aggregation at the air�water sur-face. The finding points to the possibility that thereare strong long-range repulsive forces operating overseveral microns at the oil�water interface which areabsent at the air�water surface. An example of suchan ordered monolayer is given in Fig. 3. The ��Aisotherm at the oil�water interface is unchanged bythe addition of salt to the water phase, as is themonolayer structure. It is concluded that the repul-sion acts through the oil phase, since the presence ofelectrolyte is expected to shield electrostatic repulsionin the aqueous phase. The enhanced lateral repulsionbetween particles at the oil�water interface is at-


Ž .Fig. 3. Optical microscopy image from a laser tweezer instrumentof an ordered monolayer of charged polystyrene latex particles ofdiameter 2.6 �m at a planar decane�undecane�water interface at293 K. The centre-to-centre separation is 5.8 �m. Obtained during

� � �work described by Aveyard et al. 22 .

tributed to the existence of residual surface charges atthe particle�oil surface. Such charge could easily ariseas a result of the presence of trapped or hydrationwater between particles and oil. Using a model whichdescribes the electrostatic interaction between adsor-

Žbed particles and the effect of image forces acting.across the oil�water interface , the calculated surface

charge required to account for the observed repulsionis approximately 1% of the total possible chargeŽ .number of sulfate groups on the particle surface� � �6 . A very recent experiment using a laser tweezerinstrument has succeeded in determining the repul-sive force as a function of separation between twoidentical latex particles at a non-polar oil�water in-

� � � Žterface 22 . At large separations 6�12 �m between.particle centres the force is found to decay with

distance to the power minus 4, consistent with theasymptotic form expected for a charge-dipole interac-tion.

Even at large separation between particle surfacesŽ .low surface pressures hexagonal packing is created

� �in structured monolayers 7 . Upon compression, themonolayer structure becomes distorted to give arhombohedral array, probably because the compres-sion is anisotropic. Further compression beyond � cleads first to a gentle folding of the monolayer and, athigh compression, the monolayer becomes corrugatedof wavelength �100 �m and amplitude �30 �m.

ŽImportantly, particle expulsion either singly or as.aggregates is not observed experimentally. The study

by Aveyard et al. also focused on the relationshipbetween the monolayer collapse pressure and theinterfacial tension in the absence of a particlemonolayer. The latter was varied in the range 50�4mN m�1 by addition of surfactants, to the aqueousphase, over a range of concentration. Within error,� is equal to the bare tension � in all cases. Thec ow

correspondence is explained by the fact that the sur-face pressure exerted by the repulsive particles tendsto cause expansion of the monolayer-covered inter-face, whereas the oil�water tension of the interfacebetween the particles tends to cause contraction. Atcollapse the two opposing effects become equal. Theobserved behaviour of micron-sized particles is similarto that found by molecular dynamics simulations for

� �nanoparticulates 23,24 , suggesting that the repulsionand buckling may be general features of particle

� �arrays 25 .The surface behaviour of silver colloids located at

the interface between water and dichloromethanerepresents a study bridging the gap between nano-

� � �and micron-sized particle films 11 . The particles areprepared in situ at the oil�water interface and thefilms consist of ‘soft’ flocs of silver nanoparticles ap-proximately 1 �m in size. Both ��A isotherms andUV-visible absorption-reflection spectra were ac-quired simultaneously. Upon compression, themonolayer, which appears smooth and shiny at largeareas, suddenly loses its metallic luster and developscreases parallel to the moving barrier. Further com-pression beyond collapse results in the creases be-coming more pronounced with the distance betweenthem remaining constant. As before, the bucklingoccurs at surface pressures close to the bare interfa-

Ž �1 .cial tension 23 mN m and is reversible. AnalysisŽof the spectra yields the silver particle radius 1.06

.nm in the flocs in agreement with their true size.Identical buckling of the interfacial film is alsoobserved in beakers by simply dispersing excess col-loid at the oil�water interface. Patches of parallelstraight folding lines are visible with exactly the samecrease spacing as that in the trough. Using the model

� �of Milner et al. 26 for the buckling of two-dimen-sional systems at zero interfacial tension, the bendingconstant of the monolayer can be derived. Thevalue for the silver films is of the order of 5�104 kT,which is much larger than that of monolayers of

Ž � �.soluble surfactant molecules typically kT, 27 . It isworth pointing out, however, that values for monolay-

Ž .ers of insoluble surfactant molecules behenic acidon water can be as high as 200 kT at high surface

� �pressure 28 . The difference probably arises from themuch more condensed nature of an insolublemonolayer compared with that of a soluble surfactant.Monolayers of nanoparticles of silica, coated to dif-ferent extents with dichlorodimethylsilane, formed at

� �the octane�water interface also fold above � 9 . Itcis found that very hydrophilic or very hydrophobicparticles are lost from the monolayer upon compres-sion, presumably to the aqueous and oil phases, re-spectively. Particles of intermediate hydrophobicityremain in the surface.

For particles or surfactant molecules at an inter-


face, the Volmer equation of state which takes intoaccount the finite size of a particle but assumes alllateral interactions are absent reads

Ž . Ž .� A�A �kT 2o

Ž 2 .where A is the co-area of a particle taken as roand k is Boltzmann’s constant. Rearranging the equa-

Ž .tion and plotting � vs. reduced area A�A , Fig. 4aoŽ .depicts the effect of particle radius affecting A ono

Žthe isotherms. Only for relatively large particles 10.nm in radius, lowest curve should the isotherms

exhibit virtually zero pressure until hard-sphere be-haviour sets in at A�A equal to unity. This hasorarely been observed experimentally. Upon decreas-ing the particle radius, the curves become more ex-panded and shift to higher areas. The shape for aparticle of radius 0.4 nm is typical of that measuredfor small surfactant molecules like myristic acid. It isclear from this that in order to reproduce the kinds ofisotherms measured for large particles, an alternativeequation of state is required. When lateral interac-tions within the monolayer become significant, thetwo-dimensional equivalent of the van der Waalsequation can be taken

� Ž . Ž .�� A�A �kT 3o2ž /A

where � is a two-dimensional van der Waals constantwhich allows for lateral interactions. Positive values of� signify attraction whereas negative values signifyrepulsion within the monolayers. The curves in Fig. 4bfor a particle of 1 nm in radius move to higher areasand become more expanded as the magnitude of therepulsive term is increased; the shape is reminiscentof that observed experimentally for repulsive particles

� � � �within monolayers 6 ,11 ,14 .

2.3. Determination of contact angles of particles withinterfaces

As will be apparent working with spherical particlesat interfaces, a knowledge of the contact angle theymake with the interface and how different variablesalter it is essential in interpreting some of the pheno-mena. In addition to direct horizontal microscopic

Žobservation when the particles are large enough �2.�m , three methods for determining � have become

available but have their own drawbacks and deficien-� �cies. The first suggested by Clint and Taylor 29

makes use of Langmuir trough studies of particlemonolayers at liquid surfaces. By equating � withc

Žthe work required to remove the particles in a unit

Ž .Fig. 4. a Calculated surface pressure-reduced area per particleisotherms at 298 K following the Volmer equation of state for

Ž .different particle radii. The radii in nm from top to bottom areŽ .0.25, 0.4, 1, 2 and 10. b Calculated surface pressure-reduced area

per particle isotherms at 298 K following the equivalent van derWaals equation of state for a particle of 1 nm in radius. The values

Ž 2 �36 .of the constant � in J m �10 from top to bottom are �1,�0.5, �0.2, 0 and �0.05.

