by R Petkova · 2013 · Cited by 62 — Briefly,. 15 mL of the studied foaming solution was loaded in a glass cylinder with 75 mL total volume. To generate foam, ten standard hand-.
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Colloids and Surfaces A: Physicochem. Eng. Aspects 438 (2013) 174– 185Contents lists available at ScienceDirect Colloids and Surfaces A: Physicochemical andEngineering Aspectsjourna l h o me pag e : of polymer–surfactant interactions in foams: Effects of pH and surfactanthead group for cationic polyvinylamine and anionic surfactantsR. Petkova, S. Tcholakova, N.D. DenkovDepartment of Chemical Engineering, Faculty of Chemistry and Pharmacy, Sofia University, 1 J. Bourchier Ave., 1164 Sofia, Bulgariah i g h l i g h t sEffect of cationic polymer on foam-ing properties of SDS and SDP1S isstudied.Effect of polymer depends signifi-cantly on the head group of anionicsurfactant.The stability of SDS + polymer foamsis much higher than the stability ofSDS foams.The stability of SDP1S + polymerfoams is lower, compared to SDP1Sfoams.Stronger complex SDP1S + polymer isformed in the bulk, as compared toSDS + polymer.g r a p h i c a l a b s t r a c ta r t i c l e i n f oArticle history:Received 6 November 2012Received in revised form 13 January 2013Accepted 15 January 2013Available online 26 January 2013Keywords: Foaming Foam stabilitySurfactant–polymer interactionsFoam filmsKinetics of adsorptiona b s t r a c tIn a previous study (Langmuir, 28 (2012) 4996) we showed that the foamability of mixed solutions ofthe cationic polymer polyvinylamine (PVAm) and the anionic surfactant sodium dodecyl sulphate (SDS)is strongly reduced, whereas the stability of the formed foams is strongly enhanced, as compared to thesolutions of SDS alone. Here we study in more detail the foaming properties of mixed solutions of PVAmwith anionic surfactants. The effect of surfactant head group is studied by comparing SDS with anotheranionic surfactant (sodium dodecyl oxyethylene sulphate, SDP1S) which contains an additional ethoxyfragment in the charged head-group. For changing the electrostatic polymer–surfactant interactions,we varied pH between 6 and 10, thus crossing the polymer pKa 8.6. The foam tests showed thatthe foamability of all mixed solutions is strongly reduced in the entire range of pH values studied. Thenegative effect of PVAm on solution foamability is highest at low pH, where the polymer charge densityis the highest. Model experiments revealed that the reduced foamability is due to prolonged lag-time forformation of mixed adsorption layer on bubble surfaces. Surprisingly, we found that the stability of SDP1Sfoams is also reduced strongly by PVAm at pH 6 (contrary to SDS and to conventional understanding). Theobtained results indicate that the ethoxy group in SDP1S enhances the surfactant association with thepolymer molecules, thus decreasing the concentration of free surfactant monomers, necessary to adsorbon the solution surface for foam stabilization during foaming and immediately after it. These resultsclearly demonstrate that excessively strong polymer–surfactant interactions could be a problem in theformation and stabilization of foams from mixed solutions. Moreover, the effect of cationic polymers onthe foaming properties of anionic surfactants could depend significantly on the specific head group ofthe surfactant.© 2013 Elsevier B.V. All rights reserved.Corresponding author. Tel.: +359 2 962 5310; fax: +359 2 962 5643.E-mail addresses: SC@LCPE.UNI-SOFIA.BG, (S. Tcholakova).0927-7757/$ – see front matter© 2013 Elsevier B.V. All rights reserved.

