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Colloids and Surfaces A: Physicochem. Eng. Aspects 380 (2011) 100–106

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical andEngineering Aspects

journa l homepage: www.e lsev ier .com/ locate /co lsur fa

ssociation of anionic surfactant mixed micelles with hydrophobicallyodified ethyl(hydroxyethyl)cellulose

lexandre G. Dal-Bó ∗, Rogério Laus, Arlindo C. Felippe, Dino Zanette, Edson Minattiepartamento de Química, Universidade Federal de Santa Catarina, 88040-900, Florianópolis, SC, Brazil

r t i c l e i n f o

rticle history:eceived 3 November 2010eceived in revised form 17 February 2011ccepted 18 February 2011vailable online 2 March 2011

a b s t r a c t

Some aspects of ethyl(hydroxyethyl)cellulose (EHEC) aqueous behavior in the presence of ionic sur-factants are described in the literature; however, most of the studies reported deal with moderatelyconcentrated solutions. Few studies have been carried out in the dilute regime using mixtures of anionicsurfactants sodium dodecyl sulfate (SDS) and sodium dodecanoate (SDoD). The main proposal of thiswork is to investigate the interaction of EHEC in diluted regime and to verify the mixtures of the surfac-

eywords:thyl(hydroxyethyl)celluloseurface tensiononductivity

tants SDS and SDoD. Mixtures of EHEC and SDoD were investigated using surface tension, conductivityand transmittance measurements. Parallel experiments with EHEC in mixtures with SDS were carriedout. The formation of micelles of SDoD mixed with SDS in the absence and presence of EHEC was alsoinvestigated. The transmittance vs. [SDS] and [SDoD] profiles exhibit bands that, in conjunction with con-

ion d

ransmittanceolymer–surfactant interactionnionic surfactants

ductivity and surface tenssolutions.

. Introduction

The synergism between ionic surfactants and nonionicmphiphilic polymers in aqueous solution has attracted a great dealf attention in recent years in fundamental and applied research1,2]. The growing importance of polymer surfactant mixtures in aide range of technical applications (pharmaceuticals, paints, oil

ecovery, etc.) has led to more intense research activities in thiseld [2]. Poly(ethylene oxide) (PEO) with sodium dodecyl sulfateSDS) is the most studied system and it has been extensively useds a reference for other polymer–surfactant systems [3–6]. Moreecently, cellulose derivative polymers have been widely studiedince they enhance the rheological properties and the commercialuality of emulsions and soft materials. Besides the results beingf interest in fundamental and applied research, they have beensed in industry for several applications in different areas. The mosttudied systems are, undoubtedly, neutral polymers derived fromydrophobically modified cellulose and surfactants, in particular,queous solutions of the ethyl(hydroxyethyl)cellulose (EHEC) andDS mixture [7–15].

The classical profiles of conductivity and surface tension vs. sur-

actant concentration in the presence of water-soluble polymers,enerally observed when the association phenomenon occurs,xhibit the following characteristics: (i) The onset of binding toolymer, defined by the critical aggregation concentration (cac), is

∗ Corresponding author. Tel.: +55 48 3721 9854; fax: +55 48 3721 6850.E-mail address: alexandredalbo@hotmail.com (A.G. Dal-Bó).

927-7757/$ – see front matter © 2011 Published by Elsevier B.V.oi:10.1016/j.colsurfa.2011.02.028

ata, aid a better understanding of the behaviors of EHEC–SDS–SDoD–water

© 2011 Published by Elsevier B.V.

observed through abrupt changes in the property analyzed, similarto the critical micellar concentration (cmc) of the surfactant itself.For most polymer–surfactant systems, the cac occurs at a lowersurfactant concentration than the corresponding cmc, as describedelsewhere [5,6,16]; (ii) At concentrations above the cac, the pro-files exhibit a second breakpoint which marks the saturation of thepolymer by surfactant, denoted as the polymer saturation point(psp); and (iii) The plot of surfactant concentration at psp vs. poly-mer concentration shows a linear correlation, and it is acceptedthat the surfactant amount exceeding the psp forms regular aque-ous micelles, in dynamic equilibrium with the polymer–surfactantcomplexes [17,18].

