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Mode of actionof emulsifiersManufacture of emulsions
1 Introduction to emulsions 3
1.1 Definitions 3
1.2 Emulsion types 3
1.3 Emulsifiers 31.3.1 Structure and mode of action of emulsifiers 31.3.2 Bancroft rule 41.3.3 HLB value 51.3.4 Kinetics 51.3.5 Emulsifier/coemulsifier principle 6
1.4 Properties of emulsions 81.4.1 Stability 81.4.1.1 Creaming and sedimentation 81.4.1.2 Aggregation and flocculation 91.4.1.3 Ostwald ripening 101.4.1.4 Coalescence 111.4.2 Droplet size distribution 121.4.3 Rheology 131.4.4 Electrical conductivity 13
2 Manufacture of emulsions 14
2.1 Microemulsions 14
2.2 Spontaneously emulsifying systems 14
2.3 Self-emulsifying systems 15
2.4 Mechanical emulsification 15
2.5 Emulsification using other phase boundaries 16
2.6 Phase inversion processes 17
3 Appendix 18
3.1 Marker method 18
3.2 Continuous emulsification in orifice systems 20
3.3 High-throughput screening: automated testingand optimizing system 22
3.4 Stabilization of oil/water emulsionswith alcohol ethoxylates 23
3.5 Poly dimethyl siloxane emulsions in watermade with nonionic surfactants from BASF 30
3.6 Amino modified silicon microemulsionsmade with nonionic surfactants from BASF,e.g. for textile softening 32
3.7 Microemulsions 36
2
U = � · A
1.3 Emulsifiers
Emulsions* are thermodynamicallyunstable. The droplets in the dis-persed phase tend to coalesce intolarger droplets, thus reducing theinterfacial area between the twophases and leading to a thermody-namically more favorable, i.e. lower,energy state.
The interfacial energy of anemulsion is given by:
U interfacial energy� interfacial tension between
the two phasesA interfacial area
This means that if the droplet sizedecreases and the total volume ofthe dispersed phase remains thesame the interfacial energy of theemulsion will increase because thetotal interfacial area increases.But higher energy states generallyhave lower thermodynamic stability,so that the driving force for coales-cence also increases (see alsosection 1.4.1).
Emulsifiers are used to reducethe tendency to coalescence andstabilize the droplets.
The interfacial tension of an O/Winterface is approx. 25 mN/m with-out emulsifier. Adding emulsifierlowers the interfacial tension tovalues typically around 3 – 5 mN/m.
By choosing suitable emulsifier sys-tems, even lower interfacial tensionsof below 1 mN/m are possible.
1.1 Definition
An emulsion is a dispersion of twoincompletely miscible liquids in oneanother.
1.2 Emulsion types
Simple emulsions consist of ahydrophilic (aqueous) and a lipophilic(oily) phase. In the simplest case thetwo phases are water and oil. Theinternal or dispersed phase is dis-persed in the external, continuousphase in the form of fine droplets.
Depending on the nature of thedroplet-forming phase, a distinctionis made between oil-in-water (O/W)and (W/O) emulsions.In addition to simple emulsions,there are also multiple emulsions. Anexample of this type is the W/O/Wdouble emulsion, which has anexternal phase of water and an inter-nal phase consisting of a water-in-oilemulsion.
* To distinguish them from the thermodynamicallystable microemulsions (cf. 2.1), emulsions aresometimes also called macroemulsions.
3
1 Introduction to emulsions
Electrostatic repulsion
Steric repulsion
oil-in-water( O / W )
water-in-oil-in-water(W / O / W )
water-in-oil(W/ O )
Different types of emulsions
1.3.1 Structure andmode of action ofemulsifiers
Emulsifiers are surface-active sub-stances whose molecules consist ofa hydrophilic and a lipophilic part.Because of their amphiphilic proper-ties, free emulsifier molecules accu-mulate at the interface betweeninternal and external phases. A com-peting process also occurs in whichemulsifier molecules aggregate intomicelles. Above a certain concentra-tion, known as the critical micellarconcentration (CMC), the proportionof monomer emulsifier moleculesremains constant. In practice, thebest results are obtained when theemulsifier is applied in concentra-tions well above the CMC.
The stabilizing properties ofemulsifiers are based on variousmechanisms, depending on thetype of emulsifier:
a) Electrostatic repulsion
b) Steric repulsion
I
4
1.3.2 Bancroft rule
Whether an emulsifier is better ableto stabilize an O/W or a W/O emul-sion depends on which is larger, thehydrophilic or the lipophilic portion ofthe molecule.
Emulsifiers with a larger hydrophilicportion are good O/W emulsifiers,whereas those with a larger lipophilicportion are better able to stabilizeW/O emulsions.Too large a hydrophilic or lipophilicpart, on the other hand, leads toboth low interfacial affinity of theemulsifier and poor packing at theinterface.There is therefore an optimum sizeratio between the hydrophobic andhydrophilic parts.
O
O
HO
O
O
O
HO
O
O
O
HO
O
O
O
HO
O
O
O
HO
O
O
O
HO
O
O
O
HO
O
O
O
HO
O
O
O
HO
O
O
O
HO
O
O
O
HO
O
O
O
HO
O
O
O
HO
O
O
O
HO
O
O
O
HO
O
O
O
HO
O
O O O O O O OO
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O O O O O O OO
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O O O O O O OO
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O O O O O O OO
HO HO HO HO HO HO HOHO
Lipophilic portion larger Hydrophilic and lipophilic Hydrophilic portion largerportions equal in size
Potential energy
Interdroplet distance
Steric repulsion
Electrostatic repulsion
London-vander-Waals attraction
Sum of attractive andrepulsive forces
Dependence of attractive and repulsive forces on interdroplet distance
In addition to repulsive forces,there are also attractive forces –the London-van-der-Waals forces.
According to Derjaguin,Landau, Verwey and Overbeek(DLVO theory):
�Utotal total potential energy�Uel electrostatic repulsion�Ust steric repulsion�UvdW London-van-der-Waals
forces
The resulting potential energyis the sum of the electrostatic repul-sion, steric repulsion and attractionby London-van-der-Waals forces.The figure below shows that below acertain distance, after the repulsiveforces have been overcome, onlyattractive forces operate. At thispoint the droplets coalesce.
�Utotal = �Uel + �Ust – �UvdW
5
1.3.3 HLB value
In many cases, the HLB value(hydrophilic lipophilic balance) givesan indication of the type of emulsionfor which emulsifiers are suitable.Strictly speaking, the Griffin formulaapplies only to nonionic ethoxylates,which are classified on a scale of0 to 20. Emulsifiers with low HLBvalues tend to be lipophilic mole-cules, dissolving mainly in the oilphase of an emulsion and betterable to stabilize W/O emulsions atroom temperature. Emulsifiers withmedium or large HLB values arehydrophilic. They are more solublein water and are used preferentiallyfor stabilizing O/W emulsions atroom temperature.
For other emulsifiers, especiallyionic ones, a method was developedby Davis in which the sum of experi-mentally determined increments iscalculated.
In practice, the procedure is asfollows:
1) Choose an emulsion type (O/Wor W/O)
2) Find the HLB value of the oil fromtables1
3) Select the HLB value of theemulsifier or emulsifier mixtureto be the same as the HLB valueof the oil
4) Corrections may be necessaryif the temperature differs signifi-cantly from 25°C or there are saltsin the aqueous phase. In the caseof ethoxylates, for example, theHLB is chosen to be 0.5 – 1higher for each increase of 10 K intemperature or 5 wt% NaCl.
1.3.4 Kinetics
In addition to thermodynamics, thekinetics of emulsions is very signifi-cant, especially in their manufacture.
The critical step is the fragmentationof the internal phase. Large dropletsare divided into smaller ones byintroducing energy, e.g. by shearing.The newly created surface must beoccupied by emulsifier molecules asrapidly as possible to protect thedroplets and prevent them fromcoalescing.
The rate at which an emulsifiermolecule occupies or vacates anewly created interface dependson a number of factors:
a) The rate at which emulsifiermolecules are transferredfrom the continuous or thedispersed phase to the vicinityof the interface
b) Penetration of the interfaceby emulsifier molecules
c) The orientation of emulsifiermolecules at the interface
d) Distribution of emulsifiermolecules over the interface(Marangoni flow)
e) Removal of emulsifier moleculesfrom the interface by thermalagitation
f) Removal of emulsifier moleculesfrom the interface by currents andeddies
Under turbulent flow conditions,transfer of emulsifier throughthe continuous phase is faster thandroplet breakup, irrespective of theemulsifier’s diffusion coefficient.Similarly, Marangoni flow at theinterface is usually more rapid thanthe creation of new interfaces.These two effects are thereforeseldom rate-determining.
1 e.g. Ullmann’s Encyclopedia of IndustrialChemistry, 6th edition, Wiley-VCH, Weinheim, 2000.
