Surfactants and Emulsion

Surfactants in water-borne paints

Ann-Charlotte Hellgren, Peter Weissenborn, Krister Holmberg*

Institute for Surface Chemistry, P.O. Box 5607, SE-114 86, Stockholm, Sweden

Received 15 July 1998; received in revised form 15 September 1998; accepted 1 March 1999

Abstract

The main uses of surfactants in water-borne coatings are discussed. Special attention is put on the use of surfactants in latex polymer-ization and in post-emulsification of binders, such as alkyd resins. The advantage of polymerizable surfactants as emulsifier is pointed outand the use of atomic force microscopy (AFM) to observe differences in film properties between formulations based on polymerizablesurfactants and on conventional surfactants is illustrated. The paper further discusses the problem of competitive adsorption betweensurfactants and between surfactant and associative thickener in paint formulations. 1999 Elsevier Science S.A. All rights reserved

Keywords:Paints; Water-borne; Emulsifier; Alkyd; Polymerizable; Surfactant

1. Introduction

All water-borne coatings need surfactants in order toreduce the free energy of the various interfaces of the sys-tem, thus providing kinetic stability to the formulation. Sur-factants are used as binder emulsifier and as pigmentdispersant, they are needed to improve wetting on lowenergy substrates, to control foaming during applicationand processing, and to prevent film defects caused by sur-face tension gradients. In addition, surface active polymers,often referred to as associative thickeners, are widely usedto optimize the rheological properties of the formulation,and anionic polyelectrolytes such as polyphosphates arecommonly used as pigment dispersing agents. Takentogether, a water-borne paint formulation is extremely com-plex with a plethora of low and high molecular weightcompounds competing for available surfaces, such as binderdroplets, pigment particles, and, although much smaller insurface area, the substrate to be painted. The situation isschematically illustrated in Fig. 1.

The majority of surfactants used in coatings formulationsare standard anionic and nonionic amphiphiles, such as fatty

alcohol sulfates, alkylaryl sulfonates and alcohol ethoxy-lates. Cationic and amphoteric surfactants are rarely used.A few types of speciality surfactants have found specificniches. Fluorosurfactants [1,2] and silicone surfactants[3,4] reduce surface tension to extremely low values.They are used in paint formulations to eliminate surfacetension gradients that can form due to faster evaporationof the solvent from the coating edges than from the center.Acetylenic glycols, characterized by having two short,bulky hydrocarbon chains surrounding the polar group,are another type of niche surfactant. These non-micelleforming surfactants form expanded films on water surfaceswhich can withstand high surface pressures. They arewidely used as antifoaming agents in coatings [5,6].

In recent years polymerizable surfactants have become ofinterest as emulsifiers in emulsion and suspension polymer-ization [7,8]. By using surfactants that become covalentlyattached to the latex particle, many of the problems encoun-tered with conventional emulsifiers can be avoided or atleast minimized. Positive effects may be obtained both onthe stability of the latex and on the properties of the driedfilm.

The present paper focuses on recent developments inmain stream paint surfactants used in latex dispersionsand alkyd emulsions. The advantage of polymerizable sur-factants as emulsifiers is pointed out and the versatility ofatomic force microscopy to monitor film topography during

Progress in Organic Coatings 35 (1999) 79–87

0300-9440/99/$ - see front matter 1999 Elsevier Science S.A. All rights reservedPII : S0300-9440(99)00013-2

* Corresponding author. Present address: Department of Applied SurfaceChemistry, Chalmers University of Technology, 412 96, Go¨teborg, Swe-den.

drying is illustrated. Finally, the important phenomenon ofcompetitive adsorption of surfactants in paint formulationsis discussed.

