Brine-rich corner of the phase diagram of the ternary system cetylpyridinium chloride-hexanol-brine

The brine-rich side (mass fraction of brine is 0.75 < Phi < 0.98) of the phase diagram of the ternary system cetylpyridinium chloride-hexanol-brine (1% NaCl by weight) is studied. The hexanol-to-surfactant ratios eta at which the isotropic micellar L(1)-liquid crystal lamellar L(alpha)-isotropic sponge L(3)-multiphasic state phase transitions occur on the eta versus (1 - Phi) orthogonal phase diagram vary like a hyperbola with dilution. The solubility of hexanol in brine is found to be responsible for the variation of eta values with dilution. An expression which well describes the experimental phase boundaries is found taking into account the hexanol solubility in brine. The hexanol mass fraction beta in brine outside the surfactant aggregates is deduced from the experimental phase boundaries. The beta value is somewhat lower than the hexanol solubility in brine. The hexanol-to-surfactant ratio eta(0) which describes the bilayer composition is found to be constant with dilution at the L(alpha)-L(3) phase transition but varies at the L(1)-L(alpha) phase transition. The presence of hexanol molecules in brine as well as in the surfactant aggregates implies that the surfactant aggregate has to be considered as an open system. We predict that the composition of the bilayer can be changed upon external influence.