Surfactants with similar structures of like charge usually mixideally because hydrophobic and hydrophilic parts sense the sameenvironments in the mixed aggregates. Consequently, they aresimilar to those in the pure component micelle. This ideal mix-ing behavior can be simply modeled by Eq. (1) which assumesthat the thermodynamics of mixed micelle formation obey idealsolution theory [19]. In this equation, cmc is the predicted criticalmicellar concentration of the mixtures, and cmcSDoD and cmcSDSare the cmc values for the individual components [20].Eq. (2) isrelated to the predicted ideal mixing behavior for the ternary sys-tem polymer–SDS–SDoD. The cac necessarily marks the beginningof formation of mixed polymer–SDS–SDoD complexes and Eq. (2)

provides also an excellent description of the onset of mixed com-plex formation over the entire range of surfactant composition [20].

cmcIdeal = cmcSDScmcSDoD

�SDoDcmcSDS + �SDScmcSDoD(1)

Physicochem. Eng. Aspects 380 (2011) 100–106 101

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A.G. Dal-Bó et al. / Colloids and Surfaces A:

acIdeal = cacSDScacSDoD

�SDoDcacSDS + �SDScacSDoD(2)

In this study we focus on conductivity measurements, in com-ination with surface tension and transmittance measurements, ashe principal tool to explain the various aspects of the associationf anionic surfactant mixed micelles with EHEC in diluted regime.

The basis for initiating this investigation lies in the fact that con-uctivity has not yet been systematically applied in the study of theHEC–surfactant system, and published results [8,9,13,18] providenly a partial picture. The ionic species in solution can be monitoredhrough the specific conductivity vs. surfactant concentration pro-le, a fact that is directly related with the process of the surfactantinding. In addition, mixtures of SDS and SDoD, with and with-ut polymer, have been previously investigated in our laboratory,nd this study should be regarded as an extension of our previ-us research [20–22]. Finally, from a practical point of view, thisork contributes to the search for alternative uses of surfactants

n practical formulations containing neutral polymers such as theydrophobically modified celluloses and provides information onhe ideality of mixtures of the surfactants SDS and SDoD.

. Experimental

.1. Materials

The EHEC, Bermocoll E 230FQ, was obtained from Akzo NobelB, Sweden. Sodium dodecanoate (SDoD) was prepared from dode-anoic acid (Sigma 99%) and NaOH solution. Sodium dodecyl sulfateSDS) was supplied by Sigma Aldrich and used as received. Stockolutions of 0.02 M borate buffer prepared from boric acid wereitrated with NaOH solution (pH 9.20) at 25.0 ◦C, monitored withBeckman pH meter model Ø71, equipped with a combined glasslectrode. To ensure that the experimental data would apply ton equilibrium situation, all of the aqueous EHEC stock solutionsMilli-Q water) were routinely prepared, with magnetic stirring fort least 12 h.

.2. Methods

Surface tension measurements were carried out in borate bufferolutions (pH 9.20) at 25.0 ◦C. The former was measured by usingKrüss GMBH interfacial tensiometer (model K8), equipped withPt–Ir-20 ring. The cmc, cac and psp values of the surfactants in

he EHEC–surfactant solutions were obtained from the intersectionf the two linear regions of the surface tension vs. log surfactantoncentration plot [1,2].

Electrical conductivity data were acquired by means of a water-acketed flow dilution cell, with an ATI Orion conductometrymodel 170). Routinely, aliquots of buffer surfactant stock solu-ion containing the appropriate amount of EHEC, were added ton initial volume with the same EHEC concentration to keep theoncentration constant. The parameters cac and psp, obtained fromolutions containing EHEC and SDS or SDoD, were determined fromhe inflection points in the electrical conductivity vs. surfactantoncentration plot.

Transmittance was measured using a Hewlett-Packard 8452Aiode array spectrophotometer. For each solution containing EHECnd surfactant, the transmittance bands were recorded in the wave-ength range of 400–650 nm.