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R. Petkova et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 438 (2013) 174– 1851751. IntroductionPolymer–surfactant mixtures are widely used for controllingthe colloid stability and rheological properties of complex dispersesystems in many practical applications, such as the paper indus-try, and in food, pharmaceutical, home and personal care products[1,2]. Due to their wide-spread usage and complex physico-chemical properties, these systems have attracted the attention ofresearchers and they are subject of numerous studies, aimed at rev-eling the role of the surfactant–polymer interactions, both in thebulk and on the solution surface, for the overall system behavior[1–38].In the course of these studies, the researcher have distinguishedbetween strongly interacting systems (surfactants and polymerswith opposite charges) and weakly interacting systems (usuallycomprising neutral polymer and charged surfactant) [12]. Numer-ous studies have focused on the structure and composition of themixed adsorption layers [7–8,10,12–15,22–31] and the thinningbehavior of the respective foam films [9,17–18,21,32–36]. It wasdemonstrated that surface active polymer–surfactant complexesare able to stabilize the foam films at relatively large film thickness,even at very low surfactant and polymer concentrations (whereeach of the individual components is unable to stabilize the foamfilms). Therefore, the common understanding is that the strongsurfactant–polymer interactions are important to obtain stablefoams from the mixed solutions. The specific mechanisms of foamfilms stabilization are still under debate in literature [9]. The stabil-ity of the foam films was shown to have no direct correlation withthe surface tension or surface elasticity of the solutions (as claimedin some earlier studies) [32]. Therefore, combined electrostatic-steric repulsion is usually considered as governing the stabilityof such mixed systems [32]. At high surfactant concentrations,polymer–surfactant aggregates are often trapped in the foam films,which might additionally stabilize the films [33]. The main resultsabout the behaviour and stability of foam films are summarized inRefs. [4,9].Despite the practical importance of the foams formed fromsuch mixed solutions, there are a very limited number of studiesfocused on the foamability and foam stability with these systems.In our recent study [39] we combined foam tests and model experi-ments (optical observations of foam films, ellipsometry, and surfacetension measurements) to evaluate the foamability and foam sta-bility for several polymer–surfactant mixtures, and to explain theobserved trends. Different types of surfactants (cationic, anionicand nonionic) and polymers (nonionic and cationic) were studiedto clarify the factors governing the foamability and foam stability ofthe mixed systems. Highly hydrophilic cationic and nonionic poly-mers, polyvinylamine (PVAm) and polyvinylformamide (PVFAm)were used.Our experiments [39] revealed two rather unexpected trendswhich could not be predicted from foam film studies only.First, the experiments showed that most of the mixed solu-tions, including those of cationic polymer and cationic surfactant,and nonionic polymer and anionic or cationic surfactant, mayshow enhanced foamability and foam stability under appropriateconditions. Therefore, no strong surfactant–polymer interactionsare needed for observing synergistic effects in such mixtures.Furthermore, the foam tests showed clearly that the foamabil-ity of the solutions was strongly reduced when the oppositelycharged anionic surfactant SDS and cationic polymer PVAm weremixed, whereas the foam stability was enhanced, as comparedto the individual components. The reduced foamability wasexplained with the slower formation of the adsorption layerson the bubble surface, due to the strong association of the twocomponents in the bulk solution (thus reducing the adsorptionrate).The major aim of the current study is to clarify in moredetail the role of the electrostatic interactions, and the mecha-nisms behind the observed trends, in such strongly interactingpolymer–surfactant mixtures. The effects of surfactant head groupand of polymer charge density on the foamability and foam stabil-ity were studied. Two anionic surfactants were studied – sodiumdodecyl sulphate (SDS) and sodium dodecyloxyethylene sulphate(SDP1S). The same cationic polymer PVAm was used, and pH wasvaried between 6 and 10 to modify the charge density of the poly-mer molecules. Along with the foam tests, we studied the thicknessand the stability of the respective foam films. The surface tensionwas measured and, for the most interesting systems, the surfacerheological properties were determined.The paper is organized as follows. The used methods and mate-rials are described in Section 2. Section 3.1 presents experimentalresults from the foam tests. The results from the model experi-ments are described in Sections 3.2–3.3. The mechanisms of foamstabilization are discussed in Section 3.4. The main conclusions aresummarized in Section 4.2. Materials and methods2.1. MaterialsThe cationic polyvinylamine (PVAm) was product of BASF.According to its producer, the