Phase Equilibria and Structures of Oppositely Charged Polymers and Surfactants in Water

Abstract: A new approach to the study of aqueous mixtures of oppositely charged polymers and surfactants has been introduced. Pure polyion-surfactant ion complex salts have been synthesized and used as the point of departure. The elimination of the simple ions reduces the number of components in the mixtures, thus greatly simplifying the system. By mixing the complex salt with either the polyelectrolyte or the surfactant in water, truly ternary systems are obtained. Two different complex salts have been investigated: alkyltrimethylammonium polyacrylate (CTAPA), containing a flexible and highly charged polyion, and cationic hydroxyethyl cellulose with dodecylsulfate counterions (cat-HECDS), containing a rigid polyion of low charge density. Phase diagrams of their mixtures with either polyelectrolyte or surfactant were investigated by Small Angle X-ray Scattering (SAXS) and Pulsed Field Gradient NMR (PFG NMR). The complex salts CTAPA form hexagonal and cubic Pm3n phases in concentrated mixtures with water. Phase separation appears at high water contents. The effect of changing the surfactant alkyl chain length and the polyion length was investigated. The solubility of the complex salt increases with decreasing chain lengths. Dilute mixtures of the complex salt C16TAPA30 (cetyl surfactant chains, PA- 30 repeating units), the surfactant C16TAAc and water contain spherical micelles with mixed polymeric (PA-) and monovalent (Ac-) counterions. The interaction between the aggregates can be tuned by changing the counterion composition, from attraction (polyion bridging) to repulsion (entropic double-layers). Monte Carlo simulations confirm that poyion bridging is the dominant attractive force. Self-diffusion measurements were performed in the cubic Pm3n phase of the same mixture. The complex salt cat-HECDS does not form any liquid crystalline phases due to the low charge density and the rigidity of the polyion. In contrast to CTAPA, cat-HECDS is dissolved by a small excess of surfactant due to hydrophobic binding of the excess surfactant to the cat-HEC polyion.