Polyion-Surfactant Ion Complex Salts Formed by a Random Anionic Copolyacid at Different Molar Ratios of Cationic Surfactant: Phase Behavior with Water and n-Alcohols Ana Maria Percebom, † Lennart Piculell, ‡ and Watson Loh* ,† † Institute of Chemistry, University of Campinas (UNICAMP), Caixa Postal 6154, Campinas, SP 13083-970, Brazil ‡ Physical Chemistry 1, Center for Chemistry and Chemical Engineering, Lund University, P.O. Box 124, S-221 00 Lund, Sweden * S Supporting Information ABSTRACT: The presence of acid groups with different pK a values in the anionic copolymer poly(4-styrene sulfonic acid- co-maleic acid), P(SS-Ma), allowed the preparation of complex salts with a variable fraction of anionic groups neutralized by cationic surfactant in the copolymer via controlled titration with hexadecyltrimethylammonium hydroxide, C 16 TAOH. Two new complex salts were selected for detailed phase studies, C 16 TA 2 P- (SS-Ma) and C 16 TA 3 P(SS-Ma), where both had 100% charged styrene sulfonate groups, but the fraction of charged carboxylate groups on the polyion was 50% or 100%, respectively. These complex salts thus contained both hydrophobic (styrene sulfonate) and hydrophilic (carboxylate) charged groups, and the ratio between the two could be altered by titration. These features were found to have consequences for the phase behavior in water and in ternary mixtures with water and n-alcohols for the two complex salts, which differed compared to complex salts containing homo- or copolyions with only carboxylate or styrene sulfonate charged groups. For both complex salts, binary mixtures with water produced, in the dilute region, two isotropic phases in equilibrium, the bottom (concentrated) one displaying increasing viscosity with increasing concentration. For the complex salt C 16 TA 2 P(SS-Ma), there was evidence of micellar growth to form anisometric aggregates at high concentrations. For the C 16 TA 3 P(SS-Ma) complex salt, this was not observed, and the isotropic phase was followed by a narrow region of cubic phase. In both cases, concentrations above ca. 60 wt % produced a hexagonal phase. For ternary mixtures with n-alcohols, the general trend was that a short-chain alcohol such as n-butanol acted as a cosolvent dissolving the aggregates, whereas with n-decanol, a cosurfactant effect was observed, inducing the formation of lamellar phases. Visual inspection (also between crossed polarizers), small angle X-ray scattering (SAXS) and diffusion nuclear magnetic resonance (NMR) were used in these studies. ■ INTRODUCTION Mixtures of oppositely charged polymers and surfactants in water display associative phase separation as a notable feature. 1 Because of the strong electrostatic attraction, a phase enriched in polyion and ionic surfactant molecules is formed and can exhibit different types of structure (liquid crystalline or disordered micellar systems) making these systems suitable for several applications: as templates, for drug delivery, in cosmetics, and others. 2-4 A considerable number of studies has been developed in order to understand the phase behavior and the interactions involving these species formed by polyions and oppositely charged sur- factants. In order to simplify the study of this complex phase behavior, Svensson and co-workers proposed a new strategy, 5 involving the preparation of pure polyion-surfactant ion complex salts (free of simple counterions), which has been applied in several recent studies. 5-19 These studies are important to elucidate how one may control the structure of the (meso)phases formed by these complex salts and their miscibility with water, in terms of the surfactant and polyion features as well as the nature of an added third component. The focus of the present study is on the consequences of changing the polyion properties. The chemical nature of the charged units of the polyion plays an important role in the involved interactions as verified by Hansson et al., 20 when comparing complexes formed by sodium polyacrylate, NaPA, and sodium poly 4-styrene sulfonate, NaPSS, with the same cationic surfactant, dodecyltrimethylammonium bromide, C 12 TABr. The latter study concluded that the hydrophobic con- tribution to the interaction between NaPSS and the surfactant is important, whereas for the complex salt formed with poly- acrylate, the interaction is essentially electrostatic. Although both polyions are vinyl based, their complexes presented different phase behavior in water, due to the difference in the hydro- phobicity of the pendant ionic groups. In the present work, the random anionic copolymer poly- (4-styrene sulfonic-co-maleic acid), P(SS-Ma), containing equal Received: October 27, 2011 Revised: January 5, 2012 Published: January 30, 2012 Article pubs.acs.org/JPCB © 2012 American Chemical Society 2376 dx.doi.org/10.1021/jp2103403 | J. Phys. Chem. B 2012, 116, 2376-2384