2. Emulsifiers

2.1. Lattices

The majority of water-borne paints are latex paints, i.e.,aqueous dispersions of water insoluble polymers made byemulsion polymerization using free radical initiators. In themajority of cases the polymers are based on combinations ofmonomers, often with a high content of water insolubleentities such as methyl methacrylate, butyl acrylate andstyrene and a much smaller fraction of water soluble mono-mers such as acrylic and methacrylic acid. The water solu-ble monomers give oligomeric acid segments at the latexparticle surface which improves the colloidal stability of theformulation and adhesion and curing characteristics of thefilm. Most lattices have an average particle diameter in therange 100–500 nm.

The emulsifier used in latex preparation is often a com-bination of nonionic and anionic surfactants. The nonionicsurfactant has traditionally been an alkylphenol ethoxylate,but environmental concern has caused a change over toother ethoxylated surfactants, such as fatty alcohol ethoxy-late or fatty acid monoethanolamide ethoxylate. The ethox-ylate is the surfactant mainly responsible for dispersionstabilization. The steric stabilization provided by surfac-tants with relatively long polyoxyethylene chains (. 10EO) is needed in order to retain stability at high solids con-tent in the presence of electrolytes. Steric stablization alsogives proper shear stability to the latex. The presence of

anionic surfactant, usually an alkyl sulfate or an alkylarylsulfonate, during the latex synthesis is needed to compen-sate for the reverse temperature dependence of ethoxylatedsurfactants. For nonionics an increase in temperature leadsto a decrease in water solubility and an increase in oil solu-bility. During the course of the emulsion polymerizationthere is an increase in temperature which would lead toformation of a water-in-oil emulsion if nonionic surfactantswere the sole emulsifier. Upon depletion of the monomerphase, i.e., at high conversion, there would be a phase inver-sion into an oil-in-water emulsion. Such a phase inversionleads to a very broad particle size distribution, since particlenucleation as well as reaction kinetics will be out of control[9,10]. A way to circumvent the problem is to use a semi-continuous polymerization process with the monomer beingslowly fed into the reactor during the polymerization.

Fig. 2 shows a typical surfactant monolayer at the surfaceof latex droplets. It is important to realize that the surfac-tants are not permanently adsorbed at the surface but subjectto a continuous adsorption-desorption process. As will bediscussed below, the driving force for adsorption is strongerfor nonionic surfactants than for anionics, unless the elec-trolyte concentration is very high. This means that the ratioof nonionics to anionics at the surface is usually muchhigher than the ratio in bulk solution.

Anionic surfactants provide electrostatic stabilization andsuch lattices exhibit good storage stability in formulationscontaining low or moderate salt concentration. However,lattices made with only anionic emulsifier are not stable athigh electrolyte concentration. Evaporation of water duringthe drying process leads to a continuous raise in ionicstrength of the formulation. Often the stability limit isexceeded at a relatively early stage, leading to particle coa-gulation and consequent loss in film gloss.

Fig. 1. A paint formulation containing emulsion droplets, pigment particles, associative thickener and surfactant.

80 A.-C. Hellgren et al. / Progress in Organic Coatings 35 (1999) 79–87

2.2. Post-emulsified binders

Another common approach to water-based coating for-mulations is post-emulsification of a polymer in water. Sev-eral condensation polymers, e.g., alkyds, i.e. fatty acidmodified polyesters [11–14], polyurethanes [15,16] andepoxy resins [17], have been made into dispersions by useof a suitable emulsifier and application of high shear. Forinstance, long oil alkyd resins of the type used in whitespirit-based formulations have been successfully emulsifiedusing nonionic surfactants such as fatty alcohol ethoxylates,alkylphenol ethoxylates or fatty acid monoethanolamideethoxylates [18–20]. Neutralization of alkyd carboxylicgroups helps in producing small emulsion droplets andwith the proper choice of surfactant, droplet diameters ofless than 1mm can be obtained. Such dispersions are suffi-ciently stable for most applications.