.area from a hexagonal close-packed array in theinterface the contact angle is given by

'� 2 3c Ž .cos�� 1 4( �ž /��

where the choice of sign depends on whether particlesare squeezed out of the interface into phase � orphase �. The method was exemplified using hy-drophobic calcium carbonate particles 5 nm in diame-ter spread on a pure water surface. Subsequently,good agreement was shown between contact anglesdetermined this way and those measured using the


sessile drop technique for surfactant solutions onplanar substrates, in the case of hydrophobic silica

� �particles of 2.5 �m in diameter 30 . In this case,particle expulsion was confirmed by the appearance of

Ž .either particles sinking through water low � orŽbuilding up as a thick multilayer on the surface high

.� . The method requires that particles are monodis-perse and spherical and must be small enough not tocause curving of the interface close to the contact linewith them. However, as seen above for polystyrene

� �particles 7 , it appears that the assumption of particleexpulsion from the surface at collapse is inappropri-ate. The trough method, therefore, should not beused indiscriminately for the determination of contactangles.

An alternative approach developed by Butt and� � �colleagues 31,32 involves attaching a single particle

to the cantilever of an atomic force microscope. Thecantilever is then moved gradually in an aqueous

Ž .medium towards an air bubble until contact , wheredeflection of the cantilever and position of the bubbleare measured. From the force vs. position data thecontact angle can be calculated. The initial studyinvolved measurement of the force between a particleŽ4.4 �m diameter silica coated to different extents

.with gold�alkylthiol monolayers and an air bubble inwater, from which the receding air�water contact

Ž .angle 30�90� was calculated. A systematic differencebetween angles measured using particles and on pla-nar surfaces was observed and discussed in terms ofthe line tension acting at the three phase contact line.If the influence of line tension is responsible for thedifference, its magnitude increased with the hy-drophobicity of the particle. For ��90� the linetension ranges from �2 to 8�10�8 N. A secondstudy included determining both the advancingair�water contact angle by approaching a particletowards a water drop in air, and the oil�water contactangle by bringing a particle near a water drop in

� � �hexadecane 32 . Micron-sized polystyrene particlesof three diameters were employed and hysteresis incontact angle was noted at the air�water surface. Theangles decreased with increasing particle diameterfrom which a line tension in this system of �0.3�10�6 N was derived. This negative value for angles�90� means that the true angle is less than that inthe presence of line tension, i.e. the particle behavesmore hydrophobically as a result. For an 8.76-�mdiameter latex particle, the air�water angle was 75�compared with that at the oil�water interface of 145�.The method is obviously unsuitable for sub-micronparticles whose size is below the limit of resolution ofan optical microscope. A completely different methodsuitable for determining the contact angles ofnanoparticles with interfaces uses heat flow immer-

� �sion microcalorimetry 33 . The enthalpy of immer-

sion is defined as the energy released or absorbedwhen a clean solid is immersed into a liquid anddepends on the particle wettability and the liquidsurface tension. Recent developments in the design ofmicrocalorimeters have made it possible to measurevery small changes in thermal energy for low energysurfaces with reasonable accuracy. For fumed silica

Žpowders of different hydrophobicity primary particle.diameter�12 nm , the enthalpies of mixing in sepa-

rate particle�water�air systems and particle�toluene�air systems yielded the contact angles at the air�waterand air�oil interfaces, respectively. Using thermody-namic arguments, the contact angle at the oil�waterinterface was calculated using both aforementionedangles and measured values of the partition coeffi-cient of particles between oil and water. These angleswere in the range 0�96� for a series of six silicasamples.

3. Partitioning of particles between phases

The rules are reasonably well established now fordescribing the partitioning of both monomeric andaggregated surfactant molecules between oil andwater phases. Thus, most ionic surfactants partitionexclusively in favour of water as monomers whereasnonionic surfactant molecules containing ethylenoxygroups partition into both phases as monomer, withthe distribution normally heavily in favour of the oilphase. Micelles or microemulsion droplets occur inonly one of the phases at any overall composition butchanges in temperature, electrolyte concentration orconcentration of cosurfactant can induce completetransfer of aggregates from one phase to the otheraccompanied by a change in type, e.g. from oil dropletsin water to water droplets in oil. The behaviour ofcolloidal particles and macromolecules in two-phaseimmiscible liquid mixtures is a topic of long terminterest and of considerable practical importance,

� �particularly for separating biological cells 34 . Due tothe small size of the particles present in the liquidmixture, Brownian motion is efficient in re-distrib-uting the particles to an equilibrium configurationwith the lowest total free energy. Consequently, therecan be a partition of the majority of particles into apreferred phase where the particle�liquid interactionis most favourable, or alternatively, onto the interfacebetween the two phases where, due to the reductionin the free liquid�liquid area, the total surface energyis effectively reduced. Simple thermodynamic modelsbased on arguments of this kind have been developedand extended to include effects of gravity, particleshape and particular features of the particle�liquid

� �interaction 35 .The free energy change on transferring a small


spherical particle from water to oil is simply given bythe product of the particle surface area and theinterfacial energy difference for the particle in both

2Ž .phases, i.e. 4 r � �� where s, o and w denoteso swsolid, oil and water, respectively. The term in bracketsis related to the contact angle � and the tension ofthe oil�water interface � through Young’s equationow

Ž .� �� cos� 5so sw ow

If the entropies of mixing of particles with waterand oil are neglected, the free energy of transfer isthen 4 r 2 � cos�. This is equivalent to �kT ln K,ow

Žwhere K is the partition coefficient defined as par-. Žticle concentration in oil � particle concentration in

.water . From this, the fraction of the total number ofparticles present in the water phase, f , can be writ-w

Ž .ten as 1�1�K . Fig. 5 shows the calculated variationof f with contact angle for particles of radius 10 nmwpartitioned between water and oil for two values of

�1 Ž .� . For a tension of 36 mN m toluene�water , theowdistribution changes in a stepwise manner at a contactangle of 90�, i.e. hydrophilic particles are partitionedmore or less exclusively to water and hydrophobicparticles are partitioned exclusively to oil. For low

Ž �1 .tension interfaces 0.05 mN m such as those in thepresence of certain surfactants, it can be seen that thedistribution of particles changes in a more gradualmanner with �. For larger particles, partitioning typi-cally takes place between one of the phases and theinterface and the treatment above becomes morecomplicated.

3.1. Partially miscible liquids

The partitioning of polystyrene latex particlesbetween critical mixtures of water and 2,6-lutidineŽ .dimethylpyridine has been investigated at tempera-

Fig. 5. Fraction of particles present in water in two-phase systemscontaining oil as a function of the contact angle the particles make

Ž .with the interface measured through water . The particle radius isŽ . Ž .10 nm and the curves are for � �36 full and 0.05 dashed mNow

m�1 at 298 K.

tures where the liquid mixture separates into twoimmiscible phases, and is closely linked to wetting

� �� phenomena 36 . This system has the advantage ofexhibiting very low interfacial tensions just above the

Ž .critical temperature, T 33.8 �C . Lutidine and watercmixtures display a lower consolute temperature curveabove which two phases form. Using different latexparticles originally dispersed in water and varying

Ž .separately in diameter 0.4�1.5 �m and in surfaceŽ �2 .charge density, � 0.4�5.8 �C cm , three types of

behaviour are observed for critical mixtures. For tem-peratures approximately 0.1 K above T partitioningcof the majority of particles into one of the two bulkphases occurred, as detected by light scattering, inwhich the interfaces were ‘clean’ and devoid of parti-cles. The preferred or wetting phase was sensitive tothe surface charge density of the particles and inde-pendent of their size or concentration. Particles ofhigh � preferred the water-rich phase, whereas thoseof low � preferred the phase rich in lutidine. Atsufficiently high overall particle concentrations, theparticle concentration in both phases was determinedreasonably accurately yielding the partition coefficientK. For particles of 0.55 �m diameter, K is 2300 infavour of the preferred aqueous phase. It is expectedthat K approaches unity at the critical temperaturewhere the two phases are identical.

