Elliptic phase boundaries between smectic and nematic phases

Abstract

Smectic–nematic phase boundaries that resemble arcs of ellipses can be analyzed by a quadratic expansion in temperature and pressure of the change ΔG of Gibbs function. If the change of entropy or volume is known, ΔG and its first two derivatives can be determined. Data on 8OCB and mixtures of 8OCB and 6OCB and of CB9A and CB7A fit an ellipse well, which suggests that the boundary is an ellipse. The smectic phase is therefore enclosed, which appears to be unique in single-component systems and to disagree with simple mean-field theory.