2893
Langmuir 1991, 7, 2893-2898
Phase Behavior of Systems of Polyacrylate and Cationic Surfactants Kyrre Thalberg,: Bjorn Lindman, and Karin Bergfeldt Physical Chemistry 1, Chemical Center, Lund University, P.O. Box 124, S-221 00 Lund, Sweden Received February 11, 1991. In Final Form: April 12, 1991 The phase behavior of aqueous systems of sodium polyacrylate (NaPA) and cationic surfactants of the alkyltrimethylammoniumbromide type (C,TAB) has been investigated. Phase separation occurs in dilute solution in a certain interval of surfactant concentration and may be suppressed by addition of simple salt. At very low concentratione, the polyacrylate chains can bind a considerable amount of surfactant and remain in solution, whereas at higher NaPA concentrations, phase separation occurs at low degrees of surfactant binding to the polymer. The phase diagram for systems of NaPA and C,TAB is of the same type as obtained previously for another polyelectrolyte (sodium hyaluronate, NaHy) and the same cationic surfactants. The higher linear charge density for PA as compared to Hy is reflected in a larger two-phase area for this polymer. Systems of a polycation and an anionic surfactant have also been subject to investigation. The interactions between polyelectrolyte and surfactant are seen to be enhanced in these systems as compared to systems of a polyanion and a cationic surfactant but the main features are the same.
Introduction
A precipitation reaction between proteins and cationic detergents was first reported by Kuhn in 1940.’ The phenomenon was found to be general for anionic polymers? and was recognized to be due to electrostatic interactions. Thus, addition of simple salt could lead to dissolution of the precipitate^.^^^ These findings were exploited in the purification of anionic polysaccharides from biological tissue, as has been reviewed by S ~ o t t . ~ Studies directed to the phase behavior of polyelectrolytesurfactant systems have been carried out by Saito,6 by Goddard and Hannan,’ and by Ohbu et a1.8 In all systems studied, phase separation was reported to take place and was most pronounced in solutions containing equimolar amounts of surfactant and polyelectrolyte charges. In some systems, the precipitates could readily be redissolved by additional amounts of surfactant, while in other systems, this was not the case. It may be concluded that polyelectrolyte-surfactant systems display a rather complicated phase behavior, which furthermore is largely dependent on the polyelectrolyte used. During the last 20 years, much progress has been made in the field of polymer-surfactant interactions. First, the development of specially designed surfactant selective electrodes has enabled direct monitoring of the binding of surfactant to the polymer c h a i n ~ . ~Second, J~ the application of powerful tools, such as NMR spectroscopy,11J2small angle neutron scattering (SANS),13J4and (1)Kuhn, R. Ber. Dtsch. Chem. Ces. 1940,73,1080. (2)Scott, J. E. Chem. 2nd. (London) 1955,168. (3)Scott, J. E. Ph.D. Thesis, University of Manchester, 1956. (4)Laurent, T.C.; Scott, J. E. Nature 1964,202,661.Scott, J. E. In The Chemical Physiology of Mucopolysaccharides;Quintarelli, G., Ed.; Little, Brown: Boston, MA, 1968;p 219. (5)Scott, J. E. In Methods of Biochemical Analysis; Glick, D., Ed.; Intersciences Publishers, Inc.: New York, 1960; Vol. 8,p 145. (6)Saito, S.Kolloid-2. 1955,143,66. (7)Goddard, E.D.; Hannan, R. B. J. Colloid Interface Sci. 1976,55, 73;J. Am. Oil Chem. SOC.1977,54,561. (8)Ohbu, K.; Hiraishi, 0.; Kashiwa, I. J . Am. Oil Chem. SOC.1982,59, 108. (9)Birch, B. J.; Clarke, D. E.; Lee, R. S.; Oakes, J. Anal. Chim. Acta 1974, 70,417. (10)Gilanyi, T.; Wolfram, G. Colloids Surf. 1981,3,181. (11)Cabane, B. J. Phys. Chem. 1977,81,1639.