used PVAm consists of 95 %vinylamine and 5 % vinylformamide. The pKa value for the polyviny-lamine is around 8.6 [37,38], which means that this is a highlycharged polymer at pH 6. The molecular mass of PVAm was deter-mined by static light scattering to be around 4.5 × 104.As low-molecular-mass surfactants we used two anionic sur-factants, which have similar chain length and differ in their headgroups – sodium dodecylsulfate (SDS, product of Acros) and sodiumdodecyloxyethylenesulfate with one ethoxy group, SDP1S (productof STEPAN Co., with commercial name STEOL CS-170). Accordingto its producer, SDP1S contains 68–72 wt% sodium alkyl (C10–16)ether sulfate, 24–32 wt% water and less than 2.5 wt% C12–14ethoxylated alcohols. The CMC values of these surfactants are3.5 mM for SDS and 0.25 mM for SDP1S (determined from sur-face tension isotherms at 10 mM NaCl). The used SDS sample didnot show minimum in the surface tension isotherm – therefore, itdoes not contain dodecanol as a contaminant.The two surfactants and the polymer were used as received. Theaqueous solutions were prepared with deionized water, purified byMilli-Q Organex system (Millipore). All solutions contained 10 mMNaCl as background electrolyte.To prepare mixed surfactant–polymer solutions we first pre-pared separate stock solutions with doubled concentrations ofsurfactant and polymer. Afterwards, by mixing these stock solu-tions (1:1 by weight) we obtained the final working solutions withthe desired concentrations of the two components. The stock solu-tion of the polymer was prepared by the following procedure:0.7 wt% of PVAm was added to 10 mM NaCl solution and stirredat 35C for 1 h with a magnetic stirrer.Therefore, the PVAm concentration in the solutions used forthe actual experiments was fixed at 0.35 wt %, corresponding to0.078 mM polymer molecules and to approx. 80 mM of monomerunits, included in these polymer molecules. The surfactant con-centration was varied between 0.01 and 0.1 mM, because this isthe range where the foam becomes stable in these mixed sys-tems. The polymer concentration was chosen to be well in excessto the used surfactant, in order to avoid the possible replacementof the polymer molecules on the solution surface by competitivelyadsorbing surfactant (an effect which is beyond the scope of the cur-rent study). pH of the mixed solutions was adjusted just before the

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176R. Petkova et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 438 (2013) 174– 185Fig. 1. Initial foam volume, V0, as a function of surfactant concentration for (A) SDSand (B) SDP1S-containing foams, formed from solutions of surfactant alone (redcircles) or surfactant + PVAm (blue squares), at pH 6 (empty symbols) and pH 10 (fullsymbols). All solutions contain 10 mM NaCl and 0.35 wt% PVAm. (For interpretationof the references to color in this figure legend, the reader is referred to the webversion of the article.)actual experiments with 0.06 or 0.25 M HCl solution (Merck, Cat.N1.00318), or with 3 M NaOH (Sigma, Cat.N 82730). As explainedin Section 3.4 below, the positively charged polymer groups werein a large excess to the anionic surfactants in the entire range ofsurfactant concentrations and pH values studied.2.2. Experimental methods2.2.1. Foam testTo compare the foamability and foam stability of the studiedsolutions, we used a modification of the Bartsch test [39]. Briefly,15 mL of the studied foaming solution was loaded in a glass cylinderwith 75 mL total volume. To generate foam, ten standard hand-shakes of the cylinder were applied. The initial foam volume andthe subsequent foam decay were monitored during the following15 min.The solution foamability was characterized by the volume oftrapped air, V0, immediately after shaking (at t = 0), while the foamstability was characterized by the defoaming time, tDEF, which isdefined as a time required for obtaining half of the solution sur-face free of bubbles. The experimental results for V0and tDEFweredetermined from (at least) three consecutive measurements. Thesymbols shown in Fig. 1–4 represent the average values from thesemeasurements and the error bars in Figs. 1 and 2 represent theFig. 2. Initial volume, V0, as a function of pH for foams, formed from 0.05 mMSDS (red empty circles); 0.05 mM SDP1S (pink empty diamonds) ; 0.05 mMSDS + 0.35 wt% PVAm (blue squares) or 0.05 mM SDP1S + 0.35 wt% PVAm (green tri-angles). All solutions contain 10 mM NaCl, 0.35 wt% PVAm and 0.05 mM surfactant.(For interpretation of the references to color in this figure legend, the reader isreferred to the web version of the article.)Fig. 3. Defoaming time, tDEF, as a function of surfactant concentration for (A) SDSand (B) SDP1S-containing foams, formed from solutions of surfactant alone (redcircles) or surfactant + PVAm (blue squares), at pH 6 (empty symbols) and pH 10(full symbols). All solutions contain 10 mM NaCl and 0.35 wt% PVAm. The arrowsshow that the defoaming time is longer than 15 min. The experimental data areaverage from three experiments – the scattering of the data is represented by thesymbol size. (For interpretation of the references to color in this figure legend, thereader is referred to the web version of the article.)

