Advances in Colloid and Interface Science 168 (2011) 79–84

Contents lists available at ScienceDirect

Advances in Colloid and Interface Science

j ourna l homepage: www.e lsev ie r.com/ locate /c is

Electrostatic and steric interactions in oil-in-water emulsion films fromPluronic surfactants

G. Gotchev, T. Kolarov, Khr. Khristov, D. Exerowa ⁎Institute of Physical Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria

⁎ Corresponding author at: Institute of Physical CheSciences, ‘Acad. G. Bonchev’ Str., bl. 11, 1113 Sofia, Bulga

E-mail address: exerowa@ipc.bas.bg (D. Exerowa).

0001-8686/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.cis.2011.05.001

a b s t r a c t

a r t i c l e i n f o

Available online 10 May 2011

Keywords:Interaction forcesOil-in-water emulsion filmsPEO-PPO-PEO triblock copolymersSteric forcesElectrostatic forces

Stabilization of oil-in-water emulsion films from PEO–PPO–PEO triblock copolymers is described in terms ofinteraction surface forces. Results on emulsion films from four Pluronic surfactants, namely F108, F68, P104 andP65 obtainedwith the Thin Film Pressure Balance Technique are summarized. It is found that film stabilization isdue to DLVO (electrostatic) and non-DLVO (steric in origin) repulsive forces. The charging of the oil/water filminterfaces is related to preferential adsorption of OH– ions. This is confirmed by pH-dependentmeasurements ofthe equivalent film thickness (hw) at both constant capillary pressure and ionic strength.With reducing pH in theacidic region, a critical value (pHcr,st) corresponding to an isoelectric state of the oil/water film surfaces is foundwhere the electrostatic interaction in thefilms is eliminated. At pH≤pHcr,st, the emulsionfilms are stabilized onlyby steric forces due to interaction between the polymer adsorption layers. Disjoining pressure (Π) isothermsmeasured for emulsion films from all the four Pluronic surfactants used at pHbpHcr,st show a transition to aNewton black filmwith increasingΠ. The experimental data before the NBF-transition in the disjoining pressureisotherms are fitted to the Alexander–de Gennes’ scaling theory for steric interaction between polymer brusheswith the PEO-brush thickness as a free parameter. The NBF observed are stabilizedmost probably by short-rangesteric forces that may differ from the brush-to-brush interaction.

mistry, Bulgarian Academy ofria. Tel./fax: +359 2 971 2662.

l rights reserved.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Colloid stability has been described on the basis of the forces betweenthe interacting colloidparticles. TheDerjaguin-Landau-Verwey-Overbeek(DLVO) theory [1] explains the stability of lyophobic colloids withoperation of both electrostatic (diffuse double-layer) interaction forceand dispersion (Van der Waals) one. Beyond the DLVO theory, thestabilization of colloids in the presence of adsorbing polymers isattributed to steric interactions between the colloid particles coveredwith polymer adsorption layers. This field has been extensively studiedboth in experimental and theoretical aspects and most works could befound e.g. in Refs. [2–6].

The action of repulsive forces in the thin aqueous films thatspontaneously arise between bubbles or oil droplets determines thestability of foams and oil-in-water (O/W) emulsions. A rich set ofresults for foam films e.g. reviewed in Refs. [7–14] demonstrates theapplicability of the DLVO theory for describing film stability. Theelectrostatic interaction in foam films from nonionic surfactants isattributed to charging of air/water interface due to preferentialadsorption of OH– ions [8,9,11,12,15–19]. This hypothesis is confirmed

by pH-dependent measurements that show the existence of isoelectricpoint (pH*) of air/water interface [8,15], which the surface chargedensity (diffuse double-layer potential, φ0) disappears at. Some otherexperiments give evidence that the charge of oil/water interface, beingeither bare or with adsorbed nonionic surfactants could be accountedfor by preferential adsorption of OH– ions, aswell [20–22]. Compared tofoam films there is more limited amount of data confirming theelectrostatic stabilization of O/W emulsion films from nonionic lowmolecular weight [23–25] and polymeric [14,26–28] surfactants. Someof theseworks [14,25,26] have described quantitatively the electrostaticforce in the emulsion films studied by fitting experimental disjoiningpressure isotherms to the DLVO theory.

