Langmuir 1993,9, 2933-2941
2933
(Polymer/Polymer) -Like Phase Behavior in the System Tetradecyltrimethylammonium Bromide/Sodium Polyacrylate/Water+ J. 0. Carnali Unilever Research US., Inc., 45 River Road, Edgewater, New Jersey 07020 Received May 12,1993. In Final Form: July 26,199P
The phase behavior observed in the system tetradecyltrimethylammonium bromide (TTAB)/sodium polyacrylate (Na1pPAA)lwateris in many ways similar to that observed when two types of polymers are mixed in a solvent. The Nal/2PAA (50% neutralized) interacts strongly with the cationic surfactant in water-rich systems, leading to the phase separation expected of these oppositely charged components. The surfactant binds with the polymer in the form of micelles, and the consequent release of the respective counterions allows the formation of a mutually enriched phase in coexistence with one dilute in both polyions. This phenomenon can be thought of as mimicking the behavior of two strongly interactive polymers. The composition of the concentrated and dilute phases indicates that the sodium counterion of Nal/ZPAA is largely replaced by TTA+ but that the TTA+:polyacrylateion ratio at saturation is about 2.5,strongly supporting the micellar nature of the surfactant binding species. With addition of excesses of either polyion, the entropy gain upon counterion release becomes screened and the two polyions become lesscompatible;consequently,the enriched phase swellswith solvent. Whenthe added polyion is NalpPAA, this phase undergoes a hexagonalto isotropicphase transition at about 2.5 % polymer as its TTAB content is diluted. Continuation of this process leads to reversion back into a single-phase system at about 5% polymer. With the compatibilitymechanismnow completely swamped,futher addition of polymer results in again observing phase separation. This time, the behavior is that of incompatible polymers which separate, at polyion concentrations over about 1.7 mol of ionic specieslkg, to give a phase enriched in polymer and one enriched in micelles.
Introduction Polymerlsurfactant interactions form a challenging blend of two well-established scientific fields and, as evidenced by a number of recent reviews (Goddard,1*2 Hayakawa and Kwak,3 Goddard and Ananthapadmanabhan4),have attracted considerablescientificattention. This area has traditionally been divided between work at low and a t high polymer and surfactant concentrations and then further subdivided depending on the charge (anionic, cationic, or nonionic) of both components. The majority of studiess7 have considered nonionic polymers and anionic surfactants and have been interested in detecting the onset of surfactant binding a t low polymer levels (defined here as below the critical overlap concentration). The universal finding is that surfactant binding begins after its concentration exceeds a so-called critical aggregation concentration (cac). At this point, mixed micelles begin to form with the polymer, serving to effectively lower the critical micelle concentration (cmc) of the surfactants.eand to decrease the counterion binding. As a further indication of the micellar nature of this + Presented in part at the XI1 European Chemistry at Interfaces Conference, Lund, Sweden, June 28 to July 2, 1992. Abstract published in Advance ACS Abstract& October 1,1993. (1) Goddard, E. D. Colloids Surf. 1986,19, 255. (2) Goddard, E. D. Colloids Surf. 1986,19, 301. (3) Hayakawa, K.; Kwak, J. C. T. In Cationic Surfactants: Physical Chemistry; Rubingh, D., Holland, P. M., Eds.; Surface Science Series; Marcel Dekker: New York, 1992. (4)Ananthapadmanabhan, K. P.; Goddard, E. D. Interactions of Surfactants with Polymers and Proteins; CRC Press: Boca Raton, FL,
Jones, M. N. J. Colloid Interface Sci. 1967,23, 36. Smith, M. L.; Muller, N. J. Colloid Interface Sci. 1975, 52, 507. Moroi, Y.: Akisada,H.;Saito, M.; Matuura,R. J. Colloid Interface Sci. 1977, 61, 233. (8) h a , R.;L a g , J.; Lianos, P. In Polymer Science and Technology, Vol 3 0 Microdomains in Polymer Solutions: Dubin,. P... Ed.:. Plenum Press: New York, 1986; p 357: (9) Painter,D. M.; Bloor, D. M.; Takisawa,N.; Hall, D. G.; Wyn-Jones, E . J. Chem. SOC.,Faraday Trans. 1 1988,84,2087.