0743-7463/91/2407-2893$02.50/0
fluorescence techniques,15J6has significantly contributed to the revelation of structure in polymer-surfactant systems. For systems of an uncharged water-soluble polymer and an ionic surfactant, a structure of micellelike surfactant aggregates adsorbed to the polymer chains is the general picture in dilute solution (often called a “pearl necklace” structure). This structure seems to be valid also in systems of a polyelectrolyte and an oppositely charged surfactant, as indicated from fluorescence result^,^^-^^ and is in agreement with the marked cooperativity in surfactant binding to polyelectrolytes.2e22The surfactant concentration at the onset of cooperative surfactant binding to the polymer is normally quite welldefined and will here be denoted the critical aggregation concentration, cac. The field of polymer-surfactant interactions has been thoroughly reviewed by G ~ d d a r d , ~ ~ and for systems of a cationic surfactant and an anionic polyelectrolyte, a review has recently appeared.24 We have previously investigated the phase behavior of systems containing the anionic polysaccharide hyaluro(12)Gao, Z.; Wasylishen, R. E.; Kwak, J. C. T. J.ColloidInterface Sci. 1988,126,371;J. Phys. Chem. 1990,94,773. (13)Cabane, B.; Duplessix, R. J.Phys. (Paris) 1982,43,1529; Colloids Surf. 1985,13,19;J. Phys. (Paris) 1987,48,651. (14)Leunp.. P. S.:Goddard. E. D.:. Han., C.:. Glinka. C. J. Colloids Surf. 1985,i3,47.-’ (15)Turro, N. J.; Baretz, B. H.; Kuo, P-L. Macromolecules 1984,17, 1321. (16)Zana,R. Insurfactant Solutions. New Methods oflnoestigation; Zana,R., Ed.; Surfactant Science Series; Marcel Dekker: New York, 1987; Vol. 22,p 241. (17)Abuin, E. B.; Scaiano, J. C. J. Am. Chem. SOC.1984,106,6274. (18)Chu, D.; Thomas, J. K. J. Am. Chem. SOC.1986,108,6270. (19)Chandar,P.; Somasundaran, P.; Turro, N. J. Macromolecules 1988, 21,950. (20)Hayakawa, K.; Santerre, J. P.; Kwak, J. C. T. Macromolecules 1983,16, 1642. (21)Hayakawa, K.; Kwak, J. C. T. J. Phys. Chem. 1982,86, 3866. Sanwrre, J. P.; Hayakawa, K.; Kwak, J. C. T. Colloids Surf. 1985,13,35. Malovikova, A.;Hayakawa, K.; Kwak, J. C. T. J. Phys. Chem. 1984,88, 1930. (22)(a) Shirahama, K.; Yuasa, H.; Sugimoto, S. Bull. Chem. SOC. Jpn. 1981,54, 375. (b) Shirahama, K.; Tashiro, M. Bull. Chem. SOC. Jpn. 1984,57,377,(c) Shirahama, K.; Masaki, T.; Takashima, K. In Microdomains in Polymer Solutions; Dubin, P., Ed.; Plenum: New York, 1985. (23)(a) Goddard, E.D. Colloids Surf. 1986,19,p 255,(b) p 301. (24)Hayakawa, K.; Kwak, J. In Cationic Surfactants: Physical Chemistry; Rubingh, D., Holland, P. M., Eds.; Surfactant Science Series; Marcel Dekker: New York, 1991;Vol. 37,p 189. ’
0 1991 American Chemical Society
Thalberg et al.
2894 Langmuir, Vol. 7,No. 12, 1991
nan (denoted Hy), and cationic surfactants of the alkyltrimethylammonium bromide type (denoted C,TAB, C12TAB 12 where n indicates the number of carbon atoms in the alkyl chain). The phase diagrams of these systems consist of a droplet-shaped two-phase region, anchored in the water corner, which is totally enclosed in an isotropic one-phase 1 CaTAB region.25 For a shorter surfactant, the size of the twophase region is reduced.26 This is also the case when salt 2 I . . . . . . . ., . . . . . . . . , . . . . . . In is added to the system.27 Addition of a sufficient amount 1 10 100 1000 Surfactant concentration (mM) of salt may completely suppress phase ~ e p a r a t i o n . ~ ~ ~ ~ ~ In this work, the phase behavior of some other systems Figure 1. Two-phase regions (black lines) for 1mM NaPA and containing polyelectrolyte and oppositely charged surfacC,TAB surfactants of varying chain length at room temperature (20-23 "C). The cmc vales are indicated by arrows. tant is investigated. In order to determine the influence of the polyelectrolyte charge density, systems containing intrinsic viscosity of the NaPA solution was about 15 mM-' at sodium polyacrylate, denoted NaPA, and the same cat50 mM NaC1, indicating a chain overlap concentration, c*, of ionic Surfactants have been studied. At pH 7, NaPA has about 100 mM. All viscosity measurements were carried out well below this concentration. a linear charge density of approximately one charge per Self-diffusion coefficients for the surfactant molecules were 3.2 A, which is three times as high as for NaHy. Systems determined by lH NMR measurements on a JEOL FX-60 of NaPA and C,TAB at neutral pH have been studied spectrometer, using the FT PGSE (Fourier Transform Pulsed previously by Hayakawa et a1.,20who determined binding Gradient Spin Echo) method as described earlier.28 HzO was isotherms, and by Chandar et al.,19 who investigated the replaced by DzO (Norsk Hydro, Norway) in these samples. fluorescence behavior of pyrene. Referenceto these studies Phase diagrams have been obtained by a procedure described will be made in due course. To examine further the in ref 25. In brief, the two coexistingphasesof aphase-separating generality of the phase