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R. Petkova et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 438 (2013) 174– 185177Fig. 4. Defoaming time, tDEF, as a function of pH for (A) 0.03 mM surfactant or (B)0.05 mM surfactant. The empty red circles represent data obtained with SDS alone,the blue squares represent data for SDS + 0.35 wt% PVAm, the empty pink diamondsrepresent data for SDP1S alone, and green triangles are for SDP1S + 0.35 wt% PVAmfoams. All solutions contain 10 mM NaCl and 0.35 wt% PVAm. The arrows show thatthe defoaming time is longer than 15 min. The experimental data are average fromthree experiments – the scattering of the data is represented by the symbol size. (Forinterpretation of the references to color in this figure legend, the reader is referredto the web version of the article.)respective standard deviations. For the defoaming times shownin Figs. 3–4, the scattering of the data is less than the symbolsize.2.2.2. Surface properties of the foaming solutionsThe surface tension of the foaming solutions was measuredby the pendant drop method. The measurements were per-formed on apparatus DSA100 (Krüss GmbH, Hamburg, Germany)at 25.0 ± 0.5C. The kinetics of the surface tension decrease, aftera pendant drop has been formed rapidly on the capillary tip, wasmonitored for 15 min. The dynamic surface tension of the solutionswas measured by the maximum bubble pressure method (MBPM)on tensiometer BP2 (Kruss GmbH, Germany).2.2.3. Surface rheological propertiesThe surface dilatational rheological properties of the foamingsolutions was measured by the oscillating drop method (ODM)[40]. In this method, the surface of surfactant solution is per-turbed sinusoidally, a (t) = a0sin (t), where a (t) = [A (t) – A0]/A0isthe normalized change of the drop surface areaaround the meanarea, A0, while a0is the relative amplitude of oscillations. Theresulting variation of the surface tension is measured and (for smalldeformations) is presented as:(t) = ESDa0sin() + ELDa0cos() (1)where ESDis the surface storage modulus (related to surface elas-ticity) and ELDis the surface loss modulus, which is related tosurface dilatational viscosity, SD= ELD/. The total surface dilata-tional modulus isED= (E2SD+ E2LD)1/2(2)Measurements were performed at 5 s oscillation period and theamplitude of deformation was varied between 1 and 7 %, at tem-perature T = 25C.2.2.4. Foam films in capillary cellFoam films of millimeter size were formed and observed in acapillary cell to obtain information about the film stability, equi-librium film thickness and film-thinning pattern. The observationswere made in reflected light, by using the method of Scheludko[41]. The films were formed from a biconcave drop, placed in a shortcapillary (i.d. 2.5 mm, height 3 mm), by sucking out liquid througha side orifice. The observations were performed in reflected lightby means of a microscope Axioplan (Zeiss, Germany), equippedwith a long-distance objective Zeiss Epiplan 20×/0.40, CCD camera(Sony SSC-C370P), video-recorder and monitor. The film thick-ness was determined by light interferometry, using the method ofScheludko–Exerowa [41]. The relation between the instantaneousvalues of the intensity of the reflected light I (t) and film thicknessh (t) can be expressed by the equation:h =20( + arcsinI − IminImax− Imin)where Imaxand Imindenote the maximal and minimal intensity ofthe reflected light, respectively, k = 0, 1,. . . is the order of the inter-ference maximum, is the wavelength of the incident light and n0is the refractive index of the liquid forming the film.3. Results and discussion3.1. Foamability and foam stability3.1.1. PVAm solutions (no surfactant added)Experiments at five different pH values, varied between 6 and10, were performed with polymer solutions without surfactantadded. The polymer concentration is fixed in all experiments at0.35 wt%. Under all conditions studied, the foamability of the poly-mer solutions was low – the initial foam volume was 5 ± 1 mL. Thegenerated foams contained relatively large bubbles, with diameter1–2 cm, and were very unstable – with defoaming time shorterthan 10 s. No significant effect of pH was found on the foamabilityand foam stability for these polymer systems.Concluding, PVAm alone is unable to stabilize the foams, underall conditions studied, due to its hydrophilic character (see also theexperimental data in Ref. [39]).3.1.2. Surfactant–polymer solutionsIn our previous study [39] we demonstrated a strong synergybetween SDS and PVAm with respect to foam stability, and antago-nistic effect with respect to the foamability of the mixed solutions.The experiments in the previous study were performed at the nat-ural pH, obtained after dissolution of the components (withoutadjustment) which was measured to be pH 9.2 for all PVAm-containing solutions.In the current study we performed experiments at different pHvalues for SDS and SDS + PVAm solutions. The experimental results

