The structure of lyotropic mesomorphic phases

Abstract

A comparative optical, density and X-ray diffraction study has been made of the mesomorphic (liquid crystalline) phases occurring at room temperature in a semi-polar surface-active agent/solvent system with H$_2$O and D$_2$O respectively as solvent. Since the surface-active agent investigated, N,N,N-trimethylamino dodecanoimide $(C_{11}H_{23}.CO.N^-.N^+(CH_3)_3),$ has a low melting point it was possible to test directly the theory that lyotropic mesomorphic phase structures merely represent different ways of packing various micellar units that are essentially liquid in their core configurations. The previously accepted structures for neat phase and viscous isotropic (or cubic) phase have been confirmed but a more detailed structure has been advanced for middle phase. It is proposed that the rod-like building units of the latter consist of linearly aggregated spherical micelles and, on the basis of this new structure, both the experimental data and the mesomorphic phase transformations can be rationalized. The micellar behaviour in very dilute solution has also been examined by the light-scattering method, and it is concluded that across the phase diagram, from the c.m.c. to neat phase, the fundamental structural unit is the spherical micelle, and that the different phases merely represent different aggregation states of this primary unit.