behavior of polyelectrolyte-sursample are separated macroscopicallyand their relative amounts factant systems, we also investigate a system of a cationic are assessed. From analyses of the concentrations of all ionic polymer and an anionic surfactant. species, normally performed in the supernatant solutions, the compositions of coexisting phases are obtained. By an appropriate choice of initial sample compositions,both the shape and Experimental Section the location of the two-phase region, as well as the direction of Materials. Poly(acry1ic acid) was from Aldrich, Beersee, the tie lines, can be obtained. Belgium and had a molecular weight of about 2.5 X 105. It was neutralized with NaOH to pH 7 for viscometryand phase diagram Results and Discussion studies, which gives a linear charge density of about one charge Phase Separation at a Low Polyelectrolyte Conper 3.2 A (about 75% of neutralization). The neutralized polycentration. Mixing of NaPA and cationic surfactant leads acrylate is denoted NaPA. For the self-diffusion studies, fully neutralized NaPA was used. Poly(diallyldimethy1a"onium under a wide range of conditions to phase separation and chloride),PDADMAC,a cationic polymer containingquaternary the formation of dense white precipitates. Figure 1 shows ammonium groups,was obtained fromAllied ColloidsInc., Bradthe two-phase regions for samples containing 1.0 mM of ford, Great Britain. The molecularweight was about 2 X lo6and NaPA and alkyltrimethylammonium bromides of varying the distance between adjacent groups is about 5.0 A along the alkyl chain length. The gap between the onset of phase polymer backbone. All polyelectrolyte solutions were dialyzed separation and the critical micelle concentration, cmc, extensivelyagainst pure water before use. The concentration of increases (on a logarithmic scale) with an increased chain polyelectrolyte is expressed in weight percent or as the millilength of the surfactant. It is also seen that phase molar concentration of the repeating polymer unit. For NaPA, separation is suppressed by a high surfactant concentra1.0 wt % corresponds to 106 mM, while for PDADMAC, 1.0 wt tion. We will refer to this as redissolution. A higher sur% equals 68 mM. factant concentration is required for a surfactant with a Cationicsurfactants of the alkyltrimethylammoniumbromide type were purchased from Tokyo Kasei Inc., Japan. These will longer hydrocarbon chain, in order to bring about redisbe denoted C,TAB, where n indicates the number of carbons in solution. the alkyl chain. Sodium dodecyl sulfate (SDS) was from BDH, The two-phase regions for systems with 1.0 mM sodium Poole, Great Britain and sodium octyl sulfate (SOS) was from hyaluronate (NaHy) and the same cationic surfactants Merck,Darmstadt, Germany. The Surfactantswere usedwithout were determined in a previous study.28 Exactly analogous further purification. Orange OT, i.e. l-(o-tolylazo)-2-naphthol, features were found, both regarding the onset of phase was from Tokyo Kasei, Inc., Japan. separation in relation to the cmc, and regarding redissoMethods. Solubilization of the water-insoluble uncharged lution (Figure 1 in ref 28). It thus seems as phase dye Orange OT was performed by adding an excess amount of separation is governed by the same mechanism in the two the dye powder to aqueous polyelectrolyte-surfactantmixtures systems. The pattern is, however, shifted toward shorter and shaking the test tubes for 3 days end over end at 25 "C, to ensureequilibrium. After centrifugationthe absorbancewas read surfactant chain lengths in the NaPA system; as an at 490 nm. example, CsTAB does not give phase separation with Viscosimetrictitration of polyelectrolyte with surfactant was NaHy. This shift reflects the stronger interactions in the performed by stepwise addition of small amounts of a surfactant NaPA system, which indeed may be expected as NaPA solution to a polyelectrolyte solution containing 50 mM NaBr. has a linear charge density that is about three times as At each surfactant concentration, the viscosity was measured on high as that of Hy. average8 times. The temperature was 25-26 OC. As a reference, Effect of Added Salt. Addition of electrolyte will measurements at addition of a NaBr solution, of the same ionic weaken the interactions between the polyelectrolyte and concentration as the surfactant solution were performed. The the oppositely charged surfactants. At a critical electrolyte concentration, abbreviated cec, phase separation no longer (25)Thalberg, K.; Lindman, B.; KarlstrBm, G. J.Phys. Chem. 1990, 94,4289. occurs. The cec value obviously depends on both the sur(26)Thalberg, K.; Lindman, B.; KarlstrBm, G. J. Phys. Chem. 1991, factant and the polyelectrolyte concentrations in the 95, 3370. sample. For 1.0 mM NaPA with the surfactants CaTAB (27)Thalberg, K.;Lindman,B.; KarlstrBm,G.J . Phys. Chem.,in press. or CI~TAB,the observed cec behavior (Figure 2) largely (28)Thalberg, K.; Lindman, B. J.Phys. Chem. 1989, 93, 1478.