. Results and discussion

During the past decade there has been a considerable interestn investigations on amphiphilic systems and micelle formation

echanisms and the specific problem of surfactant/polymer inter-

log [SDS], M

Fig. 1. Surface tension as a function of [SDS] in 0.02 M borate buffer solution, pH9.20, in the absence (�) and presence (©) of 0.1 wt% EHEC at 25.0 ◦C.

action has attracted much interest. However, most of the studiesreported in the literature deal with moderately concentrated solu-tions, in which the dynamics of the polymer is affected. Few studieshave been carried out on diluted regime using mixtures of anionicsurfactants.

The interaction of water-soluble polymers [3,6] and proteins[3,23–25] with anionic surfactants has been conventionally mon-itored by surface tension and specific conductivity measurementsplotted against the surfactant concentration. These plots exhibitthe features of two critical concentrations classically named “T1”,herein the critical aggregation concentration (cac), and “T2”, hereinthe polymer saturation point (psp). There follows a description ofthe surface tension and conductivity plots of EHEC–SDS–SDoD mix-tures obtained applying classical interpretation and conventionalconcepts.

Knowledge of the concentration regime adopted in the investi-gation of a polymer solution, is as important as knowledge of theconcentration, given that, in passing from one system to another,there are considerable changes in the properties of the solution[26].

In another publication, through the extrapolation of the viscos-ity profiles the parameter c* was obtained whose value at 25 ◦C was0.31 wt%. This result is in agreement with those previously obtainedby Lindman and Holmberg [10,13].

For the EHEC, c* varies within the range of 0.2–0.4 wt%. Thesevalues change with increasing temperature, the technique used andthe degree of substitution in the cellulose chain of the groups ethyland ethylene oxide.

3.1. Surface tension of EHEC–SDS–SDoD mixtures

Fig. 1 shows the profile of the surface tension of SDS and SDoDin buffer, in the absence and presence of 0.1 wt% EHEC. In thepresence of polymer the different regions are interpreted in termsof interactions between the polymer and surfactant. Initially, atlow concentrations the surfactant acts only as a monomer at theair–liquid interface, as the non-polar groups of the surfactant inaqueous solution are close to the surface minimizing contact withthe water. The polymer acts by minimizing the repulsion betweenthe polar parts of the surfactant and thus constant adsorption of thesurfactant occurs on the surface saturating the air–liquid interface

[27–30].

The beginning of the cooperative association of the surfactant isachieved by saturation of the interface forming polymer–surfactantcomplexes and is indicated by the first inflection point of the

102 A.G. Dal-Bó et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 380 (2011) 100–106

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S3

Fig. 3. Plots of specific conductivity (plotted on relative scale except for plot F) inthe absence of EHEC as a function of SDS–SDoD surfactant concentration in 0.02 M

ig. 2. Profiles of surface tension in the presence of 0.1 wt% EHEC in 0.02 M borateuffer solution, pH 9.20, as a function of surfactant concentration obtained for theollowing SDS molar fractions (©) 1.0 and (�) 0.5.

urve (cac), which occurs at 2.5 mM for SDS. The association pro-ess finishes at the point of discontinuity (psp), which occurs at.6 mM for SDS, representing the concentration at which satura-ion of the EHEC polymer chains by surfactant monomers occurs31]. At concentrations above the psp, the tension values in thebsence and presence of EHEC are identical and constant. This facts a strong indication that, above the psp, the surfactant addedree micelles which are in equilibrium with the polymer–surfactantomplex. In the absence of polymer, the cmc for SDS occurs at.7 mM.

Fig. 2 shows the effect of the 0.5 M fraction of the SDS andDoD mixture on the surface tension profile. It can be observedhat when the solution contains only SDS the surface tension athe psp is 38.5 mN m−1. When the mixture has equal fractions ofhe surfactants, the surface tension decreases to 33.7 mN m−1 andhe psp falls to 15.0 mM. Above this value, the surface tension isonstant. According to the argument applied above for the SDSnd SDoD solutions, it can be concluded that above the psp theres formation of free micelles and mixed SDS–SDoD. The fact thatbove the psp the surface tension is lower than when in pure SDS,ndicates that the SDoD has greater activity at the air–water inter-ace than the SDS and therefore, in principle, is a more efficienturfactant.

In this study, the adsorption of the surfactants at the air–solutionnterface was investigated by surface tension measurements, as aunction of the concentration of surfactant, using a ring tensiome-er.