In addition to the partitioning of particles betweenbulk phases, a striking temperature dependence ofthe concentration of particles at the liquid�liquidinterface was seen which is reversible. Beginningabruptly at a temperature called the particle wettingtemperature, T , the onset of preferential adsorptionwof particles at the interface occurs. The interfacebecomes increasingly cloudy and of non-uniform re-flectivity. The adsorption of particles at the interfaceincreases between 0.1 and 0.5 K above T , until theconset of the third temperature regime with the ap-´pearance of highly concentrated islands of particles atthe interface. As the temperature increases furtherfrom T , these islands grow in size until all of thecavailable interfacial area is occupied. By extendingthe model describing the behaviour of particles in animmiscible liquid mixture to include the temperature

Ž .dependence via K , the crossover from partition intoa bulk phase near T to preferential adsorption at thecinterface further from T is predicted, in excellentcqualitative agreement with experiment. Diffraction oflaser light from the concentrated particle layer at theinterface indicated crystalline order which extendedover 60 particle diameters, probably arising from di-rect Coulomb interactions between neighbouring par-ticles at spacings less than the Debye length. Thesefindings are no doubt linked to the effect of tempera-ture on the interfacial tension influencing the energyof attachment of particles at the interface. The ten-


Ž �1 .sion is very low close to T 0.001 mN m wherecdetachment is easy but increases to 0.017 mN m�1 atT �0.5 K allowing stronger attachment. Very inter-cesting kinetic effects were also mentioned includingthe formation of metastable emulsion films. Theyappeared at temperatures where the population ofthe interface was high and phase separation was slow.During separation, particles find the many interfacespresent and the rate of drop coalescence is reduced asa result. Emulsion films of lutidine�water�lutidine, inwhich the water layers contained high concentrationsof particles, were embedded in the upper lutidine-richphase and remained stable for several days beforefinally rupturing. Interestingly, the opposite type offilm was rarely observed presumably because of itsinherent instability.

3.2. Immiscible liquids

In addition to determining the partitioning of parti-cles between phases it is also important to be able toeffect transfer of particles from one phase to another.Developing means for directing nanoparticles intospecific environments is an essential pre-requisite fortheir arrangement into micro-assemblies for produc-ing novel materials including electronic and electro-optical devices. Two papers describe methods fortransferring either gold or silver colloidal particlesfrom water to oil phases, retaining their integrity and

� � Ž .state of dispersion. In one 37 , an o�w emulsion 1:1is prepared from an aqueous dispersion of gold parti-

Ž .cles in water diameter�14 nm and an alkane phasecontaining a low concentration of sodium oleate sur-factant. Upon addition of aqueous electrolyte withstirring, breaking of the emulsion occurs leaving anupper layer coloured red being the gold dispersion inoil and a lower colourless aqueous layer. In theabsence of salt, separation also occurred but withoutparticle transfer. Divalent electrolytes like MgCl were2

Ž .more effective than monovalent ones NaCl in pro-moting transfer. The authors suggested that salt addi-

Ž .tion causes the formation of say magnesium oleatewhich adsorbs onto the surfaces of the gold particlesvia its C�C bond. This in turn renders the particlesmore hydrophobic so that they prefer oil rather thanwater. An alternative explanation is that the initiallyhydrophilic particles help stabilise the o�w emulsionbut a change in their wettability due to oleate adsorp-tion causes the preferred emulsion to be w�o withinstability to coalescence ensuing during inversion. In

� �the second study 38 , monodisperse nanometre-sizedsilver particles were prepared in water in the presenceof sodium oleate. Transfer of the colloid to an organicphase is induced by addition of a low concentration oforthophosphoric acid or sodium perchlorate with anefficiency of �60%. The non-aqueous colloids are

stable and, after solvent evaporation, the particles canbe re-dispersed in a variety of other solvents. On thebasis of IR spectroscopy measurements, it is con-cluded that the oleate chain adsorbs onto the parti-cles via the C�C bond leaving the dissociated car-

Ž .boxylate group interacting with water hydrosol ,whereas following rearrangement the carboxylategroup serves to anchor the surfactant to the surface inthe organosol such that the hydrophobic chain isdirected towards the oil. The importance of a doublebond within the molecule is emphasised since use ofother surfactants like sodium stearate or sodium do-decyl sulfate does not result in particle transfer to theoil phase. It remains, however, to elucidate the role of

Ž .the transfer agent H PO or NaH PO in this3 4 2 4process.

Very few studies exist in which the partitioning ofparticles between completely immiscible liquids hasbeen determined in the absence of surfactant. Yan

� � �and Masliyah 39 measured in detail the partition-ing of a range of suitably treated kaolinite clay parti-

Ž .cles diameter 0.2 �m between drop surfaces in ano�w emulsion stabilised by them and the continuousaqueous phase. They found that the ratio of theparticle concentration at oil drop interfaces to that inbulk water depended strongly on both the clay con-centration in water and the contact angle of particlesat the oil�water interface, and related this to theinfluence of � on the strength of attachment of parti-cles. An alternative way to effect migration of parti-cles from one phase to another is to change in situ thehydrophobicity and hence wettability of the particles.This can be achieved, say, by an increase in aqueousphase pH in systems containing particles possessingionisable surface groups. It was known for some timethat a chargeable surface is maximally hydrophobic at

� �conditions near its point of zero charge 40 . Twoquite different studies have demonstrated that thisline of thought appears reasonable. The first by Henry

� �et al. 41 is linked to what happens during the re-moval of particles from hydrocarbon suspensions whenwater is added. Removing mineral matter from coal-derived liquids can often be a limiting process, andaddition of water to a solid�liquid suspension pro-moting separation of the dispersed particulate phaseis more economic than conventional processes such asfiltration or centrifugation. Typically, a surfactant isadded to the particle�oil slurry and water is thendispersed into the oil phase. Electrically-inducedcoalescence, used to break the resulting w�o emul-sion, separates the aqueous phase containing the min-eral matter from the mineral depleted oil phase. Theprocess is analogous in many ways to that of frothflotation, except that particles collect at oil�waterinterfaces as opposed to air bubble�water surfaces.Many of the minerals present in coal-derived liquids


Ž .are originally hydrophilic from clays, rocks, soils , butthese particles obtain a hydrophobic coating duringthe coal liquefaction process. Experiments were con-ducted in which the coal-derived liquid containing

Ž .such particles 1�10 �m was equilibrated with waterof different pH values in a simple system in theabsence of surfactant. Using gravimetric analysis, thepercentage loss of particles from oil to water wasdetermined, alongwith measurement of the contactangle of a water drop under oil on a compressed diskof the coated mineral particles. A very nice correla-tion between the transfer of particles from oil towater and a decrease in � from 120 to 30� was seenowas pH was raised from 3 to 10.3. Thus, particlesinitially hydrophobic at low pH are rendered increas-ingly hydrophilic on adding base and subsequentlypartition in favour of the aqueous phase. The likelycause of this effect is the ionisation with increasingpH of the zwitterionic asphaltene molecules adsorbedonto the mineral particles.

