WHY THE MOLECULES ARE TILTED IN ALL SMECTIC A PHASES, AND HOW THE LAYER THICKNESS CAN BE USED TO MEASURE ORIENTATIONAL DISORDER

Even though the average molecular direction in smectic A phases is perpendicular to the smectic planes, most molecules in these phases have rather large tilt angles because the orientational order parameter S in all smectic A phases is significantly less than unity. This molecular tilt inherent to all A phases accounts for the observed differences between layer thickness d and molecular length l of four quite different compounds, without any need for assuming additional tilt, interdigitation, or kinking of chains. From the differences between d and l, S values from 0. 78 to 0. 84 are obtained, which yield average molecular tilt angles from 20 degree to 17 degree . It is predicted that all smectic A phases with monomolecular layers will follow the same pattern. The concept of ″inherent tilt″ should apply to all orthogonal smectic phases, thermotropic as well as lyotropic.