Journal of Colloid and Interface Science 342 (2010) 93–102

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Journal of Colloid and Interface Science

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Polymer-induced ordering and phase separation in ionic surfactants

Ewelina Kalwarczyk a, Monika Gołos a,b, Robert Hołyst a,c,*, Marcin Fiałkowski a,*

a Institute of Physical Chemistry, PAS, Department III, Kasprzaka 44/52, 01-224 Warsaw, Polandb Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Polandc WMP-SNS Cardinal Stefan Wyszynski University, Dewajtis 5, 01-815 Warsaw, Poland

a r t i c l e i n f o a b s t r a c t

Article history:Received 23 June 2009Accepted 6 October 2009Available online 12 October 2009

Keywords:Ionic surfactantsElectrolytesPolymersPhase separationPhase orderingHexagonal phasePolyelectrolytes

0021-9797/$ - see front matter � 2009 Elsevier Inc. Adoi:10.1016/j.jcis.2009.10.016

* Corresponding authors. Address: Institute of Phyment III, Kasprzaka 44/52, 01-224 Warsaw, Poland (R

E-mail addresses: holyst@ptys.ichf.edu.pl (R. HołyFiałkowski).

We present a new method to induce phase separation in solutions of ionic surfactants. In this method, thephase separation is obtained either by addition of polyelectrolytes or nonionic polymers along with inor-ganic salt. As a result, the system separates into polyelectrolyte-rich (or nonionic polymer-rich) and sur-factant-rich phase. Four types of the mixtures were investigated: (i) anionic surfactants and anionicpolyelectrolytes, (ii) cationic surfactants and cationic polyelectrolytes, (iii) cationic surfactants and non-ionic polymers, and (iv) anionic surfactants and nonionic polymers. We found that the addition of poly-electrolyte with the charge of the same sign as that of surfactant can induce the phase separation in awide range of surfactant concentrations. The addition of nonionic polymers induces the phase separationonly in solutions of cationic surfactants. Moreover, the addition of nonionic polymers induces the phaseseparation only for relatively high total content of polymer and surfactant in the mixture. We found how-ever that the addition of inorganic salt to the mixture of cationic surfactant and nonionic polymer triggersthe phase separation even for a small concentrations of surfactant. In our experiments, water as well asmixtures of water and polar solvents were employed as solvents. Based on the optical microscopy studieswe found that the surfactant-rich phase represents hexagonal ordering.

� 2009 Elsevier Inc. All rights reserved.

1. Introduction

Solutions of surfactants exhibit a variety of self-assembledstructures [1,2]. They can aggregate to form micelles or vesiclesor condense to from ordered structures of complex morphologies.The formation of the ordered structures is based on the phase sep-aration/ordering processes [3–7]. In binary mixtures of surfactantand water the phase transition is caused by the dehydration ofthe surfactant hydrophilic heads and – at fixed temperature – iscontrolled by the concentration of the surfactant molecules inthe system. The dehydration increases the influence of the vander Waals attraction between the surfactant micelles. In the caseof ternary surfactant/polymer/water mixtures the phase separa-tion is driven by different forces. Namely, adding water-solublepolymer to the binary mixture induces attractive interactions be-tween micelles, which is also referred as to the depletion interac-tions [8]. The depletion interactions are driven by entropychanges. The entropic depletion forces are known to cause phaseseparation in colloids and emulsions composed of small and largeparticles [8,9].

ll rights reserved.

sical Chemistry, PAS, Depart-. Holyst).st), fialkows@ichf.edu.pl (M.

In the mixtures of colloids (or micelles) and polymers thedepletion forces have also geometrical origins and result fromchanges in the conformational entropy of the polymer chains[10–13]. Namely, the geometric constrains prevent the center ofmass of a polymer molecule from getting closer to the micellethan a certain characteristic distance. This distance is, approxi-mately, a sum of the radius of gyration of a polymer, RG, andthe radius of the micelle, a. The center of mass of a polymer can-not get closer to the micelle because it would result in a decreasein its conformational entropy. For this reason, when two micellesare separated by a distance smaller than 2ðaþ RGÞ, polymer coilscannot enter between them. As a result, the region between themicelles is depleted with respect to the polymer concentration.The polymers outside the depletion zone between the micelles in-duce an osmotic pressure causing an effective attraction betweenthe micelles. This osmotic pressure pushes the micelles togetherand leads to the phase separation into the colloid-rich phaseand polymer-rich phase [14].

Walz and Sharma [15] developed a model for calculating thedepletion interactions between two charged particles immersedin a solution of like-charged macromolecules. The authors pre-dicted that the electrostatic interactions can increase both themagnitude and range of the depletion forces. The effect of the poly-electrolyte on the stability of solutions of charged particles againstaggregation has also been confirmed experimentally [16–18].

94 E. Kalwarczyk et al. / Journal of Colloid and Interface Science 342 (2010) 93–102

The phase separation processes induced by the addition of non-ionic water-soluble polymers in solutions of nonionic surfactantshave recently been investigated [19–21]. It has been shown [19]that the addition of poly(ethylene glycol) (PEG) to the aqueoussolution of nonionic surfactant C12E6 (n-dodecyl hexaoxyethyleneglycol monoether) can result in the separation of the system intothe polymer-rich and the surfactant-rich phases. The surfactant-rich phase exhibits hexagonal structure which is composed ofcylindrical micelles (channels) packed in a hexagonal array. Also,it has been demonstrated [21] that the hexagonal ordering canbe induced provided that (1) the radius of gyration of the polymeradded to the system is larger than the size of the water channels inthe hexagonal phase, and (2) the polymer reduces the separationtemperature to below the temperature of the hexagonal-isotropicphase transition in a binary surfactant-water mixture.