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178R. Petkova et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 438 (2013) 174– 185for the initial foam volume, at pH 6 and pH 10, are shown in Fig. 1A,as functions of SDS concentration. One sees that there is no sig-nificant effect of SDS on the foamability of the polymer solutionat very low surfactant concentrations, CS< 0.03 mM. The volumeof the initially formed foam is V0 6 ± 2 mL, which is similar tothe initial foam volume in the absence of surfactant. At higher SDSconcentrations, CS 0.05 mM, the foam volume produced with SDSsolutions (without polymer) approaches 10 mL. However, the addi-tion of PVAm to these SDS solutions reduces the foam volume rathersignificantly (compared to SDS alone), especially at higher SDS con-centrations and low pH. Thus we see that the cationic polymerPVAm acts as pronounced foam inhibitor for the anionic surfactantSDS, in the entire range of pH values studied.To investigate in detail the effect of pH on the foamability of themixed solutions, we present in Fig. 2 the experimental results forthe initial volume V0(pH) of the foams, produced from 0.05 mMSDS + 0.35 wt% PVAm solutions. No significant effect of pH on thefoamability of 0.05 mM SDS solution (without polymer) is seen –the volume of the generated foam is 14 ± 3 mL for all pH valuesstudied. There is a weak minimum in V0around pH 8, but this effectis relatively small, compared to the other effects studied.On the other hand, the increase of pH from 6 to 10 leads toa gradual increase in V0for SDS + PVAm solutions (from 5 to 6 to10 mL, see the blue squares in Fig. 2) which evidences that the foaminhibition effect of PVAm is stronger at lower pH. The latter trendis related to the higher charge density of the polymer moleculesat low pH. Note that pKa for PVAm is 8.6, which means that thepolymer molecules are highly charged at pH 6 and 7, whereasonly 10 % of the amine groups are charged at pH 10 [39]. As aconsequence, the fraction of bound SDS molecules to the polymermolecules is expected to increase strongly with the decrease of pHbelow the pKa of the polymer.The results for the foamability of SDP1S-containing solutionsare presented in Fig. 1B. Similarly to SDS, no detectable effect ofSDP1S on the foamability of the polymer solution is observed atCS 0.03 mM, while PVAm acts as foam inhibitor at CS 0.05 mM,with the effect being bigger at higher surfactant concentrations.The effect of pH on the initial foam volume for 0.05 mM SDP1S and0.05 mM SDP1S + 0.35 wt% PVAm solutions is illustrated in Fig. 2.One sees that the foamability of SDP1S solutions (with and withoutpolymer) is not affected significantly by the pH variation. Shallowminimum at pH 8 is observed for the system without polymer andslight increase is observed for the system with polymer, but botheffects are relatively small. As in the case of SDS, the polymer is astrong foam inhibitor in the whole range of pH values studied.As explained in our previous study [39], the initial foam vol-ume for all three surfactants studied there (SDS, DTAB, C12EO23)were found to be very similar in the absence of polymers. Thisunexpected result was explained with the prevailing effect of thekinetics of surfactant adsorption at such low surfactant concen-trations on the solution foamability – this adsorption kinetics isexpected to be similar for given molar concentration of the varioussurfactants, in the case of diffusion-limited control of adsorption.In the current study we see that SDP1S also gives very similarinitial foam volume to SDS (cf. the curves for SDS and SDP1S inFigs. 