Phase separation in polyelectrolyte gels interacting with surfactants of opposite charge

Macroscopic phase separation in covalent sodium polyacrylate (PA) networks following the absorption of cetyltrimethylammonium bromide/chloride (CTAB/C) from aqueous solutions is studied experimentally and theoretically. The gels are shown to consist of a solvent-swollen polyelectrolyte network (core), surrounded by a dense surface phase (skin) of polyion/surfactant complexes. The effect of core swelling on network structure and polyion/surfactant interaction in skins is discussed. It is demonstrated that the skin limits the swelling of the core. A model of the equilibrium swelling of phase separated gels is developed, taking into account the osmotic swelling of the core due to the presence of mobile counterions, the work of deformation of the core network, and the work of deformation of the skin. The last contribution is described using the theory of rubber elasticity. The core network is described using an empirical equation of state. The model is used to calculate the volume of gels after the absorption of various amounts of surfactant, Comparison with experiments shows that the agreement is satisfactory. The skin microstructure is investigated by means of small-angle X-ray scattering, optical birefringence, and time-resolved fluorescence quenching. The size, shape, and spatial organization of surfactant micelles is found to depend on the composition of the skin. Stoichiometric polyion/surfactant complexes (free from simple ions) form an ordered cubic structure (space group: Pm3n). The incorporation of bromide or chloride ions leads to a transition to hexagonal structure. The transition is related to the corresponding transition in complexes between linear PA and CTAB. Skins with cubic structure are found to be elastic and can be deformed at constant volume. The structural basis for the rubber-like behavior is discussed. The appearance of hexagonal skin microstructure is found to correlate with an anomalous swelling/deswelling pattern leading to the formation of "balloon gels".