Fig. 3 illustrates the important fact that whereas a non-ionic surfactant needs to be added in an amount sufficient togive close packing of the emulsifier on the surface in orderto give sufficiently small droplets, an anionic surfactantgives small droplets already at concentrations that givevery low packing density [21]. This is consistent with thedifferent mechanisms by which nonionic and anionic sur-factants exert stabilization, as discussed above. Fig. 4 showsthat there is good correlation between droplet size andmechanical stability, the smaller the droplet, the higherthe shear rate needed for coalescence. As for latex disper-sions, alkyd emulsions stabilized with only anionic surfac-tants are highly sensitive to electrolytes.

The main drawback of water-borne alkyd paints is slowdrying. This is partly due to the comparatively low evapora-tion rate of water but there is also strong evidence thatcatalysis of autoxidation does not work properly in water-

borne systems. The distribution of the drier, in particularcobalt alkanoate, between the alkyd and water phases is

Fig. 2. A mixed monolayer of nonionic and anionic surfactant usually stabilze latex droplets.

Fig. 3. Effect on initial droplet size of concentrations of top: the anionicsurfactants sodium dodecylbenzene sulfonate (DoBS), sulfated hexadecy-lalcohol (C16-S) and sulfated hexadecyl/octadecylalcohol ethoxylate (2EO) (C16/C18-EO2-S); bottom: the nonionic surfactants dodecyl/tridecy-lalcohol ethoxylate (15 EO) (C12/C13-EO15), nonylphenol ethoxylate (20EO) (NP-EO20) and linseed oil fatty acid monoethanolamide ethoxylate(13 EO) (LA 13).

81A.-C. Hellgren et al. / Progress in Organic Coatings 35 (1999) 79–87

believed to influence the early stages of drying of alkydemulsions [13,18]. Fig. 5 illustrates that the distribution ofcobalt between the phases is strongly affected by the pH ofthe formulation [13]. This is a practically important obser-vation since it is known that there is often a drop in pH onstorage of paints.

2.3. Emulsification of short oil alkyds

Alkyd emulsions are also of interest in the industrial coat-ings market. The alkyds used for such applications, so calledshort oil alkyds, have a much higher viscosity and are mostconveniently emulsified in a phase-inversion process. Theemulsifier, which can either be a nonionic surfactant, ananionic surfactant or a combination, is dissolved in thealkyd at high temperature and water is added under lowshear so that a water-in-oil emulsion is formed. For alkydsof very high viscosity the process must be performed inpressurized vessels to prevent boiling of the water. By add-ing more water and/or lowering the temperature, the emul-sion is made to invert and form an oil-in-water emulsion[22].

The location of the emulsifier prior to phase inversion isof prime importance. This was demonstrated by attemptingto prepare emulsions with different ratios of emulsifier

added to the alkyd and water phases [23]. Emulsions ofsmallest droplet size (less than 1mm) were obtained whenthe emulsifier was added to the alkyd before the addition ofpure water. To explain this result attempts were made tomeasure the emulsifier concentration in the water phase.This was only experimentally possible at temperaturesbelow and approaching the inversion temperature (i.e. inthe oil-in-water emulsions). The method involved phaseseparation of the emulsions by temperature controlled ultra-centrifugation and measurement of the surface tension ofthe separated water phase to determine the emulsifier con-centration. Emulsifier concentration was converted to per-cent distribution of the total amount of emulsifier originallyadded to the alkyd, which gave an indication of the migra-tion of emulsifier from the alkyd phase to the water phaseduring phase inversion.