The mixtures of surfactants and polymers have recently beenextensively studied [2,22–25] because of their importance toindustrial applications [1,22,23]. They play an important role incosmetics, detergents, pharmaceuticals, pesticides, or enhancedoil recovery. Surfactants, in their ordered phases, can be used asend products themselves or applied as templates [26–29] thatare further processed to obtain complex micro- or nano-structuredmaterials. For instance, hexagonal phases self-assembled in surfac-tant mixtures may be filled with metal nanoparticles [30,31].

In the present work we demonstrate for the first time that theelectrostatic interactions can enhance the depletion forces in themixtures of surfactant and polyelectrolyte and lead to the phaseseparation and further ordering of ionic surfactants. It is a purposeof this work to investigate the process of the phase separation inaqueous solutions of ionic surfactants and polyelectrolytes as wellas in aqueous solutions of ionic surfactants and nonionic polymers.Four types of mixtures were studied: (i) cationic surfactant andcationic polyelectrolyte, (ii) anionic surfactant and anionic poly-electrolyte, and (iii) aqueous solutions of cationic surfactant andnonionic polymer, and (iv) aqueous solutions of anionic surfactantand nonionic polymer. In the case of cationic surfactants and non-ionic polymers the effect of inorganic salt on the phase separation

Fig. 1. Schematic representation of four types of experimental systems studied in themixture of anionic surfactant and anionic polyelectrolyte, (c) mixture of cationic surfacmixture of anionic surfactant and nonionic polymer. For the cases (a)–(c) the phase spolymer and salt. In the case (d) the phase separation was not observed.

was also investigated in details. All types of the experiments weperformed are schematically represented in Fig. 1. As a main result,we show that by adding either the polyelectrolytes or nonionicpolymers along with inorganic salts to the surfactant solutionone can induce phase separation over a wide range of initial surfac-tant concentrations. The resulting concentration of surfactant mol-ecules in the surfactant-rich phase is high enough to form orderedstructures.

2. Experimental

2.1. Chemicals

Cationic surfactants: (1) cetyltrimethylammonium bromide(CTAB, Mw ¼ 364:46, Sigma, 99%), (2) tetradecyltrimethylammoni-um bromide (TTAB, Mw ¼ 336;41, Sigma, 99%), (3) dodecyltrimeth-ylammonium chloride (DTAC, Mw ¼ 263;89, Sigma, 99%), (4)hexadecylpyridinium bromide (CPB, Mw ¼ 384;44, Fluka, 97%). An-ionic surfactants: (1) sodium dodecyl sulfate (SDS, Mw ¼ 288;38,Sigma, 98.5%), (2) dodecanesulfonic acid, sodium salt (DSS,Mw ¼ 272;38, Sigma, 99%). Cationic polyelectrolytes: (1) poly(dial-lyldimethylammonium chloride) (PDDAC, Mw from 100,000 to200,000, Sigma, 20% aqueous solution), (2) polyethylenimine (PEI,Mw � 750;000, Sigma, 50% aqueous solution). Anionic polyelectro-lytes: (1) poly(sodium 4-styrenesulfonate) (PSS, Mw � 70,000, Sig-ma), (2) poly(acrylic acid, sodium salt) (PAAS, Mw � 15,000,Sigma, 35% aqueous solution). Nonionic polymers: (1) poly(ethyleneglycol) (PEG, Mw � 20,000, Fluka), (2) polyvinyl alcohol (PVA,Mw � 27,000, Fluka). Inorganic salts: (1) sodium chloride (NaCl,Chempur), (2) nickel nitrate, hexahydrate ðNiðNO3Þ2 � 6H2O, Sig-ma), (3) copper sulfate (CuSO4, POCh), (4) cobalt sulfate, heptahy-drate ðCoSO4 � 7H2O; POChÞ, (5) manganese sulfate, pentahydrate(MnSO4 � 5H20, Wako Pure Chemicals Industries), (6) sodium ni-trate (NaNO3, Chempur). Polar solvents: (1) acetone (99.5%, Chem-pur), (2) ethanol (96%, Linegal Chemicals). The chemicals wereused as delivered. Chemical structures of the surfactants and thepolymers are shown in Fig. 2.

present work: (a) mixture of cationic surfactant and cationic polyelectrolyte, (b)tant and nonionic polymer – either undoped or doped with inorganic salt, and (d)eparation occurs for sufficiently high mass fraction of polyelectrolyte or nonionic

Fig. 2. The chemical structures of the surfactants and polymers used in the experiments: cetyltrimethylammonium bromide (CTAB), tetradecyltrimethylammonium bromide(TTAB), dodecyltrimethylammonium chloride (DTAC), hexadecylpyridinium bromide (CPB), sodium dodecyl sulfate (SDS), dodecanesulfonic acid, sodium salt (DSS),poly(diallyldimethylammonium chloride) (PDDAC), polyethylenimine (PEI), poly(sodium 4-styrenesulfonate) (PSS), poly(acrylic acid, sodim salt) (PAAS), poly(ethyleneglycol) (PEG), polyvinyl alcohol (PVA).

E. Kalwarczyk et al. / Journal of Colloid and Interface Science 342 (2010) 93–102 95

The values of the critical micelle concentrations (the concentra-tions of surfactants above which the micelles are spontaneouslyformed in the solution) for the surfactants used in experimentsare the following: CTAB: 0.9 mM [32]; TTAB: 3.86 mM [33]; DTAC:21.5 mM [34]; CPB: 0.65 mM [35]; SDS: 8.16 mM [36]; DSS:8.55 mM [37].