� �� In a similar vain, Binks and Lumsdon 42 showedhow changes in pH affect the partitioning of nanome-tre-sized, partially hydrophobic silica particles between

Ž .toluene and water. At low pH �9 , virtually all theparticles distributed in favour of oil, whereas at suf-

Ž .ficiently high pH �12.5 they distributed in favour ofwater. At intermediate pH, particles were present inboth oil and water in addition to a third phase formedat the interface. The particles, possessing residual

Ž .silanol SiOH groups on their surface, become in-Ž �.creasingly charged through dissociation SiO in-

creasing their wettability by water. The findings al-lowed the first verification of Bancroft’s rule in solid-stabilised emulsions prepared from equilibrium multi-phase systems. As predicted, emulsions were w�o atlow pH and phase inverted to o�w at high pH. As inmost surfactant-stabilised emulsions, the continuousphase of the emulsion is the one containing thehighest concentration of particles.

4. Emulsions stabilised by particles

The fact that finely divided solid particles can act asthe stabiliser in emulsions has been known since thebeginning of the last century. The credit is usually

� �given to Pickering 43 , hence the term ‘Pickeringemulsions’, who noted that particles which were wet-ted more by water than by oil acted as emulsifiers foro�w emulsions by residing at the interface. However,in a paper 4 years earlier and cited by Pickering,

� �Ramsden 44 described the formation of a membraneof solid particles enveloping both air bubbles in waterand oil drops in water, giving rise sometimes to ‘per-sistently deformed sharply angular and grotesqueshapes of the emulsified globules’! It was not until the

� �work of Finkle et al. 45 that the relationship betweenŽ .the type of solid and emulsion type o�w or w�o was

recognised. They stated that in an emulsion contain-ing solid particles, one of the liquids will probably wetthe solid more than the other liquid, with the morepoorly wetting liquid becoming the dispersed phase.The importance of the wettability of the particles atthe oil�water interface, quantified by the contact an-gle � that the particle makes with it, was thereforenoted. Thus, water-wet particles, e.g. silica, shouldstabilise o�w emulsions and oil-wet particles, e.g.carbon black, should stabilise w�o emulsions. Theseideas were given strong support by the experiments of

� �Schulman and Leja 46 using barium sulfatecrystals�powders and surfactant, measuring the rele-vant contact angles and determining the emulsion

Žtype and stability. For conditions such that � mea-.sured through the aqueous phase was slightly �90�,

particles stabilised o�w emulsions, but for ��90�particles were still held at the interface but nowstabilised w�o emulsions. However, if the particles

Ž .were either too hydrophilic low � or too hy-Ž .drophobic high � they tended to remain dispersed in

either the aqueous or oil phase respectively, givingrise to very unstable emulsions.

4.1. Simple emulsions

It is surprising that, since the early studies men-tioned above, little work in this area appeared in theopen literature until quite recently. Clues and under-standing can be gained from the food science areahowever, since it has been known for some time that

Ž .in many food emulsions and foams stabilised primar-ily by phospholipids or proteins, particles are neces-sary for the required stabilisation, e.g. ice crystals inice cream and fat particles in whipping cream. Theprecise role of the particles is gradually being unrav-

� � �elled and a recent review by Rousseau 47 discussesthe effect of fat crystal wettability and microstructureon emulsion stability, interfacial rheology and particlelocation in food systems. Further, the importance offat crystal concentration on the stability of water-in-triglyceride emulsions stabilised by monoglycerides

� �was emphasised by Johansson et al. 48 . At lowconcentrations, flocculation and eventual coalescenceof the water drops were induced, whilst at higherconcentrations both were inhibited. The results wereexplained by a crossover in the mode of action of thecrystals, in which they behaved as bridging particles

Ž .between drops when dilute destabilisation butformed a protective layer around drops when concen-

Ž .trated stabilisation . A wide variety of solid particleshas been used as stabilisers of either o�w or w�oemulsions including iron oxide, hydroxides, metal sul-fates, silica, clays and carbon. The effectiveness of the


solid in stabilising emulsions depends, inter alia, onparticle size, particle shape, particle concentration,particle wettability and on the interactions betweenparticles. Particle wettability may be altered by ad-sorption of suitable surfactants, in some cases leadingto emulsion phase inversion. Several studies concludethat stable emulsions can only be formed if the parti-cles are weakly flocculated to some extent, achieved

� �by addition of salt for o�w 49 or by surfactant for� �w�o emulsions 50 . Much less stable emulsions re-

sulted when the particles were completely flocculated.Depending on the exact system, there are at least

two mechanisms by which colloidal particles stabiliseemulsions. In the first, the particles are required toadsorb at the oil�water interface and remain there

Ž .forming a dense film monolayer or multilayer aroundthe dispersed drops impeding coalescence. In the sec-ond, additional stabilisation arises when theparticle�particle interactions are such that a three-di-mensional network of particles develops in the contin-uous phase surrounding the drops. This has beeninvoked particularly in clay-containing systems inwhich the oil drops become captured and more or lessimmobilised in the array of clay platelets in water� � �51 ,52 . The enhanced viscosity of the continuousphase reduces the rate and extent of creaming.Amongst the teams involved in this field, Tambe and

� � � � �Sharma 53 , Masliyah and colleagues 39 , Zhai and� � � � �Efrima 54 , Midmore 55 ,56 and Lagaly and col-� � �leagues 51 ,52 have made particularly important

contributions recently to the understanding of parti-cle-stabilised emulsions. Even millimetre-sized drops,coated by particles, are shown to be extremely stable

� �to coalescence 54 �a situation which has never beenrealised in the case of surfactant-stabilised emulsions.Such large drops are seen to roll over each otherwithout deformation in just the same way that truesolid spheres behave. The potential for exploitingsuch stability at these length scales is high.

In a recent series of papers, Binks and colleagueshave systematically investigated the formation, stabil-ity and structure of emulsions stabilised entirely bysolid particles in an attempt to elucidate unambigu-ously the role of the particles in such systems� �� 42 ,49,57�64 . Such surfactant-free emulsionsrepresent a novel alternative to conventional onesand may prove to be more advantageous too. It issignificant that at least two patents, assigned toBeiersdorf, have been published recently describing

� � � �the properties of both stable o�w 65 and w�o 66emulsions using particles alone for cosmetic and der-matological applications. In the work of Binks and

� �� colleagues 42 and 57�64 , the particle typeswere silica, Laponite clay and polystyrene latex, and arange of oils were used including alkanes, silicones,alcohols and esters. Simple emulsions of both types

are possible but their stability is crucially dependenton the wettability of the particles in situ at theoil�water interface. A major objective of part of thework was to investigate and understand the relation-ship, if any, between the properties of the particledispersions before emulsification and the ensuingemulsion characteristics. A new method for preparingmonodisperse solid-stabilised emulsions has also beendescribed involving the sequential replacement of sur-factant by particles at drop interfaces, using dialysis� �63 . Such emulsions are more amenable to study andtreat using theoretical models than the more commonpolydisperse ones.