HLB = 20 (1 – Mlipophilic/Mtotal)
According to Griffin’s formula, the HLB value is calculated from the massof the lipophilic portion as a fraction of the total mass of the molecule:
Mlipophilic mass of lipophilic portionMtotal total mass of molecule
lipophilic hydrophilic
0 5 10 15 20
W/O emulsifiers O/W emulsifiers
HLB scale according to Griffin
Mechanical energy
Deformationand breakup “Slow”
emulsifier
“Fast”emulsifier
Coalescence
Stabledroplets
Continuous phase+ emulsifier+ phase to be dispersed
Effect of adsorption rate on stabilization
6
1.3.5 Emulsifier/coemulsifier principle
As described in the previous section,small emulsifier molecules oftenhave an advantage when it comesto rapid stabilization of the internalphase during droplet fragmentation.
However, small emulsifier moleculesalso have a disadvantage. Sincethey are generally less tightlyadsorbed on to the interface thanlarger emulsifier molecules, espe-cially polymers, they are more readilyremoved from the interface again byBrownian motion. Furthermore, theirsmaller molar mass often meansthey have smaller repulsive groupsand therefore, according to theDLVO theory, do not stabilizedroplets so well against coales-cence.
This is not true of transfer throughthe dispersed phase. In the caseof O/W emulsions, hydrophobic,readily oil-soluble emulsifiers havethe advantage here. They becomeconcentrated in the oil droplets,where they diffuse rapidly to thephase interface.
Little is known about the kinetics oftransfer of emulsifier molecules tothe interface from micelles in the twophases. Removal of small emulsifiermolecules from the interface isdominated by thermal motion. Inthe case of polymers with molecularweights above approx. 100,000daltons, eddies and currents playan increasingly important role.
It has been found empirically thatsmall emulsifier molecules stabilizenewly generated interfaces morerapidly than large ones.
Mechanical energy
Slowemulsifier
Fastemulsifier
Coalescence occursbefore interface isoccupied
Coalescence can occurbecause emulsifiermolecules leaveinterface again
Good long-termstability
Poor long-termstability
Dispersed phase
Time scale
Different long-term stabilities of rapid and slow emulsifiers
7
It has proved advantageous tocombine small and large emulsifiermolecules. The rapid adsorption ofthe small molecules means theyimmediately occupy a newly createdsurface and then gradually makeroom for the slower but more tightlyadsorbing large emulsifier mole-cules. In such combinations of twoor more emulsifiers, the emulsifier(s)present in smaller quantities is (are)known as the coemulsifier(s).
It has been found in practice thatemulsifiers consisting of a mixture ofsmaller and larger molecules as aresult of the synthesis process, forexample ethoxylates with a broadEO distribution, do not needcoemulsifiers to be able to stabilizeemulsions well.
A further advantage of emulsifier/coemulsifier mixtures is that higherpacking densities can be achieved atthe interface. Higher packing densi-ties have the effect of increasing therigidity and thickness of the emulsi-fier film.
Coemulsifier
Emulsifier
log M
dNN·d log M
Inte
nsity
Molar masses (M) of emulsifier and coemulsifier
O
O
OH
O
O
O
O
OH
O
O
O
O
OH
O
O
O
O
OH
O
O
O
O
OH
O
O
O
O
OH
O
O
O
O
OH
O
O
O
O
OH
O
O
O
O
OH
O
O
O
O
OH
O
O
OH OH OHOH
Without coemulsifier Higher packing densitywith surfactant alcohol as coemulsifier
GPC of an ethoxylate with a broad molar mass distribution
1.4.1.1 Creaming andsedimentation
In addition to thermodynamic insta-bility, there are other emulsion agingmechanisms of significance to theuser, for example creaming and sed-imentation. Creaming is reduced inemulsions containing small droplets,small density differences and a high-viscosity continuous phase. It canbe quantified by optical techniquesor ultrasonic scattering. Very smalldroplets (< 100 nm) are in favorablecases also stabilized thermodynami-cally against creaming by Brownianmotion.
1.4 Properties of emulsions
1.4.1 Stability
Emulsions have a much larger inter-facial area between the two liquidsthan the corresponding unemulsifiedmixtures. Most emulsions are there-fore thermodynamically unstableeven in the presence of emulsifiers.Only in emulsions where the inter-facial tension is extremely smallcan the thermal energy exceed theinterfacial energy. Such emulsionsare known as (thermodynamicallystable) microemulsions (see 2.1).
Density differences in emulsions lead to creaming (see figure) or sedimentation of droplets
8
v =gd2 · ��
18�
Stokes law gives the creamingrate for a droplet dispersed ina very dilute emulsion:
d droplet diameterg acceleration of gravity�� difference in density
between dispersed andcontinuous phases
� viscosity of continuousphase
Analogous but more complexequations are used to describecreaming for dispersed dropletswith finite surface viscosity,for distributions of droplet diame-ters and for more concentratedemulsions. But the parametersderived from the simple Stokeslaw apply here, too.
The thermodynamic stability of the emulsion can be derived from theGibbs equation:
G free energy interfacial tension between the two phasesA interfacial areaS entropyT temperature
Normally, �G > 0, i.e. the emulsion is thermodynamically unstable.However, if the interfacial tension is very small, the greater disorder(entropy) of the emulsified state leads to a thermodynamically stablemicroemulsion (�G < 0).
�G = Gemulsified – Gunemulsified = � · �A – T · �S
1.4.1.2 Aggregation andflocculation
Flocculation (or aggregation) refersto the formation of clusters of two ormore emulsion droplets that behavelike distinct particles but in which theidentity of the individual droplets isretained. Such clusters can even bein dynamic equilibrium with singledroplets, individual droplets leavingthe cluster while new ones join it.Flocculation is therefore a processthat is easily reversible, in contrastto coalescence, though flocculationoften results in coalescence. Ther-modynamically, flocculation is basedon a secondary energy minimum, asdescribed for example by the DLVOtheory (see 1.3.1 and 1.4.1.4).
Attractive forces between droplets lead to aggregation or flocculation
9
Emulsion with droplets> 100 nm
Emulsion with droplets<<100nm
Droplet density N/V
Height ofliquidcolumn h
Emulsions with very fine droplets and small density differences can be entropy-stabilized againstcreaming
The kinetics of irreversible flocculation in monodisperse, unstirredemulsions can be described by the Smoluchowski equation:
W probability of a successful collisiond droplet diameterDTr diffusion constant of droplet� viscosity of continuous phaseN/V droplet densitykT measure of thermal energy
It should be noted that to a first approximation flocculation kineticsis independent of droplet size.
N= – W · 4�d · DTr
N 2
= –kT N 2( )�t V ( ) ( )V 2� V
�
The degree of thermodynamicstabilization is obtained from theBoltzmann distribution law:
h height of liquid columnk Boltzmann constantT temperatured droplet diameter�� difference in density between
dispersed and continuousphases
g acceleration of gravity
�d3 · �� · g · h �� 6kT
10
1.4.1.3 Ostwald ripening
Another phenomenon of emulsionaging is the growth of large dropletsat the expense of smaller ones: Ostwald ripening. It arises from thefact that small droplets dissolvemore readily in the continuous phasethan large ones. The phenomenonwas already described by LordKelvin in 1871.
Ostwald ripening causes large droplets to grow at the expense of small ones
Kelvin’s equation describes the effect of droplet size on the solubility of the dispersed phase:
c(d) saturation concentration of the dispersed phase in the continuous phase for a droplet of diameter d
interfacial tensionRT measure of thermal energyVMol molar volume of dispersed phase
RTln c(d) = 4 � · VMol + RTln c(d = ∞)
d
In very dilute emulsions, the rate at which large droplets grow at theexpense of small ones is given by the Lifshitz-Slezov-Wagner (LSW)equation.
D=diffusion coefficient of the dispersed in the continuous phase
The third power of the mean diameter d–increases in proportion to
the interfacial tension and the solubility c of the dispersed in the continuous phase. This growth is accelerated by the Brownian motion of the droplets, the dispersed phase fraction and the presence ofmicelles in the continuous phase.
�(d)3 =
64� · D · c (d = ∞) · VMol
�t 9 RT
11
Stabil
The main way of reducing Ostwaldripening is to ensure that the solubil-ity of the dispersed in the continuousphase is as low as possible. To stabilize an emulsion in which thedispersed phase is too soluble, aninert, very poorly soluble auxiliarycan be added – in the case of water,for example, an extremely hydro-phobic substance such as a hydro-carbon. Adding sufficient auxiliaryallows a stable equilibrium to beestablished between the droplets.The emulsion can then be technicallyconsidered an emulsion of the auxil-iary, the droplets of which containthe original internal phase in a dissolved state.
Koaleszenz
1.4.1.4 Koaleszenz
Coalescence is the merging of twoor more droplets into a single largedroplet, driven by the reduction ininterfacial area and hence interfacialenergy. Below a distance of approx.100 nm, two oil droplets in an aque-ous solution experience a percepti-ble attractive force that increases as the droplets approach oneanother. It is suspected that even atthese distances the water betweenthe droplets adopts structures oflower entropy. The addition of a sta-bilizer such as an emulsifier super -imposes on this attractive force arepulsive force that derives from thespatial requirement or charge of thehydrophilic groups on the emulsifier.The balance of these forces isdescribed by the theory of Derjaguin, Landau, Verwey andOverbeek (DLVO, see 1.3.1). An energy barrier keeps the aggre-
Adding a very poorly soluble auxiliary can stop Ostwald ripening
gated droplets at a distance and theheight of this barrier is critical for thestability of the emulsion.
