Structural studies of nonionic lyotropic liquid crystals

The nonjonic surfactants n-C 1 6113 3 (OCH2CH2) 60H (C 1 6E06 ) and CiS-C9H18=C 1 3H2 5 (0CH2CH2 ) 60H (C22E06) have been studied in binary mixtures with heavy water, using techniques including optical microscopy, nuclear magnetic resonance and x-ray and neutron scattering. The liquid crystalline phases have been characterized and their dimensional parameters determined. A phase diagram of C 16E06 has been produced covering the range of surfactant concentrations from 24 to 60% by weight. This system presented a nematic phase associated with calamitic mesogenic units, the first one to be characterized in this kind of system. Both systems showed a lamellar phase at high concentration and temperatures, while a hexagonal phase at lower concentrations and temperatures. Between these phases a different lamellar region was found, but with smaller interplanar distances than the L. phase. It was called fluid phase, F. This was the first observation of this phase in such kind of system. The structure of the fluid phase was concluded to be disrupted planes of amphiphile molecules with water filling the inplane spaces. No correlation was found between mesogenic units in different planes, therefore no regular structural lattice in the plane normal to the lamellae is expected. The C 16E06 system also showed an ambiguous V 1 cubic/intermediate phase between hexagonal and fluid phases, the cubic being the more stable state.