Self-Assembly of Polyion–Surfactant Ion Complex Salts in Mixtures with Water and n-Alcohols

Phase behavior and structural features were investigated for "complex salts", consisting of the cationic hexadecyltrimethylammonium (CTA) surfactant with polyacrylate (PA(n), n = 30 or 6000) counterions, mixed with water and different n-alcohols (ethanol, butanol, hexanol, octanol, and decanol). The liquid crystalline structures formed were identified by small-angle X-ray scattering measurements, which provided information about the changes in the geometry of the aggregates as functions of the concentration and chain length of the added n-alcohol. The obtained results were compared with a previous work on similar ternary mixtures of the same cationic surfactant but with the monomeric bromide counterion, CTABr (Fontell, K; Khan, A.; Lindstrom, B.; Maciejewska, D.; Puang-Ngem, S. Colloid Polym. Sc., 1991, 269, 727). In general, the same phases were detected in systems with the complex salts CTAPA(n) as in systems with CTABr, but the swelling of the various liquid crystalline phases by water was much more limited in the complex salt systems. An isotropic alcoholic phase was observed with all alcohols and the size of this region of the phase diagram increased for the shorter alcohols, except for ethanol. For mixtures with octanol and ethanol, in particular, the extensions of the disordered isotropic phases were larger for the complex salt with the shorter polyacrylate ions

A new approach to the phase behavior of oppositely charged polymers and surfactants

The complex salt (ionic surfactant + polymeric counterion) cetyltrimethylammonium polyacrylate (CTAPA) has been synthesized, and its aqueous mixtures with cetyltrimethylammonium bromide (CTABr) have been studied. These mixtures differ from conventional oppositely charged polymer/surfactant mixtures in that the conventional counterion of the polyion (usually sodium, for the polyacrylate) is absent, which simplifies the studies and their interpretation considerably. The phase diagram of the CTAPA/CTABr/water system at > 20 wt % water and at 40 degreesC has been established, representing the first truly ternary phase diagram of an oppositely charged polymer/surfactant pair in water. The two dimensions of the phase diagram may be chosen as the water content (in weight percent) and the fraction of bromide counterions, x(Br) (in units of charge equivalents). The phase diagram is characterized by a large hexagonal phase (at low water contents and for all values of x(Br)), a small cubic phase (at 55 wt % water content and for x(Br) 0.9), and a large multiphase region (at water contents > 50 wt %) containing two or three of the cubic, hexagonal, or isotropic phases in coexistence. The cubic and hexagonal phases are connected to the corresponding phases that separate out from aqueous NaPA/CTABr mixtures. The maximum water uptake of the hexagonal phase is remarkably constant at ca. 50 wt % over a large CTAPA/CTABr composition range (x(Br) < 0.9). The study confirms previous conclusions that the polyacrylate counterions favor a higher aggregate curvature (leading to smaller aggregates) than do the bromide counterions

Soluble Aggregates in Aqueous Solutions of Polyion-Surfactant Ion Complex Salts and a Nonionic Surfactant.

Water-soluble aggregates based on two polyion-surfactant ion "complex salts", consisting of hexadecyltrimethylammonium (C16TA(+)) and polyacrylate (PA(-)) with either 25 or 6000 repeating units, with added nonionic surfactant octaethylene glycol monododecyl ether (C12E8) have been investigated. A previous phase study has shown that added C12E5 or C12E8 can solubilize complex salts in aqueous systems, and that increasing the poly(ethylene oxide) chain length of the nonionic surfactant and/or decreasing the polyion length favors dissolution. In this work we report on dynamic light scattering, NMR diffusometry, small-angle X-ray scattering, and isothermal titration calorimetry measurements performed to characterize the solubilized composite aggregates in dilute aqueous solution in terms of size and stoichiometry. It was found that mixed aggregates of polyacrylate, C16TA(+) ions, and C12E8, with almost constant stoichiometry, coexist with free micelles of C12E8 at all investigated mixing ratios. The length of the polyion only weakly affects the stoichiometry of the mixed aggregates while strongly affecting their size and water solubility