Apart from stabilizers, emulsions canalso be stabilized by restricting themobility of the droplets by a continu-ous phase of very high viscosity.Such a high viscosity can, for exam-ple, be obtained by adding thicken-ers. Emulsions can also be stabilizedhydrodynamically. In this case,strong turbulence reduces the con-tact time between two collidingdroplets to the point where drainageof the liquid film of continuous phaseseparating the droplets is impededand coalescence prevented.
The merging of separate droplets into a single larger droplet is called coalescence
The Higuchi-Misra relation describes the effect of using an auxiliary:
Adding an auxiliary that is very poorly soluble in the continuous phasestabilizes an emulsion against Ostwald ripening if for every droplet i the effects of its diameter di and the activity ai of the more soluble component in the auxiliary counterbalance one another until the aboveequation is fulfilled.
interfacial tensionVMol molar volume of dispersed phaseRT measure of thermal energydi , ai diameter of droplet i and associated activity of dispersed phase
in auxiliary
4� · VMol + RTln ai = constdi
12
As a rule, emulsion droplets arepresent not in monodisperse formbut as a relatively broad size distri-bution. Only by applying specialtechniques, such as membraneprocesses (see below) or micromix-ers, can a virtually monodispersedistribution be obtained. Varioustechniques are available for measur-ing the size distribution, for examplelaser diffraction methods.
1.4.2 Particle size distribution
Typical emulsions have droplet diameters ranging from 0.5 to 50 µm. The size and size distributionof the particles to a large extentdetermine the properties of emul-sions, for example their stability andappearance. Thus emulsions tend toappear white or milky because oflight scattering, unless the refractiveindices of the dispersed and contin-uous phases happen to be thesame.Emulsions whose particles are muchsmaller than the wavelength of visible light appear opaque to clear.
Typical examples of broad and narrow particle size distributions (measured by laser diffraction)
9
8
7
6
5
4
3
2
1
0
Cumulative volume distribution Q3 (x) (%)
100
80
60
40
20
00.1 0.5 1 5 10 50 100 1000
Volume density% distributionq
3(x) (µm-1)
Particle size (µm)
8
7
6
5
4
3
2
1
0
100
80
60
40
20
00.1 0.5 1 5 10 50 100 1000
Cumulative volume distribution Q3 (x) (%) Volume density% distributionq3(x) (µm-1)
Particle size (µm)
13
1.4.3 Rheology
The rheological behavior of emul-sions is determined mainly by thefraction of the dispersed phase.Dilute emulsions are characterizedby Newtonian flow.
As the fraction of the internalphase increases, the emulsionstarts to show non-Newtonian flowbehavior and the droplets arepacked more and more closelytogether. At even higher internal-phase concentrations, the dropletsare deformed into polyhedra andrheologically the emulsion behaveslike a foam. It exhibits a yield point and pro-nounced shear thinning caused bydeformation and relaxation of the droplets. Such emulsionsare known as gel or high-internal-phase emulsions.
1.4.4 Electrical conductivity
The specific conductance of emul-sions is determined by the conduc-tivity of the continuous phase (orphases in the case of bicontinuousemulsions). It is therefore a simplemethod for distinguishing W/O fromO/W emulsions. Moreover, the transition from O/W to W/O, or to abicontinuous emulsion, is generallyaccompanied by an abrupt changein specific conductance, which istherefore an indicator of the forma-tion of new phases or of emulsioninversion.
The viscosity of an emulsion depends on the viscosity of the continuous phase. This relationship is described by Einstein’s equation:
� viscosity� phase fraction of dispersed phase
The equation takes into account the contribution of momentum transfer due to the droplets, which are regarded as rigid spheres. The equation does not take into account further increases in viscosity due to deformed, deformable or aggregated droplets, dropletcharges, polymers and thickener additions, or surfactant layers at the interface. In concentrated emulsions the viscosity increases morerapidly as a function of the phase fraction � of the dispersed phase than described by Einstein, and site exchange similar to that encountered in diffusion phenomena in solids becomes the dominant factor.
�emulsion = �cont. phase · (1 + 2.5�)
Phase fraction of oil (%)
Specific conductance (µS/cm) 60
50
40
30
20
10
00 20 40 60 80 100
O/W
W/O
Bicontinuous
Discontinuities in specific conductance indicate fundamental changes in emulsion type. O/W emulsions generally have good conductivity, bicontinuous emulsions are much poorer conductors, and W/O emulsions are practically nonconducting.
2 Manufacture of emulsions
14
2.1 Microemulsions
Microemulsions are a special casebecause they are thermo -dynami-cally stable. By cleverly combiningthe incompletely miscible phasecomponents and the emulsifier sys-tem or using very high surfactantconcentrations, extremely low inter-facial tensions are obtained, so thatthe emulsion is entropy-stabilized.
In addition, the emulsifiers must notbe very soluble in either of the twoliquid phases at the application tem-perature, so that they remain almostentirely at the interface and an emul-sion forms spontaneously, creatingnew interface in proportion to theamount of emulsifier present. In practice, the three-phase region(Winsor III region) in the phase diagram is sought, in which water, oil and microemulsion coexist. Theamount of emulsifier, via its inter -facial area requirement, determinesthe volume fraction of the micro -emulsion.
2.2 Spontaneously emulsifying systems
Even systems that do not ultimatelyform stable microemulsions canemulsify spontaneously. The neces-sary interfacial energy is supplied bythe entropy of mixing. For example,a mixture of oil, surfactant andethanol emulsifies spontaneously inwater because the ethanol diffusesinto the water. Another mechanisminvolves intermediate microemul-sions that form as a result of highsurfactant concentrations at theinterface and break down in the continuous phase. Turbulence at theinterface can in both cases lead tothe formation of a (metastable) emul-sion instead of two macroscopicallyseparate water and oil phases.
Microemulsions are thermodynamically stable. The interfacial tensionbecomes so small – approx. 1 to 100 nN/m – that the increase in entropyon emulsification exceeds the surface energy and the emulsion formsspontaneously.
Since the total energy �G determined by the Gibbs equation is < 0, the microemulsion is thermodynamically stable.
�G = G emulsified – Gunemulsified = � · �A – T · �S < 0
When “emulsifiable concentrates” are added to water, chaotic turbulence at the interface leads to the formation of an emulsion instead of two macroscopic phases.
Water
Oil
0.1 µm
Emulsifiable concentrate
Mole fractionof waterincreases
Entropy ofmixing leadsto formationof interface
Electron micrograph of a frozen microemulsionSource: Science, Vol. 240, Microemulsions, Manfred Kahlweit. Copyright 1988, AAAS
(wt. % surfactant)
2 phases
2 phases
1 phase(Microemulsion)
3 phases(microemulsion,
oil, (water)
Tem
pera
ture
Typical phase diagram of a microemulsion
15
2.3 Self-emulsifying systems
In the case of microemulsions andspontaneously emulsifying systems,emulsions are formed without energybeing supplied from the outside, but with self-emulsifying systemsthis is not strictly true. However, onlya small power input is required toturn self-emulsifying systems intoemulsions, for example from a slowlyturning stirrer or simply by shaking.Important for this process are smalldynamic interfacial tensions, so thateven a small power input will resultin a sufficiently high critical Webernumber to produce small droplets.Generally speaking, oil-soluble surfactants with a low HLB value, or mixtures containing such surfactants, tend to be used forspontaneously or self-emulsifyingsystems.
2.4 Mechanical emulsification
In mechanical emulsificationprocesses involving high power densities, very low interfacial tension is helpful but not essential.High-turbulence zones, laminarshear flows and cavitation induceemulsion formation even without anemulsifier. Much more important insuch processes is good stabilizationof the resulting emulsion, since theoutlet of dispersion machines isoften characterized by turbulent flow conditions and a high dropletcollision rate.
The Weber number is defined as the quotient of the external and internal forces acting on a droplet. It describes the emulsion process in terms of laminar forces.
v�l laminar shearing field�c viscosity of continuous phased droplet diameter interfacial tension between phases
The external forces are transmitted by the viscosity of the continuousphase, while the internal restoring forces are caused by the interfacialtension. The critical Weber number Wecr is obtained by substituting into the above equation the diameter of the smallest droplet that can just still be (or can no longer be) broken up.Critical Weber numbers can also be calculated from the type of laminarflow (extensional or shear flow) and from the viscosities of the continuousand dispersed phases. The diameter of the smallest attainable droplet isthen derived from the above equation.
We = �v/�l · �c · d
2�
The Reynolds number relates inertial and viscous forces to one another:
v flow rate�c viscosity of continuous phasel characteristic dimension (see below)�c density of continuous phase
If the dimensions of the stirrer or other emulsifying apparatus are sub -stituted for l, then there is laminar flow up to Re � 103 and turbulent flowabove Re � 104. Between these values is a tran -sition range. Substitutingthe droplet diameter for l allows the Reynolds number in the vicinity of adroplet to be calculated. Below ReTr � 1 droplet breakup (ReTr ) is causedmainly by laminar forces; above ReTr � 1 mainly by inertial forces.
