Micellar aggregates near the isotropic-cubic liquid crystal phase transition

Abstract

Micellar solutions of the palmitoyl-lyso-phosphatidyl choline (PLPC)/water system are studied by small angle x-ray scattering (SAXS) from the diluted isotropic (I) phase up to the concentrated I phase near the I→cubic (Q223) phase transition, for concentrations C in the range 5⩽C⩽37.5 wt % of PLPC. The particle distance distribution function p(r) is calculated for the less concentrated solution. A model for the product P(q)S(q), where P(q) is the micellar form factor and S(q) is the intermicellar interference function calculated using a hard sphere interaction potential, is fitted to the SAXS curves. It is found that PLPC micelles remain with a prolate ellipsoidal shape of constant anisometry ν=(1.80±0.05) in the whole concentration range. Such finding contrasts whit that found for systems that show I→hexagonal (H) phase transitions. In the latter, similar SAXS curves revealed that micelles grow to slightly higher anisometry (ν∼2.4). Results are analysed in terms of a recent model based on the micellar elastic bending energy, through the surfactant parameter p0=vpar/alc(vpar and lc are the volume and the critical length of the paraffin chain, a the polar head surface area). It is concluded that the formation of the Q223 phase requires that micelles remain spheroidal and avoid the spherocylindrical shape.