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Interactions between Water-Soluble Polymers and Surfactants: Effect of the Polymer Hydrophobicity. 2. Amphiphilic Polyelectrolytes (Polysoaps) Olivier Anthony and Raoul Zana* Institut Charles Sadron (CRM), CNRS, 6, rue Boussingault, 67083 Strasbourg-Cedex, France Received February 29, 1996. In Final Form: May 2, 1996X The binding of DTAC (dodecyltrimethylammonium chloride) to amphiphilic polyelectrolytes of differing hydrophobicity, the alternated copolymers poly(maleic acid-co-alkyl vinyl ether) with alkyl ) hexyl, octyl, decyl, dodecyl, and hexadecyl, referred to as PS6, PS8, PS10, PS12, and PS16, respectively, has been investigated by potentiometry with a surfactant ion-specific electrode, by dialysis, by time-resolved fluorescence quenching, by fluorescence anisotropy, and by viscosimetry. In aqueous solution, in the absence of surfactant these polymers can form hydrophobic microdomains and are referred to as polysoaps. The surfactant binding isotherms, the number of surfactants (N) per hydrophobic aggregate, the lifetime of pyrene (τ2) in these aggregates, and their microviscosity (ηi) have been obtained as a function of the surfactant concentration, C, and the copolymer neutralization degree R. The binding is not cooperative, but the binding constants are very large so that most of the surfactant was bound to the copolymer, under the experimental conditions used, and N is proportional to C. These results differ strongly from those characterizing the interaction between DTAC and the hydrophilic poly(maleic acid-co-methyl vinyl ether) at all R’s and poly(maleic acid-co-butyl vinyl ether), or PS4, at R g 0.50, where binding is cooperative and N is independent of C. The results indicate that the added surfactant can be considered to be partitioned between the aqueous phase and the microdomains formed by the amphiphilic copolymers, simply swelling these microdomains. They permitted us to obtain the number of polymer repeat units per microdomain in the absence of surfactant. The large values of the pyrene fluorescence lifetimes have been related to the high microviscosity of the mixed surfactant-polysoap microdomains, which reduces the efficiency of quenching of pyrene fluorescence by molecular oxygen solubilized in the microdomains. The binding of DTAC to PS4 at R ) 0.25 has some of the characteristics encountered with hydrophilic polyelectrolytes (N changes only little with C) but is not cooperative and shows no critical aggregation concentration, as for polysoaps. The results are discussed in terms of the structure adopted by polysoaps in solution, prior to the addition of surfactant, and of the models of binding for the two limiting cases presented.
Introduction This series of papers investigates the effect of the polymer hydrophobicity on the properties of charged polyelectrolyte-oppositely charged surfactant systems, with particular emphasis on the aggregation number and microviscosity of the aggregates present in the system. Part 1 in this series reported on the study of the interaction between the cationic surfactant dodecyltrimethylammonium chloride (DTAC) and two hydrophilic polyanions, the copolymers poly(maleic acid-co-methyl vinyl ether) and poly(maleic acid-co-butyl vinyl ether), referred to as PS1 and PS4, by means of potentiometry with a surfactant ion-specific electrode, time-resolved fluorescence quenching, and fluorescence anisotropy.1 The surfactant binding isotherms, the number of surfactants (N) making up polymer-bound aggregates, the lifetime of pyrene (τ2) in these aggregates, and their microviscosity (ηi) were obtained as a function of the DTAC concentration, C, and copolymer neutralization degree, R. Surfactant binding occurred only above a surfactant concentration referred to as the critical aggregation concentration (cac) and was cooperative at all R’s for PS1 and at R g 0.5 for PS4, with the amount of bound surfactant increasing rapidly with C, above the cac. Surfactant aggregation numbers, pyrene lifetimes, and aggregate microviscosities were all found to be independent of C but to depend strongly on R. N increased with R for PS1 but decreased for PS4. For PS1, τ2 and ηi increased when R decreased, i.e, with decreasing micelle size. The comparison of the τ2 values obtained with DTAC and dodecyltrimethylam* To whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, June 15, 1996. (1) Anthony, O.; Zana, R. Langmuir 1996, 12, 1967.