1 and 2) and, hence, to the other two surfactants studied inRef. [39] (DTAB and C12EO23).In contrast, when comparing the mixed polymer–surfactantsolutions, we see that the foamability of SDP1S + PVAm solutionsis somewhat lower than the foamability of SDS + PVAm solu-tions, especially at high pH, see Fig. 2. This comparison suggeststhat a stronger attraction between SDP1S + PVAm (compared toSDS + PVAm) leads to more pronounced binding of the SDP1Smolecules to the polymer backbone in the bulk, which decreasesthe adsorption rate and the foamability of the mixed PVAm-SDP1Ssolutions. This explanation is supported by the measurements ofthe adsorption kinetics, which is slower for SDP1S + PVAm, com-pared to SDS + PVAm (see Fig. 9A).We studied also the stability of the generated foams, as a func-tion of pH and surfactant concentration. As mentioned already, thefoams formed from the polymer solutions (without surfactants)were very unstable and disappeared for less than 10 s. The effectsof SDS and SDP1S concentration on the defoaming time at pH 6 and10 are compared in Fig. 3. One sees a qualitative difference betweenthe trends for SDS-containing foams and SDP1S-containing foams.First, the foams generated from solution of SDS (without polymer)are unstable at both pH values (6 and 10) in the entire range of SDSconcentrations studied, CS 0.1 mM, see the red circles in Fig. 3A.In contrast, the SDP1S foams (without polymer) are stable for morethan 900 s at CS 0.05 mM for pH 6 and at CS 0.03 mM when pH10, see Fig. 3B. We recall that the initial volumes of the foam, formedfrom SDS and SDP1S solutions, are very similar (cf. Figs. 1 and 2above) which means that these two surfactants stabilize the bub-bles with similar efficiency under dynamic conditions (during foamgeneration). However, as seen from Fig. 3, the SDS foams are muchless stable under static conditions.The effect of PVAm on the stability of SDS and SDP1S foamsdepends significantly on the pH, especially around the thresholdsurfactant concentration which separates the stable from unsta-ble foams. As seen from Fig. 3, this concentration is between 0.03mM and 0.05 mM for both SDS and SDP1S solutions. To clarify bet-ter the effect of pH on the stability of surfactant + PVAm foams, weperformed additional experiments at these two surfactant concen-trations, varying pH between 6 and 10 – see Fig. 4.Let us discuss first the foam stability in the absence of polymer.The stability of foams, formed from 0.03 mM and 0.05 mM SDS doesnot depend on pH – the defoaming time for all these foams was lessthan 60 s. Even at the highest concentration studied, 0.1 mM SDS,the defoaming time was <100 s. On the other hand, for 0.03 mMSDP1S solution, the defoaming time increased from 50 s up to 900 swith the increase of pH from 9 to 10. Similarly, tDEFincreased from50 to 265 s upon increase of pH from 9 to 10 for 0.01 mM SDP1S solu-tion (data not shown). The defoaming time tDEFfor 0.05 mM SDP1Ssolutions was longer than 900 s in the entire range of pH valuesstudied. These results demonstrate once again that SDP1S foamsare significantly more stable than the SDS foams in the transitionalrange of concentrations (in the absence of polymer).The effect of PVAm on the stability of SDS and SDP1S foams isqualitatively different. The addition of 0.35 wt% PVAm to 0.05 mMSDS leads to very stable foams, tDEF> 900 s, for all pH values studied,see Fig. 4B, despite the very low stability of SDS foams at this sur-factant concentration. In contrast, the addition of 0.35 wt% PVAmat the same surfactant concentration of SDP1S, leads to unstablefoams with tDEF< 50 s at pH 6 and 7, despite the high foam stabil-ity in the absence of polymer, see Fig. 4B. At the lower surfactantconcentration (0.03 mM) the foams formed from SDS and PVAmare all stable at pH 7, whereas the addition of PVAm to the SDP1Ssolutions does not affect noticeably the foam stability. Therefore,there is a significant difference in the interactions of the anionicsurfactants SDS and SDP1S with the cationic PVAm polymer, whichdeserves more detailed analysis.The main results from all these experiments can be summa-rized as follows: (1) Under all conditions studied, PVAm solutionsproduce unstable foams with initial volume of 3–5 mL, deforamingtime <10 s, and relatively large bubbles. (2) The addition of PVAmto SDS and SDP1S solutions decreases the foamability for both sur-factants. This effect is strongest at lower pH where the polymer ishighly charged. (3) The stability of SDS foams is strongly enhancedin the presence of PVAm at CS 0.03 mM. (4) The stability of SDP1Sfoams is strongly reduced after addition of PVAm at pH between6 and 8. The most intriguing results here are the observed syn-ergy for SDS + PVAm foams and the opposite antagonistic effect PAGE - 6 ============ R. Petkova et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 438 (2013) 174– 185179Fig. 5. Surface tension as a function of time for 0.01 mM SDS (green squares);0.03 mM SDS (red circles); 0.05 mM SDS (blue diamonds) and 0.1 mM SDS (pinktriangles) without (empty symbols) and with 0.35 wt% PVAm (full symbols), at (A)pH 6 and (B) pH 10. All solutions contain 10 mM NaCl. The symbols