Re = v · ll · �c
�c
16
2.5 Emulsification using other phase boundaries
Another method of producing emul-sions is to use other phase bound-aries, for example the solid-liquidinterfaces of membranes and micro -mixers. Here the large interface ofwhat is to be the dispersed phase isgenerated by appropriate differentialpressure across a membrane that is not wettable by emulsifier. The process is very energy-efficient.Nature makes use of it, for example,in producing milk. A gas-liquid interface, such as inaerosol and condensation processes,can also be used to produce emul-sions.
Significant criteria for selecting anemulsifier for such a mechanicalemulsification process are that itrapidly occupies the newly formedinterface, is not carried away againby eddies, and protects dropletsfrom coalescence in the turbulentdischarge zone of the emulsifyingapparatus. Low-molecular “fast”emulsifiers are generally superior inthis regard to polymeric emulsifiers(see 1.3.4).
A large number of emulsifyingmachine types are available on the market. Typical are rotorstatorsystems such as toothed disc dis-persing machines, reaction mixingpumps and colloid mills. Othermachines consist of pumps com-bined with static mixers, nozzles ororifices, for example the very effi-cient high-pressure homogenizers.Others again use ultrasonicsonotrodes.
Fine emulsion
Crude emulsion
Rotor-Stator-System
High-pressure homogenizer
High-pressure homogenizers and rotor-stator systems are commonly used to manufacture emulsions
Fine emulsion
Crude emulsion
Rotor-Stator-System
High-pressure homogenizer
Droplet formation at membrane pores
Mechanical energy
Continuous phase+ emulsifier
Phase to be dispersed
Mechanical energy
Membrane
“Very slow”emulsifier
“Fast”emulsifier
Membranes are very energy-efficient emulsifying apparatuses. However, they are mostly too expensive for wide-scale industrial application.
17
2.6 Phase inversionprocesses
A simple way to obtain very fineemulsions with very little energyexpenditure is the phase inversionmethod. It is particularly simple toapply with ethylene oxide based surfactants. Three basic cases aredistinguished:
1) In the first, the surfactant is mixedwith, for example, oil and waterand heated to above, or justbelow, the phase inversion tem-perature (PIT). The mixture is then emulsified and the emulsionis stabilized by cooling. Thisapproach takes advantage of thelow interfacial tension at the PIT,and only a small amount of energyis required to produce a fine emul-sion. Emulsification above the PITand subsequent cooling is themore efficient method, but caremust be taken to ensure that theemulsion does not break duringinversion. The risk of this occur-ring is smaller if the mixture isheated to just below the PIT*,which assumes, however, that the PIT is known exactly.
2) In the second case, the surfactantis mixed with, for example, oil andvery little water and emulsified.Then, without altering the temper-ature, water is added to inducephase inversion. (Such inversionof an emulsion containing a verylarge fraction of the future internalphase shows hysteretic behaviorwhen the emulsion is redilutedwith oil and can be described bycatastrophe theory; thus it iscalled “catastrophic phase inver-sion”. Inversion of small regions of the emulsion ultimately leads to inversion of the emulsion as awhole.) Fine emulsions can beproduced very simply by thismethod, but it requires an emulsi-fier that tends to be soluble in oil rather than water. Such emulsi-fiers usually have small hydrophilicgroups and are therefore, accord-ing to the DLVO theory, poorerstabilizers.
3) The third case is a combination of the first two. A water-in-oilemulsion or microemulsion, forexample, is produced at high temperature and high surfactantconcentration and then invertedby adding cold water. This is asimple way to produce very fine,stable emulsions.
These phase-inversion techniquescan also be used with ionic surfac-tants, but it should be rememberedthat, because of the expan sion oftheir Debye ion clouds, ionic surfac-tants become more hydrophobic at low temperatures and morehydrophilic at high temperatures –the exact opposite of ethylene oxidebased surfactants.
* It is usual to emulsify approx 10 – 15K below the PIT.
The phase inversion method produces fine emulsions with little energy expenditure. It is illustrated here for ethylene oxide based emulsifiers.Case 1 involves the inversion of a W/O emulsion into an O/W emulsion by cooling.Case 2 is isothermic phase inversion, and Case 3 is a combination of the other two cases.
Case 1
Oil fraction of the emulsion
PIT
Case 3
Case 2
W/O
O/W
Temperature
18
3 Appendix
3.1 Marker method
A technique used at BASF to deter-mine the stabilizing properties ofemulsifiers is the marker method*,based on the Danner dyeingmethod**. In this technique, the dis-persed phase is marked with markersubstances in such a way that afteremulsion coalesced and non-coa-lesced droplets can be distinguishedand determined quantitatively.
Here we describe how the method is used to analyze O/W emulsionswith oil-soluble dyes as marker substances.
First, two coarsely dispersed rawemulsions are prepared with identicalformulations but a different-coloredoil phase in each (one blue and oneyellow). The two emulsions are mixedby slowly stirring them together.
The emulsion mixture is then sub-jected to a coalescence experiment.This may, for example, be storage,temperature change, shearing, centrifuging or addition of chemicals.The emulsion is subsequently examined under a microscope andthe proportion of droplets of mixedcolor determined. Droplets contain-ing the mixed color (in this case
green) are formed by coalescence,i.e. the droplets combine becausethey are not sufficiently stabilized by the emulsifier. The higher the pro-portion of droplets of mixed color,the more poorly the emulsifier hasstabilized them against coalescence. If only a few green droplets – or noneat all – are found in the sample, itcan be concluded that the emulsionis well stabilized by the emulsifier.
The emulsion is evaluated quantita-tively by digital image analysis,which allows the area fraction ofeach droplet color to be determined.
Quantitative determination of thestabilizing properties under actualemulsification conditions is thereforepossible.
* DE 10247086 ** Dr. Thomas Danner, Tropfenkoaleszenz in Emulsionen, dissertation at the University of Karlsruhe, GCA-Verlag, Herdecke 2001
Crude emulsion A
Crude emulsion B
Fine emulsification
1:1 mixtureMicrograph showing
coalesced droplts
Principle of the marker method
Source: GCT, Dr. Tho
mas Danner
Evaluation of the micrographsLeft image sequence: no coalescence, two signals, one for each of the yellow and blue color areasRight image sequence: 100% coalescence, only one signal is obtained for the green color area
Original image
� Droplet separation
� Color evaluation
19
Advantages of the markermethod:
– Coalescence can be isolated fromother effects such as Ostwaldripening and flocculation.
– Coalescence can be accuratelymeasured at an early stage, reducing development time.
– Coalescence can be measuredunder shearing/stirring conditions.
20
3.2 Continuous emulsification in orificesystems
An emulsification technique whoseresults can be readily evaluated bythe marker method (see separateappendix) is emulsification in orificesystems, as used for example inhigh-pressure homogenizers. In alaboratory test, an autoclave is filledwith two differently colored crudeemulsions with a particle size of 20 – 30 µm. While being slowly
stirred, the crude emulsion with different colored droplets is forcedthrough an orifice by gas pressure.
Before entering the orifice, the crudeemulsion passes through a region of laminar extensional flow, whichdeforms the emulsion droplets.Within the orifice is a small zonewhere laminar shear flow predomi-nates. Here the droplets are furtherdeformed and some of them breakup. Most of the droplets, however,break up in the turbulent outflow ofthe orifice, where, depending on thepower input, cavitation may occur
and produce shock waves. Duringthe emulsification process, thedroplets collide with one another.Where droplets are created by divi-sion of larger droplets, the emulsifiermust rapidly occupy the newlyformed interface or they will coa-lesce. Details of these processes arediscussed in numerous monographson emulsification.
Autoclave with orifice in outlet
To obtain a measure of the perfor -mance of the emulsifier in terms of coalescence, the coalescenceprobability of the droplets is deter-mined from the measured proportionof green droplets. The lower the coalescence probability, the betterthe emulsifier stabilizes the emulsionagainst coalescence in turbulentregions and other shearing zones.
Laminarextensional flow
High pressure,low flow rate
Low pressure,high flow rate
TurbulenceOrifice
Emulsification zones in flow through an orifice
Laminar
Extensionalflow
Shearing,turbulence
Cavitation
Extension of droplets and occupation of newly formed surface by emulsifier
21
We recommend the emulsifiers listedbelow for preparing oil-in-wateremulsions in laminar flow under high shear with a high power input(> 105 – 106 W/m3). These conditionsare typically present in screen mills,rotor-stator colloid mills and high-pressure homogenisers. Thedroplets that are formed in this typeof apparatus are usually smallenough, but the emulsifier moleculeshave to be able to migrate quickly tothe newly formed interfaces and theyhave to be capable of preventing theoil droplets from coalescing in thezones of turbulent flow in the outletports of the machinery.
Of course, the size of the dropletsalso depends on the type of emulsifi-
cation process and the on the powerthat is introduced into the system.Depending on the type of processthat is involved, much smaller orlarger droplets can be obtained ifrequired, but the ranking of theemulsifiers in the diagrams stillapplies.
Because of the similarity of theseprocesses, these recommendationsalso apply without restriction tomembrane processes, and they also apply to a certain extent toultrasonic processes and turbulentemulsification processes with a highpower input, such as in high-speed stirrers, toothed-ring dissolvers or in large rotor-stator-type homo -genisers.
