Solubilization of oil in a mixed cationic liquid crystal

The mixing fraction of didodecyldimethylammonium bromide (DDAB) in dodecyltrimethylammonium bromide + DDAB to produce a lamellar liquid crystal (L _α) abruptly decreases upon addition of a small amount of m-xylene, whereas the mixing fraction becomes constant at high m-xylene content. Similar results were obtained in saturated hydrocarbon systems. It is considered that oil molecules in the surfactant palisade layer increases the effective cross-sectional area per surfactant head group, a_s, whereas a_s is constant if the oil molecules are solubilized in the core of the liquid crystal. The volume fraction of penetrating oil in the total solubilized oil is defined as a penetration parameter, Pe, which is calculated from small-angle X-ray scattering data. Pe is high in the m-xylene system, whereas it is low in the n-decane system. Even in the same oil system, Pe decreases dramatically with increasing solubilization. Hence, most of the oil added penetrates into a palisade layer at an early stage of oil addition. This causes a change in the mixing fraction of surfactant in the L _α phase. Thereafter the oil is solubilized in the core of the bilayer with further addition of oil. Received: 20 April 1998 Accepted: 16 July 1998