are experimentaldata, obtained by drop shape analysis of pendant drops, whereas the curves are bestfits according Eq. (3) for SDS solutions and Eq. (4) for SDS + PVAm solutions. (Forinterpretation of the references to color in this figure legend, the reader is referredto the web version of the article.)for SDP1S + PVAm foams, as well as the fact that these effects aremore pronounced in the region of pH where the polymer is highlycharged.3.2. Surface propertiesThe surface properties of SDS and SDP1S-containg solutionswere characterized by measuring their dynamic surface tensionby MBPM, their surface tension at long surface age by drop shapeanalysis, and their surface rheological properties by oscillating dropmethod.Experiments with the mixed PVAm + SDS and SDP1S + PVAmsolutions, at pH 6 and 10, were performed. The complete set ofresults, obtained by drop shape analysis of pendant drops fromthe studied solutions, is shown in Figs. 5 and 6. The experimen-tal results for PVAm alone at pH 6 and 10 are presented in FigureS1 in supporting information. One sees that the shape of the curve (t) for SDP1S + PVAm solution is similar to that of SDS + PVAmsolutions and the surface tension at long adsorption times is ratherlow (<30 mN/m). Therefore, similarly to SDS + PVAm, the mixtureSDP1S + PVAm behaves as strongly interacting system with respectFig. 6. Surface tension as a function of time for 0.01 mM SDP1S (green squares);0.03 mM SDP1S (red circles); 0.05 mM SDP1S (blue diamonds) and 0.1 mM SDP1S(pink triangles) without (empty symbols) and with 0.35 wt% PVAm (full symbols),at (A) pH 6 and (B) pH 10. All solutions contain 10 mM NaCl. The symbols are exper-imental data, obtained by drop shape analysis of pendant drops, whereas the curvesare best fits according Eq. (3) for SDP1S solutions and Eq. (4) for SDP1S + PVAm solu-tions. (For interpretation of the references to color in this figure legend, the readeris referred to the web version of the article.)to surface tension. Thus we conclude that the reduced foam sta-bility for SDP1S + PVAm mixture requires deeper analysis to clarifythe mechanisms and the factors, which lead to such qualitativelydifferent behavior, when compared to SDS + PVAm.To compare quantitatively the kinetics of adsorption for SDS andSDP1S containing solutions (with and without PVAm) we tried tofit the kinetic data for (t) by appropriate equations and to extractvalues for the characteristic parameters, such as the characteristicadsorption time.For the data obtained with SDS and SDP1S solutions (no poly-mer) we used the kinetic equations for diffusion-limited control ofadsorption [42]:(t) = EQ+ (0− EQ)exp(ttD)erfc(ttD) (3)Here tDis the characteristic diffusion time, EQis the equilib-rium surface tension, and 0is the initial surface tension after dropformation which might be much lower than the surface tensionof the clean solution surface, if rapidly adsorbing components arepresent. The characteristic adsorption time tDin Eq. (3) is the time PAGE - 8 ============ R. Petkova et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 438 (2013) 174– 185181(similar relations have been studied in detail and explained withemulsions – see Refs. [50–51] for detailed discussion).On the other hand, the experimental data for the foam volumeof the surfactant + polymer mixtures lay much lower than the datafor the single surfactants. This comparison shows that the molecu-lar mechanism of foam film stabilization, during foam generation,is different for the low molecular mass surfactants and for thesurfactant–polymer mixtures. As discussed in our previous study,the surfactants can ensure rapid stabilization of the foam films byMarangoni effect, due to their ability to rapidly adsorb and spreadon the solution surface. This explanation is supported by the factthat both the initial surface tensions and the initial foam volumesare very similar for the SDS and SDP1S solutions (see Figs. 1 and 2),as one may expect for dynamic foams, stabilized by Marangonieffect. This dynamic stabilization of the foam films is less efficient instrongly interacting polymer–surfactant mixtures, due to the asso-ciation of the surfactant molecules with the big and slowly diffusingpolymer molecules.The results for the equilibrium surface tension of these solutions,EQ, are compared in Fig. 7. We note first that, as seen in Figs. 5 and 6,for most of the systems studied, the surface tension continues todecrease slowly even after 15 min of surface age. Because we wereprimarily interested in the foam stability up to 15 min of foamlifetime, we did not measure the