For a fatty alcohol ether sulphate emulsifier containing 30oxyethylene units (C12EO30-S) about 75% migrated fromthe alkyd phase to the water phase. This value remainedsteady over the temperature range of 25–80°C (the inver-sion temperature was 90°C). For a series of fatty alcoholether sulphate emulsifiers migration from the alkyd to waterphase increased with the length of the polyoxyethylenechain from 51% for the EO5 compound, via 66% for theEO10 surfactant to 75% for the EO30 compound. A stableemulsion was only achieved with the latter emulsifier. Whenincreasing the polyoxyethylene chain further, the level ofmigration decreased significantly (to 36% for the EO50surfactant), and the droplet size was slightly larger thanfor the EO30 compound. These results suggest that mobility(or migration) of the surfactant during phase inversion iscontrolled by both the hydrophilicity (water solubility) ofthe surfactant and its molecular size, a very high molecularweight surfactant diffusing too slowly across the interface.However, bulky emulsifiers with very long polyoxyethylenechains may still be able to stabilise the droplets due to theirability to induce a long range steric repulsive force.

Two fatty alcohol ethoxylates were also tested, C16/18EO25 and C16/18EO80. The former surfactant producedan emulsion which phase separated upon standing. Analysisshowed that only 11% had migrated from the alkyd phase.

Fig. 4. Mechanical stability of emulsions stabilized by top: the anionicsurfactant sodium dodecylbenzene sulfonate; bottom: the nonionic surfac-tant linseed oil fatty acid monoethanolamide ethoxylate (13 EO). Theinitial droplet sizes are given in the figures. Increase in droplet size onshearing is a sign of coalescence. The decrease in droplet size for the 7mmdroplets of the bottom figure reflects shear-induced disintegration of thelarge aggregates formed.

Fig. 5. Concentration of cobalt in the alkyd and water phases as a functionof pH. Cobalt 2-ethylhexanoate was used in an amount of 0.2 wt.% onalkyd. The oil-in-water alkyd emulsions were made without emulsifier.


The latter product produced a stable emulsion (similar toC12EO30-S) and 25% had migrated from the alkyd phase.Again the differences seem to be a reflection of the hydro-philicity of the emulsifiers. Compared to the fatty alcoholether sulphates, the fatty alcohol ethoxylates would beexpected to have a higher solubility in the alkyd phase atthe emulsification temperature and, therefore, their drivingforce to migrate to the water phase is much smaller.

The results from the migration studies show that theemulsifier must be in the alkyd phase prior to emulsificationand that any migration of the emulsifier from the alkydphase to the water phase occurs primarily during phaseinversion. Despite the anionic emulsifiers giving morestable emulsions, their high concentration in the waterphase would be detrimental to the water resistance of thedry alkyd film. Therefore, a blend of nonionic and anionicsurfactants is often suitable.

Post-emulsification is simplified if the polymer itself issurface active. This can be achieved, e.g., by using polymersof high acid values or by using polymers with noncharged,hydrophilic segments such as polyoxyethylene chains. Suchpolymers can often be emulsified with considerably lesssurfactant, but the trade-off is water and chemical resistanceof the paint film. If the polymer is sufficiently polar, noemulsifier at all is needed. However, such binders need tobe crosslinked during curing in order to give acceptable filmproperties [24].

3. Competitive adsorption of surfactants

Competitive adsorption of surface active agents is a com-mon problem in paints. In fact, in a paint formulation, withits many different surface active species and its variety ofinterfaces, it is virtually impossible to maintain full controlof the surface interactions. Uncontrolled desorption/adsorp-tion of surfactants frequently gives rise to unexpected rheo-logical effects and lack of dispersion stability. For instance,the nonionic surfactant needed as steric stabilizer of latexparticles may preferentially adsorb on a hydrophobic pig-ment surface where it replaces the original dispersant. Thenet result will be that the latex particle will no longer befully covered with nonionic surfactant, leading to reducedstability, particularly in formulations of high electrolyteconcentration.