2.2. Sample preparation

The samples were prepared in ambient conditions at the roomtemperature (about 25 �C). The solutions were prepared by dissolv-ing surfactants and polyelectrolytes/polymers in a solvent inappropriate weight proportions. As a solvent we applied (i) eitherpure water or (ii) a mixture of water and one of the two organic po-lar solvents: acetone or ethanol. To calculate the mass fraction ofsurfactant, Cs, and polymer, Cp, in the sample, the following formu-las were applied:

Cs ¼ms

ms þmsolv; ð1Þ

Cp ¼mp

mp þmsolv; ð2Þ

where ms, mp, msolv are, respectively, the mass of surfactant, poly-mer, and the solvent. If the solvent was a mixture of water and a po-lar solvent (acetone or ethanol), the mass of the solvent wascalculated as a sum of the mass of water and the mass of the polarsolvent, msol ¼ mH2O þmpol. The amount of the polar solvent, m, wasexpressed as the mass fraction,

m ¼ mpol

msol: ð3Þ

In experiments where the system was doped with inorganicsalt, the amount of salt was measured relative to the content ofthe surfactant in the solution, as the molar fraction

/salt ¼Nsalt

Ns; ð4Þ

where Nsalt and Ns denote, respectively, the number of moles of thesalt and the surfactant.

To obtain homogeneous solutions the samples were first heatedup to about 60 �C and thoroughly blended using a magnetic stirrer.After that, in order to remove gas bubbles from the bulk of thesolution, the samples were ultracentrifuged (using the Hettich Uni-versal 32 centrifuge). Before the examination, the samples were al-lowed to relax at the room temperature. The mixtures wereinitially clear (transparent). The process of phase separation wasaccompanied by the formation of a cloudy phase. The cloudy andthe clear phases were separated by a sharp interface and corre-sponded, respectively, to the surfactant-rich and the polymer-richphases. If the process of the phase separation took place, the cloudyphase showed up usually 1 or 2 h after the sample preparation. Ifthe sample remained clear for more than 10 h we assumed thatthe phase separation does not occur. Typically, the volume of thesample was about 10 cm3.

To determine the threshold concentrations of salt in the exper-iments with the solutions of CTAB and PEG doped with (NaCl), thesalt was added successively to the system starting from the molarfraction /salt ¼ 0. The amount of salt was increased by either 0.05or 0.1. Each time after the addition of NaCl the solutions wereblended using magnetic stirrer at the room temperature and thenrelaxed for a couple of hours. A photograph of the sample – a mix-ture of CTAB (Cs = 15%) and PEG ðCp ¼ 5%Þ doped with NaClð/salt ¼ 0:6Þ – taken before and after the phase separation is shownin Fig. 3.

2.3. Optical studies

When the separation process took place, the cloudy (surfactant-rich) phase was further investigated with the optical microscopywith the purpose of determining whether it represents an ordered

Fig. 3. The mixture of CTAB ðCs ¼ 15%Þ and PEG ðCp ¼ 5%Þ doped with NaCl ð/salt ¼ 0:6Þ before (left) and after (right) the phase separation process. The cloudy and the clearregion in the right picture corresponds to the surfactant-rich and the polymer-rich phase, respectively. The time difference between the pictures is about 5 h.

96 E. Kalwarczyk et al. / Journal of Colloid and Interface Science 342 (2010) 93–102

or disordered phase. The samples for the microscope studies wereprepared in the following way: First, with the aid of a syringe, asmall portion of the cloudy phase was placed between two glassplates of the diameter of about 15 mm. Prior to use, the plates werecleaned in the ultrasound cleaner and washed in acetone andmethanol. The distance between the plates – controlled with theuse of aluminum spacers – was 100 lm. Finally, the sample wassealed with the glue ‘‘Poxipol” to prevent water from evaporatingduring the observations. The sample was investigated using theoptical microscope Nicon Eclipse E 400 equipped with crossedpolarizers. The images were acquired and processed with the useof the image analyzer software Lucia G. To determine the type ofthe ordering, the texture of the sample was compared to the tex-tures of the hexagonal phases formed by aqueous solutions ofthe surfactant (with known type of ordering, based on the phasediagrams [1,2,38]). An example of the texture of the surfactant-richphase obtained from the CTAB/PDDAC/water ternary mixture andthe texture of the hexagonal phase observed in CTAB/water binarymixture (25 wt.%) is shown in Fig. 4.

3. Phase separation in mixtures of ionic surfactants andpolyelectrolytes

3.1. Cationic surfactant and cationic polyelectrolyte

To investigate the phase separation in the system composed ofcationic surfactant and cationic polyelectrolyte we performed asystematical study of aqueous solutions of CTAB and PDDAC. Weemployed the CTAB/PDDAC mixtures for the following seven massfractions of the surfactant: 3%, 5%, 7%, 10%, 12%, 15%, and 20%. Foreach of the above mass fractions of CTAB, we determined the

threshold mass fractions, CH

p , of PDDAC. The threshold mass frac-tion was estimated in the following way: For the mass fractionsof the polyelectrolyte lower than CH

p the mixture did not separate,for the mass fractions higher than CH

p it separated into the surfac-tant-rich and the polyelectrolyte-rich phases. We found that thephase separation can be induced by addition of the polyelectrolytefor a wide range of the surfactant mass fractions. We observed theseparation in the systems in which the mass fraction of CTAB wasclose to that corresponding to the spontaneous ordering of the hex-agonal phase in the binary mixture CTAB/water (about 20 wt.%) aswell as in the mixtures of low mass fraction of the surfactant. Wefound that the amount of the polyelectrolyte needed to induce thephase separation process decreased with the mass fraction of thesurfactant. In the mixture of high surfactant mass fraction, contain-ing 15% CTAB, the phase separation was induced by a small admix-ture (of the order of 2.5%) of PDDAC. In the mixture of lowsurfactant mass fraction (�3%), the phase separation took placewhen the mass fraction of the polyelectrolyte reached about18.5%. The dependence of the threshold mass fraction of the poly-electrolyte on the surfactant mass fraction for the CTAB/PDDAC/water ternary mixture studied is shown in Fig. 5.

The analysis of the textures, carried out using the opticalmicroscopy under crossed polarizers, revealed that for all massfractions of CTAB used, the surfactant-rich phases exhibited thehexagonal order. This result is not surprising in view of the factthat at the temperatures higher than 20 �C the first ordered phaseobserved in the CTAB/water binary mixture is the hexagonal phase[2], which occurs for the mass fraction of CTAB about 25% (cf.Fig. 4).