-I
200r;'x:
Systems of Polyacrylate and Cationic Surfactants
aE
E 5
." 0 i
Langmuir, Vol. 7, No. 12, 1991 2895
300
1.oo
x
.f .-x
6 2
.-
100
100
o
0
10
20 200 400 600 Surfactant concentration (mM)
fi 0.00 0.40 i
"
,
0
~
~
0.5
~
~
~
~
~
~
~
,U , , ~,
1.5 2 2.5 C, JAB concentrauon [mM] 1
0.94
0.92'
0
Figure 2. Critical electrolyte concentrations (cec) of NaBr in samples of 1.0 mM NaPA as a function of the surfactant concentration. Open symbols refer to clear one-phase solutions, and filled symbols to two-phase dispersions.
Phase reparailon
0.96
ia
0
SO0
0.98
1 ~
'
'
'
'
'
1 2 3 4 Surfactant concentration [mM]
Figure 4. Measured viscosity relative to the viscosity of the initial polyacrylate solution of 20 mM NaPA, pH 7, and 50 mM NaBr, at addition of &TAB (@), and at addition of a NaBr solution of the same ionic concentration (+I. The cac and the observed onset of turbidity, are indicated. Table I. Measured and Calculated Decrease in Surfactant
Self-Diffusion
~
,
~
3
Figure 3. Solubilization of Orange OT in samples of 1.0 mM NaPA and C12TAB. Above a surfactant concentration of 1.5 mM, phase separation occurs (as indicated by the arrow), and the data presented refer to the supernatant solution. parallels that observed with NaHy as a polyelectrolyte;28 i.e., the cec rises abruptly from the point where phase separation is first observed up to a plateau, whereafter it remains essentially constant until rather high surfactant concentrations are attained, whereafter a successive decrease toward the redissolution point in the absence of added salt is seen. The stronger interactions in the NaPA system are reflected in a higher cec plateau level for a surfactant of a given chain length (250 mM for C12TAB with NaPA, as compared to 120 mM in the NaHy-ClzTAB system). The cec behavior of the NaPA-C8TAB system is slightly different, as a gradual increase in the cec is observed with increasing surfactant concentration before the maximum is reached. Dye Solubilization. The solubilization of the uncharged water-insoluble dye Orange OT was investigated in samples containing 1.0 mM NaPA and C12TAB (Figure 3). At about 1.5 mM C12TAB,phase separation is observed. Above this point (the arrow in Figure 3), data refer to the supernatant phase. The dye dissolves in samples containing0.1 mMC12TAB or more, indicating that the critical aggregation concentration, cac, is close to this value. (The presence of the dye may slightly influence the cac.) The results furthermore indicate a structure with hydrophobic domains in the system. It thus seems likely that the surfactant molecules occur as micelle-like aggregates associated to the polyacrylate chains. This is in agreement with the cooperativity in the binding of C12TAB to PA, reported by Hayakawa et a1.,20and also with the fluorescence probe investigation by Chandar et al.'9 Viscometry. In Figure 4, the viscosity behavior of a dilute solution of NaPA a t addition of &TAB is shown. The viscosity of the NaPA-surfactant solution starts to deviate from the behavior of the reference solution at a certain concentration of added surfactant. The viscosity thereafter decreases rapidly and the solution turns opalescent. As we are in the dilute regime, the viscosity mainly reflects the dimensions of the polymer chains. In the first place, binding of surfactant leads to a decreased total
,
~
sample composition 50 mM NaPA, 7.0 mM CsTAB 50 mM NaPA, 11.7 mM CsTAB 4.1 mM CeTAB ,10,mM , NaPA, O 10 mM NaPA, 6.1 mM CsTAB 10 mM NaPA, 31.7 mM CsTAB 10 mM NaPA, 50 mM NaBr, 4.0 mM &TAB 10 mM NaPA, 50 mM NaBr, 7.9 mM CloTAB 10 mM NaPA, 50 mM NaBr, 11.9 mM CloTAB a
DBJDo calcda
exptl 0.76 0.77 0.76 0.80 0.95 0.94 0.96 0.96
0.68 0.70 0.72 0.75 0.90 0.93 0.94 0.94
Parametersused in the calculations: linear charge density = 2.75
