Experimental Study of a Lyotropic Lamellar Phase Swollen with Polymer Solutions

(Received 27 January 1995, accepted 27 February 1995)

Abstract
We report on the effect of a hydrosoluble neutral polymer on the stability of a lamellar phase made of charged surfactant bilayers. In the system investigated, the smectic structure exists for a wide range of dilutions, with smectic periods continuously varying between 30 and 300 Å. Our results show that the lamellar structure still exists when pure water is replaced with aqueous solutions of polyethyleneglycol (PEG: radius of gyration  Å, molecular weight  g/mol). Large amounts of PEG can be solubilized in the mesophase: up to 50% of the water can be replaced by the polymer. However, the added polymer causes a reduction in the swelling of the phase and most remarkably produces at intermediate polymer and bilayer concentrations a phase separation between two lamellar phases of different periods. We have studied the system with static (X-ray or neutron) and dynamic (light) scattering techniques. The polymer gets partially associated to the surfactant bilayer, as revealed by the small decrease of the smectic spacing as a function of PEG concentration at constant surfactant cosurfactant volume fractions. Besides, the interactions between bilayers are less repulsive in the presence of PEG than the weakly-screened electrostatic repulsion expected with our ionic surfactant. Interactions may even become attractive, which leads to a phase separation between two lamellar phases. The decrease in the layer compression modulus is studied through its consequences on Bragg peak linewidth, small angle scattering and baroclinic mode relaxation frequency. The eventual presence of critical points on the border of the closed-loop smectic-smectic miscibility gap is investigated.