Results on O/W emulsion films from a homopolymer (poly(vinylalcohol)) [29], a statistical copolymer ((poly(vinyl alcohol)-poly(vinylacetate)) [30] or polymeric surfactants [14,22,26–28] show thatindependently from the electrostatic force, an additional repulsiveforce arises in the films due to interaction between the polymeradsorption layers. Experiments with emulsion films from triblock- andgraft- copolymers in the presence of electrolytes demonstrate thattransition from DLVO (electrostatic) to non- DLVO (steric in origin)interaction in the films occurs through critical electrolyte concentrationvalue (Cel,cr) [14,26–28]. This finding is similar to that for foam filmsfrom polymeric surfactants [8,9,14,31].

Pluronics are triblock copolymers of the A-B-A type with A beinghydrophilic poly(ethyleneoxide) (PEO)blocksandBbeingahydrophobicpoly(propylene oxide) (PPO) block. The interfacial properties of aqueous

80 G. Gotchev et al. / Advances in Colloid and Interface Science 168 (2011) 79–84

Pluronics solutions in contact with a fluid or solid hydrophobic phasehave been extensively explored bymeans of different methods andmostof the results could be found for instance in Refs. [32–40]. Pluronicsmacromolecules adsorb at the hydrophobic/aqueous solution interfaceby anchoring the PPO block with the two PEO chains protruding into thewater.

Following the Alexander–de Gennes’ scaling approach [41,42],polymers terminally attached to an interface should form a brush (athigh grafting density and good solvent conditions). In this line, somestudies have dealt with the cases of aqueous PEO-PPO-PEO triblock- andPEO-poly(butylen oxide) diblock copolymers, proving by surface forcetechniques and ellipsometry that a PEO-brush is formed at the air/water[8,9,36–38] and various solid/water [35,43,44] interfaces. Pluronicsmacromolecules are supposed to take brush-like conformation at theoil/water interface, as well. Evidence for that has been derived fromresults on the variation of the brush thickness with changing the PEO-chain length obtained from neutron reflection measurements [32,36]and zeta-potential data for oil droplets in aqueous solution [39].Experiments with O/W emulsion film [14,22,28] have been involved instudying the steric repulsion between two parallel brushes at the oil/Pluronics aqueous solution interface by fitting experimental disjoiningpressure isotherms to the Alexander–de Gennes’ scaling theory forbrush-to-brush steric interactions [41,42] with the brush thickness as afree parameter.

In the present article, we summarize previous [14,22,28] and somemore recent (unpublished) results on four Pluronic surfactants inorder to describe the nature of the repulsive interaction forces(electrostatic force due to specific adsorption of OH– ions and stericforce due to brush-to-brush interaction) that stabilize O/W emulsionfilms from PEO–PPO–PEO triblock copolymers. Schematic structure ofPluronics O/W emulsion films is illustrated in Fig. 1.

2. Experimental

2.1. Methods

O/W emulsion films are studied with the Thin Film PressureBalance Technique (described in details e.g. in Refs. [8,9,13,14]) withtwo measuring cells: i. a cell with a glass tube, where the films are atconstant capillary pressure, PC≈2σ1,2/R (σ1,2 is the interfacial tensionat oil/water interface and R=2 mm is the radius of the glass tube) andii. a porous plate cell whereby PC could be altered. At equilibrium, PC isequal to the total disjoining pressure (Π) in the film. Film thickness ismeasured inteferometrically making use of experimentally accessibleequivalent film thickness (hw) [10,45]. Thus, hw vs. either NaCl

Fig. 1. Schematic structure of Pluronics O/W emulsion films: a. Thick film with aqueous corethe total film thickness and δ is the brush thickness.

concentration (CNaCl) or pH plots (Π=const) and Π vs. hw plots(disjoining pressure isotherms) for the emulsion films are obtained.

2.2. Materials

Four PEO–PPO–PEO triblock copolymers of the Pluronic (BASF,Germany) series F108, F68, P104 and P65 are used as received. Themolecular weight (Mw) and the average number (n) of EO and POmonomers in the different copolymers are known from the manufac-turer and are displayed in Table 1 alongwith other quantities,whichwillbe discussed later in the text. The electrolytes used are NaCl (Merck,Suprapur) and HCl (Sigma–Aldrich). Doubly distilled water (specificconductivity of about 1 μS.cm-1) is used for the preparation of allsolutions. pH of the solutions used to obtain the films is adjusted by theNaCl/HCl ratio while the ionic strength (I, corresponding to 1:1electrolyte) is maintained constant and controlled by conductivitymeasurements. The emulsion films studied are obtained between twoorganic phases of isoparafinic oil Isopar M (mixture of C11–C15isoalkanes), which is an Exxon Mobil Chemicals product (used asreceived). All the experiments are performed at constant temperature of22 °C.