Emulsifiers recommended for process with a high power inputSmall droplets with a low tendency to coalesce can be prepared with the following emulsifiers:
Paraffin oil Emulan® P, AT 9, NP 3070
Naphthenic mineral oil Emulan® TO range, NP 3070, EL
Aromatic mineral oil Emulan® TO range, OU, OC, OG Emulan® NP 3070Emulphor® FAS 30
Triglycerides Emulan® EL
Silicone oil Emulan® TO range, ELEmulan® NP 3070
– Good emulsifier
– Stable emulsion
– No coalescence
– Poor emulsifier
– Unstable emulsion
Coalescence
Difference between emulsifiers with good and poor stabilizing properties
22
3.3 High-throughput screening: automated testing and optimizing system
The emulsification step can, forexample, be carried out using anultrasonic probe or a rotor-statorsystem (both laminar and turbulentflow). The stabilizing effect of emulsifiers can be evaluated bytransmittance measurements or by automatic measurement of the particle size distribution using laserdiffraction. Other significant variables,such as the viscosity and creamingproperties of the emulsion obtained,can also be determined fully auto-matically.
HTS apparatus with robot
An emulsifier selection process thatis rational and above all reproducibleis the high-throughput screeningmethod (HTS).
This fully automated process, con-sisting essentially of a robot withmetering, emulsifying and analysisunits, makes it possible to vary alarge number of different parameters(concentration, temperature, emulsi-fier type, composition of formulation,etc.) in very extensive screeningtests.
Pipetting station for emulsifiers
23
3.4 Stabilization of oil/water emulsions with alcohol ethoxylates
Emulsion preparation and methodology
The stability against coalescence atroom temperature of O/W emulsionswas measured with two model oils:
– Thin fluid paraffin oil with a visco -sity of 30 mPa·s (23°C) and an Mn of about 300 as a model ofhydrophobic hydrocarbons
– Sunflower seed oil with a viscosityof 56 mPa·s (23°C) and an acidnumber of maximum 0.15 mgKOH/g as a model of native triglycerides
Demineralized water was used asthe aqueous phase
Eleven classes of alcohol ethoxylatesserved as emulsifiers. For each class,a set of ethoxylates was prepared in steps of 1 – 2 HLB units andmeasured.
O O O OO OH O O
O OO OH
? ?
Why can one surfactant class stabilize while another cannot?
Ethoxylates of linear alcohols
– n-Decanol ethoxylate
– C12C14 coconut fatty alcoholethoxylate (Lutensol® A ..N)
– C13C15 Oxo alcohol ethoxylate(Lutensol AO)
– C16C18 Tallow fatty alcohol ethoxy-late (Lutensol AT)
Ethoxylates of oxo alcohols fromthe oligomerization of higherolefins
– iso C10 Oxo alcohol ethoxylate(Lutensol ON)
– iso C13 Oxo alcohol ethoxylate,medium branched (Lutensol TO)
– iso C13 Oxo alcohol ethoxylate,highly branched (Lutensol TDA)
– iso C17 Oxo alcohol ethoxylate,highly branched
Other alcohol ethoxylates
– C10 Guerbet alcohol ethoxylate(Lutensol XP)
– C10 Guerbet alcohol ethoxylate containing small quantities ofhigher alkylene oxides (Lutensol XL)
– Nonylphenol ethoxylate (LutensolAP)
Macroemulsions were prepared witha low power input.
– Quantity ratios: 30% oil, 1% surfactant, 69% water
– Emulsification procedure: Propeller agitator, P/V=104 W/m3,23°C, 15 minutes
Then the emulsions were stored at23°C, the stability index S deter-mined by the marker method andplotted for each ethoxylate class as a function of the HLB value.
Introduction
The HLB concept gives some indication of which hydrophile of the emulsifier is suitable for an exist-ing emulsification task. On the otherhand, which emulsifier class is bestsuited – in the case of optimization of the HLB value – cannot be givenby the concept, since it oversimplifiesthe relevant physical-chemical effects.Ultimately, this is only possible empirically.
Simple, industrial-grade alcoholethoxylates are especially suited for a systematic study of the stability ofoil-water emulsions. They are simpleto produce from a great variety of Hactive compounds. Their HLB value is easily variable for any hydrophobicpart and they react only slightly toimpurities in both the oil and aqueousphases, such as ions, for example.
24
t in month
r = 0.079 1/month
HLB value (= 20* wt.% EO)
A hyperbola was adapted to the measured data by using the least squares method
S = - log (r x month) is repre-sented as a function of the HLB
From repetitions it follows that s(S) ≈ 0.25. A 2s bar is found in each diagram.
From time to time the average green content is determined with 30 different microscopic images
2.62.52.42.32.22.1
21.9
2
1
0
-1
-2
-310 2 3 4 5 6 7 76 8 9 10 11 12 13 14 15 16 17 18
200 % / (100 % – green %) S = - log (r * month)
Visualized process for calculation of the stability index S
Values of S = 1.5 correspond to roughly 10% coalesced droplets within ahalf-year, values of S = -1.5 to about 10% coalesced droplets in five hours.
Emulsions of paraffin oil in water – ethoxylates of linear alcohols
In the case of emulsions of paraffinoil in water stabilized by ethoxylatesof linear alcohols, one recognizes,on the one hand, that the suitableHLB range depends on the chainlength of the alcohol:
– C10 Alcohol ethoxylates:HLB = approx. 13
– C12C14 to C16C18 alcohol ethoxy-lates: HLB= approx. 9 – 13
On the other hand, the resistance tocoalescence increase in the order
In each case by about one order ofmagnitude.
HLB value
8 9 10 11 12 13 14 15 16 181776
10 % of the dropshave coalesced afterStability index
Mon
ths
Wee
ksD
ays
Hou
rsM
inut
es
2
1
0
-3
-1
-2
≈ 2s
Lutensol® AT
C20C22 x 7.3 EO
Lutensol® A..N, AO
n-Decanol ethoxylate
less suitable well suited
C10 � C12C14 = C13C15 �C16C18 A comparison with C20C22 x 7.3 EO(HLB = 10) shows that an additionalincrease in resistance to coales-cence by chains longer than C18 is
not possible; therefore, among theethoxylates of linear alcohols C16C18
ethoxylates such as Emulan® AT 9 or Lutensol AT 11 are optimal.
25
HLB value
8 9 10 11 12 13 14 15 16 181776
10 % of the dropshave coalesced afterStability index
Mon
ths
Wee
ksD
ays
Hou
rsM
inut
es
2
1
0
-3
-1
-2
≈ 2s
Lutensol® TDA
iso C17 Ethoxylate
Lutensol® TO
Lutensol® ON
Emulsions of paraffin oil inwater – ethoxylates ofbranched alcohols
Among the ethoxylates of branchedoxo alcohols one finds HLB rangessimilar to those among the linearalcohol ethoxylates
– iso C10-Alcohol ethoxylates, Lutensol ON: HLB = approx. 14
– iso C13-Alcohol ethoxylates,medium branched, Lutensol TOHLB = approx. 10 – 15
– iso C13-Alcohol ethoxylates, highlybranched, Lutensol TDA HLB = approx. 12 – 14
– iso C17 Alcohol ethoxylates, highlybranched HLB = approx. 10 – 15
One can also recognize a somewhatbetter stabilization than with theethoxylates of linear alcohols. There-fore, Lutensol TO, for example, stabilizes just as well as ethoxylatesof branched or linear alcohols with16 to 18 C atoms in the hydrophobicpart. Surprisingly, the medium-
branched Lutensol TO types offer abroader HLB window than the highlybranched Lutensol TDA brands,which is an advantage, since withLutensol TO one has to make fewercompromises with regard to othertarget values such as foam or formu-lability.
As opposed to the linear alcoholethoxylates, among which C16C18
ethoxylates clearly stabilize betterthan C12C14 ethoxylates, in the caseof the branched alcohols, the longerchains, such as in the analogous iso C17 alcohol ethoxylates, do notprovide any significant extra advan-tage.
Emulsions of paraffin oil in water – direct comparison of C10 ethoxylates
In the direct comparison of the C10 alcohol ethoxylates – linear, Guerbet andiso C10 oxo alcohol – one finds ideal HLB values around 13 – 14, but aboveall, the following surprising order in the coalescence stabili zation:
HLB value
8 9 10 11 12 13 14 15 16 181776
10 % of the dropshave coalesced afterStability index
Mon
ths
Wee
ksD
ays
Hou
rsM
inut
es
2
1
0
-3
-1
-2
≈ 2s
Lutensol® XL
Decanol ethoxylateLutensol® XP
Lutensol® ON
less suitable well suited
Lutensol ® ON � Lutensol ® XP = linear decanol ethoxylate � Lutensol ® XL
26
HLB value
8 9 10 11 12 13 14 15 16 181776
10 % of the dropshave coalesced afterStability index
Mon
ths
Wee
ksD
ays
Hou
rsM
inut
es
2
1
0
-3
-1
-2
≈ 2s
Lutensol® A..N, AO
Lutensol® TO
Lutensol® TDA
less suitable well suited
Lutensol ® A..N = Lutensol ® AO � Lutensol ® TDA = Lutensol ® TO
Emulsions of paraffin oil in water – direct comparison of C12-C15 ethoxylates
In the direct comparison of the C12 to C15 alcohol ethoxylates one recognizesonce more the above mentioned order with slight advantages on the part ofLutensol TO due to the larger HLB window:
HLB value
8 9 10 11 12 13 14 15 16 181776
10 % of the dropshave coalesced afterStability index
Mon
ths
Wee
ksD
ays
Hou
rsM
inut
es
2
1
0
-3
-1
-2
≈ 2s
Nonylphenol ethoxylate
Lutensol® TO
Lutensol® XL
Emulsions of paraffin oil in water – substitution ofnonylphenol ethoxylate
In the direct comparison of fattyalcohol ethoxylate vs. nonylphenolethoxylate, C16 to C18 ethoxylatescan be excluded because of theirtotally different behavior in terms ofsurfactant properties and formulationbehavior, and Lutensol TDA becauseof its poor biodegradability. There-fore one can consider
Lutensol TO andLutensol XL
to be optimal substitutes for nonyl -phenol ethoxylate for stabilizingparaffin oil emulsions.