S0743-7463(96)00184-9 CCC: $12.00
monium bromide (DTAB) in the presence of PS1 revealed that the surface of polymer-bound aggregates is nearly free of surfactant counterions. This led to a model of binding where polymer chains tightly wrap around aggregates with their charged groups in contact with surfactant charged groups. The difference of behavior between PS1 and PS4 was attributed to the contribution of hydrophobic interactions between PS4 butyl side chains and surfactant alkyl chains. This contribution is small for PS1, where binding is essentially due to short range electrostatic interactions. The high microviscosity of PS1bound aggregates relative to free DTAC micelles was attributed to these interactions and also to the presence of the polymer main chain at the aggregate surface. This large microviscosity slows down the diffusive motion of reactants (probe and quencher, or any other reactant) in the aggregates, thus making quenching and other processes less efficient than in free micelles and resulting in the observed high pyrene fluorescence lifetime values. This paper reports on the interaction of DTAC with the more hydrophobic (amphiphilic) homologues of the poly(maleic acid-co-alkyl vinyl ether) series (PSX, where X is the side chain carbon number), the alkyl chain being a hexyl (PS6), octyl (PS8), decyl (PS10), dodecyl (PS12), or hexadecyl (PS16) group. In aqueous solution all these copolymers form hydrophobic microdomains which persist up to a certain value of the pH or of the neutralization degree R, that increases with X.2-7 For the sake of completeness the interaction of DTAC with PS4 at R ) (2) Pefferkorn, E.; Schmitt, A.; Varoqui, R. C. Acad. Sci., Ser. C 1968, 268, 349. (3) Dubin, P.; Strauss, U. P. J. Phys. Chem. 1967, 71, 2757; 1970, 74, 2482; 1973, 77, 1427. (4) Strauss, U. P.; Varoqui, R. J. Phys. Chem. 1968, 72, 2657. (5) Strauss, U. P.; Vesnaver, G. J. Phys. Chem. 1975, 79, 1558.
© 1996 American Chemical Society
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0.25, where this polymer forms hydrophobic microdomains, was also investigated. In a separate investigation of aqueous PSX solutions we have evaluated the fraction of side chains forming microdomains.8 Also, the PSX-DTAB interaction was shown to become less cooperative as X increases, but its strength increased.9 As in Part 1, this study focuses mainly on the surfactant binding isotherm, the aggregation number of the bound surfactant, and the microviscosity of the mixed PSX-surfactant aggregates. A few measurements of viscosity have also been performed. As shown below, the interaction between DTAC and the amphiphilic PSX used presents very different characteristics from that with hydrophilic PSX. This was anticipated because the PSX used can form hydrophobic microdomains even in the absence surfactant. The binding resembles a comicellization between surfactant and PSX repeat units. Materials and Methods Materials. The samples of PS4, PS6, PS8, PS10, PS12, and PS16 were the same as in previous investigations.1,6-9 Their degrees of polymerization were determined by light-scattering measurements in tetrahydrofuran and found to be 260 for PS4, 300 for PS6, unknown but low ( 0.75 mM. This behavior can be attributed to the binding of DTAC to PS16, which decreases the microdomain electrical charge and increases the hydrophobicity. These two effects favor associations between macromolecules and may be responsible for the decrease of the minimal CPS value necessary to obtain viscoelastic solutions. It would be interesting to examine the PS16-DTAC system by transmission electron microscopy at cryogenic temperature to check whether DTAC additions induced the formation of entangled threadlike micelles by acting as glue between polymer ends or if some microdomains now involve more than two PSX molecules (network formation). In order to quantify the above effect, the viscosity η of PS16-DTAC systems at R ) 1 and CPS ) 2 mM was measured as a function of C and shear rate, γ˘ . A rapid decrease of η was observed upon increasing γ˘ , i.e., a behavior typical of polyelectrolyte in the absence of salt.28 For this reason, it was impossible to obtain the value of η at γ˘ f 0. The measurements were thus performed at γ˘ ) 20 s-1. The results expressed as specific viscosity, ηsp, are plotted in Figure 8. ηsp increases about linearly with C and becomes about twice as large when C is increased from 0 to 2 mM. For the sake of comparison DTAC was replaced by tetramethylammonium chloride (TMAC), an electrolyte that is chemically identical to DTAC, except for the dodecyl group which is replaced by a methyl group. Additions of TMAC are seen to result in a small decrease (26) Zana, R.; Kaplun, A.; Talmon, Y. Langmuir 1993, 9, 1948. (27) Kamenka, N.; Kaplun, A.; Talmon, Y. Zana, R. Y. Langmuir 1994, 10, 2960. (28) Bohdanecky, M.; Kovar, J. In Viscosity of Polymer Solutions; Jenkins, A. D., Ed.; Polymer Science Library 2; Elsevier: Amsterdam, 1982; Chapter 4.