A (correspondingto release of 90 % of the polyacrylicacid protons); radius of the polyelectrolyte chain = 3 A. charge of the polymer chains, which in turn leads to less extended conformations. Moreover, a structure with surfactant micelles adsorbed to the polyelectrolyte chains is likely to entail coiling up of the polyacrylate chains around the micelles to some extent, which will also lead to reduced dimensions of the P A chains in the solution. We may therefore interpret the decrease in viscosity relative to the reference solutions as due to surfactant binding to the polyacrylate chains, and the cac is obtained as the point where the deviation starts. (It should be noted that the reference solutions are included to show the effects of dilution and altered ionic strength.) A t 20 mM NaPA and 50 mM NaBr, a cac value of approximately 0.9 mM &TAB was obtained (Figure 4). In this system, turbidity was discovered at about 1.9 mM C12TAB. At 50 mM NaPA and 50 mM NaBr, the cac was 1.0 mM and phase separation was detected at 2.2 mM CuTAB. Both these results indicate that phase separation starts from solutions with a very low degree of surfactant binding to the polyelectrolyte (less than 5 % calculated as bound surfactant charges per polyelectrolyte unit). Measurements for samples of NaPA and CloTAB, give exactly analogous results. At 50 mM of NaPA, phase separation started from a solution with less than 2% of surfactant binding to PA. (These results certainly do not exclude a high degree of surfactant binding to polyacrylate in the concentrated phase formed, see below.) Surfactant Self-Diffusion. The self-diffusion of the surfactant molecules was investigated for samples containing fully neutralized NaPA in the range 10-50 mM, and C8TAB or CloTAB in concentrations from 2 mM up to the phase-separation limit. Some typical results are given in Table I, expressed as D,/Do; i.e., the measured surfactant diffusion coefficient has been divided by the diffusion coefficient for the surfactant monomer ions in the absence of polyelectrolyte. As is seen, the presence of NaPA induces only small reductions in the self-diffusion
Thalberg et al.
2896 Langmuir, Vol. 7, No. 12, 1991
of the surfactant molecules (in the range 4-2596). The reduction is larger at a higher NaPA concentration, and decreases at addition of NaBr. If there would have been significant binding of the surfactant molecules to the polymer,a marked decrease in the surfactant self-diffusion would have been o b s e r ~ e d . ~The ~ ? observed ~~ diffusion behavior is similar to that displayed by ordinary polyelectrolyte counterions, which can be referred to general electrostatic interactions between the counterions and the polyions.30 In order to further investigate this aspect, the reduction in the self-diffusion due to the presence of polyacrylate chains has been calculated by using the Poisson-Boltzmann-Smoluchowski theory30 (Table I). As is seen, the observed D,/& values are similar or even somewhat lower than predicted by electrostatic theory. In fact, the theory often underestimates the reduction in self-diffusion, as the diffusion in B direction parallel to the polyelectrolyte chain is assumed to be unaltered. One reason for the high diffusion rates obtained may be the bulkiness of the surfactant ions, preventing them from coming close to the polyelectrolyte surface. Rymd6n and Stilbs obtained higher D/Dovalues for tetramethylammonium ion than for lithium and cesium ions in solutions of p ~ l y a c r y l a t e . ~ ~ As we here have both small sodium ions and the bulky alkyltrimethylammonium ions, there may for steric reasons be preferential enrichment of the sodium ions close to the polyelectrolyte, which will further suppress the decrease in surfactant self-diffusion. It is anyhow quite clear that the self-diffusion results speak against a structure of surfactant molecules adsorbed to the polymer chains in the studied samples, as such surfactant binding would further decrease the observed diffusion relative to that predicted from general electrostatic theory. In conclusion, the results indicate no or a very low fraction of surfactant molecules, occurring as micelle-like aggregates, adsorbed to the polymer chains in the solutions subject to self-diffusion measurements (i.e., they are located below the cac curve; cf. Figure 5 ) . As some of these solutions have a surfactant concentration just below the phase-separation limit, the results imply that phase separation starts at very low degrees of surfactant binding to the polyelectrolyte in the range of P A concentrations studied. Thus, both viscosity and self-diffusion data indicate that, for NaPA in the concentration range 10-50 mM, phase separation starts from solutions with very low degrees of surfactant binding to PA. Relation between Critical Aggregation Concentration and Phase