Fig. 6 gives a good illustration of competitive adsorptionof relevance to latex paints. A mixture of sodium dodecylsulfate and nonylphenol ethoxylate (10 EO) is a commonsurfactant combination in latex preparation. Additional non-ionic surfactant is often introduced into the formulation asmill base dispersant or as wetting agent. Additional anionicsurfactant may be introduced as pigment or filler dispersant.As can be seen from the Figure, at low total surfactant con-centration almost all of the nonionic surfactant is present onthe latex particle surface. As the surfactant concentration isincreased the ratio of nonionic to anionic surfactant at the

surface approaches that in the bulk. The surfactant compo-sition at the surface clearly varies with the total amount inthe formulation and the preferential adsorption is greatest atthe onset of micellization [25]. It has also been shown thatthe presence of polar solvents, which are commonly used tofacilitate coalescence, affect the ratio of nonionic to anionicsurfactant at the surface [26]. The experimentally deter-mined surface composition from such surfactant mixturesagrees well with values predicted by treating the system as apseudo three-phase system, consisting of a surface phase, amicellar phase, and a monomer phase, with only monomersadsorbing on the surface. The micelle-monomer equilibriumin the aqueous phase is given by the regular solution theoryfor mixed micelles [25].

The problem related to competitive adsorption has beenaccentuated with the incorporation of associative thickenersin the formulation. These polymers, being highly surfaceactive, have a strong driving force for hydrophobic surfacessuch as latex particles. Adsorption behavior of hydrophobi-cally modified polyurethanes, so-called HEUR thickeners,have been investigated in some detail. If the latex surface isnot fully covered by surfactant, these surface active poly-mers adsorb at the particle surface, often resulting in gels[27,28]. Addition of nonionic surfactant often results influid, uniform dispersions, suggesting that the nonionic sur-factant displaces the polymer from the latex surface.

In a systematic study on adsorption of mixtures of asso-ciative thickeners of the polyurethane type and nonylphenolethoxylates on a hydrophobic latex it was demonstrated thatpolymers with hydrophobic side chains along the backbonecould displace the nonionic surfactant provided the concen-tration of polymer hydrophobe units was high enough. Forassociative thickeners with only terminal hydrophobicchains it was found that the size of these chains is a decisivefactor in competitive adsorption [29]. Also the distancebetween the hydrophobic end groups is of importance; the

Fig. 6. Composition at the particle surface (B) and in the aqueous phase(X) as a function of total surfactant concentration in the aqueous phase.Adsorption from an aqueous solution of a 84 : 16 molar ratio mixture ofsodium dodecyl sulfate and nonylphenol ethoxylate (10 EO) on a poly-(butyl methacrylate) latex. The lines are predicted compositions (see text).(From Ref. [25])


shorter the distance, i.e., the higher the concentration ofhydrophobic chains, the more effective the polymer is indisplacing the nonionic surfactant [30].

Competitive adsorption at solid surfaces is complex and itis, therefore, difficult to predict the outcome when newmixtures of amphiphiles or a new particle surface is intro-duced. For instance, the same mixture of nonionic surfactantand hydrophobically modified polyurethane showed a verydifferent adsorption behavior on alpha olefin-maleic acidstabilized titanium dioxide than on the above-mentionedlatex. On the coated pigment particle, the surfactant didnot replace the associative thickener regardless of the struc-ture of latter species [31].

4. Polymerizable surfactants

4.1. Polymerizable surfactants for lattices

By using surfactants that become covalently bonded tothe latex particle, many of the problems encountered withconventional surfactants can be avoided or at least mini-mized [7,8]. Positive effects on the properties of both thedispersion itself and the dried film are often obtained[20,32].

The surfactant-related problems in lattices, as well as inmany other dispersions, arise from the fact that surfactantsphysically adsorbed on the particle surface may desorb intothe bulk aqueous phase and that the equilibrium betweensurface and bulk surfactant concentration is governed byfactors such as particle concentration, temperature, ionicstrength and pH, all of which may be changed during sto-rage, use and film formation. Since a certain surface con-centration of surfactant is needed to give proper latexstabilization, a change in the adsorption-desorption equili-brium may severely affect rheology and stability of the dis-persed system.