For emulsions stabilised by silica particles, the ef-fect of particle hydrophobicity has been explored sincea series of particles of the same size distribution butincreasing coating extent was synthesised specially.Although the primary particle diameter varies between10 and 30 nm, silica particles easily aggregate or fusecompletely forming larger structures of non-sphericalshape. The hydrophobicity of the particles is de-

Ž .scribed in terms of the percentage of silanol SiOHgroups on their surface. Thus, raw hydrophilic silicapossesses 100% SiOH and the most hydrophobic has14% SiOH. Particles are rendered hydrophobic byreaction of native silica with dichlorodimethylsilane inthe vapour phase. As a result, the coating is assumedto be homogeneous over the particle surface. More

Ž .hydrophilic particles �65% SiOH can be dispersedin water, whereas more hydrophobic ones are easilydispersed in oil. Preferred emulsions, i.e. those formedat an oil�water volume ratio of unity, are o�w for thehydrophilic silicas and w�o for the hydrophobic ones� � � � �57 , in line with previous findings 67 . However, inone and the same system, emulsions of both types canbe prepared depending on the oil:water ratio. In theexample given in Fig. 6a, emulsions are of low con-

Ž .ductivity and disperse in oil at low w�o butwbecome highly conducting dispersing in water at high

Ž . o�w . Importantly, for particles of intermediatewhydrophobicity, both emulsion types are stable tocoalescence for periods in excess of 3 years, despitebeing of very different average drop diameter, namely1 and 100 �m. The ability to make both o�w and w�oemulsions with the same kind of particles represents asignificant advantage compared with single surfactantsystems which do not invert at extremes of volumefraction but which form gel emulsions instead. Thecatastrophic inversion brought about by changing the

Žoil:water ratio occurs without hysteresis increasing or.decreasing in the particle case, in contrast tow

surfactant systems where the hysteresis can be asmuch as 0.3 in volume fraction. In addition, emulsions

� � �are most stable at conditions near inversion 57 ,unlike those of surfactants which are notoriously veryunstable. The different behaviour is linked to the very


Ž .Fig. 6. a Conductivity of water�toluene emulsions stabilised byŽ .hydrophobic silica particles 50% SiOH as a function of the

volume fraction of water at 298 K. Oil contained 2 wt.% particles;Ž .water added sequentially to oil open points or oil added sequen-

Ž .tially to water filled points . Results from drop tests includedŽ � � �. Ž .Re-drawn from earlier work 57 . b Conductivity of

Ž .water�toluene emulsions �0.5 containing 2.5 wt.% total silicawparticles as a function of the weight fraction of hydrophobic parti-

Ž .cles 50% SiOH, initially in oil at 298 K. Hydrophilic particlesŽ .100% SiOH are initially in water. Results from drop tests in-

Ž � �.cluded redrawn from earlier work 58 .

high energies of attachment of particles to interfaces.It is found that the value of at inversion increaseswprogressively with particle hydrophobicity, varyingfrom 0.35 to over 0.8. However, it remains to under-stand why both types of emulsion occur in systemscontaining the same three components.

Another type of emulsion phase transformation,known as transitional inversion, is brought about insurfactant systems by changing the system HLB insome way at fixed oil:water ratio, e.g. by increasingthe concentration of hydrophobic co-surfactant inmonolayers of hydrophilic surfactant. Using mixturesof particles of different hydrophobicity, one of whichprefers o�w emulsions and the other w�o emulsions,

offers the possibility of effecting this inversion bychanging the average wettability of the particles at thedrop interfaces. Three ways of achieving this are bythe addition of hydrophilic silica to w�o emulsionsstabilised by hydrophobic silica, by addition of hy-drophobic silica to o�w emulsions of hydrophilic silicaand by varying the weight fraction of one of the

� �particle types at constant particle concentration 58 .An example demonstrating this principle is seen inFig. 6b in which emulsions invert from o�w to w�oupon increasing the weight fraction of hydrophobicsilica. As the fraction of hydrophobic silica particlesincreases in the mixed particle monolayers, the aver-age wettability of the particles changes from mainlywater wettable to mainly oil wettable accompanyinginversion.

In a detailed study into the effect of particle hy-drophobicity on the type and stability of toluene-con-

� �� taining emulsions 59 , it was shown that predic-tions based on considerations of the energy of attach-ment of a single particle to the oil�water interface� Ž .�Eq. 1 relate directly to the stability of emulsions.Emulsions stabilised by very hydrophilic or very hy-

Ž .drophobic particles contain large drops 100 �mand are unstable to coalescence, presumably as aresult of their facile displacement from interfacesduring drop collisions. Those with particles of inter-mediate hydrophobicity, in which particles are heldstrongly at interfaces, contain sub-micron sized dropsand are stable to coalescence indefinitely. Unlikecoalescence that either occurs or does not within theparticle series, the stability to gravity-induced separa-

Ž .tion creaming for o�w and sedimentation for w�opasses through a sharp maximum upon increasing theparticle hydrophobicity. In line with this, the averageemulsion drop size passes through a minimum. Thiscorrespondence is shown to be universal and indepen-dent of both and the type of emulsion formed.w

The effect of oil type on emulsions stabilised byŽ .partially hydrophobic silica particles 67% SiOH was

combined with measurements of the contact angles ofwater drops under oil phases on hydrophobised glass

� �� plates 42 . Since the energy of attachment of parti-cles anchored at interfaces depends on both � and theoil�water tension, � , the influence of both parame-owters on emulsion behaviour could be assessed. Theoils ranged from non-polar hydrocarbons of relativelyhigh � to polar alcohols and esters of relatively lowow� . It turns out that particles are more hydrophobicowŽ .higher � at polar oil�water interfaces preferring

Ž .w�o emulsions, and are more hydrophilic lower � atnon-polar oil�water interfaces with preferred emul-sion type being o�w. The combination of a low ten-

Ž �1 . Ž .sion 9.5 mN m and high contact angle 160� inthe case of undecanol as oil leads to an unusualphenomenon in which w�o emulsions destabilise


Ž . Ž .Fig. 7. Calculated ordinate and experimental abscissa contactangles of water drops under different oils on a partially hy-drophobised glass substrate at 298 K. The diagonal line represents

Žagreement between theory and experiment redrawn from earlier� � �.work 62 .

completely into the parent liquid phases with time.The breaking of the emulsion is such that it contractsvia coalescence in a way that maintains the shape ofthe vessel. A recent theoretical treatment has been

� � �developed to shed light on these findings 62 . Itinvolves using surface energy considerations to inter-pret the interactions between the solid and the twoliquid phases, and hence to predict � in the casesowmentioned above. Calculated oil�water contact anglesfor a solid of given hydrophobicity with a range of oilsof different polarity show good agreement with exper-imental data, as seen in Fig. 7. The approach is also

Žused to show that to effect inversion � to passow.through 90� , silica surfaces of increasing hydrophilic-

ity require oils of increasing polarity. A strength ofthe simple approach is that the type of emulsion,stabilised only by solid particles of known surfaceenergy components, as judged by � , can be pre-owdicted knowing only the surface tension of the oil inair and its interfacial tension with water.