The concentration of excess surfactant monomers at the sur-ace is a useful measure of the effectiveness of the surfactantdsorption, since this is the maximum that the adsorption canttain.

The surface tension graphs were used to estimate the molecu-ar area occupied by one molecule of surfactant at the air–waternterface. The area of the monomeric surfactant is a measure of theegree of packing and orientation of surfactant molecules at their–water interface, thus indicating the effectiveness of the surfac-ant adsorption at the air–water interface [32].

According to the results in Table 1, the surface concentration� ) decreases with an increasing fraction of SDS in the mixture.he area occupied by each molecule on the surface is higher when

he mixture is richer in SDS. This behavior is probably due to greaterteric and electric exclusion promoted by the polar part of the sur-actant SDS, since the sulfate ion is larger and has a charge densityreater than that of the carboxylate ions.

borate buffer solution, pH 9.20, at 25.0 ◦C, obtained for the following SDS molarfractions: (A) 1.0; (B) 0.75; (C) 0.6; (D) 0.5; (E) 0.25; (F) in pure SDS. The arrowsindicate the cmc.

3.2. Conductivity measurements

The conductivity is a property widely used in the study of theinteraction between polymers and ionic surfactants. It has also beenused to estimate the degree of ionization (˛) of ionic micelles, aswell as to study the processes and complementary association ofmicelles of ionic surfactants and a mixture of water-soluble neutralpolymers [20,33–35]. The conductivity results are analyzed as pro-files of electrical conductivity vs. surfactant concentration, in theabsence and presence of 0.1 wt% EHEC, varying the concentrationof EHEC in molar fractions of SDoD.

The cmc values of SDS and SDoD mixtures are very similar tothose found for surface tension (Table 1). These values are lowerthan those obtained in pure water, which vary depending on themethod used due to the effect of saline buffer [18]. The increasein the ionic strength shields the electrostatic repulsion betweenthe charged groups in the Stern layer. This phenomenon favors themicellization of the surfactant and occurs at lower concentrations,inducing the growth of micellar aggregates [25].

Fig. 3(A) shows the profile of specific conductivity vs. [SDoD]in the absence of EHEC. At low [SDoD], it was observed that as theconcentration is increased, the electrical conductivity increases lin-early until it reaches the cmc inflection point at ≈23 mM SDoD.At low concentrations, Fig. 3(F), there is a linear increase in theconductivity up to the cmc at around 5.7 mM. This behavior ischaracteristic of strong electrolytes and the slope varies accord-ing to the molar conductivity of the species in solution, that is, thecounter-ion and the organic group.

From Fig. 4 the experimental cmc values and the degree of micel-lar ionization (˛1) were obtained for mixed surfactants (Table 2).Fig. 4 shows the profiles of the electrical conductivity for differentmolar fractions of the SDS–SDoD mixture in the presence of 0.1 wt%EHEC. In the presence of EHEC polymer and mixtures of surfactants,the profiles of electrical conductivity show a similar behavior forthese systems. The first break in the curve is the beginning of thecooperative association between the polymer and the surfactant(cac), and the second break represents the saturation of the poly-

mer by surfactant molecules (psp). These characteristics have twolinear regions: (i) below the cac; and (ii) above the psp, and a thirdregion between cac–psp which is dependent on the linearity of thesystem and the experimental conditions [36].

A.G. Dal-Bó et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 380 (2011) 100–106 103

Table 1Parameters obtained from profiles of surface tension for SDS–SDoD mixtures in the presence of 0.1 wt% EHEC in 0.02 M borate buffer solution, pH 9.20, at 25.0 ◦C.

EHEC(wt%)

�SDoD cmc or cac(mM)

Surface tensionat cmc or cac(mN m−1)

10−6�(mol m−2)

w (A2

molecule−1)psp(mM)

Surfacetension at psp(mN m−1)

0 1.0a 22.0 27.0 7.1 23.40 0.0b 5 38.50.1 1.0 16.6 31.6 6.88 24.1 28.0 27.80.1 0.75 6.3 39.2 3.22 51.4 22.0 31.00.1 0.50 4.8 39.0 2.68 62.1 15.0 33.70.1 0.25 3.0 41.0 2.56 64.9 10.8 36.00.1 0.0 2.5 41.0 2.29 72.6 7.6 38.5

a cmc value for SDS in 0.02 M borate buffer pH 9.20, at 25.0 ◦C.b cmc value for SDoD in 0.02 M borate buffer pH 9.20, at 25.0 ◦C.