binding, the driving forceappearsto be largely hydrophobic in naturelGl3 as demonstrated by the fact that the cac varies with surfactant chain length in the same way as does the cmc. Also, the effect of addition of electrolyte is to lower the cac, again the same effect as is observed with the cmc.14J5 The decrease in counterion binding in these systems, presumably due to the interspacing of polymer chains between the surfactant headgroups a t the micellarsurface,6JoJ6leads to smaller aggregationnumbers than in free m i c e l l e ~ . ~ J ~Association J~ of this type continues as the surfactant concentration increases until the availablepolymer chains are completely saturated with micelles. Beyond this second critical concentration, additional surfactant is thought to form free micelles. Analogous behavior at low polymer concentrations is also found when the polymer is a polyelectrolyte and the surfactant bears a charge of opposite sign. In this case, binding is a consequence of several driving force^:^^^^^ electrostatic attraction between the oppositely charged species,release of the small polyelectrolyte and surfactant counterions with the associated gain in entropy,2l and the above hydrophobic interactions with the hydrocarbon tails of the surfactant already adsorbed onto the polymer. These (10) Cabane, B. J. Phys. Chem. 1977,81, 1639. (11) Shirahama, K.; Ide, N. J. Colloid Interface Sci. 1976,54,460. (12) Shirahama, K.; Himuro, A.; Takieawa, N. Colloid Polym. Sci. 1987,
m,96.
(13)Shirahama,K.Colloid Polym. Sci. 1974, 262, 978. (14) Hoffmann, H.; Huber, G. colloids Surf. 1989, 40, 181. (15) Shirahama,K.; Oh-Ishi,M.; Takisawa,N. Colloids Surf. 1989,40, 261. (16) Nagarajan, R. Colloids Surf. 1985, 13, 1. (17) G h y i , T.; Wolfram, E. In Polymer Science and Technology, Vol 3 0 Microdomains in Polymer Solutions; Dubin, P., Ed.; Plenum Press: New York, 1985; p 383. (18) Witte, F. M.; Engberta, J. B. F. N. Colloids Surf. 1989, 36,417. (19) Chu, D.; Thomas, J. K. J. Am. Chem. SOC.1986,108,6270. (20) Shmizu, T.; Seki, M.; Kwak, J. C. T. Colloids Surf. 1986,20,289. (21) Almgren, M.; Hansson, P.; Mukhtar, E.; van Stam,J. Langmuir 1992,8, 2405.
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driving forces make binding considerably more favorable than free micelle formation, as is illustrated by the fact that the cac is orders of magnitude below the cmc. The interaction at such low bulk surfactant levels is often discussed in terms of the binding of individual surfactants. However, this interaction in oppositely charged systems is decidedly cooperative-commencing and saturating over a narrow range of surfactant c o n ~ e n t r a t i o n .Kiefer ~ ~ ~ ~et~ aLZ4have further shown that the majority of the observed binding occurs via clusters of surfactant monomers-as in the nonionic polymer case. But in contradiction to that case, binding is postponed to higher surfactant concentrations in the presence of background electrolyte,serving to emphasize the dominating role of electrostatic^.^^^^^^^^ As a further example,the binding of the cationic surfactant tetradecyltrimethylammonium bromide (TTAB) occurred preferentially to anionic polyelectrolytes of higher charge density.27.28 Much less work has been reported regarding polymer/ surfactant interactions at higher polymer concentrations. One reason for this dearth of studies is that for oppositely charged systems a precipitate phase usually forms which is difficult to characterize. As a typical example, Goddard et aLZ9found that adding anionic surfactant to a cationic modified cellulose ether gave peak precipitation at 1:l charge stoichiometry. The precipitate could be redissolved with excess surfactant, and the observed reversal of the electrophoretic mobility upon redissolution suggested adsorption of excess s u r f a ~ t a n t . Fluorescence-probe ~~?