Being of intermediate hydrophobicity, particlescontaining 67% SiOH groups can be dispersed sepa-rately in either oil or water phases using ultrasound.It is then of interest to investigate the effect of initial

� �� location of particles on emulsion type 42 . Sinceparticles are significantly larger than surfactantmolecules, the question arises whether they equili-brate between the bulk phases and interface to thesame extent in both situations. It was found that forparticles originally dispersed in toluene, the preferredemulsions at �0.5 were w�o, compared with o�wwwhen dispersed in water. The continuous phase of the

emulsion thus becomes that in which particles arefirst dispersed. It was also established that preferredemulsions were composed of smaller drops and were

Žmore stable than non-preferred emulsions o�w if.starting in oil and w�o if starting in water . An

important conclusion arising from this is that maxi-mum stability can be obtained for either emulsiontype simply by changing the initial particle location.In contrast, evidence was given that emulsion type isindependent of initial location for surfactant-stabi-lised systems due to the faster distribution of surfac-tant between phases occurring during formation. An-other expected difference between particles and sur-factant molecules as emulsifiers lies in what happens

� �during the course of Ostwald ripening 60 . The latterŽ .is the result of the solubility differences of say oil

contained in drops of different sizes. As a conse-quence of its increased solubility, oil contained withinsmaller drops tends to diffuse through the aqueousphase and re-condense onto larger drops. The accom-panying decrease in interfacial area provides the driv-ing force for the growth of the drops. Unlike surfac-tant molecules which are reversibly adsorbed at inter-faces, certain types of particles are effectively irre-versibly adsorbed. Thus, in the initial stages, it isexpected that diffusion of oil from small drops resultsin compression of the adsorbed particle layer until iteither buckles or resists further area reduction, sohalting ripening. As larger drops swell, excess parti-cles present in either the dispersed or continuousphase may adsorb to maintain overall stability tocoalescence. For toluene-in-water emulsions stabilisedby disc-shaped clay particles and stable to coales-cence, the initial increase in average drop diameter

� �with time was attributed to Ostwald ripening 60 .However, in agreement with predictions, the processceased after 3 days as the emulsion reached a quasi-equilibrium state. Another difference between thetwo systems is that surfactant micelles, if present inthe continuous phase, can act as carriers of oilbetween drops leading to an enhancement in theripening rate. No such aggregates can form in thecase of particles however.

Since sub-micron sized particles adsorbed at emul-sion drop interfaces cannot be visualised using opticalmicroscopy, the technique of freeze fracture scanning

Ž .electron microscopy SEM has been applied to these� �systems 64 . For w�o emulsions stabilised by

polystyrene latex particles, a monolayer of close-packed particles was observed. This contrasts withtheir more dilute arrangement at planar oil�water

Ž .interfaces earlier , implying that particle�particle re-pulsions can be overcome with the input of energyduring emulsification. In the case of silica, the ar-rangement of particles at drop interfaces was either inthe form of flocs of particles separated by particle-free


Fig. 8. Freeze fracture SEM image from a triglyceride-in-waterŽ .emulsion stabilised by silica particles 76% SiOH . The close-packed

arrangement of particles on the external surface of a drop is viewedŽfrom the continuous phase. Scale bar represents 100 nm redrawn

� �.from earlier work 64 .

regions or close-packed particle layers over the entiresurface, depending on the oil. An example given inFig. 8 shows the external surface of a triglyceride oildrop viewed from the continuous aqueous phase. Theabsence of any holes in the particle layer is mostlikely due to the polydispersity in size and to thedifferent shapes of the particles leading to very effi-cient packing. It should be noted however that the

Ž .average ‘size’ of the individual particles 60�80 nm isŽ .larger than the primary particle diameter 20 nm

suggesting fusion has taken place between particles inclose proximity. This is a common occurrence withfumed silica particles either in the vapour phase or inliquid dispersions. These preliminary micrographs ofparticles at curved emulsion drop interfaces raisemany questions which remain to be answered.

4.2. Multiple emulsions

In addition to acting as emulsifiers in simple emul-sions, the possibility exists that particles may be ef-fective in the stabilisation of multiple or double emul-sions. In the surfactant case, two different surfactantsare normally required in order to prepare such emul-sions, one of which adsorbs primarily at the interfaceof inner drops whilst the other adsorbs mainly at theouter globule interface. Since the curvatures of thetwo types of monolayer are opposite, a mixture of ahydrophilic and a hydrophobic surfactant is used to

Ž .satisfy this requirement. Tween high HLB numberŽ .and Span low HLB number surfactants are popular

choices in this respect. Thus, for oil-in-water-in-oilŽ .o�w�o emulsions, it is common practice to preparean o�w emulsion with the hydrophilic surfactant whichis then re-emulsified gently into an oil phase contain-ing dissolved hydrophobic surfactant. One of the main

problems associated with surfactant-stabilised multi-ple emulsions is their inherent instability as a resultof coalescence leading ultimately to the formation ofa simple emulsion. Various attempts have been madeat improving the shelf-life stability of multiple emul-sions by incorporating small solid particles into thesurfactant formulation. The idea, as before, is thatparticles act as a mechanical barrier to coalescence if

� � �adsorbed at interfaces. Oza and Frank 68 were thefirst to develop this by using colloidal microcrystalline

Ž .cellulose MCC as the water-soluble emulsifier inw�o�w emulsions containing oil-soluble Span surfac-tants. Emulsions, stable for up to 1 month, wereshown to have a network of MCC adsorbed at the

� �outer oil�water interface. Garti et al. 69 added sub-micron crystalline fat particles as a co-stabiliser of theinner interface of water-in-soybean oil-in-water emul-sions, good stability to coalescence and release ofelectrolyte ensuing. A similar enhancement in stabil-

� �ity was reported by Sekine et al. 70 but for o�w�oemulsions in which the external w�o interface waspartially coated with a layer of hydrophobically modi-fied clay particles causing it to become rigid. Theorganoclay particles also caused gelling of the outeroil phase preventing sedimentation of the waterglobules. More relevant to what follows is the paper

� �by Midmore and Herrington 71 on both w�o�w ando�w�o emulsions stabilised by a mixture of polymer,

Žsurfactant and colloidal silica particles hydrophilic.and hydrophobic . Both drops and globules were stable

to coalescence over a 6-month period.Unlike the previous studies outlined above in which

both surfactants and particles have been employed in� �� combination, work described by Binks et al. 72

was aimed at preparing and stabilising multiple emul-sions using particles alone as emulsifier. Since it hasbeen shown that many similarities exist between sur-factant molecules and particles with respect to emul-sion stabilisation, it is predicted that multiple emul-sions should form in oil�water mixtures containingtwo types of particles differing only in their hy-drophobicity. This turns out in practice to be the caseand extremely stable multiple emulsions of both typescan be prepared using a mixture of silica particlesdiffering by only 25% in SiOH content. The micro-graph in Fig. 9 is of an o�w�o emulsion, in whichinner oil drops are stabilised by hydrophilic silicaparticles and outer water globules are stabilised byhydrophobic particles. Despite the close proximity ofthe oil drops to each other and to the globule inter-face, they are completely stable due to the adsorbedparticle layer around them. The effects of inner andouter particle concentration, drop and globule volumefractions and oil type have been investigated and theresults correlated with the properties of the simpleemulsions stabilised by either type of particle alone.


Fig. 9. Optical microscopy image of an o�w�o multiple emulsionstabilised by two types of silica particles. The system contains

Ž .toluene and both hydrophilic 80% SiOH and hydrophobic silicaŽ .51% SiOH . Inner oil drops are approximately 4 �m in diameter.