Note also that the molar concentration corresponding to thesurfactant mass fraction Cs ¼ 3%, at which the phase separationwas observed in our experiments is, approximately, only 80 times

Fig. 4. The photograph of the texture of the surfactant-rich phase which emerged inthe mixture of CTAB ðCs ¼ 5%Þ and PDDAC ðCp ¼ 16%Þ as a result of the phaseseparation (top), and (bottom) the texture of the hexagonal phase observed in theaqueous solution of CTAB at the surfactant mass fraction 25 wt. %, at thetemperature 25 �C. Pictures obtained from the optical microscopy under crossedpolarizers. As seen, the concentration of the surfactant molecules in the surfactant-rich phase is high enough to induce the hexagonal ordering.

0 10 20 30 40Cs

0

5

10

15

20

25

30

C∗ p

SDS/PSSCTAB/PDDAC

Fig. 5. Phase diagram for the aqueous solutions of CTAB/PDDAC (cationic surfac-tant/cationic polyelectrolyte) and SDS/PSS (anionic surfactant/anionic polyelectro-lyte) plotted in the surfactant mass fraction ðCsÞ – polyelectrolyte mass fraction ðCpÞplane. The data points represent the threshold mass fractions, CH

p , of the polyelec-trolytes needed to induce the phase separation process. The threshold massfractions were determined for selected mass fractions of the surfactant at thetemperature 25 �C. For the mass fractions of the polyelectrolytes higher than CH

p themixtures separate into the surfactant-rich and the polymer-rich phases. The dashedlines represent least-squares fit of Eq. (15) to the experimental data.

Table 1Examples of the polymer-induced phase ordering observed in different cationicsurfactant/cationic polyelectrolyte systems. The symbols ‘‘w”, ‘‘w + a”, and ‘‘w + e”refer, respectively, to water, mixture of water and acetone, and mixture of water andethanol. The numbers in brackets denote the amount of acetone/ethanol, m, in thesolvent mixture (see Eq. (3)). The polymer mass fractions, Cp , are not the thresholdmass fractions. The values of the threshold mass fractions have been investigated indetail only for the CTAB/PDDAC/water system. Here we only show that the sameeffect is observed for other systems.

Surfactant Polyelectrolyte Solvent Cs (wt. %) Cp (wt. %)

TTAB PDDAC w 20 20TTAB PDDAC w 30 10TTAB PEI w 30 20TTAB PDDAC w + a (5%) 30 10CPB PDDAC w 15 5CPB PDDAC w + a (5%) 15 10CTAB PDDAC w + a (5%) 10 15CTAB PDDAC w + a (95.5%) 15 15CTAB PDDAC w + e (5%) 15 10

E. Kalwarczyk et al. / Journal of Colloid and Interface Science 342 (2010) 93–102 97

bigger than the critical micelle concentration, CMCCTAB = 0.9 mM[32]. Thus, by addition of the polyelectrolyte we can induce thephase separation in a wide range of the surfactant concentrations.

To demonstrate the robustness of the effect studied we also car-ried out a number of experiments with other cationic surfactants:TTAB, CPB, and CTAB, and two other cationic polyelectrolytes:PDDAC and PEI. As a solvent either water, mixtures of ethanoland water, or mixtures of acetone and water was used. We didnot perform detailed studies of the phase diagram for these sys-tems. That is, in contrast to the CTAB/PDDAC/water system, wedid not determine the threshold mass fractions of the polyelectro-lytes. The aim of the experiments was simply to show that it is pos-sible to find the mass fraction of the polyelectrolyte, Cp, at whichthe phase separation occurs for given mass fraction of the surfac-tant, Cs. The results of the experiments are summarized in Table1. The values of Cp listed in the last column represent the massfractions of the polyelectrolyte that was sufficient to induce thephase separation.

3.2. Anionic surfactant and anionic polyelectrolyte

To study the phase separation in the system composed of anio-nic surfactant and anionic polyelectrolyte we employed aqueoussolutions of SDS and PSS. The SDS/PSS/water ternary mixture was

investigated for the following nine mass fractions of SDS: 3%, 5%,10%, 15%, 20%, 25%, 30%, 35%, and 40%. For each of the above massfractions of the surfactant, we determined the threshold mass frac-tions of the polyelectrolyte, CH

p , above which the phase separationwas observed. We found that for all the surfactant mass fractionsthe phase separation process can be induced by increasing the con-tent of the polyelectrolyte in the mixture. As in the case of the cat-ionic surfactant/cationic polyelectrolyte system discussedpreviously, we observed that the amount of the polyelectrolytenecessary for the phase separation process to occur decreased withthe mass fraction of the surfactant. For example, in the mixture ofhigh SDS content ðCs ¼ 35%Þ, the phase separation was induced bya relatively small amount of PSS (Cp � 3:5%Þ in the solution. In themixture of low surfactant mass fraction ðCs ¼ 3%Þ, the phase sepa-ration occurred for the mass fraction of the polyelectrolyteCp � 26%. The dependence of the threshold polyelectrolyte massfraction on the surfactant mass fraction for the system studied ispresented in Fig. 5. Based on the optical microscopy studies ofthe textures, we found that for all the mass fractions of SDS, the

0 0.25 0.5 0.75 1 1.25φ

3

6

9

12

C∗ p

Fig. 6. The threshold mass fraction, CH

p , of the polyelectrolyte (PSS) needed toinduce the phase separation/ordering process in the SDS/PSS/water system as afunction of the molar fraction, ð/Þ, of the NaCl added. The mass fraction of thesurfactant (SDS) is Cs ¼ 20%, the experiment was conducted at the temperature25 �C. For the mass fractions of the polyelectrolyte higher than CH

p the mixturesseparate into the ordered (hexagonal) surfactant-rich and the polymer-rich phases.The solid lines are plotted as guides for the eye.