Summary: Paraffin oil in water
In conclusion, we can say thatamong the ethoxylates tested, thetypes
Lutensol TO,Emulan/Lutensol AT andLutensol XL
are the best suited for stabilizingemulsions of paraffin oil in water.
27
HLB value
8 9 10 11 12 13 14 15 16 181776
10 % of the dropshave coalesced afterStability index
Mon
ths
Wee
ksD
ays
Hou
rsM
inut
es
2
1
0
-3
-1
-2
≈ 2s
n-Decanol ethoxylate
Lutensol® AT
Lutensol® A..N, AO
C20C22 x 7.3 EO
Emulsions of sunflower oil inwater – ethoxylates of linearalcohols
In the case of emulsions of sunfloweroil in water stabilized by ethoxylatesof linear alcohols, one finds a totallydifferent picture than in the case ofparaffin oil. Ethoxylates of lineardecanols cannot stabilize the emul-sion at all, only from C12 on does onerecognize a slight stabilization aroundHLB = 11. But good stabilization isonly achieved by the ethoxylates ofthe long-chained C16C18 fatty alcohols.
– C16C18-Alcohol ethoxylates, Emulan/Lutensol AT: HLB = approx. 10 – 12
decreases again beyond C18. This isdue to the incompatibility betweenthe waxlike C20C22 alcohol and thetriglyceride.
A comparison with C20C22 x 7.3 EO(HLB = 10) shows that the stabiliza-tion of triglyceride in water withethoxylates of linear alcohols clearly
Now one finds a very surprising picture with sunflower oil andethoxylates of branched alcohols.The order C10 � C13 � C17, antici-pated from the ethoxylates of linearalcohols is found only at a signifi-cantly higher level, although themedium branched iso C13 ethoxy-lates – Lutensol TO – display highlyreproducibly a very sharp maximumin the stabilization around HLB = 11,which corresponds approxmately toLutensol TO 6. Here the ethoxylatesexceed highly branched iso C13
ethoxylates by more than an order of magnitude in stabilization, so thatthe following order now exists:
Emulsions of sunflower oil in water – ethoxylates of branched oxo alcohols
less suitable well suited
Lutensol ® ON � Lutensol ® TDA � iso C17 ethoxylate � Lutensol ® TO
HLB value
8 9 10 11 12 13 14 15 16 181776
10 % of the dropshave coalesced afterStability index
Mon
ths
Wee
ksD
ays
Hou
rsM
inut
es2
1
0
-3
-1
-2
≈ 2sLutensol® TO
Lutensol® ON
iso C17 Ethoxylate
Lutensol® TDA
28
HLB value
8 9 10 11 12 13 14 15 16 181776
10 % of the dropshave coalesced afterStability index
Mon
ths
Wee
ksD
ays
Hou
rsM
inut
es
2
1
0
-3
-1
-2
≈ 2s
Lutensol® XL
Lutensol® ON
Lutensol® XP
n-Decanol ethoxylate
Emulsions of sunflower oil in water – direct comparison of C10 ethoxylates
In the direct comparison of the C10 ethoxylates, one recognizes an unambiguous order in the stabilizing effect
where only the surfactants based on C10 Guerbet alcohol act well againstcoalescence.
less suitable well suited
Linear C10 ethoxylate � Lutensol ® ON � Lutensol ® XL = Lutensol ® XP
less suitable well suited
Lutensol ® A..N, Lutensol ® AO � Lutensol ® TDA � Lutensol ® TO
HLB value
8 9 10 11 12 13 14 15 16 181776
10 % of the dropshave coalesced afterStability index
Mon
ths
Wee
ksD
ays
Hou
rsM
inut
es
2
1
0
-3
-1
-2
≈ 2s
Lutensol® TDA
Lutensol® A..N, AO
Lutensol® TO
Emulsions of sunflower oil in water – direct comparison of C12-C15 ethoxylates
In the direct comparison of the C12-C15 alcohol ethoxylates the great differences in performance are again especially striking.
29* Ethoxylates of long-chained alcohols with 16 – 18 C atoms in the hydrophobic part stabilize against coalescence so well that no correlation with the degree of branching is recognizable.
HLB value
8 9 10 11 12 13 14 15 16 181776
10 % of the dropshave coalesced afterStability index
Mon
ths
Wee
ksD
ays
Hou
rsM
inut
es
2
1
0
-3
-1
-2
≈ 2s
Lutensol® AT
Nonylphenol ethoxylate
Lutensol® TO
Emulsions of sunflower oil in water – nonylphenol substitution
Compared with nonylphenol ethoxylate, one will note that the latter – likeLutensol TO, but with a slightly higher HLB – displays a surprisingly goodstabilization and is exceeded only by Lutensol TO or long-chained ethoxy-lates such as Emulan AT or Lutensol AT.
Effect of the degree of branching on the emulsion stabilization
If the maximally attainable stabilityindex with optimal HLB value for agroup of commercial and experimen-tal ethoxylates is plotted against thenumber of branches per C atom inthe hydrophobic part, then one findsa surprising correlation for the groupof C10 and C12 to C15 alcohol ethoxy-lates*.
The optimal window containsethoxylates of alcohols with about0.15 branches per C atom in thehydrophobic part, such as the com-mercial ethoxylates Lutensol TO or Lutensol XP. The reason for this window is not clear; obviously anoptimum is achieved here in terms of packing and flexibility in the inter-facial film.
It is further favorable that the ethoxy-lates of these medium-branchedalcohols are usually readily biode -gradable and lowly aquatoxic. Theseproducts are therefore the bestchoice if one is searching for shortor medium chained non-ionic surfac-tants that are to stabilize emulsionswell.
Number of branches per C atom in the hydrophobic part
0.05 0.1 0.15 0.2 0.25 0.3 0.350-3
-2
-1
0
1
2
Lutensol® XP
C12 – C15 Ethoxylates of paraffin oil
Lutensol® TO
C12 – C15 Ethoxylates of sunflower oil
C10 Ethoxylates of paraffin oil
C10 Ethoxylates of sunflower oil
3.5 Poly dimethyl siloxane emulsions in water made with nonionic surfactants from BASF
30
Three principal methods are used to produce polydimethyl siloxane (PDMS) emulsions:
Microemulsions can only be made if the silicon oil is of very lowviscosity – e.g. short chains or cyclicoligomers – or by using silicon-based emulsifiers. Here, simple mix-ing both the oil and water phasestogether with a well-tuned emulsifierpackage leads to the microemul-sions; a special emulsificationmachine is not required.
High-energy emulsification isused to emulsify normal (viscous)PDMS oils. For oils with viscositiesup to ca. 500 mPa·s rotor-statormachines can be used, whilst high-pressure homogenizers can emulsifyoils with viscosities up to ca.100,000 mPa·s. Emulsification ofviscous oils at elevated tempera-tures – as done with e.g. hydrocar-bons – is usually not helpful,because of the low temperaturedependence of the PDMS viscosi-ties.
Emulsion polymerization isanother common process to manu-facture PDMS emulsions. For targetoil viscosities beyond 100,000mPa·s emulsion polymerization isthe only feasible process.
Standard process to emulsifyPDMS
– Charge your vessel with 40 partsof water
– Upon stirring add 10 parts ofemulsifier
– Then add 50 parts of the PDMSoil, mix thoroughly
– Subject the mixture to the emulsification procedure
– Fine-tune the emulsifier contentand HLB value for optimum stability.
The following charts illustrate the stability of aqueous PDMS emul-sions against coalescence for vari-ous alcohol ethoxilates as a functionof emulsifier chemistry and HLBvalue as determined by the Markermethod.
Lutensol® AT, TO and XL offer bestcoalescence stabilization. Lutensol XLis preferred, as it forms small dropletsat rather low power input and stabi-lizes across a wide HLB range.
