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Influence of Hydrophobic Cosolutes on the Associative/ Segregative Phase Separation of Aqueous Cationic Surfactant-Polymer Systems S. Nilsson,*,† A. M. Blokhus,‡ S. Hellebust,‡ and W. R. Glomm‡ Rogaland Research, Box 8046, N-4068 Stavanger, Norway, and Department of Chemistry, University of Bergen, Allegatan 41, N-5007 Bergen, Norway Received February 18, 2002. In Final Form: June 5, 2002 The effect of hydrophobic cosolutes on the phase behavior for a cationic surfactant and charged polymer in water has been studied. With a similarly charged polymer (cationic), the phase separation is of the segregative type with one surfactant-rich phase in equilibrium with a polymer-rich phase. Addition of small amounts of octane to such a system significantly increases the compatibility; that is, the two-phase region is reduced. If a long-chain alcohol like octanol is added, the incompatibility is enhanced. The effect is attributed to changes in the micellar aggregation number and is essentially a molecular weight effect. Octane reduces the micellar molecular weight, while octanol causes an increase. With an oppositely charged polymer (anionic), the phase separation is associative with a concentrated polymer/surfactant phase in equilibrium with a water phase. For a system that has phase-separated associatively, the addition of hydrophobic cosolutes produces little effect.
Introduction An ionic surfactant when mixed with a polymer (in aqueous solution) often phase-separates into two distinct phases.1-7 With similarly charged systems, the phase separation is referred to as segregative with each phase enriched with its respective macromolecule, polymer or surfactant micelles.3,7 If the surfactant and polymer are oppositely charged, the phase separation is associative with a concentrated polymer-surfactant phase in equilibrium with a water phase depleted of macromolecular components.1,2 In this study, we have focused on how the above types of phase behavior are influenced by hydrophobic cosolutes such as octane and octanol. In an earlier study,7 it was demonstrated that the segregative phase behavior (for the system polystyrenesulfonate-sodium dodecyl sulfate (SDS)-water) was influenced by the presence of hydrophobic cosolutes. The finding was that alkanes increased the compatibility (i.e., the two-phase region decreased), whereas alcohols decreased the compatibility. The phenomenon was attributed to changes in the micellar molecular weight (aggregation number) induced by the hydrophobic cosolutes. Long-chain alcohols such as octanol induce micellar growth to long rodlike aggregates, thus effectively increasing the micellar molecular weight. Alkanes such as octane, on the other hand, reduce the micellar aggregation number by inducing rod to sphere transitions. The effect of hydrophobic cosolutes on micellar aggregation behavior is known in the * To whom correspondence should be addressed. Present address: Institute for Surface Chemistry, Box 5607, SE-114 86 Stockholm, Sweden. † Rogaland Research. ‡ University of Bergen. (1) Goddard, E. D. Colloids Surf. 1986, 19, 301. (2) Thalberg, K.; Lindman, B.; Karlstro¨m, G. J. Phys. Chem. 1990, 94, 4289. (3) Thalberg, K.; Lindman, B. Colloids Surf., A 1993, 76, 283. (4) Wormuth, K. R. Langmuir 1991, 7, 1622. (5) Clegg, S. M.; Williams, P. S.; Warren, P.; Robb, I. D. Langmuir 1994, 10, 3390. (6) Piculell, L.; Bergfeldt, K.; Gerdes, S. J. Phys. Chem. 1996, 100, 3675. (7) Nilsson, S.; Blokhus, A. M.; Saure, A. Langmuir 1998, 14, 6082.
