A simple molecular theory of smectic-C liquid crystals

Smectogenic organic compounds are made of rod-like molecules, which often h ave dipolar groups with lateral components. We argue that since the groups are generally off-axis, the electrostatic contribution to the intermolecula r interaction between the neighbours in a smectic layer favours a tilting o f the molecules. We develop a mean-field theory of the smectic-C (SmC) phas e based on a single-particle potential of the form u proportional to sin 2 theta cos phi, consistent with the biaxial nature of the phase, where theta and phi are the polar and azimuthal angles, respectively. The hard-rod int eractions which favour the smectic-A (SmA) phase with non-zero tilt are als o included to construct phase diagrams involving isotropic, nematic, SmA an d SmC phases. The topology of the phase diagrams, the calculated temperatur e variation of tilt angle in the SmC phase and the specific-heat jump acros s the SmA-SmC transition compare favourably with experiments.