Why the Molecules are Tilted in all Smectic A Phases, and How the Layer Thickness Can be Used to Measure Orientational Disorder*

Abstract

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° to 17°. 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 lytropic.