Table 2Values of cmc or cac and psp and slopes of the linear region below the cac (S1), between cac and psp (S2) and above the psp (S3), data in units �−1cm2 mol−1, obtained fromprofiles of Fig. 4 in the presence of 0.1 wt% EHEC. The values of ˛1 and ˛2 were determined from the ratios S3/S1 and S2/S1, respectively.

EHEC(wt%)

�SDoD cmc(mM)

cac(mM)

psp(mM)

S1 S2 S3 ˛1 ˛2

0.0 0 5.7 59.0 23.0 0.390.0 1 23.0 58.0 26.5 0.460.1 0 2.3 62.5 36.3 23.6 0.38 0.580.1 0.15 2.5 8.3 62.4 39.7 24.2 0.39 0.640.1 0.25 3.0 10.5 59.1 37.6 25.6 0.43 0.640.1 0.5 4.2 14.7 58.5 39.7 26.6 0.45 0.680.1 0.75 6.3 20.5 58.3 41.6 27.2 0.47 0.710.1 0.85 8.3 22.5 56.7 43.4 26.8 0.47 0.77

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0.1 0.88 9.3 23.20.1 0.92 10.5 24.40.1 0.95 11.0 25.00.1 1.00 15.5 27.0

With the addition of a neutral polymer, as in the case of EHEC, tosolution containing a surfactant, there is no significant interfer-

nce in the electrical conductivity of the solution and changes in theonductivity profile therefore indicate interaction. These profilesere similar for all fractions studied, indicating that the SDS and

DoD interact with the polymer. These aspects are elucidated as fol-

ows: (i) The first linear region occurs at concentrations below theac. Table 2 lists the values for the angular coefficients S1 which areractically independent of the composition of the mixture obtained

n the absence and presence of EHEC. Considering the limit of accu-

100 20 30 40

1000

1500

2000

2500

3000

3500

Surfactant concentration, mM

Speci

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E

D

C

B

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ig. 4. Plots of specific conductivity (plotted on relative scale except for plot A) inhe presence of 0.1 wt% EHEC as a function of SDS–SDoD surfactant concentrationn 0.02 M borate buffer solution, pH 9.20, at 25.0 ◦C, obtained for the following SDS

olar fractions: (A) 1.0; (B) 0.75; (C) 0.5; (D) 0.25; (E) in pure SDoD. The arrowsndicate the cac and the dotted lines indicate the psp.

58.8 46.7 27.5 0.47 0.7958.0 28.0 0.4856.8 26.1 0.46

racy of the technique, this fact indicates that, below the cac the firstmodel of the mixture is formed by surfactant monomers and in thepolymer there is no connection process and cooperative binding onthe polymer occurs only at specific cac values. (ii) Above the psp,S3 is interpreted as a region where only free surfactant micellesare formed in dynamic equilibrium with the complexes formed atthe psp. Strong evidence for this conclusion comes from the factthat above the psp the values for S3 for the EHEC–SDS–SDoD com-plex are very similar to those obtained in the absence of EHEC. (iii)The region located between the points of discontinuity, cac-psp,is defined as a stage where aggregates are formed with differentcharacteristics of the mixed micelles of SDS–SDoD. It was notedthat the linearity of this region is dependent on the experimentalconditions, mainly the increase in polymer concentration, wherethere is a synergistic effect caused by the connection process anddesorption [36].

Table 2 summarizes the parameters of the polymer mixtureswith SDS and SDoD obtained by electrical conductivity at 0.1 wt%EHEC. The parameters ˛1 and ˛2 were determined by the slopemethod, that is, the ratio between S3/S1 and S2/S1 estimated fromthe linear regions of the conductimetric profiles [21,37,38].