98 E. Kalwarczyk et al. / Journal of Colloid and Interface Science 342 (2010) 93–102

surfactant-rich phases represented the hexagonal ordering. Wenote also that the amount of surfactant in the mixture ðCs = 3%)at which the phase separation was induced is, approximately, only13 times bigger than the critical micelle concentration, CMCSDS =8.16 mM [36].

We investigated also three other systems: SDS/PAAS/water,DSS/PAAS/water, and a solution of SDS and PSS in the mixture ofwater and acetone. Our goal was to confirm the effect of polyelec-trolyte on the phase separation. Our goal was only to demonstratethat it is possible to find the mass fraction of the polyelectrolyte,Cp, at which the phase separation takes place for given mass frac-tion of the surfactant, Cs. In contrast to the SDS/PSS/water system,we did not perform detailed studies of the phase diagram and didnot determine the threshold mass fractions of the polyelectrolytes.The results of the experiments are summarized in Table 2.

3.3. Effect of inorganic salt

To investigate the effect of salt on the phase separation, we con-ducted series of experiments on both the SDS/PSS/water and CTAB/PDDAC/water systems employing NaCl as the inorganic salt. Theexperiments were carried out at the temperature 25 �C. We foundthat in each system the addition of the salt resulted in lowering ofthe mass fraction of the polyelectrolyte, CH

p , needed to induce thephase separation. However, for the systems studied, the salt addedwithout the polyelectrolyte did not induce the hexagonal orderingin the surfactant-rich phase. The dependence of CH

p on the molarfraction of NaCl for the SDS/PSS/water system for the surfactantmass fraction Cs ¼ 20% is shown in Fig. 6. As seen, CH

p decreaseswith the increasing content of the salt added and saturates at about/ = 1.0. Similar effect was observed for the CTAB/PDDAC/water sys-tem. For this system, for the surfactant mass fraction Cs ¼ 10%, wefound that the addition of NaCl lowers the threshold mass fractionof the polyelectrolyte from CH

p ¼ 9:5% (observed for / ¼ 0:0) toCH

p = 2.5% for / ¼ 1:0.

0.5

1

1.5

φ*N

aCl

CCTAB = 10%

CCTAB = 15%

CCTAB = 20%

4. Phase separation in mixtures of ionic surfactants andnonionic polymers

4.1. Cationic surfactant and nonionic polymer doped with inorganicsalt

Aqueous solutions of CTAB and PEG were used to investigate thephase separations in cationic surfactant/nonionic polymer system.We studied three surfactant mass fractions of CTAB: 10%, 15%, and20%, and the mass fractions of PEG in the range from 1% to 50%. Foreach the mass fractions we examined the effect of the salt on thephase separation. We found that for each of the mass fractions ofCTAB studied there existed two threshold mass fractions of PEG:(i) the upper threshold mass fraction, Cup

PEG, above which the phaseseparation takes place without the addition of the salt, and (ii) thelower threshold mass fraction, Clow

PEG, below which the phase separa-

Table 2Examples of the polymer-induced phase ordering observed in different anionicsurfactant/anionic polyelectrolyte systems. The symbols ‘‘w”, ‘‘w + a”, stand for waterand mixture of water and acetone, respectively. The numbers in brackets denote thecontent, m, of acetone in the solvent mixture (see Eq. (3)). The polymer mass fractions,Cp , are not the threshold mass fractions. The values of the threshold concentrationhave been investigated in detail only for the SDS/PSS/water system. Here we onlyshow that the same effect is observed for other systems.

Surfactant Polyelectrolyte Solvent Cs (wt. %) Cp (wt. %)

SDS PAAS w 10 20SDS PSS w + a (5%) 15 20DSS PAAS w 10 15

tion could not be induced by the addition of the salt. The values ofCup

PEG decreased with the mass fraction of CTAB. For the mass frac-tions of CTAB Cs = 10%, 15%, and 20%, the upper threshold massfractions of PEG were Cup

PEG estimated as 50%, 40%, and 30%, respec-tively. The value of the lower threshold mass fraction was approx-imately the same for the three mass fractions of CTAB studied. Itwas estimated as Clow

PEG � 2%.For each mass fraction of CTAB, for the mass fractions of PEG in

the range between ClowPEG and Cup

PEG, we determined the threshold mo-lar fraction of NaCl, /H

salt , representing the minimal amount of thesalt needed to induce the phase separation. If the amount of NaClwas lower than /H

salt the mixture did not separate; for /salt P /H

salt

the mixture separated into the surfactant-rich and the polyelectro-lyte-rich phases. The values of /H

salt as a function of the mass

0 10 20 30 40 50CPEG

0

Fig. 7. The threshold molar fraction, /H

salt , of NaCl needed to induce the phaseseparation in the aqueous solutions of CTAB/PEG/NaCl at the temperature 25 �C,plotted as a function of the mass fraction of PEG for three mass fractions of CTAB:10%, 15%, and 15%. The solid lines are drawn as guides for the eye. For the molarfractions of the salt above /H

salt the system separates into the surfactant- and thepolymer-rich phases.

E. Kalwarczyk et al. / Journal of Colloid and Interface Science 342 (2010) 93–102 99

fraction of PEG are plotted for three mass fractions of CTAB inFig. 7. From the optical microscopy studies of the textures, wefound that for all mixtures, the surfactant-rich phase representedthe hexagonal ordering.

We also confirmed the robustness of the effect of inorganic salton the phase separation by studying variety of other systems beingaqueous solutions of cationic surfactants and nonionic polymers.The aim of our experiments was to find – for a given mass fractionof surfactant and polyelectrolyte – the molar fraction of inorganicsalt that was sufficient to induce the phase separation. In contrastto the CTAB/PEG/water system, we did not find the threshold molarfraction of salt needed to induce the phase ordering. Results of theexperiments are presented in Table 3. Note that for each mixturelisted in Table 3 the phase separation did not take place in the ab-sence of inorganic salt.