Separation. By means of a surfactant-specific electrode, Hayakawa et al. obtained a cac of about 30 pM for the binding of &TAB to 0.5 mM NaPA at 30 0C.20The cac at low NaPA concentrations was found to be extremely sensitive to addition of salt; addition of 10 mM NaCl resulted in a tenfold increase in the cac. Both with and without salt, 0.5 mM NaPA was reported to bind more than 60 '31of surfactant (calculated as bound surfactant moleculesper repeating polymer unit), without phase separation taking place. A high degree of surfactant binding to PA is also consistent with the solubilization behavior at 1.0 mM NaPA (Figure 3). Obviously, the NaPA-C,TAB systems behave differently at different polyelectrolyte concentrations. At very low PA concentrations (51.0 mM), the PA chains can bind a considerable amount of surfactant and still remain in (29) Carlsson,A.; Karlstrom, G.; Lindman, B. J.Phys. Chem. 1989,93, 3673. (30) Nilsson, L. G.; Nordenskiold, L.; Stilbs, P.; Braunlin, W. H. J. Phys. Cltem. 1985, 89, 3385. (31)RymdBn, R.; Stilbs, P. J. Phys. Chem. 1985,89, 2425.
100.00
Figure 5. Cac values and phase-separation data for the system NaPA-C12TAB. Cac values are from ref 19 (A),ref 20 ( 0 ) , and from solubilization ( 0 )and viscometry (0; at 50 mM NaBr) data of this work. Phase-separation data are from this work in the absence of added salt).( and at 50 mM of NaBr (0).The entire cac behavior (dotted lines) and the phase boundaries (full lines) are schematically indicated. solution. At somewhat higher PA concentrations (110 mM), however, phase separation occurs at a low degree of surfactant binding to the polyelectrolyte. The phase-separation limit was determined for 0.5 mM NaPA and C12TAB. Both without salt and with 10 mM salt, phase separation was observed at about 3 mM ClzTAB, which is consistent with the data of Hayakawa et al. In Figure 5, reported cac values for the NaPA-Cl2TAB system without added salt are shown together with cac and phase-separation data obtained in this work. In addition, the general cac behavior (dotted line) and the phase-separation curve (full line) are schematically outlined. Clearly, the cac has to decrease from the initial cmc value in the absence of polyelectrolyte, but at higher polyelectrolyte concentrations, an increase in cac with increasing polyelectrolyte concentration is inferred. The phase-separation line has to increase rapidly when the polyelectrolyte concentration approaches zero as the binary surfactant-water system is a one-phase system (up to very high surfactant concentrations). It is thus seen that there exist different solubility regimes for the polyelectrolyte-surfactant complexes. At low polyelectrolyte concentrations, the complexesare soluble even at relatively high degrees of surfactant binding to the polyelectrolyte, while a t higher polyelectrolyte concentrations, binding of surfactant to the polyelectrolyte almost directly results in phase separation. For the NaPA-ClZTAB system, the transition between the two regimes occurs at about 2 mM NaPA. Another change in behavior is present a t very high concentrations of polyacrylate, where again PA-surfactant complexes are stable. This is referred to the fact that phase separation is restricted to the waterrich part of the phase diagram, as will be seen below. Phase Diagram for the System of NaPA and C12TAB. Samples with 2.0 w t % NaPA and C12TAB in the range from 25 to 600 mM were mixed and carefully equilibrated. Phase separation was observed in all samples and resulted in the formation of two clear and isotropic phases, of which the bottom phase was highly viscous. Systems of a polyelectrolyte and an oppositely charged surfactant are four-component systems, as four different ionic species are present in addition to the solvent component, and the requirement for charge neutrality reduces the number of independent variables by one. A complete (three-dimensional)representation for this kind of system has been devel~ped.~'For many purposes, however, a (two-dimensional) pseudo-three-component phase diagram has been proven The system is then treated as consisting of solvent (water), polyelec-
Langmuir, Vol. 7, No. 12, 1991 2897
Systems of Polyacrylate and Cationic Surfactants