3.3. Ideal mixing behavior of SDS and SDoD in the absence andpresence of EHEC

Fig. 5 shows the variation in cmc as a function of the variationin the molar fraction in the SDS and SDoD mixture in the absenceof EHEC. It can be observed that the experimental values are con-sistent with the values expected for an ideal mixture, determinedby equation 1, where the ideal cmc is the cmc of the mixture.

This was, in fact, to be expected because the surfactants have thesame counter-ion, a hydrophobic group without ramifications anda homologous series of 12 and 11 carbons and, finally, the values ofthe S1 slopes are similar. It can also be noted that the values of cacare all lower than those of cmc when considering the same frac-

104 A.G. Dal-Bó et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 380 (2011) 100–106

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ig. 5. Variation in cac and cmc for the ternary system EHEC–SDS–SDoD in theresence of 0.1 wt% EHEC as a function of SDoD molar fractions. Dotted lines arealculated from Eqs. (1) and (2) and points (�) and (�) are the values of cmc andac, respectively.

ions of the mixture, indicating that there is a polymer–surfactantssociation for both the EHEC–SDoD and the EHEC–SDS.

The experimental points, in this case, also show good agreementith the curve representing an ideal system (dotted line), obtained

rom Eq. (2) modified by replacing the ideal cmc with the ideal cac,he SDS cmc with the cac of the EHEC–SDS system, and the SDoDmc with the EHEC–SDoD cac, indicating the ideality of the system.he presence of EHEC in the mixture does not affect the interactionetween surfactant molecules, which may indicate that the twourfactants interact similarly with the polymer.

Thus, both mixtures in the presence and absence of polymer,ehave ideally. It is interesting that the mixtures behave similarlyo mixtures of SDS–SDoD and PEO, indicating that the associationrocess is similar even with a polymer which is structurally quiteifferent [20].

The variation of cmc to simulate the behavior of mixed micellesf SDS–SDoD and cac was achieved here by applying the theory ofdeal solution (see Eqs. (1) and (2)). Electrical conductivity methodsave largely been used to estimate the degree of ionization (˛) of

onic micelles and have also been extended to polymer–surfactantssemblies. In particular, this is a fundamental parameter in thenterpretation of micellar catalysis by ionic micelles [20].

Although several techniques have been used to estimate theegree of ionization for ionic micelles, the simplest is conductom-try which consists of estimating the value from the ratio of thelopes of linear plots above and below the cmc. This method canlso be applied to surfactant–polymer systems in which the ratiof the slopes of linear regions above (S2) and below (S1), the cac˛1), is attributed to the degree of ionization of EHEC–surfactantomplexes, and above (S3), the psp, and below the cac (˛2) it iselated to free micelles [1,20,39].

It was observed that the values of ˛2 are greater than thosef ˛1 indicating that the aggregates in the micellar complexHEC–SDS–SDoD are more ionized than regular micelles, althoughhe values are dependent on the molar fraction. Similar results werebtained for a PEO–SDS–SDoD complex by Zanette and Frescura20].

The values of the degree of ionization (˛1) for the

HEC–SDS–SDoD complex are very similar to the values obtainedor mixtures of SDS and SDoD in the absence of EHEC (Table 2),ndicating that the second point of discontinuity found on theonductivity profiles is the beginning of polymer saturation. Notelso that the values for the angular coefficients ˛1 listed in Table 2

Fig. 6. Variation in psp for the the SDS–SDoD mixture, measured through specificconductivity (�), and surface tension (�) as a function of SDoD molar fractions at0.1 wt% EHEC in 0.02 M borate buffer solution, pH 9.20, at 25.0 ◦C.

are almost independent of the composition of the mixture ofsurfactants.

3.4. Measurement of psp through electrical conductivity andsurface tension

The psp values obtained from the profiles of electrical con-ductivity and surface tension in the presence of 0.1 wt% EHEC arerepresented in Fig. 6. In all fractions, both surfactants interact withthe polymer and the interaction is highly influenced by the com-position of micellar aggregates, that is, for the fraction of eachsurfactant in the mixture. Note that the values for psp obtained withthe two techniques are similar, varying linearly with increasingmolar fraction of SDoD. The linear relation obtained has been usedto predict the saturation of the polymer surfactant for different sys-tems [1,20,39]. Note that the linear relation obtained by applyingEqs. (3) and (4) for the electrical conductivity and surface tension,respectively, are similar, indicating that both techniques indicatethe same phenomenon of saturation by the polymer surfactants.