4.2. Anionic surfactant and nonionic polymer

To study the phase separation in the system composed of anio-nic surfactant and nonionic polymer we used solutions of SDS andPEG. We investigated mixtures of the following percentage compo-sitions of the surfactant and the polymer: (10%, 0.5–60%), (20%, 5–30%), (30%, 5–35%), (35%, 5–20%), and (45%, 5–30%). The first termsin the brackets refer to the mass fractions of SDS, and the secondterms denote the ranges of the mass fractions of PEG employed.In the experiments, the mass fractions of the polymer were sam-pled with DCp = 5%. Note here that the viscosity of the mixture in-creases rapidly with the content of PEG. For this reason we did notuse samples with the polymer mass fractions higher than about60%. For any of the mixtures studied the phase separation processdid not occur. Also, in order to check possible effect of inorganicsalt on the phase separation, for selected mass fractions of SDSNaCl was added to the mixture. We found that the salt inducedthe phase separation only for the highest total mass fractions ofPEG and SDS, when the salt was added in a relatively high molarfraction ð/salt ¼ 2:0Þ.

5. Discussion

5.1. Ionic surfactant/polyelectrolyte

The mechanism of the polyelectrolyte-induced phase separa-tion in the ionic surfactant/polyelectrolyte system can be ex-plained in terms of the chemical potential of water in the

Table 3Examples of the phase ordering induced in different cationic surfactant/nonionicpolymer systems doped with inorganic salt. For each mixture listed, the phaseseparation did not occur in the absence of the salt. The molar fractions, /, are not thethreshold fractions. The values of the threshold molar fractions have been investi-gated in detail only for the mixture CTAB/PEG/water doped with NaCl. Here we onlydemonstrate that the same effect is observed for other systems.

Surfactant Polymer Salt Cs (wt. %) Cp (wt. %) /salt

CTAB PEG NiðNO3Þ2 � 6H2O 10 10 0.7CTAB PEG CuSO4 10 10 0.7CTAB PEG CoSO4 � 7H2O 10 10 0.7CTAB PEG MnSO4 � 5H2O 10 10 0.7CTAB PEG NaNO3 10 10 0.7CTAB PVA NaCl 20 30 1.0TTAB PEG NaCl 10 10 1.3TTAB PEG NaCl 15 10 0.6TTAB PVA NaCl 30 3 1.0CPB PEG NaCl 10 10 1.0CPB PEG NaCl 15 10 0.6CPB PVA NaCl 18 3 1.0CPB PVA CoSO4 � 7H2O 18 3 1.0CPB PVA NiðNO3Þ2 � 6H2O 18 3 0.8DTAC PEG NaCl 35 15 1.5

surfactant- and polymer-rich phases. It was found in Ref. [19] thatthe interface between these two coexisting phases acts as a semi-permeable membrane [39]. That is, this interface is permeable towater (and small ions) but not to large polyelectrolyte molecules.Therefore, the thermodynamic equilibrium in the system is deter-mined by the equality of the chemical potential of water in the sur-factant-rich phase, lsurf

H2O, and in the polymer-rich phase, lpolyH2O,

lsurfH2O xsurf

H2O

� �¼ lpoly

H2O xpolyH2O

� �; ð5Þ

where xsurfH2O and xpoly

H2O denote, respectively, molar fraction of water inthe surfactant- and the polymer-rich phase. The chemical potentialsare the following functions of the activities, asurf

H2O and apolyH2O, of water

in the surfactant-rich and the polymer-rich phase:

lsurfH2O xsurf

H2O

� �¼ l0

H2O þ RT ln asurfH2O; ð6Þ

lpolyH2O xpoly

H2O

� �¼ l0

H2O þ RT ln apolyH2O; ð7Þ

where l0H2O denotes the chemical potential of water in the standard

state, T is the temperature, and R is the gas constant. The wateractivities are linked with the molar fractions by the relationsasurf

H2O ¼ csurfH2Oxsurf

H2O, and apolyH2O ¼ cpoly

H2OxpolyH2O, where csurf

H2O cpolyH2O denote,

respectively, the activity coefficients of water in the surfactant-and polymer-rich phase. The molar fractions of water both in thesurfactant- and polymer-rich phase are close to unity, and, conse-quently, csurf

H2O � cpolyH2O � 1. Thus, from Eqs. (5)–(7) one gets

ln xsurfH2O ¼ ln xpoly

H2O: ð8Þ

The molar fraction of water in the surfactant-rich phase can beestimated based on the molar fraction, xsurf of the surfactant. Thespace between the charged micelles is filled with water and thecounterions released by the surfactant molecules (Na+ for SDSand Br� for CTAB). Some fraction of the counterions are bound tothe surface of the micelles and form an immobile layer (referredalso to as the Stern layer). The rest of the counterions are weaklyassociated with the micelles and form a diffuse layer. Only the lat-ter affect the molar fraction of water in the surfactant-rich phase.Denote the fraction of the diffuse ions by gðg 6 1Þ. Then, xsurf

H2O is gi-ven by

xsurfH2O � 1� gxsurf : ð9Þ

In the polymer-rich phase two types of ions are present: thecharged polyelectrolyte molecules and the accompanying counte-rions (Na+ for PSS and Cl� for PDDAC). If we denote the averagenumber of counterions released from the dissociation of the poly-electrolyte molecule as n ðn� 1Þ the molar fraction of water in thepolymer-rich phase is given by

xpolyH2O ¼ 1� ð1þ nÞxpoly � 1� nxpoly; ð10Þ

where xpoly is the molar fraction of the polyelectrolyte molecules inthe polymer-rich phase. From Eqs. (8)–(10), one gets the followingcondition for the quality of the chemical potentials of water in boththe phases:

gxsurf ¼ nxpoly: ð11Þ

We note that, in general, the exchange of small ions betweenthe polymer- and surfactant-rich phases can occur. That is, thecounterions released from the surfactant molecules can migrateto the polymer-rich phase and these released from the dissociationof the polyelectrolyte molecules migrate to the surfactant-richphase. However, the number of polyelectrolyte molecules in thepolymer-rich phase and the number of surfactant molecules inthe surfactant-rich phase remains unchanged. Thus, to maintainthe electrical neutrality, the total number of counterions of eithertype is constant in both the phases. It follows that the molar frac-

100 E. Kalwarczyk et al. / Journal of Colloid and Interface Science 342 (2010) 93–102

tions of the counterions, xpoly and xpoly, are fixed. Consequently, thecondition (11) is valid even if the surfactant and the polyelectrolytereleases different types of counterions, provided they aremonovalent.