Physicochemical Characteristics of the Surfactant Binding to Polysoaps. For hydrophilic polyelectrolyte-surfactant systems, several results lead to the conclusion that polymer-bound surfactant aggregates are very similar to classical micelles.1 This was indeed the case for PS1-DTAC systems at all R’s and PS4-DTAC systems at R g 0.50, which showed the existence of a cac, a cooperative surfactant binding, and nearly no dependence of N on C, as for DTAC micellar solutions. This result indicated that addition of more surfactant results in the formation of more polymer-bound surfactant aggregates, of the same composition.1 These similarities disappear when the hydrophilic polyelectrolyte is replaced by an amphiphilic polyelectrolyte (polysoap). Then, no cac is observed and binding is not cooperative, but the surfactant binding constants are very large (almost all added surfactant binds to the polymer, even at very low C). Besides, N is proportional to the concentration of bound surfactant, which indicates that adding more surfactant does not generate additional microdomains. Nevertheless, with both hydrophilic polyelectrolytes1 and polysoaps (this work), the aggregate microviscosity values are high and almost all surfactant counterions appear to be expelled from the aggregate surface. The differences between hydrophilic polyelectrolytes and polysoaps obviously arise from the capacity of the latter to form intramolecular hydrophobic microdomains in the absence of surfactant.6-9 Microdomain concentration and size depend on the side chain carbon number X, neutralization degree R, and polymer concentration CPS.8 The successive microdomains in a polysoap molecule are connected by polysoap segments of variable length, not organized in microdomains and where the side chains are exposed to water.7,8,14,29 No new aggregates or microdomains are formed upon addition of DTAC to polysoaps, in spite of the fact that most of the surfactant binds to the polymer, clearly because the surfactant binds preferentially to polysoap microdomains rather than to nonorganized parts of polysoap molecules. Such a binding is indeed highly favored from the energetic point of view, as it allows hydrophobic interactions between bound surfactant alkyl chains and polysoap side chains constituting microdomains and electrostatic interactions between PSX and surfactant oppositely charged groups. The absence of cooperativity in binding comes from the fact that once a first DTA+ ion incorporates into a microdomain, the next one tends to incorporate into another microdomain for entropic reason and, to a lesser extent, because binding of the first DTA+ ion has reduced the microdomain electrical charge. The absence of a cac is also due to the preexisting microdomains. Indeed the incorporation of surfactant ions into polysoap microdomains is equivalent to a partition (29) Cochin, D.; Candau, F.; Zana, R.; Talmon, Y. Macromolecules 1992, 25, 4220.