3. Results and discussion

3.1. Electrostatic interaction forces

Electrostatic force in thin liquid films originates from theinteraction between the diffuse electric double-layers at film surfacesas explained by the DLVO theory. Plenty of experiments on foam filmse.g. reviewed in Refs. [8–13] give evidence for the action of double-layer force in the films and demonstrate the applicability of the DLVOtheory for describing film stability. Compared to foam films, there is amore limited amount of data confirming the electrostatic stabilizationof O/W emulsion films from nonionic low molecular weight [23–25]and polymeric [14,26–28] surfactants.

Experiments with O/W emulsion films from Pluronic surfactants,namely F108 and P104, show that at constant capillary pressure thefilm thickness changes with varying CNaCl in the surfactant solutions(natural pH of around 5.5) used to obtain the films [14,28]. It isevident in Fig. 2a that film thickness for both surfactants decreaseswith increasing CNaCl up to a certain value denoted Cel,cr (marked witharrows in the same figure). Obviously, such variation in film thicknessis due to the screening effect of electrolytes on the electric diffusedouble-layers interaction, which effect is well described by the DLVOtheory. As far as the film thickness in Fig. 2a remains virtually thesame at/above Cel,cr, the double-layer force (electrostatic component

stabilized by electrostatic forces; b. Film stabilized by brush-to-brush steric forces. h is

Table 1Values of the average molecular weight (Mw); the average number (n) of EO and POmonomers; the thickness (hw) of the emulsion films obtained in the glass tube cell atpH≤pHcr,st; the PEO-brush thickness (δ) obtained by fitting the experimental ΠST vs. hdata to Eq. (1), for details see Section 3.2.

Pluronic Mw [g.mol−1] n (EO) n (PO) hw [nm] δ [nm]

F108 14,600 128 54 41±2 9.6F68 8400 75 30 27±1 7.6P104 5900 31 54 13.5±0.5 7.2P65 3400 18 30 12.5±0.5 6.5

Fig. 3. Equivalent thickness vs. pH plots for O/W emulsion films obtained from aqueoussolutions of 70 μM F68 (□) or 0.5 mM P65 (○) and I=1 mM at constant capillarypressure of 32 Pa and 17 Pa, respectively. The arrows represent the pHcr,st-values andthe dashed lines represent the final film thickness at pH≤pHcr,st.

81G. Gotchev et al. / Advances in Colloid and Interface Science 168 (2011) 79–84

of the disjoining pressure,ΠEL) in the films is suppressed. Under theseconditions (absence of ΠEL at CNaCl≥Cel,cr) the films are considered tobe stabilized only by repulsive non-DLVO forces, which could berelated to steric interaction between the two polymer adsorptionlayers at film surfaces. Then the film thickness is determined by thethickness of the polymer layers. The same behavior is also observed forPluronics foam films [8,9] and O/W emulsion films from inulin basedgraft copolymeric surfactants [26,27]. It has also been found that thetype of the electrolyte have influence on film thickness, as expected bythe DLVO theory, but does not affect the general trend of the hw (Cel)dependence for emulsion films from inulin based surfactants [26]. Thus,Cel,cr represents transition from DLVO (electrostatic) to non-DLVO(steric in origin) film stabilization.

Another type of experiments are performed with emulsion filmsobtained from aqueous solutions of the same Pluronic surfactants(F108 and P104) [22]; the surfactant solutions are of constant ionicstrength (corresponding to 0.5 mM 1:1 electrolyte that is well belowCel,cr) and various pHs. Fig. 2b shows the variation of film thicknesswith changing pH for the emulsion films studied. A tendency of filmthickness decrease is observed with reducing pH down to a certaincritical value for every copolymer (marked with arrows in Fig. 2b),such value being denoted as pHcr,st. At pHcr,st the lowest film thickness(for every copolymer) is reached and it remains virtually constantwith further decrease in pH below pHcr,st. It is evident in Fig. 2a and bthat there is a good correlation between the final film thickness values(markedwith dashed lines) obtained either at/above Cel,cr or at/belowpHcr,st and this will be discussed later in Section 3.2.

