Thermal Pressure Coefficients and Specific Volumes of Cyanobiphenyls and their Transition Entropies at Constant Volume

Abstract

Specific volumes and thermal pressure coefficients (∂p/∂T)_v of 5CB, 7CB, 50CB, and 80CB were measured from temperatures near their crystalline melting points to above their nematic-isotropic transitions. At T_NI, volume changes ΔV of 0.52, 0.67, 0.25, and 0.48 cm³ mol⁻¹, respectively, were found. A discontinuity of 0.02 cm³ mol⁻¹ in the volume of 80CB was observed at its smectic A-nematic transition. The entropy change at constant volume ΔS_v was determined at T_AN and T_NI by subtracting the entropy of dilation (∂p/∂T)_v ΔV from the entropy change at constant pressure. The latter quantity was obtained from the slope of the phase boundary curve according to the Clapeyron equation. This was judged to be a more reliable procedure than using transition heats from DSC. Values of ΔS_v vary from 0.03Rto 0.10R for the nematic-isotropic transition, where R is the gas constant.