literature,8-11 but the consequences for aqueous two-phase polymer-surfactant systems have received little attention. Here, our previous study7 is extended to cationic surfactants considering both associative and segregative phase behaviors. Experimental Section Chemicals. The surfactant TTAB, tetradecyltrimethylammoniumbromide (∼99%), was delivered by Merck. The polymers polyDMAC, poly(diallyldimethylammonium)chloride supplied as a 20% solution in water with a molecular weight of 100-200 000 g/mol, and PSS, sodium polystyrenesulfonate with a molecular weight of ca. 500 000 g/mol, were both delivered by Aldrich. PAA, sodium polyacrylate from Polysciences, was supplied as a 20% solution with a molecular weight of ca. 250 000. The cosolutes were octane (>96%) from Fluka and 1-octanol (>99%) from Merck. The water was Millipore filtered. Methods. The binodials in the phase diagram were determined by a titration method. For systems that phase-separated segregatively (see below), water was added dropwise to a two-phase sample until a clear one-phase sample was obtained. The procedure was repeated with different compositions of the starting sample until most of the binodial curve was mapped out. With associative systems (see below), the procedure was essentially the same except that the water was added to a one-phase system until the two-phase region was reached. The accuracy in the location of the binodial was estimated to be approximately (0.5%. The hydrophobic cosolutes were dissolved in a stock solution with TTAB. The surfactant with added cosolute was then used as a pseudo-one-component system. The surfactant/cosolute ratio is therefore constant in all the phase diagrams, being 24.6 and 22.5 for octane and 1-octanol, respectively. All the phase diagrams were obtained at 25.0 °C. Viscosity measurements (at 25.0 °C) were made using a simple Ostwald viscosimeter. Since the viscosities of the current systems are (8) Israelachvili, J. Intermolecular & Surface Forces, 2nd ed.; Academic Press: San Diego, 1992; Chapter 17. (9) Nagarajan, R.; Ruckenstein, E. Langmuir 1991, 7, 2934. (10) Bayer, O.; Hoffman, H.; Ulbricht, W.; Thurn, H. Adv. Colloid Interface Sci. 1986, 26, 177. (11) Hoffman, H.; Ulbricht, W. J. Colloid Interface Sci. 1989, 129, 388.
10.1021/la025636d CCC: $22.00 © 2002 American Chemical Society Published on Web 07/27/2002
Influence of Cosolutes on Phase Separation
Figure 1. Phase diagram for the three-component system water-polyDMAC-TTAB with no cosolute added (1), with added octane (9), and with added octanol (b). The TTAB/cosolute ratio was 24.6 for octane and 22.4 for 1-octanol. shear rate dependent, the reported values should be considered as efficient viscosities.
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Figure 2. Viscosity of TTAB solutions as a function of added NaBr with no cosolute added (1), with added octane (9), and with added octanol (b). The TTAB/cosolute ratio was 24.6 for octane and 22.4 for 1-octanol.
Results and Discussion The result for similarly charged systems, TTAB and polyDMAC in water (Figure 1), show the segregative phase behavior similar to that found earlier using anionic systems (PSS and SDS). Further, it can be seen in Figure 1 that addition of octane significantly increases the compatibility whereas addition of octanol decreases the compatibility. One difference, as compared to the previously studied anionic systems,7 is that the cationic systems investigated here are more sensitive to the addition of hydrophobic cosolutes, meaning that the magnitude of the effect of adding hydrophobic cosolutes on the phase separation is more pronounced. The maximum water content is shifted from about 70 to 62 wt % by addition of octane. The addition of approximately the same amount of octanol produces a shift from 70 to about 80 wt %. The corresponding shift for the PSS-SDS system was about 3-5 wt % units.4 The cosolute is practically insoluble in water and almost completely solubilizes in the micelles.12 The surfactant with cosolute is therefore effectively a pseudo-one-component system. The effect of cosolutes on the phase behavior therefore must be attributed to changes in the micellar aggregation number and shape. The changes in aggregation number and shape also correlate with viscosity changes of the surfactant solution. The viscosity of a TTAB solution at different salinities is illustrated in Figure 2. As can be seen, the effect of the additives is limited at low salinities but at NaBr concentrations higher than about 0.3 M the change in viscosity can be substantial. Addition of octanol increases the viscosity, indicating aggregate growth, whereas addition of octane causes a decrease. It is the high-salinity range that in this case is relevant for the phase behavior since polyDMAC exerts a salt effect. A concentration of 5 wt % corresponds to a stoichiometric ionic concentration of about 0.5 M. However, the ionic activity factor is significantly less than unity because polyDMAC is a polyelectrolyte and the “salt effect” is therefore less than that given by the ionic concentration. The corresponding viscosity measurements on surfactant solutions in the presence of polyDMAC did not show any effect of added cosolute since the polymer contribution dominated the solution viscosity. The reason for the larger shift in the cationic surfactant system as compared to the anionic one is probably that (12) Høiland, H.; Blokhus, A. M. In Handbook of Surface and Colloid Chemistry; Birdi, K. S., Ed.; CRC Press: New York, 1997; Chapter 8.