The linear correlations for the two profiles are 0.997 and 0.990for Eqs. (3) and (4), respectively.

[SDS + SDoD]t = (0.0212)�SDoD + 0.0048 (3)

[SDS + SDoD]t = (0.0210)�SDoD + 0.0062 (4)

3.5. Transmittance of EHEC–SDS–SDoD mixtures

We have applied an alternative technique to follow the changesin the macroscopic properties of the EHEC–SDS–water systemby monitoring the transmittance of the solutions [1,2]. Thetransmittance results in conjunction with the conductivity andsurface tension data, support our explanation of the behavior ofEHEC–SDS–SDoD–water solutions. As shown in previous studies,some details in relation to the transmittance can be highlighted, asshown in Fig. 7A and B (insertion): (i) the decrease in transmittancecoincides with the cac obtained using the techniques of electrical

conductivity and surface tension (Tables 1 and 2), and therefore,provides a further indication that the polymer–surfactant interac-tions begin at cac; (ii) these data show that the psp coincides withthe recovery of the maximum transmittance ≈22.5 and 8.3 mM,for 0.1 wt% EHEC in SDoD molar fractions of 0.85 and 0.15, respec-

A.G. Dal-Bó et al. / Colloids and Surfaces A: Physi

Fig. 7. Transmittance (�) and (©) specific conductivity measured as a function ofSDoD molar fractions of 0.85 (A) and 0.15 (insert B), for 0.1 wt% EHEC in 0.02 Mborate buffer solution, pH 9.20, at 25.0 ◦C.

0.0 0.2 0.4 0.6 0.8 1.0

4

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16

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ig. 8. Variation of cac for the ternary system EHEC–SDS–SDoD in the presence of.1 wt% EHEC in 0.02 M borate buffer solution, pH 9.20, at 25.0 ◦C as a function ofDoD molar fractions, measured by specific conductivity (�) and transmittance (�).otted lines are calculated from Eq. (2).

ively, leading to a clear coincidence with the values obtained usinghe conductivity and surface tension techniques for all fractions ofhe ternary mixture of EHEC–SDS–SDoD (Tables 1 and 2 and Fig. 8);iii) as previously indicated by the viscosity measurements [40], theransmittance band areas are dependent on the polymer concentra-ion, meaning that the observable change in the transmittance mayven vary according to the amount of polymer–surfactant inter-olecular cross-linking, as in the model described for EHEC–SDS

omplexes using several rheological techniques [1,2].

. Conclusions

It can be concluded from both the electrical conductivity andurface tension results that the mixture formed by the surfactantsDS and SDoD is an ideal system, shown through the variation in

mc and cac.

The second point of discontinuity on the electrical conductivitynd surface tension curves in the presence of 0.1 wt% EHEC variesinearly for all fractions of the surfactants. The interaction is influ-nced by the composition of micellar aggregates, that is, the fraction

[

cochem. Eng. Aspects 380 (2011) 100–106 105

of each surfactant in the mixture implying that the process of asso-ciation between the SDS–SDoD surfactant mixture and the polymeris strongly cooperative.

The degrees of ionization of the micelles and the complexes withthe polymer indicate that certain aggregates are more ionized inrich mixtures of SDoD. The polymer does not influence the degreeof ionization below the cac and above the psp, but only in the regionbetween cac and psp.

The decrease in the transmittance of the solutions found usingthe spectrophotometric method in the region of cac is consis-tent with the rheological behavior of these mixtures. This result,together with the cac values obtained by electrical conductiv-ity and surface tension, provides evidence that the SDS andSDoD surfactant mixture behaves optimally, indicating that thepolymer–surfactant interactions start only at the cac. The spec-trophotometric method thus proved to be a good tool for theinvestigation of mixtures of cellulose ethers with mixtures ofanionic surfactants.

Acknowledgement

The authors wish to thank Conselho Nacional de Pesquisa eDesenvolvimento (CNPq–Brazil) for financial support.

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