concentrated phase of the NaPA-&TAB system, in particular in samples with a low surfactant-to-polyelectrolyte ratio. In the NaHy-C,TAB systems, the Brconcentration is always enhanced in the concentrated phase, which indicates that Br- ions constitute a large fraction of the counterions to the micellar aggregates.25~33 Some samples were also prepared with NaPA and CgTAB or &TAB. Also here, two clear and isotropic phases result. For a shorter surfactant, less dense and less viscous bottom phases are formed. The apparent viscosity of the bottom phase also decreases with an increased surfactantto-polyelectrolyte ratio, just as is observed in the NaHy- - NaPA C,TAB system.32 The phase behavior of the NaPA H20 10 20 30 40 50 systems thus seems to be quite analogous to that displayed % NaPA by NaHy and the same cationic surfactants, indicating Figure 6. Pseudethree-componentphase diagramfor the system that it is of the same physical origin. The most important NaF'A-ClzTAB, as described in the text. Open symbols refer to difference is a considerable reinforcement of the intercompositionsof the initial samples,and filled symbolsconnected by tie lines, to compositions of coexisting equilibrium phases. actions between polyelectrolyte and surfactant with NaPA, The dashed line indicates a larger uncertainty of the phase as discussed above, and a result of this is the dense opaque boundary. The charge neutralization line refers to equal conprecipitates formed at low polyelectrolyte concentrations. centrations of C12TA+ and PA- charges. The precipitates formed in samples with 1.0 mM NaPA and C12TAB (Figure 1)did not transform into a transparent gel-like phase, as was the case in the NaHy system. When we used a NaPA concentration of 100 mM or more, however, two transparent phases in equilibrium always resulted. The reason for this difference in behavior, is that the polyelectrolyte, and in particular the Na+ counterions, contribute to the ionic strength in the sample. The interaction between the polyelectrolyte and the cationic surfactant is therefore more screened at a high NaPA concentration,and a less dense concentrated phase results. (This dependence of the phase behavior on the polyelectrolyte concentration clearly shows the limitations of the .- Polyternary representation.) "20 electrolyte Systems of a Polycation and an Anionic Surfac% Polyel. tant. For comparison, we have studied the interaction of Figure 7. Comparison of the phase diagrams for the systems the cationic polyelectrolyte poly(diallyldimethy1ammoNaHy-&TAB (from ref 26) and NaPA-CIzTAB (with tie lines nium chloride), abbreviated PDADMAC,with anionic suromitted). The charge neutralization lines for the two systems factants. Samples with 2.0 w t % PDADMAC (Le., 135 are included. mM in the repeating unit) and SDS in the range from 50 to 300 mM were mixed and equilibrated as described above. trolyte, and surfactant; i.e., the distribution of the salt At low SDS concentrations,a one-phase solution,although ions is disregarded. Such a phase diagram for the NaPAwith a slight opalescence, was observed. At higher SDS C12TAB system is presented in Figure 6. A two-phase concentrations (Le., from 90 to 300 mM SDS), phase region is seen, which is anchored in the water corner. The separation into one clear supernatant and a white pretie lines are directed from this corner and toward the cipitate, which was found to be concentrated in both opposite side of the diagram. The phase diagram resemand SDS, was observed. The appearance of bles those obtained in the NaHy-C,TAB s y s t e m ~in~ ~ , ~PDADMAC ~ the precipitate was different in the different samples. In all principal aspects, such as the shape and location of the a sample containing about equimolar amounts of SDS and two-phase region, and the direction of the tie lines. PDADMAC repeating units (130 mM SDS, 135 mM In Figure 7, the phase diagrams for C12TAB with NaPA PDADMAC), the precipitate was coherent and resembled and with NaHy are compared. The area of the two-phase a piece of bubble gum. In the other samples, which were region is larger with PA than with Hy, which reflects the of both higher and lower surfactant-to-polyelectrolyte stronger interactions in the former system. Another ratios, the precipitates, on the contrary, were very loose interesting difference between the two systems is that the in consistency. The coherent precipitate contained less two-phase region extends in the direction of charge than 50% water, while the others contained about 70% neutrality between surfactant and polyelectrolyte charges water. for PA, while for Hy more surfactant than Hy charges are present in the concentrated phase. If we assume a Our explanation of these findings is that the strong structure of micellelike surfactant aggregates in the interaction between polyelectrolyte and oppositely charged concentrated phase of the NaPA system (this is known to surfactant has lead to precipitation and formation of flocs apply in Hy-C,TAB system^^^.