Resent unpublished experimental results are shown in Fig. 3,presenting measurements of the hw vs. pH dependence (PC=const,I=1mM) for O/W emulsion films from other two Pluronic surfac-tants, namely F68 and P65. The same trend as in Fig. 2b of reduction in

Fig. 2. Equivalent thickness vs. a. NaCl concentration at constant pH 5.5 (redrawn from Ref. [2W emulsion films obtained from aqueous solutions of 7 μM F108 (▼) or P104 (▲) at constantof the final film thickness at/above Cel,cr or at/below pHcr,st. The arrows indicate the Cel,cr- a

film thickness with decreasing pH until pHcr,st is observed. Again, thefinal film thickness for every copolymer (marked with dashed lines inFig. 3) is independent on pH at/below pHcr,st.

A common feature of the data in Figs. 2b and 3 is that the filmthickness is pH-dependant down to pHcr,st and any further decrease ofpH results in films with virtually the same thickness. At pHNpHcr,st,decrease in pH leads to reduction of film thickness, which indicatesthat double-layer force operates in these films. Consequently,independence of the film thickness on pH leads to the conclusionthat ΠEL in the films at pH≤pHcr,st is eliminated. The initial decreasein film thickness at pHNpHcr,st should be accompanied by a reductionof the surface charge density, i.e. diffuse double-layer potential of thefilm interfaces down to an isoelectric state (φ0→0) corresponding topHcr,st. This finding is similar to the one for foam films from polymericsurfactants of the PEO–PPO–PEO type [8,9,46,47].

Interfacial charge could be modified by adsorption of ionicsurfactants. On the other hand some experiments give evidence thatinterfaces, being either bare or with adsorbed nonionic surfactants, ofthin liquid films [8–19,23–28,31,46,47] as well as those of oil droplets[21,48,49] or gas bubbles [47,50–52] in water are negatively charged.Different studies provide arguments for the accepted explanation that

8]) and b. pH at constant ionic strength of 0.5 mM (redrawn from Ref. [22]) plots for O/capillary pressure of 23 Pa and 15 Pa, respectively. The dashed lines are guide to the eyend pHcr,st- values.

Fig. 4. Experimental disjoining pressure isotherms for O/W emulsion films obtainedfrom aqueous solutions of 7 μM F108 (▼), 70 μM F68 (□), 20 μM P104 (▲) and 5 mMP65 (○) at pH below pHcr,st. The arrow indicates film rupture; the dotted lines representthe experimental pressure limit (for details see the text in Section 3.2).

82 G. Gotchev et al. / Advances in Colloid and Interface Science 168 (2011) 79–84

the origin of the charges at oil (air)/water interface is due to specificadsorption of OH– ions [8,9,11–13,15–22,49]. Recent investigations onmolecular dynamics simulations are also in line with this hypothesis,demonstrating that the OH– ions exhibit positive affinity to astructureless hydrophobic wall [53]. It should be noted however,that there are some arguments e.g. in Refs. [54,55] excluding suchbehavior and even showing that the water surface is acidic.

For both cases of bare oil (air)/water interfaces or in presence ofadsorbed nonionic surfactants, a marked dependence of the surfacecharge density on pH is observed either by zeta-potential measure-ments for oil droplets inwater [21,48] or by calculations for the diffusedouble-layer potential of foam films [8,9,11,12,15–19,46,47]. For air/water interface, decrease in pH leads to reduction of surface chargedensity until it vanishes at a critical pH-value denoted pH* [8,15].Therefore, the electrostatic interaction in a foam film from lowmolecular weight nonionic surfactants is eliminated at pH* and thefilm either ruptures or transforms into a Newton black film (NBF)depending on the surfactant concentration. For the latter case, thecritical pH-value of NBF transition is denoted pHcr[8]. The existence ofan isoelectric point of thin liquid films from polymeric surfactants hasbeen reported for thefirst time for foamfilms fromPluronic copolymers[8,9]. It is found that at/below certain pH-value, polymer-induced stericrepulsion (steric component of the disjoining pressure, ΠST) stabilizesthese foam films, which are not of NBF type and because of that anotation ‘pHcr,st’ has been introduced to emphasize this difference. Onthe grounds of the above results and considerations, a conclusion couldbe drawn that the double-layers interaction detected in both foam andO/W emulsion films from Pluronic surfactants is pH-dependent and itdisappears at pHcr,st. Thus, pHcr,st represents the transition from DLVO(electrostatic) to non-DLVO (steric) film stabilization just like Cel,cr doesin the hw (Cel) dependence discussed above.