HLB value30% oil, 1% emulsifier, 69% water, storage at 23°C
minutes
hours
days
weeks
month
Performance of alcohol ethoxilatesCoalescence stability at room temperature
1% o
f th
e dr
ople
ts c
oale
sced
aft
er
5 6 7 8 9 10 11 12 13 14 15 16
Lutensol®/Emulan® AT
Lutensol® XL
Lutensol® XP
Lutensol® ON
Lutensol® TO
APEO (for comparison)
HLB value30% oil, 1% emulsifier, 69% water, storage at 23°C
minutes
hours
days
weeks
month
Performance of alcohol ethoxilatesCoalescence stability at room temperature
1% o
f th
e dr
ople
ts c
oale
sced
aft
er
5 6 7 8 9 10 11 12 13 14 15 16
Lutensol®/Emulan® AT
Lutensol® XL
Lutensol® XP
Lutensol® ON
Lutensol® TO
APEO (for comparison)
At elevated temperatures the longerchain ethoxilates Lutensol AT and TOoffer significant better stability than the medium chain nonionics Lutensol XL and XP.
31
HLB value30% oil, 1% emulsifier, 69% water, storage at 70°C
minutes
hours
days
weeks
month
Performance of alcohol ethoxilatesCoalescence stability at elevated temperature
1% o
f th
e dr
ople
ts c
oale
sced
aft
er
5 6 7 8 9 10 11 12 13 14 15 16
Lutensol®/Emulan® AT
Lutensol® XL
Lutensol® XP
Lutensol® ON
Lutensol® TO
APEO (for comparison)
HLB value30% oil, 1% emulsifier, 69% water, storage at 23°C
minutes
hours
days
weeks
month
Performance of alcohol ethoxilatesCoalescence stability at room temperature
1% o
f th
e dr
ople
ts c
oale
sced
aft
er
5 6 7 8 9 10 11 12 13 14 15 16
Lutensol®/Emulan® AT
Lutensol® XL
Lutensol® XP
Lutensol® ON
Lutensol® TO
APEO (for comparison)
Conclusion– For maximum emulsion stability we recommend the emulsifierEmulan® AF. Lutensol TO 5 to 7 isthe second best robust alter native.
– For emulsions that are not sub-jected to elevated temperaturesLutensol XL 50 works perfectly.Also, Lutensol XP 50 suits nicelyhere.
In addition to perfect coalescence stabilization, the special nonionic emulsifier Emulan® AF offers good resistance against creaming plus a low odor.
* for extremely low odor choose Lutensol XA, the narrow range versions of Lutensol XL
Performance of nonionic emulsifiers in emulsification of PDMS
Low Low Small Low Ecologycoalescence creaming droplets odor
Emulan® AF ++ + o ++ ++
Lutensol® TO 5-7 ++ o o o ++
Lutensol® XL 50 + o + +* ++
Lutensol® XP 50 o o + o ++
Emulan® AT 9 ++ – o ++ ++
Lutensol® AT 11 ++ – o ++ ++
APEO (for comparison) ++ – – ++ –
32
3.6 Amino modified silicon microemulsions made with nonionic surfactants from BASF, e.g. for textile softening
Amino modified silicones are perfectly suited for textilesoftening
The ammonium group ...... exhibits good affinity to the fabric,
thus the wash permanence isgood.
... offers the opportunity to make silicon oil microemulsion concen-trates with perfect shelf stability.
... offers the opportunity to make silicon oil microemulsion concen-trates which upon dilution formfine nanoemulsions. Those areable to penetrate the fiber bundlevery uniformly.
Making the microemulsion concentrate
The literature cites a large number ofmethods for making amino modifiedsilicon oil microemulsions. Thoseinclude high- and low-energy input,cold, hot and sometimes even multi-step processes, with cofeed of water, emulsifier or acids inbetween. All these processes are optimized to overcome gel formation and toachieve stable emulsions with smalldroplet sizes. However, the simplestprocesses can be sketched roughlyas follows:
Method 1
– dissolve the nonionic surfactantwith water.
– add the amino silicon oil and emulsify as finely as possible
– upon vigorous stirring add acid to charge the amino oil. Themicroemulsion will form automati-cally.
Method 2
– mix the non-aqueous nonionic surfactant with the silicon oil and stir
– upon stirring add acetic acid to charge the amino oil
– upon vigorous stirring add waterand emulsify
In both cases elevated temperatureshelp to overcome the formation ofgels or gel particles.
Typical start values for the finalmicroemulsion are: 25% silicon oil,12% nonionic emulsifier (HLB = ca. 10 – 12), water to 100%,adjust pH to 4.5. Subsequently, optimize oil and emulsifier content plus HLB.
33
Clear, nonviscous microemulsionconcentrate
Gel formation due to choice ofless suitable emulsifier
Lutensol® TO
HLB
18
17
16
15
14
13
12
11
10
% S
urfa
ctan
t
9 10 11 12 13 14
5 – 23°C
50°C
Tolerance windowEfficiency
Microemulsion
o/w Emulsion
Silicon oil A: Silicon oil B: 1.0 Pa·s, 0.3 mmol N/g 1.0 Pa·s, 0.6 mmol N/g
NPEO solid/gel solid/gel
Lutensol® TO ca. 40,000 mPa·s ca. 9,000 mPa·s
Lutensol® XL ca. 6,000 mPa·s ca. 5,000 mPa·s
Lutensol® XP ca. 2,000 mPa·s ca. 3.000 mPa·s
Ranking of the nonionics –microemulsion
Definitions:Efficiency is the required minimumamount of nonionic emulsifier tomanufacture a microemulsion withphase stability between 5 and 50°C.
Tolerance window is the width ofthe HLB window of the nonionicemulsifier leading to a microemulsionwith phase stability between 5 and50°C.
Gelling tendency is the degree offormation of gel lumps duringmicroemulsion manufacture, whichare difficult to dissolve.
High oil content suitability is thepossibility to form microemulsionswith both high oil content and lowviscosity.
Examples:Microemulsions with different typesof amino oils are checked in a tem-perature interval of 5 to 50°C todetermine both the efficiency andthe tolerance window of the nonionicemulsifiers.
Recipe:25% Silicon Oil 1.0 Pas/
0.6 mmol N/gx% Lutensol® TO75-x% WaterpH = 4.2
Please note that the microemulsionphase diagram is highly dependendon the oil type and needs to be optimized for every formulation.
High oil content suitabilityMicroemulsions with high oil contenthave been optimized with respect toboth emulsifier content and HLBvalue and their viscosity have beendetermined. Systems with low vis-cosity offer the possibility to processviscous amino oils or manufacturemicro emulsions of high oil concen-tration.
Experimental conditions:40% oil, ca. 20% surfactant, HLB ca. 10. Brookfield Spindle 4 – 6, 30 rpm, 23°C.
34
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ark of BASF group
Low gelling High oil content Efficiency &tendency* suitability tolerance window
Lutensol® TO ++ o +
Lutensol® XL ++ + +
Lutensol® XP ++ ++ +
Lutensol® ON + ++ ++
For comparison:
Nonyl phenol ethoxilate o – – +
Fatty alcohol ethoxilate o – –* Microemulsions are checked for the degree ofintermediate formation of gel particles and theirdissolution kinetics.
Conclusion:ranking of the nonionic surfactants with respect tomicroemulsion formation
Please note that the table exhibitsthe average trend. From oil to oilslight variations may occur.
Silicon Oil 1.0 Pas/0.6 mmol N/gwith APEO with Lutensol® TO
storage at pH 10
Ranking of the nonionics – diluted microemulsion under application conditions
Application
The microemulsion concentrate isdiluted to ca. 5 g/l silicon oil to beapplied on the fabric. Upon heating,addition of salts, bases or shearstress this diluted emulsion must be stable, otherwise staining of thefabric or silicon residues in themachinery are possible. Emulsifierswith additional high-speed wettingpower help to deaerate the fabricduring the impregnation step.
Stability against high temperatures
The diluted microemulsion is subjected to high temperatures and stored or sheared. Instableemulsions lead to fogging, creaming or coalescence.
Stability against high pH
The diluted microemulsion is subjected to high pH and stored or sheared. Instable emulsions lead to fogging, creaming or coalescence.
Please note that the table exhibitsthe average trend. From oil to oilslight variations may occur.
Stability of the diluted emulsionhigh temperature MgCI2** high pH
Lutensol® TO ++ ++ ++
Lutensol® XL + + +
Lutensol® ON ++ o o
Lutensol® XP ++ – –
For comparison:
Nonyl phenol ethoxilate + + to ++ + to ++
Fatty alcohol ethoxilate ++ ++ ++
Performance under application conditions
**see next page
35
Silicon Oil 1.0 Pas/0.6 mmol N/gwith APEO with Lutensol® TO
storage in 1 w% MgCI2
Stability against MgCl2
The diluted microemulsion is subjected to MgCl2 and stored or sheared. Instable emulsions lead to fogging, creaming or coalescence.
Conclusion amino modified silicon oil microemulsions
For the manufacture of amino modified silicon oil microemulsionsfor textile softening we recommendLutensol TO and XL types. Bothoffer easy formation of the micro -emulsion. Lutensol TO additionallyoffers optimum robustness in theapplication plus good wetting and
Please note that the table exhibitsthe average trend. From oil to oilslight variations may occur.