ŽThe scale bar represents 20 �m obtained during work described� �� .earlier 72 .

Since the migration of particles from inner to outerinterfaces or vice versa is expected to be minimalafter emulsion formation, the major cause of instabil-ity occurring in surfactant emulsions is removed andthese emulsions are stable to coalescence for over 1year. The kinetics of release of electrolyte from innerdrops to the outer aqueous phase has also beendetermined in the w�o�w emulsions of toluene. Amodel in which the diffusion of salt through the oilphase including its partitioning between water and oilhas been put forward. This new class of multipleemulsions should prove of great benefit in the phar-maceutical field.

5. Solid-stabilised foams

Solid particles have been incorporated into surfac-tant-stabilised aqueous foams for many years, andtheir influence on the formation and stability of thefoam is very dependent on the surfactant type, parti-

� �cle size and concentration 73,74 . In some cases,particularly if the particles are fairly hydrophilic, foamstability is enhanced since particles present in theaqueous phase of the foam films collect in theirPlateau borders slowing down film drainage. In othercases, hydrophobic particles enter the air�water sur-faces of the foam and cause destabilisation via theso-called bridging-dewetting mechanism. This is

Žthought to be the mode of action of particles fre-.quently in the presence of oil drops contained within

traditional antifoam formulations. It is worth pointingout that some of the principles involved in the use ofparticles in antifoams and in the process of froth

� �flotation are similar. A recent study by Ip et al. 75describes the effect of particle concentration, size andhydrophobicity on the stability of both aqueous andmolten aluminium foams, both in the presence ofsurfactant. Metallic foams are designed to possessspecial properties and are produced by introducing airinto molten aluminium containing finely dispersedsilicon carbide particles. The foam so produced is verystable and can be manipulated and solidified to forma slab of aluminium foam. For the aqueous system

Ž .utilising silica particles 45 �m , only particles ofŽthe correct wettability varied by adsorption of cationic

.surfactant were found to stabilise the foam, andfoam stability increased with decreasing particle sizeand increasing concentration. Partially wetted parti-cles were shown to accumulate at bubble surfaces,providing a barrier preventing rupture and coales-cence. In the high temperature aluminium system,extremely stable foams were formed above a critical

Ž .concentration of SiC particles 10 �m , and highquality SEM micrographs confirmed the presence ofparticles at air�liquid surfaces both in planar filmsand in Plateau borders, in addition to being present inthe bulk aluminium phase.

The literature concerned with the ability of parti-cles to act as foam stabilisers in the absence of any

� �� other surface-active material is very sparse 76 ,77 .� �� However, work by Wilson 76 , not yet published

fully, is worthy of comment. The thesis was aimed atdetermining the important factors involved in theformation of particle-stabilised aqueous foams. Usinga combination of foaming experiments, contact anglemeasurements, zeta potential determinations and thinfilm studies, a relatively simple picture emerges inthese systems. The particles were those of polystyrenelatex containing charged sulfate groups at their sur-face. When dispersed in pure water, such particles donot cause foaming. Particulate foams were onlyformed when the dispersion approached the condi-tions required for coagulation in bulk. This was ef-fected by addition of either electrolyte reducing re-pulsion between particles or cationic surfactant whichchanges their hydrophobicity through adsorption. Asan illustration, a variety of results have been collectedfor one particular system, that of particles of diameter

Ž .3.88 �m and surface charge density in water 0.059 Cm�2 . In the foaming experiments, 5 cm3 of concen-trated latex dispersion was shaken with 5 cm3 of

Ž .aqueous salt solution or surfactant in a reproduciblemanner at 20 �C. The type of foam produced wasdescribed in the following way: no foam�if formed,bubbles collapsed within 2 s; slight foam�collapsedwithin 3 min; good foam�definite volume of foamstable for 5 min; �ery good foam�high ratio of foamto liquid volume stable for 12 h. The zeta potentialsof dilute dispersions were measured using elec-


Fig. 10. Effect of salt concentration on the foaming of aqueousdispersions of polystyrene latices of diameter�3.88 �m, concen-

Ž .tration �6.7 wt.% �, left hand ordinate and on the contact angleŽ .of a particle at the air�water surface �, right hand ordinate at

293 K. Values of the zeta potential of particles in water are givenon the upper abscissa above the particular salt concentration, and

Ž .the dotted line signifies the transition between dispersed left andŽ .coagulated right systems in bulk. The foaming scale refers to: 0

�no foam; 33�slight foam; 67�good foam; 100�very good foamŽ . Ž � �� .see text data taken from Wilson 76 .

trophoresis and the contact angles of particles withthe air�water surface were determined using the tilt-ing plate method. As seen in Fig. 10, addition ofsodium chloride to the dispersions causes a progres-sive increase in the quality and stability of foam

Ž .formed filled points , alongside a decrease in theŽ .charge on the particles upper numbers and an in-Ž .crease in contact angle open points . The vertical

dotted line represents the salt concentration beyondwhich dispersions become coagulated in bulk. Theclose correspondence between particle hydrophobicityand foaming is clear; very good foams are possibleonce the contact angle reaches above �85�. Thecombination of particles of low charge and of rela-tively high hydrophobicity seems an essential require-ment for making it energetically favourable for themto be situated at the air�water surface compared withbulk.

The same conclusion was reached when the chargeon the particles was reduced by lowering the pH, inthe absence of salt. Very good foams could be pre-pared around pH 1, again close to the condition atwhich bulk coagulation occurred. Particle charge canalso be modified by addition of cationic surfactantŽ .DTAB capable of adsorption. At low concentrations,no foam was formed but at concentrations corre-sponding to charge neutralisation and bulk coagula-tion, a very good foam appeared of greater volume

and stability than that formed in the presence ofsurfactant alone. Further increase in surfactant con-centration resulted in a decrease in foam quality asparticles re-dispersed in bulk on acquiring a net posi-tive charge. The structure of the foam films wasconfirmed by light diffraction as being composed of abilayer of particles separated by water, with particlesin each monolayer arranged in a hexagonally close-packed array. The monolayers were found to be rigidand solid-like in character. The above findings may be

� �linked to earlier work by Heller and de Lauder 78on the phenomenon of ‘surface coagulation’, in whichan otherwise stable colloidal dispersion of particles oflow charge density was caused to coagulate at anair�water surface. The destabilisation was broughtabout by addition of electrolyte and by bubbling N2gas through the liquid. The critical amount of saltrequired to force particles to bubble surfaces wasfound to be less than that needed for bulk coagula-tion in dispersions at rest.

Over the last 10 years a number of non-DLVOsurface forces have been discovered experimentally.One of them, the oscillatory structural force, appearsduring the thinning of thin liquid films containingcolloidal ‘particles’, e.g. surfactant micelles, macro-

� �molecules or solid particles 79 . This force affects theŽ .stability of foam vapour�liquid�vapour and emul-

Ž .sion liquid�liquid�liquid films. At high particle con-centrations this force stabilises the films, whereas atlower concentrations it degenerates into the depletionforce which is found to destabilise films. One way tomeasure directly the structural force is provided bythe phenomenon of stratification, in which thinningfilms stabilised by surfactant change thickness in aregular, stepwise manner before a final stable state isreached. The height of the steps corresponds to theeffective diameter of the particles contained withinthe film. Stratification is universal and has beenobserved with particle diameters varying between a

Ž .few nm anionic micelle, silica particles and 2 �mŽ .latex . It is due to the formation of a crystal-likestructure within the film and a layer-by-layer thinningof such an ordered structure. This ordering occursbecause charged particles are mutually repulsive butare forced into the restricted volume of the thinningfilm.