Figure 3. Phase diagram for the three-component system water-PSS-TTAB with no cosolute added (1) and with added octanol (O). The TTAB/octanol ratio was 22.4.
cationic surfactants usually are more prone to micellar growth than anionic surfactants13 and as a consequence the aggregation number of cationic surfactants is also more sensitive to the addition of additives. The effect on the phase behavior is essentially a molecular weight effect. As is well studied, two polymers or macromolecules in a common solvent often phase-separate segregatively. When the molecular weight is increased, the concentration range where the polymers are miscible decreases as qualitatively described by the Flory-Huggins theory.14,16 In this case, addition of a long-chain alcohol increases the micellar aggregate size and decreases the miscibility range for the polymer-surfactant mixture in analogy with increasing the polymer molecular weight for polymer-polymer systems. Similarly, octane decreases the micellar size and therefore increases the miscibility range. The reasons for these solubilizate-driven changes in the surfactant aggregation numbers are outlined in the literature8-11 and were also discussed in our previous study7 on hydrophobic (13) Ananthapadmanabhan, K. P. In Interaction of Surfactants with Polymer and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1993; Chapter 2. (14) Flory, P. In Principles of Polymer Chemistry; Cornell University Press: Ithaca, NY, 1953. (15) Thalberg, K.; Lindman, B.; Karlstro¨m, G. J. Phys. Chem. 1991, 95, 3370. (16) Thalberg, K.; Lindman, B.; Karlstro¨m, G. J. Phys. Chem. 1991, 95, 6004.
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Figure 4. Phase diagram for the three-component system water-PSS-TTAB with no cosolute added (1) and with added octane (0). The TTAB/octane ratio was 24.6.
Nilsson et al.
polymer-surfactant complex, at least not to the same extent as it does for free micelles in the segregative systems. A possible reason for the contrasting behavior observed upon cosolute addition is that a rather stable micellar-polymer configuration is produced by the strong electrostatic attraction. In addition, the investigated polymers are hydrophobic and may therefore be incorporated into the micelle, thus acting as a cosolute. Changes in aggregation number or geometry for polymer-surfactant complexes by the addition of hydrophobic cosolutes therefore have to overcome an energy barrier that does not exist for free micelles. In the case of PSS-cationic surfactant systems, it is known that the micellar aggregation number is smaller than for the free micelles.17,18 For PAA-cationic surfactant systems, on the other hand, the aggregation number18 is about the same as for free micelles. The micelles are therefore less influenced by PAA polymer than by PSS. The addition of cosolutes gives a larger effect in the PAATTAB system than in the PSS-TTAB system, which is consistent with the above interpretation. It seems natural that the addition of cosolutes to systems where the micelles are less perturbed by the presence of polymer responds in the same way as for free micelles, that is, solubilizationdriven significant changes of the aggregation number. Conclusions
Figure 5. Phase diagram for the three-component system water-PAA-TTAB with no cosolute added (1), with added octane (0), and with added octanol (O). The phase diagram is only partial; the system is two phase above the indicated lines (toward the water corner).
cosolute effects on surfactant-polymer phase behavior and will therefore not be discussed here. In the case of oppositely charged systems such as PSS and TTAB, the phase separation is of the well-known associative type and the contrasting feature of this type of system is that the addition of hydrophobic cosolutes produces little effect, as seen in Figures 3 and 4. For associative systems, it has been shown that increasing the molecular weight or micellar aggregation number increases the two-phase area.15 The same result was also obtained with the associative system PAA-TTAB (Figure 5), where addition of cosolutes produced little effect. Possibly the addition of octanol gives some enhancement of the two-phase region, as is compatible with micellar growth, but the effect is small compared to that of the segregative systems. On the basis of the phase behavior, it seems that addition of cosolutes does not influence the micellar size in the
The segregative phase behavior of similarly charged surfactant-polymer systems in water is sensitive to the addition of small amounts of hydrophobic cosolutes. The effect is that alkanes such as octane increase the compatibility and that long-chain alcohols reduce the compatibility. The effect is attributed to solubilizate-driven changes in the aggregation number, thus effectively changing the micellar molecular weight. Alkanes reduce the micellar aggregation number, and long-chain alcohols increase the aggregation number. Cationic surfactant systems are more sensitive than anionic surfactants because cationic surfactants usually are more prone to micellar growth and as a consequence are also more sensitive to additives that influence the micellar growth/ reduction. For oppositely charged surfactant-polymer systems that phase-separate associatively, addition of hydrophobic cosolutes produces almost no effect on the phase behavior. A possible reason is that the surfactant aggregates are being stabilized by the complex formation with polymer and are therefore less sensitive to hydrophobic cosolutes. Acknowledgment. Assistance to the laboratory work by O. K. Fagerlid is acknowledged. LA025636D (17) Almgren, M.; Hansson, P.; Mukhtar, E.; Van Stam, J. Langmuir 1992, 8, 2405. (18) Hansson, P.; Almgren, M. Langmuir 1994, 10, 2115.
