Thermodynamic measurements on three 5-n-alkyl-2-(4′-isothiocyanatophenyl)-1,3-dioxanes (6DBT, 8DBT, 10DBT) at elevated pressures

Abstract

The phase diagrams of three 5-n-alkyl-2-(4′-isothiocyanatophenyl)-1,3-dioxanes (6DBT, 8DBT and 10DBT), have been established using differential thermal analysis (DTA) up to 250 MPa. All three compounds have a smectic A₁ phase. For 8DBT the melting curve splits with increasing pressure. 10DBT exhibits a solid–solid transition and a splitting of the clearing line above 170 MPa. Additionally pVT data have been measured for 6DBT in the same pressure range at temperatures between 323 and 373 K. Volume and enthalpy changes accompanying the phase transitions of 6DBT are also presented. The configurational part of the entropy change at the S_A–I transition of 6DBT amounts to ≈40%. Using the pVT data and recently published dielectric relaxation results the isochoric activation energy was calculated (being ≈50% of the activation enthalpy) and is compared with analogous results for 6CB, 7CB and 8CB.