~~ and dye solubilization studies by Ananthapadmanabhan et al.32confirmed that this process could be considered as a micellar solubilization,and similar behavior was observed by Somasundaran and MoudgiP with ionic modified polyacrylamide and oppositely charged surfactant. Redissolution in this manner has been treated from a thermodynamic viewpoint by Hall.34 This phenomenon is dependent on the polymer backbone flexibility and hydrophobi~ity3l~~~ and is more difficult with polymers of high charge density.36 Although the interaction at high polymer and surfactant concentrations has again often been discussed in terms of complexation with individual surfactant monomers,Dubin and c o - ~ o r k e rhave s ~ ~ emphasized the micellar nature of the association in their studies with mixed micelles of sodium dodecyl sulfate (SDS) and Triton X-100. Strong (22)Shirahama, K.; Yuasa, H.; Sugimoto, S.Bull. Chem. SOC.Jpn. 1981,54,375. (23)Shirahama, K.; Tashiro, M. Bull. Chem. SOC.Jpn. 1984,57,377. (24)Kiefer, J. J.; Somasundaran, P.; Ananthapadmanabhan, K. P. In Polymer Solutions, Blends, and Interfaces; Noda, I., Rubingh, D. N., Eds.; Elsevier: Amsterdam, 1992. (25)Benrraou, M.; Zana, R.; Varoqui, R.; Pefferkom, E. J.Phys. Chem. 1992,96,1468. (26)Malovikova, A.; Hayakawa, K.; Kwak, J. C. T. J. Phys. Chem. 1984,88,1930. (27)Hayakawa, K.; Santerre, J. P.; Kwak, J. C. T. Macromolecules is83,16,i642. (28)Satake, I.; Takahashi, T.; Hayakawa, K.; Maeda, T.; Aoyagi, M. Bull. Chem. SOC.Jpn. 1990,63,926. (29)Goddard, E. D.; Phillips, T. S.; Hannan, R. B. J. SOC.Cosmet. Chem. 1976,26,461. (30)Goddard, E. D.; Hannan, R. B. J. Colloid Interface Sci. 1976,55, "" I J. (31)Ohbu, K.; Hiraishi, 0.; Kashiwa, I. J. Am. Oil Chem. SOC.1982, 59, 108. (32)AnanthaDadmanabhan.. K. P.:.Le-. _.P. S.:.Goddard. E. D. Colloids Surf. 1986,13,83. (33)Somasundaran, P.; Moudgil, B. M. InMacromolecular Solutions; Sewour, R. B.. Stahl. G. A.. Eds.: Pereamon Press: New York. 1982. i34) Hall, D:G. J . &em. Soc., Faraaay Trans. 1 1986,81,g5. (35)Leung, P. S.;Goddard, E. D.; Han, C.; Glinka, C. J. Colloids Surf. 1986,13,41. (36)Goddard, E. D.; Hannan, R. B. J. Am. Oil Chem. Soc. 1977,54, 561. (37)Dubin, P. L.; Oteri, R. J. Colloid Interface Sci. 1983,95,453.
interaction with the cationic poly(dimethyldially1a"onium chloride), as characterized by gross precipitation, was observed when the SDS fraction in the micelle exceeded a critical value and this value increased with increasing ionic ~ t r e n g t h , implying 3 ~ ~ ~ ~ a critical electrostatic potential for binding the entire micelleto the polymer chain. The nature of the precipitate was typically that of a liquid coacervate,electrically neutral on the macroscopic scale but nonstoichiometric locally and solvent swollen.40 Most of our knowledge concerning the phase behavior of these types of systems comes from the systematic study by Thalberg et aL41A3 involving the weakly anionic polysaccharide sodium hyaluronate (NaHy) and a series of alkyltrimethylammoniumbromides. At polymer concentrations on the order of 1% ,addition of TTAB results in separation of a phase virtually devoid of polymer from one concentrated in both polymer and surfactant (butstill containing -70% water). This phase separation seemed to be maximized not along a line of 1:l bulk charge stoichiometry but rather toward slightly TTAB rich compositions. A limited amount of work has also been reported by Thalberg et al." dealing with sodium polyacrylate (NaPAA) replacing NaHy. The fully neutralized NaPAA showed phase behavior similar to that of NaHy, but with an accentuated tendency toward phase separation due to a stronger interaction (stronger electrostatic driving force and the ability of the flexible NaPAA to wrap around micelles). In the work reported here, a systematic study of the phase behavior in polymer/surfactant systems is begun in which the charge density of the polyelectrolyte is capable of continuous variation. By varying the degree of neutralization of PAA, the separation between charge groups can be varied without altering the chemical nature of the backbone.45 It is expected that such a study will isolate the effect of polymer charge density on the phase behavior. It is also inevitablethat the effects of polyelectrolyte counterion concentration will become evident and the two effects will partially counterbalance one another. The use of TTAB as the cationic surfactant in this study further introduces the complication of liquid crystalline phases into the relevant portion of the phase diagram. Thiscomplication adds to the complexity of the analysis but yields some unexpected and interesting results.