Until recently, all of the studies of film stratifica-Ž .tion involved particles micelles or solid which did

not adsorb on either side of the film but remained� �confined within 79 . A couple of papers, however,

report on foam film behaviour in cases where solidparticles become attached to the film surfaces. Ve-

� �likov et al. 80 investigated the influence of thefilm-forming surfactant on the dynamics of film thin-ning in the presence of negatively charged polystyrene

Ž .latex particles 7 �m . For plane parallel films


stabilised by the anionic surfactant SDS, all of theŽ .hydrophilic particles were expelled from the filminterior into the surrounding meniscus due to theirrepulsion with the film surfaces. For cationic surfac-

Ž .tant-stabilised films CTAB however, the same parti-cles become partially hydrophobic due to the couplingbetween the surface and the positively charged head-groups. Following opening and closing of the films afew times, a layer of particles simultaneously adsor-bed at the two film surfaces is formed, stericallyinhibiting any further film opening and thinning. Theparticles within this bridging monolayer displayed ex-cellent hexagonal ordering over large distances withalmost no defects. An increase in surfactant concen-tration led to a lower quality of the particle arraysdue to some lateral coagulation of the particles, butto a higher rigidity and resistance to further manipu-lation. Capillary attraction between the particles con-fined within the film was shown to account for suchtwo-dimensional crystallisation.

In contrast to the above, the stability of curvedfoam films containing small monodisperse particlesalone in the absence of surfactant has been investi-

� �gated 81 . Spherical, hydrophilic silica particles ofdiameter between 8 and 39 nm and dispersed in purewater are shown to stabilise thin films by a layeringmechanism within the film since none of the particlesadsorb to the film surfaces. The number of stepwisetransitions increases with the volume fraction of parti-cles within the film. Eventually a black spot appearsindicative of a part of the film containing no particles.The subsequent growth of the spot with time leadingto film rupture can be explained using the diffusiveosmotic model which takes into account the diffusionof particles from the film to the meniscus leavingbehind vacancies. The rate of film thinning was foundto be high when the particle concentration was lowand when both the particle size and film diameterwere large. Monte Carlo simulations showed that athigh particle concentrations the energy needed towithdraw a particle from the film is high, due to theimproved in-layer structure, thus inhibiting particlediffusion and enhancing film stability. This is an im-portant result and could have significant practicalimplications in the stabilisation of foams and emul-sions using colloidal particles. It would be very inter-esting to continue this kind of work using small parti-

Žcles of increasing hydrophobicity as used in emul-.sions above in order to study the influence of adsor-

bed particles on the thinning and eventual stability ofthe particle- stabilised foam films.

6. Conclusions

There are many similarities in the behaviour of

small particles and surfactant molecules at fluid�fluidinterfaces and in various kinds of dispersions. Impor-tant differences however also exist. The field of parti-cles at interfaces, both flat and curved, is an excitingone and is beginning to form part of the research ofmany groups. Future challenges include designing amethod for measuring the contact angles sub-micronsized spherical particles make with interfaces andsynthesising and studying particles of different shapes.The high stability encountered in solid-stabilisedemulsions is of great advantage in terms of the shelf-life of formulations including them. Their rheologyand deposition characteristics on surfaces, e.g. skin,need to be investigated. Such emulsions may act astemplates for the ordering of particles into new struc-tures following evaporation of the liquid components.

Acknowledgements

It is a pleasure to thank Dr Herbert Barthel ofŽ .Wacker-Chemie GmbH, Burghausen Germany for

the continued supply of fumed silica powders of vary-ing hydrophobicity, without which some of our workon solid-stabilised emulsions would not have beenpossible. I also thank Prof. P.D.I. Fletcher for usefuldiscussions.

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� �61 Binks BP, Lumsdon SO. Pickering emulsions stabilised bymonodisperse latex particles: effects of particle size. Lang-muir 2001;17:4540�4547.

� �62 Binks BP, Clint JH. Solid wettability from surface energy� components: relevance to Pickering emulsions. Langmuir

2002;18:1270�1273.A theoretical treatment is developed for the wetting of a particle atan oil�water interface in terms of the components of the surfaceenergies of all three phases. Calculated oil�water contact angles

� �show good agreement with experimental data given in 42 .� �63 Giermanska-Kahn J, Schmitt V, Binks BP, Leal-Calderon F.

A new method to prepare monodisperse Pickering emulsions.Langmuir 2002, in press.

� �64 Binks BP, Kirkland M. Pickering emulsions studied by lowtemperature field emission scanning electron microscopy. PhysChem Chem Phys 2002, in press.

� �65 Artz D, Christiansen M, Schonrock U, Steinke S.Ëmulsifier-free, finely disperse cosmetic or dermatological

Žformulations of the oil-in-water type patent assigned to.Beiersdorf AG . US 1998, 5,833,951.

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Žand at high water content patent assigned to Beiersdorf.AG . JP 2001, 245977.

� �67 Koretskii AF, Kruglyakov PM. Emulsifying effect of solidparticles and their wetting energetics at the water�oil inter-face. Izv Sib Otd Akad Nauk SSSR Ser Khim Nauk1971;2:139�141.

� �68 Oza KP, Frank SG. Multiple emulsions stabilized by colloidal� microcrystalline cellulose. J Disp Sci Technol 1989;10:

163�185.The stability of multiple w�o�w emulsions is enhanced by usingcolloidal microcrystalline cellulose as one of the emulsifiers inmixtures with surfactant. Electron microscopy revealed the pres-ence of a network of particle fibres on the surface of oil globules.� �69 Garti N, Aserin A, Tiunova I, Binyamin H. Double emulsions

of water-in-oil-in-water stabilized by �-form fat microcrystals.Part 1: Selection of emulsifiers and fat microcrystalline parti-cles. J Am Oil Chem Soc 1999;76:383�389.

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Žfumed silica particles European patent assigned to Wacker-.Chemie GmbH . Filed March 2002.

Hydrophilic silica particles are used to stabilise o�w drops�glob-ules and hydrophobic silica particles are used to stabilise w�odrops�globules in multiple emulsions of either o�w�o or w�o�w.Emulsions are very stable to coalescence as a result of adsorbedparticles.� �73 Kruglyakov PM, Taube PR. Syneresis and stability of foams

containing a solid phase. Colloid J 1972;34:194�196.� �74 Pugh RJ. Foaming, foam films, antifoaming and defoaming.

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Ž .�� University of Bristol UK .The work was supervised by Prof. R.H. Ottewill and describes the

conditions necessary to achieve foam stabilisation using latex parti-cles alone. Addition of salt, lowering of pH or addition of cationicsurfactant to aqueous dispersions all lead to an increase in foamstability and an increase in contact angle. Foaming is linked toparticle coagulation in bulk.� �77 Bindal SK, Nikolov AD, Wasan DT, Lambert DP, Koopman

DC. Foaming in simulated radioactive waste. Environ SciTechnol 2001;35:3941�3947.

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� �79 Nikolov AD, Wasan DT. Dispersion stability due to structuralcontributions to the particle interaction as probed by thinliquid film dynamics. Langmuir 1992;8:2985�2994.

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