Solubilization of oil in discontinuous cubic liquid crystal in poly(oxyethylene)(n) oleyl ether systems

The structural change of the discontinuous cubic (I,) phase upon addition o f oil (heptane, decane, and hexadecane) is investigated by small-angle x-ra y scattering. In polyoxyethylene oleyl ether (C18:1EOn) systems having long polyoxyethylene chain(n=19.2, 30.1, and 50.8), the I-1 phase is formed in a wide concentration range. Only in C18:1EO19.2 systems, the structure of t he I-1 phase changes from body-centered to face centered cubic lattice upon addition of oil. According to the analysis of the change in effective cros s-sectional area, the solubilization of hexadecane increases the repulsion between polyoxyethylene chains, while solubilization of heptane makes it de crease. The solubilization of decane keeps the repulsion constant. Since th e repulsion hampers the curvature change due to the solubilization, the sol ubilization capacity is in the order, heptane>decane>hexadecane. The oil pe netration and nonpenetration into the palisade layer are also discussed in term of the change in the effective volume of the lipophilic part in the su rfactant molecule.