The relation (11) allows to predict the amount of polyelectro-lyte needed to induce the phase separation in the system. Denoteby Cm

s the minimal mass fraction of the surfactant in the surfac-tant-rich phase needed to get an ordered (hexagonal) phase. Ifthe mixture separates into the polymer- and the surfactant-richphase, the ordering takes place in the latter phase when the massfraction, Cs, of the surfactant in this phase is larger than or equal toCm

s . In the following, we find the mass of the polyelectrolyte, mp,that has to be added to the binary solution of surfactant/water ofthe mass fraction Cs to induce the ordering in the surfactant-richphase after the phase separation. At equilibrium, water distributesbetween the two coexisting phases according to Eq. (11). The massof the surfactant, ms, that is sufficient to induce the phase orderingcorresponds to the mass fraction of the surfactant equal to Cm

s . Themass of water, msurf

H2O, in the surfactant-rich phase is then given by

msurfH2O ¼

1� Cms

Cms

ms: ð12Þ

The total mass, mH2O, of water in the system is

mH2O ¼1� Cs

Csms: ð13Þ

The conservation of the mass of water yields

msurfH2O þmpoly

H2O ¼ mH2O; ð14Þ

where mpolyH2O is the mass of water in the polymer-rich phase. Com-

bining Eqs. (11)–(14) one gets the following relationship betweenthe threshold mass fraction, CH

p , of the polyelectrolytes needed toinduce the phase separation/ordering process and the surfactantmass fraction, Cs, in the mixture:

CH

p

1� CH

p

¼ jMpoly

Msurf

Cms

1� Cms

� Cs

1� Cs

� �; ð15Þ

where j ¼ g=n; Mpoly and Msurf are, respectively, the molar mass ofthe polyelectrolyte and the surfactant.

We fitted the relation (15) to the experimental data obtained forthe SDS/PSS/water and CTAB/PDDAC/water systems. The quantityj and the value Cm

s were the fitting parameters. The results ofthe fitting are shown in Fig. 5. As seen, the dependencies obtainedbased on Eq. (15) agree fairly well with the experimental data. Forthe SDS/PSS/water mixture we obtained jan ¼ 2:21� 10�3 andCm

s ¼ 35:8% (slightly lower than the value �40% following fromthe phase diagram of the SDS/water mixture[1]). Assuming a com-plete dissociation, one PSS molecule releases the number n ofcounterions equal to the number of the composing monomers.For the average molar mass MPSS = 70,000, one gets n � 340. Thisvalue, combined with the ratio jan obtained, gives reasonable esti-mate of the fraction of the diffuse ions in the surfactant-rich phasegan ¼ jann ¼ 0:75. For the CTAB/PDDAC/water system the fit of Eq.(15) yielded jcat ¼ 2:47� 10�3, and Cm

s ¼ 18:6% (close to the value�20% that follows from the phase diagram of the CTAB/water mix-ture[2]). Large mass polyelectrolytes do not dissociate completelybut some part of the counterions remain bounded with polyions[40]. additionally, PDDAC is known [41] to entrap water molecules.The polymer-fixed water contains the Cl� anions that do not con-tribute to the water activity in the polymer-rich phase. Thus, theeffective number, neff , of the anions released into the solvent byone PDDAC molecule is lower than the number n of the monomerscomposing the PDDAC molecule. In the experiments we usedPDDAC of the average molar mass MPDDAC = 150,000, correspondingto n � 929. To estimate neff we assume that the fraction of the dif-

fuse anions in the surfactant-rich phase is of the order of unity,gcat � 1:0, From the value jcat obtained we get neff ¼ gcat=jcat

� 400. That is, the PDDAC molecule releases into the solventroughly a half of all possible counterions.

5.2. Ionic surfactant/nonionic polymer

In the following, we discuss the phase separation occurring inthe mixture of ionic (cationic) surfactant and non-ionic polymer(PEG) without the addition of salt. As in the case of the ionic surfac-tant/polyelectrolyte system, we consider the chemical equilibriumbetween water molecules in the surfactant-rich and the polymer-rich phase. The main contribution to the chemical potential ofwater in the surfactant-rich phase is due to the presence of theions. It is written as (cf. Eq. (6))

lsurfH2O ¼ l0

H2O þ RT ln asurfH2O � l0

H2O � RTgxsurf : ð16Þ

In the PEG-rich phase the main contribution to the chemical po-tential comes from the interactions of water molecules with oxy-gen atoms in the polymer chains[19]. If we assume that one PEGmolecule consists of Z oxygen atoms (Z � 457 for PEG 20,000)the chemical potential of water in the PEG-rich phase can be writ-ten as

lpolyH2O � l0

H2O � RTxPEG � �ZxPEG; ð17Þ

where xPEG is molar fraction of the PEG molecules, and � is someconstant of the dimension of J/mol. In Eq. (17), the product of Zand xPEG approximates the average number of the oxygen atomsper one water molecule in the PEG-rich phase. Equating (16) and(17) we get

xsurf ��

RTZg

xPEG: ð18Þ

The last term in Eq. (17) was neglected since xPEG is negligiblysmall compared with xsurf . Combining Eq. (18) with Eqs. (12)–(14), we obtain the following relation between the mass fractionof PEG, Cup

PEG, above which the phase separation occurs withoutthe addition of salt for a given surfactant mass fraction, Cs:

CupPEG

1� CupPEG

¼ RTgZ�

MPEG

Msurf

Cms

1� Cms

� Cs

1� Cs

� �; ð19Þ

where Cms is the minimal mass fraction of the surfactant needed to

induce the phase ordering in the surfactant/water system. Accord-ing to Eq. (19), Cup

PEG decreases with the increasing Cs. This agreeswell with the results of the experiments on the CTAB/PEG/watersystem (Fig. 7).