Experimental Section Materials. Tetradecyltrimethylammonium bromide was obtained from Sigma Chemical Co. This material was recrystallized from acetone and dried under vacuum at 30 OC. Poly(acrylic acid) was a product of Polysciences and was purified by precipitation from methanolic solution with ethyl ether followed by extensive dialysis (using an 8OOO MW cutoff membrane) against distilled water and fiially freeze drying. The polymer molecular weight was estimated at 60 OOO on the basis of intrinsic viscosity measurements made in 2 N NaOH.'bs'B Salt (NaBr) and standard acid (HC1) and base (NaOH) solutions were all Fisher reagent grade. ~~~~~~
(38)Dubin, P. L.; Rigsbee, D. R.; McQuigg, D. W.J. Colloid Interface Sci. 1985,105,509. (39)McQuigg, D. W.;Kaplan, J. I.; Dubin, P. L. J.Phys. Chem. 1992, 96,1973. (40)Dubin, P. L.; Davis, D. Colloids Surf. 1986,13,113. (41)Thalberg, K.; Lindman, B. J. Phys. Chem. 1989,93,1478. (42)Thalberg, K.; Lindman, B.; Karlstrcm, G. J. Phys. Chem. 1990, 94,4289. (43)Thalberg, K.; Lindman, B.; Karlstr6m, G. J. Phys. Chem. 1991, 95, 3370. (44)Thalberg, K.; Lindman, B.; Bergfeldt, K. Langmuir 1991,7,2893. (45)Kowblansky, M.;Zema, P. Macromolecules 1981,14, 166. (46)Molyneux, P. Water-solublesyntheticpolymers:properties and behaoior; CRC Press: Boca Raton, FL, 1984.
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nm was employed along with a three-point calibration plot to relate emitted intensity to Na+ concentration. less than the desired amount of distilled water, followed by The relative amounts of each phase in multiphase systems titration of a small aliquot with NaOH to a phenolphthalein end were estimated as follows. When both phases were fairly fluid, point. With the PAA content of the solution accurately estabthe height of each phase in the sample tubewas carefully measured lished, the amount of NaOH (50% weight solution) required to (to the nearest 0.5 mm) and the corresponding volume was read reach the desired a was added and then the solution diluted to off a calibration plot of height versus volume determined with the required concentration. The high viscosity of these solutions water in identical tubes. This determination was judged to be led to a practical upper concentration limit of about 30%. The accuratetowithinf0.05g (aasumingunitdensity) fora5-gsample. degree of neutralization was chosen as 50% for this study, and Since the individual phase volumes were sometimes as small as this material is referred to as NalpPAA. However,concentrations 0.5 g, an appreciable error could thus unavoidably be present. of this polymer, given in terms of a weight percent, are based on When one of the phases-usually the lower-was very viscous, the acid (PAA) form. The concentrations of charged species the thinner phase could be quantitatively poured or drawn off from the polymer (referred to as RCOO-) are expressed as moles and the weight of each phase determined directly. per kilogram of solution. For t4e carboxylicacid groups on PAA, With the compositions of both the upper and lower phases the degree of neutralization does not differ significantly from individually determined, a check could be performed to estimate the degree of ionization except at low a (