surface tension at longer times.Therefore, the values of EQshown in Fig. 7 and discussed below, areobtained by fitting the experimental data with Eqs. (3) or (4), andpresent extrapolated values at long adsorption times. The secondimportant notice is that the values of EQfor SDS and SDP1S solu-tions are very different, contrary to their initial surface tensions.The equilibrium surface tension of the SDS solutions is very closeto 0(the difference is <2 mN/m), whereas EQfor SDP1S solutionsis reduced by more than 15 mN/m, as compared to the initial one(except for the lowest SDP1S concentration). The third importantnotice is that no significant effect of pH is seen for SDS solutions(except for the highest SDS concentration, where we observe alsoan increase in the defoaming time, cf. Fig. 3A), whereas the increaseof pH from 6 to 10 leads to a significant decrease of EQfor theSDP1S solutions. Note that no such effect was observed for 0. Onthe other hand, the large difference in EQof the SDS and SDP1Ssolutions practically disappears when 0.35 wt% PVAm is added tothe solutions. Interestingly we see that EQfor SDP1S + PVAm solu-tions at pH 6 is the lowest, while the stability of the respective foamsis also very low, cf. with Fig. 3B. Also, there is no significant effectof pH on EQ, whereas we observe large effect of pH on the foamstability for these solutions, cf. with Fig. 4. Concluding, no any cor-relation between the equilibrium surface tension and the stabilityof the formed foams is observed for the mixed polymer-surfactantsystems (same conclusion was drawn in Ref. [39] with the othersurfactants studied there).Let us clarify that the observed low surface tensions insurfactant–polymer mixtures (Fig. 7) indicate the formation ofdenser adsorption layers in the presence of PVAm. Indeed, thevariations in the surface tension upon changes of the solution com-position are described by Gibbs adsorption isotherm [42]: = −11− 22(5)where index 1 refers to the surfactant and index 2 – to the poly-mer. Eq. (5) shows that the decrease of the surface tension isrelated to increase of the surfactant (polymer) chemical potentialand/or to increase of surfactant (polymer) adsorption. The attrac-tion of the surfactant and the polymer in the bulk solution certainlyreduces the chemical potential of the surfactant. Therefore, wecould explain the observed lower surface tension in the surfactant-polymer mixtures (EQ30 mN/m) only by formation of denseradsorption layer in these mixtures – the increase of 1(and possi-bly of 2) compensates for the decrease of 1which (alone) wouldFig. 9. (A) Characteristic time for SDS + PVAm (blue squares) and SDP1S + PVAm(green triangles), as a function of surfactant concentration, at pH 6 (empty sym-bols) and pH 10 (full symbols). (B) Defoaming time, as a function of characteristicadsorption time, for PVAm + SDS (blue squares) and PVAm + SDP1S (green triangles)solutions, containing 0.35 wt% PVAm and 10 mM NaCl. The two vertical lines indi-cate the range of adsorption times, in which a sharp decrease of the defoaming timeis observed. (For interpretation of the references to color in this figure legend, thereader is referred to the web version of the article.)lead to increase of . The latter explanation is in a good agreementwith the experimental results obtained by direct methods for mea-suring the adsorption in Ref. [12]. It was shown for SDS/PDMDAACmixtures that dense surfactant adsorption layer was formed on theair–water interface when the surface tension was low, 35 mN/m,even at very low surfactant concentrations (see Fig. 15 in Ref. [12]).To check how important is the kinetics of adsorption for thefoam stability in such slowly-adsorbing polymer-surfactant mix-tures, we determined the characteristic adsorption time for thevarious solutions using Eq. (4), see Fig. 9A. The characteristic times,tADS, were found to decrease rapidly with surfactant concentration,following a power-law function, for both mixtures studied. Thepower-law index is −2 for SDS-PVAm mixtures and −1.5 forSDP1S-PVAm mixtures, which is in a reasonably good agreementwith the theoretical prediction [42] for diffusion limited adsorp-tion, tADS C−2. The characteristic adsorption times are about 3–5times shorter for SDS + PVAm (corresponding to faster adsorption),as compared to SDP1S + PVAm solutions. Note also that the char-acteristic adsorption times vary between ca. 5 and 200 s in theintermediate range of surfactant concentrations (between 0.03and 0.05 mM), which illustrates rather well the excessively slowadsorption in these systems. 161 KB – 12 Pages