Obviously, the above considerations bring arguments that thecharge of the oil/water interfaces of O/W emulsion films from Pluronicsurfactants should be accounted for with preferential adsorption ofOH– ions. At pHcr,st corresponding to the isoelectric state of the filminterfaces, the diffuse double-layer potential tends to zero, i.e. theelectrostatic interaction in the emulsion films is considered to betotally eliminated.

3.2. Steric interaction forces

It is evident that there is a good correlation between the data inFig. 2a and b in respect to the final film thickness obtained either at/above Cel,cr or at/below pHcr,st. In these cases, as mentioned above, thestabilization of the emulsion films should be related to repulsive non-DLVO forces due to the interaction between the polymer adsorptionlayers atfilm surfaces. The hw vs. pHplots in Figs. 2b and 3 show that thefinal film thickness varies with changing the copolymers kind. Thiscould be due to alteration of PC according to changes of the interfacialtension of the different surfactant solutions used to obtain the films.However, such small PC-variations, as in the range of several Pa, couldhardly explain the observed significant difference in the film thickness(see Table 1). At pH≤pHcr,st, i.e. in absence of double-layers interaction,the thickness of the emulsion films should be relevant to that of theinteracting polymer adsorption layers in the films. It is evident in Table 1that hw changes with varying the PEO-chain length for the differentcopolymers and a trend of hwdecreasewith reducing PEO-chain length isobserved. This finding can be considered as a qualitative result givingevidence that the stabilization of the emulsion films at/below pHcr,st isdue to steric interaction between the layers of extended PEO-chains(brush-like conformation).

Detailed information aboutfilm interaction forces canbe acquiredbymeasurements of the film disjoining pressure isotherm. Fig. 4 showsΠvs. hw plots for O/W emulsion films from the four copolymers studied atpHwell belowpHcr,st, i.e. in absence of double-layer interaction. Initially,the film thickness gradually decreases with increasingΠ and after that

transition to a lower thickness occurs in every case for films from theindividual copolymers. This lower thickness (average values fromseveral measurements with estimated error bars are shown in Fig. 4)remains virtually constant with further increase in Π.

A comparison between the experimental disjoining pressure iso-therms and those calculated using the Alexander–de Gennes’ scalingtheory for steric interactions between polymer brushes [41,42] ispossible as has been previously reported for Pluronics foam [8,9,14] andemulsionfilms [14,22,28]. In the absence of electrostatic interaction, thetotal disjoining pressure in the films is a sum of two components:Π=ΠVW+ΠST, Van der Waals attraction and steric repulsion,respectively. For the emulsion films either at Cel≥Cel,cr[14,28] or atpH≤pHcr,st[22], ΠVW is evaluated by the equation ΠVW=A/6πh3, withthe Hamaker constant A=−5×10−21 J (an average value taken on thebasis of the literature data [56] for C11-C15 isoalkanes that constitute theIsoparM oil used in the experiments). Following Alexander–de Gennes’scaling approach, end-grafted flexible–neutral–linear chains (at highgrafting density and good solvent conditions) should form a brush andthe disjoining pressure between two brushes scales as [41,42]:

∏ST e h=2δð Þ−9=4

– h=2δð Þ3=4h i

; ð1Þ

where h is the total film thickness and δ is the brush thickness. Aspreviously discussed [14,22,26,28], in the case of O/W emulsion films hcan be approximated by the experimentally measured equivalentthickness (the PPO-blocks are situated in the surrounding oil phaseand the PEO-blocks are probably strongly hydrated).

