Volumetric study of n-octyloxy-cyanobiphenyl (8OCB)

Abstract

Volumetric measurements were carried out on n-octyloxy-cyanobiphenyl (8OCB) over temperatures from ambient to 150°C at pressures up to 200 MPa, encompassing the liquid, nematic and smectic liquid crystalline, and crystal states. From the step changes in volume at the transitions, the phase boundaries and the associated activation enthalpies and energies were determined. The results are consistent with prior studies that were limited to a narrower range of thermodynamic conditions. The thermodynamic potential parameter, Γ, found for the isotropic–nematic transition, =3.15, is significantly smaller than the scaling exponent γ (=4.4) that superposes the longitudinal relaxation times in the nematic state. From this non-equivalence, we conclude that the order parameter must vary with pressure along the clearing line. By comparing the phase behaviour of the nematic state for isochoric and isobaric conditions, the relative contribution of volume was determined to be less that the effect of thermal energy on the thermodynamic stability.

Keywords:

• specific volume
• high pressure
• interaction potential
• 8OCB

Acknowledgements

DR acknowledges a National Research Council postdoctoral fellowship. The work at NRL was supported by the Office of Naval Research. The authors thank Prof. Albert Würflinger for providing the dielectric relaxation times in numerical form and for valuable remarks.