Formulations containing a latex in combination withanother dispersion, such as a pigment slurry, constitute aparticular problem from a stability point of view. The phy-sically adsorbed latex surfactant may have higher affinityfor the pigment than for the latex, a situation which oftenleads to latex instability. The surfactants used to stabilizethe pigment are usually of a different type to those used forthe latex. Hence, the two surfactants will then compete forboth surfaces, the latex and the pigment, and the surfacecomposition and coverage obtained in the equilibrium situa-tion may be very different from that of the two componentsbefore mixing [33,34]. This type of competitive adsorptionmay drastically affect rheology and stability of a formula-tion.

The presence of surfactant in the dried latex film may alsoimpair film properties. During drying the surfactant isadsorbed on the latex particles. As the particles coalesceduring the annealing process, the surfactant migrates outof the bulk phase and concentrates at the interfaces [35].

It has been shown that surfactant molecules preferably go tothe film-air interface, where they align with their hydropho-bic tails pointing towards the air. Calculations from ESCAspectra show that a lacquer film containing 1% surfactantmay have an average surface surfactant concentration ofaround 50% [36]. Such a high concentration of a non-che-mically incorporated, water-soluble component at the filmsurface will adversely affect adhesion properties and waterresistance of the film.

Furthermore, atomic force microscopy (AFM) studieshave shown that during the film forming process many con-ventional surfactants phase separate from the binder. Whenthe surfactant has phase separated, the water flux may carryit to the film surface. Alternatively, it may accumulate in theinterstices between the particles from where it will migrateto the film-air or film-substrate interface through a long termexudation process. Eventually the surfactant will be presentin aggregates of considerable size, seen by AFM as ‘hills’.After treatment with water, the surfactant aggregates arewashed away and the ‘hills’ are replaced by distinct ‘val-leys’. The rough surface will give rise to poor gloss.

It has been shown that many of the surfactant-relatedproblems in latex paints can be minimized by the use of apolymerizable surfactant as emulsifier in the emulsion poly-merization process [37–39]. Several types of reactive sur-factants have been used for this purpose, some of which areshown on Fig. 7. Block copolymers of ethylene oxide andpropylene or butylene oxide with a polymerizable group atone end (Structures I and II) have become popular due toeasy preparation. Of particular interest from performancepoint of view are surfactants which preferably undergocopolymerization rather than homopolymerization. Agood example of such surfactants are maleate half estersof fatty alcohols, such as Structure III of Fig. 7 [38,39].However, even surfactants based on highly reactive groupssuch as maleate do not become quantitatively copolymer-ized during the emulsion polymerization [38].

In a recent work AFM was used to monitor and comparethe topography during film formation of a butyl acrylate–styrene–acrylic acid latex stabilized with either the maleateSurfactant III of Fig. 7 or the non-polymerizable surfactantsodium dodecylsulfate (SDS) [38]. Figs 8–12 show thetopography of films prepared from the two formulations.The same scale is used in all the figures but one shouldnote that in each picture thez-dimension is plotted at 20times larger magnification than thex- andy-dimensions.

Fig. 8 shows a film formed from SDS-stabilized latex.The film has a smooth, wavy surface as would be expectedafter annealing for 48 h. at 55°C above the film Tg. Afterrinsing with water, large pits were created, Fig. 9. Thischange in the film morphology is an effect of migratingsurfactant. During the drying stage SDS moves with theevaporating water towards the film surface where it crystal-lizes to form a continuous separate phase, covering the totalsurface area. Upon rinsing with water, the highly water-soluble SDS is washed away. The roughness of the remain-


ing film surface is caused by disruption of particle packingby the migrating surfactant phase.

Figs. 10 and 11 show the topologies of films cast frommaleate-stabilized latex before and after rinsing, respec-tively. The film before rinsing had a ‘hills and valleys’appearance, indicative of surfactant aggregates at the sur-face. After the film was rinsed with water, holes appeared ina regular pattern, as shown in Fig. 11. This is indicative ofremoval of surfactant from the surface. The situation ismuch improved compared to the appearance of the filmsfrom the SDS-based latex but, evidently, even with thereactive maleate surfactant, a substantial portion is notanchored to the latex particle. This observation is in agree-ment with the level of incorporation of 64%, as determinedby two-phase titration.