Following this approach, the experimental data in Fig. 4 can becompared with the Alexander–de Gennes’ scaling theory. Making useof h approximated by the experimental hw-values, ΠVW calculated asexplained above andΠmeasured, the differenceΠ–ΠVW vs. h, i.e.ΠST

vs. h is fitted to Eq. (1) with δ as a free parameter.Fig. 5 represents the individual components (ΠVW and ΠST) of the

disjoining pressure isotherms in Fig. 4. It is worth noting that thetransitions in the isotherms are not predicted in the Alexander–deGennes’ scaling theory. Thus, only the experimental data for the filmsbefore the transitions are included in Fig. 5. As it is evident from theΠST vs. hmodes, the range of the steric force in the Pluronics emulsionfilms differs between the films from all four surfactants. This factshould be accounted for with the variation of the PEO-chains length ineach copolymer. The comparison between the experimentallyobtained ΠST vs. h data and the Alexander–de Gennes’ scaling theorygives satisfactory results and the fit yields the δ-values entered in

Fig. 5. Van der Waals attraction (ΠVW, the solid curve of negative Π-values) and stericrepulsion (ΠST, same symbols as in Fig. 4) vs. h plots evaluated from the experimentaldata in Fig. 4 (for details see the text in Section 3.2). The solid curves through thesymbols are the best fit of Eq. (1).

83G. Gotchev et al. / Advances in Colloid and Interface Science 168 (2011) 79–84

Table 1. Obviously, there is a trend of reduction in the brush thicknesswith decreasing the PEO-chain length. This finding is in agreementwith the observation that the range of the steric force at a givenΠST isdetermined by the PEO-chain length, i.e. brush thickness. Resultsobtained by other authors involving neutron reflectionmeasurements[36] and zeta-potential data for oil droplets in aqueous solution [39]also confirm that the size of the PEO-block affects the polymer layerthickness as the latter increases with increasing the PEO-chain length.

The experimental data in Fig. 4 show that all the emulsion filmsfrom the different Pluronic surfactants undergo a spontaneoustransition to a lower thickness and this thickness remains virtuallyconstant with further increase inΠ. Such behavior gives evidence thatthe emulsion films from the Pluronic surfactants studied could reachequilibrium states with different film thickness at one and the samedisjoining pressure. This could be accounted for with the formation ofa Newton black film. It is evident from Fig. 4 that the thickness of theNBFs differs for all four copolymers. Though not as pronounced asdemonstrated by the disjoining pressure isotherms modes for thefilms before the NBF transition, this difference in thickness could alsobe attributed to the different PEO-chain length. Whatever themechanism, it seems that the reason for the formation of such NBFmust be due to short-range steric forces between the PEO-chains.However, these short-range steric forces may differ from the brush-to-brush interaction.

The emulsion NBF from F108, F68 and P104 remain stable notshowing rupture or thickness changeup to thepressure limit1 (indicatedwith dotted lines in Fig. 4) for each of these three copolymers. Thethickness of the NBF from P65 remains constant with increasing Π, aswell, butfilm rupture is observed at around 7 kPa (markedwith arrow inFig. 4), which is well below the pressure limit fixed for this copolymer.On the other hand, the NBF transition for P65 occurs at significantlylower Π in comparison with those for the films from the other threecopolymers. Therefore, one has to distinguish between the properties ofthe emulsion films from the different Pluronic surfactants. Both thelower stability against rupture and the lower transition pressure for thefilms fromP65 could be related toweaker steric force due to the shortestPEO-chains, i.e. lowest brush thickness for this copolymer.

1 Note that for adequate experiments, the maximum pressure that could be appliedwithin the technique conditions should not exceed the capillary pressure in the porousplate used. At a given pore size, this capillary pressure depends proportionally on theinterfacial tension of every surfactant solution and that is why the pressure limit alterswith the different copolymers.

All these lead to the inference that the different copolymers usedaffect the steric force in the emulsion films studied by varying thebrush thickness. Thus, it can be safely concluded that the repulsivesteric force in the emulsion films before the transition to a NBF in thedisjoining pressure isotherms is due to brush-to-brush interaction.

