Micellar aggregates near the isotropic-cubic liquid crystal phase transition

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 (Q(223)) phase transition, for concentrations C in the range 5 less than or equal to C less than or equal to 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 nu=(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 (nu similar to 2.4). Results are analysed in terms of a recent model based on the micellar elastic bending energy, through the surfactant parameter p(0)=upsilon(par)/al(c)(upsilon(par) and l(c) 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 Q(223) phase requires that micelles remain spheroidal and avoid the spherocylindrical shape. (C) 1997 American Institute of Physics.