The maleate-stabilized latex was cleaned by dialysisagainst methanol before the film was cast. The annealed

film was smooth, as seen in Fig. 12. This indicates theabsence of free surfactant on the surface.

4.2. Polymerizable surfactants for alkyd emulsions

Surfactants capable of participating in autoxidative dry-ing are of interest for the post-emulsification of alkyd resins[20,32]. Ethoxylated monoethanolamides of unsaturatedfatty acids are one such type of surfactants that can bechemically incorporated into the network during drying ofan alkyd based coating film. Fig. 13 illustrates the differencein surface composition with respect to surfactant for a poly-merizable amide ethoxylate and a conventional nonionicsurfactant of similar hydrophilic-lipophilic balance [40].

Fig. 7. Examples of polymerizable surfactants for lattices.

Fig. 8. AFM image of a film cast from SDS-stabilized latex. Fig. 9. AFM image of a rinsed film cast from SDS-stabilized latex.


Surfactant concentrations at the film-air interface were mea-sured by ESCA. As can be seen, both the conventionalsurfactant, a nonylphenol ethoxylate, and the amide ethox-ylate accumulate at the surface and the concentrationincreases with time. Whereas the concentration versustime curve is almost linear for the nonylphenol ethoxylate,it levels off for the amide ethoxylate. For the latter species,the distribution of surfactant in the film seems to be estab-lished within 3 days of drying.

The difference in behavior between the nonylphenolethoxylate and the amide ethoxylate is probably due to thefact that the latter surfactant becomes immobilized throughcoupling to binder molecules during the drying process.Once covalently incorporated into the network, the migra-tion process will cease. Another contributing factor for thelow degree of migration of the amide ethoxylate could bethat this surfactant is likely to be very compatible with thebinder, a long oil alkyd resin. Surfactant-polymer compat-ibility has previously been found to be decisive in determin-ing surfactant distribution in films [41,42]. Surfactants arecarried towards the surface by the flux of water during filmdrying and this process is particularly effective when thereis poor compatibility between surfactant and polymer.

The effect on surface composition of soaking the dried

film in water is also shown in Fig. 13. Whereas more thanhalf of the nonylphenol ethoxylate disappears from the out-ermost surface layer (approximately 5 nm), the effect on theamide ethoxylate is small, in spite of the fact that bothsurfactants have about equal water solubility. This is afurther indication of the amide ethoxylate being immobi-lized during the drying process, although one must keep inmind that the evidence shown in Fig. 13 is only indirect. Thesensitivity of ESCA is not sufficient for monitoring disap-pearance of carbon–carbon double bonds, which wouldhave been the most direct way of studying surfactant poly-merization. However, studies on cobalt initiated autoxida-tion of ethyl esters of unsaturated fatty acids have shownthat oleate ester does not polymerize over 110 days, whereaslinoleate ester polymerises almost completely over 3 days[43]. These findings support the view that amide ethoxylatesbased on fatty acids with a high degree of unsaturationbecome covalently incorporated in the dried film.

Fig. 10. AFM image of a film cast from maleate-stabilized latex.

Fig. 11. AFM image of a rinsed film cast from maleate-stabilized latex.

Fig. 12. AFM image of a film cast from maleate-stabilized latex dialyzedagainst methanol.

Fig. 13. Relative surface concentration of surfactant as a function of dryingtime as determined by ESCA. The NP ethoxylate is nonylphenol ethox-ylate (12 EO) and the amide ethoxylate is linseed oil fatty acid monoetha-nolamide ethoxylate (14 EO). The experimental conditions are given in[40].


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Surfactants and Emulsion