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equilibrium of the surfactant between the aqueous phase and the microdomain pseudophase. Besides, as pointed out above, surfactant ions tend to bind preferentially to microdomains in those polysoaps where microdomains coexist with nonorganized PSX segments. The results indicate that the bound surfactant ions just swell the preexisting microdomains. As for the PS1-DTAC or DTAB systems, the surfactant counterions (Cl- or Br-) are removed from the aggregate surface and replaced by PSX negatively charged groups that stabilize the aggregates. This explains the precipitation of a surfactantPSX complex when β is close to the neutralization degree of the polymer (when R e 0.50) or close to β ) 0.50 for R > 0.50. Fluorescence Lifetimes and Microviscosities. The interpretation of the variations of τ2 and ηi with surfactant concentration C is more delicate. These variations are correlated. Thus the high values of ηi in the absence of surfactant are caused by the chemical bonds connecting consecutive repeat units constituting a microdomain. These bonds hinder the motion of molecules solubilized in aggregates, in the present case pyrene and its quencher, molecular oxygen, thereby reducing the efficiency of quenching of pyrene by O2. As discussed in part 1, this effect increases the lifetime of pyrene solubilized in aggregates. This increase can be significant, since τ2 reaches 270 ns in surfactant-free PSX solutions (see Figure 6). DTAC additions to PS16 at R ) 1.00 induce a rapid decrease of ηi and the expected decrease of τ2 (Figures 5 and 6 (top)). The surfactant ions incorporated in PS16 microdomains are more free to move than the polymer repeat units, and the average microviscosity of mixed PS16-surfactant aggregates should be reduced with respect to surfactant-free aggregates. Note that in PS16 nearly all repeat units are under the form of microdomains8 and DTAC additions result in mixed aggregates where the repeat unit mole fraction decreases as more surfactant ions are bound. For the PS10-DTAC system at R ) 1, Figure 5 shows a near constancy of ηi. It is likely that this effect is partly caused by an increase in the number of decyl side chains per microdomain as more and more surfactant ions are bound. Indeed at R ) 1.00 only about 15% of the PS10 side chains are involved in microdomains, contrary to the case for PS16, and these microdomains are rather small, comprising 10-15 decyl chains.7,8 Thus as DTA+ ions start incorporating a preexisting microdomain, some free repeat units immediately adjacent to this microdomain may also be incorporated into it. The decrease of microdomain electrical charge upon DTA+ binding tends to favor this process. The simultaneous incorporation of free repeat units, which tend to increase ηi, and of DTA+ ions, which tend to decrease ηi, may result in the observed small change of ηi with C. Interaction between Polyelectrolytes and Surfactants. The nature of the interactions between polyelectrolytes and surfactants appears to depend much on the polymer hydrophobicity. In the case of PS1, i.e. the most hydrophilic PSX investigated, the two dominant interactions are (i) the electrostatic attraction between PS1 and surfactant charged groups and (ii) the hydrophobic attraction between surfactant alkyl chains.1 This situation is characterized by a cooperative aggregation of the surfactant on the polymer at C > cac and a surfactant aggregation number determined by the polymer charge fraction.1 For PSX-DTAC systems with X g 6, the cac and cooperativity disappear, indicating that a third interaction, the hydrophobic attraction between surfactant and PSX
Anthony and Zana
Figure 9. Schematic representation of the binding of a cationic surfactant to a polysoap which forms intramolecular microdomains in aqueous solution. The surfactant charged groups are located close to the polymer charged groups replacing the surfactant counterions at the aggregate surface while the surfactant alkyl chains swell the microdomains.
Figure 10. Binding isotherm of DTAC to PS4. CPS ) 2 mM; 25 °C. The vertical dotted line indicates the onset of precipitation.
alkyl chains within microdomains, is now at play. This attraction clearly dominates the other two, and the surfactant then behaves almost like a hydrophobic molecule partitioned between microdomains and the bulk phase. Figure 9 gives a schematic representation of the binding of a cationic surfactant to negatively charged polysoap microdomains. While the process of formation of PSXDTAC aggregates depends much on the polymer hydrophobicity, the structure of the mixed PSX-surfactant aggregates is somewhat independent of the PSX hydrophobicity. In all instances the aggregates are made up of surfactant and polymer alkyl chains. They are viscous enough to reduce the mobility of solubilized molecules (fluorescent probes, quenchers, etc.). Last, the surfactant counterions are nearly completely expelled from the aggregate surface. Case of the PS4 r ) 0.25-DTAC System. In the presence of DTAC, PS4 at R ) 0.25 behaves very specifically. The binding isotherm in Figure 10 shows a noncooperative binding and no cac. Besides, at low C, almost all the surfactant is bound. These are characteristic of polysoap-surfactant systems. On the other hand, Figure 11 shows that the surfactant aggregation number N and the fluorescence lifetime τ2 of pyrene solubilized in the aggregates depend very little on C, if at all, a behavior
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Figure 11. PS4-DTAC system: variation of the surfactant aggregation number (0) and of the pyrene fluorescence lifetime (O) with the DTAC concentration. CPS ) 2 mM; 25 °C.
typical of hydrophilic PSX-surfactant systems.1 This dual behavior of PS4, like a hydrophilic or like a hydrophobic polyelectrolyte, depending on the property under consideration, can be understood by recalling that PS4 behaves like a polysoap at low R (