4. Conclusion

The results summarized above are in evidence that the repulsiveforces in oil-in-water emulsion films from PEO–PPO–PEO triblockcopolymers are due to electrostatic and steric interactions. The originof the double-layer interaction in the films is related to preferentialadsorption of OH– ions at oil/water interface. This is confirmed by thedependence of the film thickness on pH at both constant capillarypressure and ionic strength. pHcr,st found, corresponding to anisoelectric state of the oil/water film surfaces represents the transitionfrom electrostatic to steric stabilization of the emulsion films studied.The good fit between the experimental disjoining pressure isothermsfor the films at pH≤pHcr,st and the Alexander–de Gennes’ scalingtheory confirms that the steric force in the films originates from theinteraction between the PEO-brushes at film surfaces. Formation ofemulsion NBF is observed for all four copolymers and these NBF aremost probably stabilized by short-range steric forces that may differfrom the brush-to-brush interaction.

References

[1] Derjaguin BV, Landau LD. Acta Physicochim USSR 1941;14:633.Verwey EJV, Overbeek TG. Theory of the Stability of Liophobic Colloids. Elsevier;1948.

[2] De Gennes PG. Scaling Concepts in Polymer Physics. Cornell University Press;1979.

[3] Napper D. Polymeric Stabilization of Colloidal Dispersions. Academic Press; 1983.[4] Israelachvili JN. Intermolecular and Surface Forces. Academic Press; 1985.[5] Fleer GJ, Cohen Stuart MA, Scheutjens JMHM, Cosgrove T, Vincent B. Polymers at

Interfaces. Chapman & Hall; 1993.Fleer GJ, Cohen Stuart MA, Leermakers FAM. In: Lyklema J, editor. Fundamentals ofInterface and Colloid Science. Soft ColloidsElsevier; 2005. Chapter 1.

[6] Tadros ThF. In: Tadros ThF, editor. Colloid Stability: The Role of Surface Forces -Part 1, vol. 1. WILEY-VCH; 2007. p. 10. Chapters 1.

[7] Kruglyakov PM. In: Ivanov I, editor. Thin Liquid Films. Fundamentals andApplicationsMarcel Dekker; 1988. Chapter 11.

[8] Exerowa D, Kruglyakov PM. Foam and Foam Films. Elsevier; 1998.[9] Sedev R, Exerowa D. Adv Colloid Interface Sci 1999;83:111.[10] Bergeron V. J Phys Condens Matter 1999;11:R215.[11] Karraker KA, Radke CJ. Adv Colloid Interface Sci 2002;96:231.[12] Stubenrauch C, von Klitzing R. J Phys Condens Matter 2003;15:R1197.[13] Platikanov D, Exerowa D. In: Lyklema J, editor. Fundamentals of Interface and

Colloid Science. Soft ColloidsElsevier; 2005. Chapter 6.Sjöblom J, editor. Emulsions and Emulsion Stability; Surfactant Science Series, vol.132. CRC Taylor & Francis; 2005. Chapter 3.

[14] Exerowa D, Platikanov D. Adv Colloid Interface Sci 2009;147–148:74.[15] Exerowa D. Kolloid Z 1969;232:703.[16] Manev ED, Pugh RJ. Langmuir 1992;7:2253.[17] Bergeron V, Waltermo A, Claesson PM. Langmuir 1996;12:1336.[18] Khristov Khr, Exerowa D, Yankov R. Colloids Surf A Physicochem Eng Aspects

1997;129–130:257.[19] Stubenrauch C, Cohen R, Exerowa D. Langmuir 2007;23:1684.[20] Becher P, Schick MJ. In: Schick MJ, editor. Nonionic Surfactants: Physical

Chemistry, Surfactant Science Series, vol. 23. Marcel Dekker; 1987. Chapter 8.[21] Marinova K, Alargova R, Denkov N, Velev O, Petsev D, Ivanov I, et al. Langmuir

1996;12:2045.[22] Gotchev G, Kolarov T, Khristov Khr, Exerowa D. Colloids Surf A Physicochem Eng

Aspects 2009;354:56.[23] Sonntag H, Netzel J, Klare JrH. Kolloid Z Z Polym 1966;211:121.

Sonntag H, Netzel J, Unterberger B. Spec Discuss Faraday Soc 1966;1:57.[24] Velev O, Gurkov T, Chakarova B, Dimitrova B, Ivanov I, Borwankar R. Colloids Surf A

Physicochem Eng Aspects 1994;83:43.[25] Binks BP, Cho W-G, Fletcher PDI. Langmuir 1997;13:7180.[26] Exerowa D, Gotchev G, Kolarov T, Khristov Khr, Levecke B, Tadros ThF. Langmuir

2007;23:1711.Gotchev G, Kolarov T, Levecke B, Tadros ThF, Khristov Khr, Exerowa D. Langmuir2007;23:6091.