Microemulsion Robustness Wetting,during application deaeration
Lutensol® TO ++ ++ +
Lutensol® XL ++ + ++
Lutensol® ON ++ + +
Lutensol® XP ++ o ++
For comparison:
Nonyl phenol ethoxilate o + to ++ o
Fatty alcohol ethoxilate – ++ o
penetration. Lutensol XL offers high-speed wetting and thus very uniformpenetration, the formulation ofmicroemulsions of low viscosity plusgood robustness under applicationconditions.
Electron microphotograph of a frozen microemulsion, Source: Science vol. 240, Microemulsions, Manfred Kahlweit. Copyright 1988, AAAS[caption poorly legible]
*cmc = critical micelle concentration
36
3.7 Microemulsions In the microemulsion, oil and water phases are still molecularlyseparated by the surfactant film; frequently bicontinuous structuresare formed, but lamellar and otherstructures are also known.
Macroscopically one observes aswelling of the surfactant phase byoil and water phase; one thereforesays that the surfactant has solubi-lized the oil and water, and onereports the solubilization parametersSO and SW which describe the ratioof the volumes of oil and waterphase, respectively, to the pure surfactant in the microemulsion.These values in efficient systemsmay amount to more than S = 10and correlate with the interfacialtensions in the system. For high solubilization parameters S micro -emulsions appear opaque due tothe Tyndall effect; they are never -theless thermodynamically stable.
When an oil phase, an aqueousphase and one or more surfactantsare mixed, then microemulsions canbe formed under certain conditions:
– The hydrophobic parts of the surfactants are miscible with the oil phase
– The hydrophilic parts of the surfactants are miscible with theaqueous phase
– The surfactants are scarcely miscible with the oil or waterphases on the molecular level i.e. when the cmc* in oil and water phase are exceeded, the surfactants are behaving“amphiphobically”.
– Hydrophilic and hydrophobic partof the surfactant or surfactant mixture are approximately of equalsize; the surfactant film thereforeprefers planar geometries
– The Krafft temperature of the surfactants is exceeded
Water
Oil
0.1 µm
Substantial differencesbetween microemulsions andconventional (macro-) emulsions
Microemulsions are thermo -dynamically stable phases thatform spontaneously after the mixing of all components withoutan input of energy.
In microemulsions ultra-low inter-facial tensions occur between theoil and water phases.
Microemulsions react moststrongly to a change in intensivemagnitudes in the system, e.g.,the temperature, the compositionof the oil and water phases as wellas the surfactant mixture. Whenthe range of existence of micro -emulsions is exceeded, they oftentransform into macroemulsions,and this is an effective means ofproducing the latter.
Microemulsions, as opposed toconventional emulsions, separateexcess oil and water phases outrelatively quickly.
Microemulsions have maximal stability at the phase inversionpoint of the system.
Microemulsions are very dynamic;boundary surfaces form and disappear, usually within micro -seconds.
Examples of applications of microemulsions
Stable formulation of two non-miscible liquid phases.
Formulation for production ofmacroemulsions via spontaneousemulsification.
Creation of a large contact areabetween two non-misciblephases, e.g. to facilitate materialtransfers during dissolving andcleaning processes, reactions orextractions.
Generation of ultra-low interfacialtension between two phases, e.g.,to mobilize the one in the other.
Simultaneous transporting of two non-miscible phases througha porous medium, a fabric orthrough narrow capillaries.
For use in the following areas:
Analytics
Soil sanitation
Fuels and propellants
Chemical reactions
Extraction
Corrosion protection
Cosmetics
Varnish
Leather industry
Metal working
Foods
Nanotechnology
Oil field
Plant protection
Pharmaceuticals
Cleaning
Textile industry
Environmental protection
37
38
Berührung mit der Hand Bereiche mit w/o EmulsionIsotherm 23 °C
Practical rules for the preparation of micro emulsions
The microemulsion is a stable phasein the oil-phase, water-phase andsurfactant phase space. The practicalexploitation of this phase space,however, is frequently made difficultby kinetic inhibition of coalescenceby other, surfactant-richer phases,often of higher viscosity, and precipi-tations of particularly ionic surfac-tants.
It has been shown empirically thatthe simplest way to produce micro -emulsions is with ethoxylates ofmedium-chain branched alcohols.The Lutensol® ON brands are parti -cularly useful for this, but also theLutensol XA, Lutensol XL or Lutensol XP brands. It is best tostart with a 45% oil phase, 45%water phase and 10% Lutensol; as the starting point for water andalkanes an HLB around 9 – 11 issuitable. An addition of co-surfac-tants is also helpful; here branched,short-chain alcohols with 4 – 6 carbon atoms are suitable.
Then the temperature is varied untila single-phase region – the micro -emulsion – or a three phase region –the microemulsion with additional oilor water phase is established. Hereit is essential to distinguish betweenthe “real” three-phase region andcreamed, partially coalesced emul-sion, which may have a similarappearance.
One has now reached the phaseinversion. By varying the HLB valueof the surfactant or surfactant mix-ture, now the range of existence ofthe microemulsion can be adjustedto the desired temperature. A 10Kelvin shift downward in the case ofethoxylates corresponds to an HLBvalue lower by about 0.5 – 1.0 unitsand vice versa. The volume of themicroemulsion phase can finally beexpanded by increasing the surfac-tant content in the formulation untilthe one-phase region is reached.
If the optimal HLB value of the surfactant mixture is determined with the medium-chain ethoxylatesLutensol XA, Lutensol XL, Lutensol XPor Lutensol ON, these may be successively replaced with longer-chain surfactants, such as, e.g.,Lutensol TO, Emulan® A, Emulan AT 9 or Lutensol AT in orderto optimize the properties of themicro emulsion.
Example of a microemulsion
In a 1 liter upright cylinder at 23°Cone adds:
– 436.4g H2O– 112.8g NaCl– 50g Lutensol TO 6 (HLB = 11)– 5g sec-butanol– 354.4 g n-dodecane
Then the components are thoroughlymixed and allowed to stand over -night. Three phases are formed: A dodecane phase on top, a saltsolution at the bottom and anopaque microemulsion in the middle.
By varying the temperature, one canobserve how the ratios of dodecaneand salt solution in the microemul-sion vary. For example, if onetouches the upright cylinder with the hand in the region of themicroemulsion phase, the latter will break up suddenly into a w/oemulsion due to the warming effect.
The microemulsion can also beremoved and a stable, finely dividedO/W emulsion produced by injectioninto the tenfold quantity of water.
39
Temperature-insensitivemicroemulsions
To formulate microemulsions with abroad thermal existence range, onerequires a surfactant mixture thathas the same space requirement for its hydrophobic and hydrophilicparts irrespective of the tempera-ture. Here one will exploit the factthat when ethylene oxide chains areheated they successively lose theirhydrate shell and therefore occupyless space, while in the case of ionicgroups, the Debye radius increasesand with it the space requirement. A mixture of ethoxylate and ionicsurfactant is therefore recommendedfor the formulation of such micro -emulsions. Empirically, Lutensit® A-BO hasbeen shown to be of value as an ionic partner. By combining Lutensit A-BO with ethoxylates,microemulsions with existenceranges over 60 Kelvin, and more,can be produced.
Example of a three phasesystem expanded from 10 to 50°C– 19.23g of 1% aqueous
NaCI solution– 14.62g n-dodecane– 0.67g Lutensit A-BO– 0.60g Lutensol AO 3
Microemulsions with extremelypolar oils
Even with extremely polar oils,microemulsions can be formulatedprovided that surfactants with a correspondingly high HLB value are selected.
– 43.5g water– 3g NaCl– 66g ortho-sec-butylphenol– 37.5g Lutensit TC-CS 40
(HLB = 41)
At 23°C one obtains three phases,the middle phase is the micro -emulsion.
Further reading:
How to Study Microemulsions –Kahlweit, Strey, Haase, Kunieda,Schmeling, Faulhaber, Borkovec,Eicke, Busse, Eggers, Funck, Richmann, Magid, Södermann,Stilbs, Winkler, Dittrich und Jahn, J. Coll. Surf. Sci., 118, 2, 1987, 436
General Patterns of the PhaseBehavior of Mixtures of H2O, Nonpolar Solvents, Amphiphiles,and Electrolytes. 1 – Kahlweit,Strey, Firman, Haase, Jen,Schmäcker, Langumir 1988, 4, 499
General Patterns of the PhaseBehavior of Mixtures of H2O, Nonpolar Solvents, Amphiphiles,and Electrolytes. 2 – Kahlweit,Strey, Schomäcker, Haase, Langumir 1989, 5, 305
Nonionic Surfactants with Linearand Branched Hydrocarbon Tails:Composition Analysis, PhaseBehavior, and Film Properties inBicontinous Microemulsions –Frank, Frielinghaus, Allgaier, Prast,Langumir 2007, 23, 6526
Interfacial Tensions and Solubiliz-ing Ability of a MicroemulsionPhase that Coexists with Oil andBrine – Huh, J. Coll. Interface Sci.,71, 2, 1979, 408
Interfacial Tensions in Micro -emulsions – Wennerström, Balogh,Olsson, Coll. Surf. A: Physicochem.Eng. Aspects, 291, 2006, 69
Uses and Applications of Micro -emulsions – Paul, Moulik, Current Science, 80, 8, 2001, 990
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