As we found, the nonionic polymer can induce the phase sepa-ration and ordering for cationic surfactant (CTAB) only. It is, how-ever, not clear why the phase ordering is not induced in thesolution of anionic surfactant (SDS) by the addition of the nonionicpolymer (PEG). This fact can be attributed to the presence of theinteractions between PEG and the sulfate groups of the SDS mole-cules [23,25,27,42,43]. The SDS molecules bind to the PEG chains toform aggregates. This effect can prevent the micelles from con-densing into ordered structures. Further studies are needed to elu-cidate this phenomenon.

5.3. The effect of salt

To discuss the effect of salt on the phase separation we invokethe Donnan effect (equilibrium) [44]. The Donnan equilibriumestablishes between two ionic solutions that are separated by asemipermeable membrane allowing the passage of selected ions.To illustrate the Donnan effect assume that, initially, one side (I)of the semipermeable membrane contains a solution consisting

E. Kalwarczyk et al. / Journal of Colloid and Interface Science 342 (2010) 93–102 101

of permeable cations such as Na+ with some impermeable large an-ions, and the other side (II) contains pure solvent. After addition ofNaCl (that dissociates into Na+ and Cl� ions, which are permeableto the membrane) an equilibrium between ion concentrations onboth sides establishes. The resulting concentration of NaCl is high-er on the side II of the membrane. This uneven distribution of thesalt is due to the presence of the immobile anions on side I that cre-ate an electrostatic potential repelling the Cl� anions. Overall, thesalt added tends to equalize the total concentrations of ions onboth sides of the membrane and reduces the osmotic pressure.

In the system composed of ionic surfactant and polyelectrolyteor ionic surfactant and nonionic polymer, the phase boundary be-tween the polymer- and the surfactant-rich phase acts as a semi-permeable membrane. That is because the interface is permeablefor small ions and is impermeable neither for the charged polyelec-trolyte molecules, polymer molecules, nor for the charged surfac-tant micelles. In the ionic surfactant/polyelectrolyte system, dueto the Donnan effect the inorganic salt added to the system doesnot distribute evenly to reduce the difference between the molarfractions xsurf and xpoly. Consequently, as observed in the experi-ments (Fig. 6), the amount of polyelectrolyte needed to inducethe phase separation/ordering is lower compared to that of un-doped system. Similarly, in the ionic surfactant/nonionic polymersystem, the ions of the salt added are repelled by the counterionspresent in the surfactant-rich phase. As a result, the salt distributesunevenly and concentrates mainly in the PEG-rich phase. The pres-ence of the salt in the polymer-rich phase counterbalances the ini-tial excess of the ions in the surfactant-rich phase. Thus, for a givenmass fraction of the polymer and the surfactant, the salt can equil-ibrate the chemical potentials in both the phases and induce theordering in the system, as observed in the experiments (Fig. 7).

6. Conclusions

In the present work we have studied the phase separation infour types of surfactant/polymer systems: (i) anionic surfactantsand anionic polyelectrolytes, (ii) cationic surfactants and cationicpolyelectrolytes, (iii) cationic surfactants and nonionic polymerseither undoped or doped with different inorganic salts, and (iv) an-ionic surfactants and nonionic polymers. As a solvent, we haveused either pure water or mixtures of water and polar solvents.We have demonstrated experimentally that in the systems studiedthe phase separation can be induced in two ways: First, by additionof ionic polyelectrolyte having the charge of the same sign as thatof surfactant, and, second, by addition of nonionic water-solublepolymer alone or along with inorganic salt. In each case the systemseparates into polyelectrolyte-rich and surfactant-rich phase withhexagonal ordering. The first method can be applied for both cat-ionic and anionic surfactants. The second method works well onlyfor cationic surfactants. To demonstrate the robustness of ourmethod we studied a variety of ionic surfactants and polymers.

We have found that the effect of nonionic polymers on thephase separation is significantly smaller than the effect of ionicpolymers, as they can induce the phase separation only in solutionsof cationic surfactants, for high mass fraction of the polymer. Foranionic surfactants the addition of nonionic polymers does not re-sult in the phase separation. We have found that the addition ofinorganic salt to the mixture of cationic surfactant and nonionicpolymer can induce the phase separation even for a small massfraction of surfactant. Inorganic salt has however significantlyweaker effect on the phase separation for solutions of anionic sur-factants and nonionic polymers.

The method of the induction of the phase separation we devel-oped is versatile and facilitates formation of surfactant-rich or-dered phases in a broad range of surfactant mass fractions.

Remarkably, the addition of ionic polyelectrolyte can trigger thephase separation even for very small surfactant mass fraction –only one order of magnitude larger than the critical micelle con-centration. This makes the presented method potentially usefulin industry, especially in water purification processes to removesurfactant contamination. It can be also applied in material engi-neering to produce hexagonal matrices that can be further pro-cessed and employed as templates to fabricate structuralfunctional materials [26,28,29].

Acknowledgments

This work was supported by the Ministry of Science and HigherEducation as a scientific project (2007–2010) and by the projectoperated within the Foundation for Polish Science Team Pro-gramme co-financed by the EU ‘‘European Regional DevelopmentFund” TEAM/2008-2/2 and also by Research Grant from the HumanFrontier Science Program Organization. EK and RH acknowledgethe MISTRZ stipendship from FNP. We are grateful to prof. P. Gars-tecki, K. Urbaniak, M. Pyzalska, and J. Keska for their useful com-ments and discussions.

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