[27] Exerowa D, Gotchev G, Kolarov T, Khristov Khr, Levecke B, Tadros ThF. ColloidsSurf A Physicochem Eng Aspects 2009;334:87.

[28] Exerowa D, Gotchev G, Kolarov T, Khristov Khr, Levecke B, Tadros ThF. ColloidsSurf A Physicochem Eng Aspects 2009;335:50.

[29] Sonntag H, Unterberger B, Zimontkowsky Z. Colloid Polym Sci 1979;257:286.

84 G. Gotchev et al. / Advances in Colloid and Interface Science 168 (2011) 79–84

[30] Mondain-Monval O, Espert A, Omarjee P, Bibette J, Leal-Calderon F, Philip J, et al.Phys Rev Lett 1998;80:1778.Espert A, Omarjee P, Bibette J, Leal-Calderon F, Mondain-Monval O. Macro-molecules 1998;31:7023.

[31] Exerowa D, Kolarov T, Pigov I, Levecke B, Tadros ThF. Langmuir 2006;22:5013.[32] Phipps JS, Richardson RM, Cosgrove T, Eaglesham A. Langmuir 1993;93:530.[33] Li J-T, Caldwell KD, Rapoport N. Langmuir 1994;10:4475.[34] Alexandridis P, Hatton TA. Colloids Surf A Physicochem Eng Aspects 1995;96:1.[35] Shar JA, Obey TM, Cosgrove T. Colloids Surf A PhysicochemEngAspects 1998;136:21.[36] Clifton BJ, Cosgrove T, Richardson RM, Zarbakhsh A, Webster JRP. Phys B

1998;248:289.[37] Sedev R. Colloids Surf A Physicochem Eng Aspects 1999;156:65.[38] Muñoz MG, Monroy F, Ortega F, Rubio RG, Langevin D. Langmuir 2000;16:1083

ibid 1094.[39] Prestidge CA, Barnes T, Simovic S. Adv Colloid Interface Sci 2004;108–109:105.[40] Noskov BA. Curr Opin Colloid Interface Sci 2010;15:229.[41] Alexander S. J Phys France 1977;38:983.[42] De Gennes PG. Adv Colloid Interface Sci 1987;27:189.[43] Wang A, Jiang L, Mao G, Liu Y. J Colloid Interface Sci 2002;256:331.[44] SchillénK, ClaessonPM,MalmstenM, Linse P, Booth C. J Phys ChemB1997;101:4238.

[45] Scheludko A. Adv Colloid Interface Sci 1967;1:391.[46] Exerowa D, Churaev NV, Kolarov T, Esipova NE, Itskov SV. Kolloidn Zh 2006;68:155.[47] Krasowska M, Hristova E, Khristov Khr, Malysa K, Exerowa D. Colloid Polym Sci

2006;284:475.[48] Balzer D. Langmuir 1993;9:3375.[49] Beattie JK, Djerdjev AM, Warr GG. Faraday Discuss 2008;141:31.

Beattie JK, Djerdjev AM. Angew Chem Int Ed 2008;43:3568.[50] Usui S, Sasaki H, Matsukawa H. J Colloid Interface Sci 1981;81:80.[51] Yoon R-H, Yordan J. J Colloid Interface Sci 1986;113:430.[52] Li C, Somasundaran P. J Colloid Interface Sci 1991;146:215 ibid 148: 587.

Colloids Surf A Physicochem Eng Aspects 1991;81:13.[53] Zangi R, Engberts J. J Am Chem Soc 2005;127:2272.

Kudin K, Car R. J Am Chem Soc 2005;130:3915.[54] Petersen P, Saykally R. Chem Phys Lett 2008;458:255.[55] Vácha R, Horinek D, Berkowitz ML, Jungwirth P. Phys Chem Chem Phys 2008;10:

4975.Winter B, Faubel M, Vácha R, Jungwirth P. Chem Phys Lett 2008;474:241 ibid 481:19.

[56] Lyklema J, editor. Fundamentals of Interface and Colloid Science. FundamentalsA-cademic Press; 1991. Appendix 9.2.