^^), we may infer that the in the system. These are not necessarily in thermodynamic surfactant micelles of this system are highly covered with equilibrium with the supernatant, but nevertheless seem polyacrylate, as the polyelectrolyte ions provide a major to be very stable. fraction of the micelle counterions. This is further In order to reduce the interactions between polyelecsupported by the low enrichment of Br- ions in the trolyte and surfactant, we studied a surfactant of shorter hydrocarbon chain, namely sodium octyl sulfate, SOS. (32) Thalberg, K.; Lindman, B. Langmuir 1991, 7, 277. Samples containing 1.0 wt % PDADMAC and 80 to 350 (33) Thalberg, K.; van Stam,J.; Lindblad, C.; Almgen, M.; Lindman, B. J . Phys. Chem., in press. mM SOS were prepared. All samples showed phase c 1 2 y
I
-
-
-
~
2898 Langmuir, Vol. 7, No. 12, 1991
Thalberg et al.
sos 5 0 A s O
% PDADMAC
-
I
I
I
10
20
30
% PDADMAC
40
PDADMAC 50
Figure 8. Phase diagram for the system PDADMAC-SOS without added salt. Symbols are as in Figure 6.
Figure 9. Phase diagram for the system PDADMAC-SOS in the presence of 1.0 M NaBr. Symbols are as in Figure 6.
separation, and the concentrated bottom phases were rather transparent, although a slight turbidity persisted also after several days. We believe that the samples are relatively close to thermodynamic equilibrium. The compositions of supernatants and bottom phases are indicated in Figure 8. As is seen, the four samples trace out part of a two-phase region. Even for the Cg surfactant, this region is inferred to be very large. In order to further reduce the interactions, salt was added to the samples. Addition of 250 mM NaCl did not induce any significant changes in phase behavior. A t addition of 1.0 M NaBr, a swelling of the bottom phase was observed, and a phase diagram more akin to the ones of polyanion-cationic surfactant systems resulted (Figure
surfactants interact more strongly than cationic ones with oppositely charged polyelectrolytes. This is manifested in the denser bottom phases formed, when Surfactants of the same chain length are compared, and also in the smaller effects of added salt on the phase behavior, for the anionic surfactant-polycation systems. (It should be noted that the linear charge density is lower for PDADMAC than for NaF'A.) A reason for this difference in behavior may be the presence of other interactions, besides the pure Coulombic ones, in systems of an anionic surfactant. For a moderately strong interaction between polyelectrolyte and oppositely charged surfactant (which may be effected by addition of salt),however, phase diagrams of the previously obtained type,25 here reported for the NaPA-C,TAB system, seem to apply generally.
9).
Anionic surfactants are known to interact stronger than cationic ones with uncharged polymer^.^^*,^ This has been attributed to the smaller size of the surfactant headgroup for anionic as compared to cationic surfactants.35 There seem, however, also to be other reasons for the observed difference in behavior.36 The above results in systems of PDADMAC and anionic surfactants, suggest that anionic (34) Shirahama, K.; Himuro, A.; Takisawa, N. ColZoidPolyn. Sci. 1987, 265, 96. (35)Nagarajan, R. Colloids Surf. 1985, 13, 1.
Acknowledgment. Svante Nilsson and Lennart Piculell are acknowledged for help and advice concerning the surfactant self-diffusion part and Ingegerd Lind is thanked for skillful technical assistance and for help with the figures. This work was financiallysupported by Pharmacia AB, Uppsala, Sweden. Registry No. NaPA, 9003-04-7; CsTAB, 2083-68-3; CloTAB, 2082-84-0; C12TAB, 1119-94-4;orange OT, 2646-17-5. (36) Witte, F. M.; Engberta, B. F. N. Colloids Surf. 1989,36, 417.
