Phase Behavior and Solution Properties of Aqueous Polyion-Surfactant Ion Systems

Abstract: Polyelectrolytes and oppositely charged surfactants interact through attractive electrostatic forces, which in many cases lead to phase separation already at low total concentrations. During the past 10-15 years investigations of these systems have been simplified through the use of complex salts that consist of surfactants with polymeric counterions in a 1:1 charge stoichiometry (no excess simple counterions present). The approach of using complex salts enables these systems to be described in two dimensions in truly ternary phase diagrams, which is not possible if the polyelectrolyte and the surfactant, including their original simple counterions, are mixed. One of the aims of this thesis was to investigate how mixtures of water-insoluble complex salts are affected by addition of nonionic surfactants in aqueous solution. Both the ternary phase behavior and the different liquid crystalline phases that form as well as the solution behavior have been investigated as nonionic surfactants are added. It was demonstrated that the prescence of ethoxylated entities in the polyion-surfactant ion aggregates promote solubility and that the addition of nonionic surfactants to insoluble complex salts give rise to a rich phase behavior. In fact, the presence of liquid crystalline phases in equilibrium with a dilute aqueous phase facilitated the formation of stable dispersions of nanoparticles with a bicontinuous cubic internal structure. Moreover, it was possible to tune this structure by means of temperature. Soluble complex salts consisting of a cationic surfactant and oppositely charged copolymers with grafted poly(ethylene oxide) side chains were also investigated. These complex salts were found to form micelles with a hydrodynamic radius of around 200 nm in size, which suggests that they mostly contain only one polyion. It was also observed that these systems displayed a well-defined CMC equivalently to surfactants in aqueous solution. The release of a polyion from oppositely charged mixed micelles was investigated in one of the studies. For this purpose a cationic and cleavable surfactant (decyl betainate) which is hydrolyzed at high pH, was used. Decyl betainate formed insoluble aggregates together with the polyion. These aggregates were then solubilized by C12E8. It was revealed that the rate of hydrolysis of decyl betainate was decreased with the polyion present in the system. It was also shown that the polyion was released into solution as the surfactant was degraded.