Thermobarometry for 4′-n-Octylbiphenyl-4-carbonitrile in Metal Tube Berthelot Method and Polymorphism in Crystalline Phase of 4′-n-Octylbiphenyl-4-carbonitrile Found through Cooling Paths in Negative-Pressure Range

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The Berthelot method has been used for generating negative pressure in a liquid sealed in a container by a temperature cycle. We introduce its thermodynamic principles and experimental procedures for observing the first-order phase transitions between the condensed phases of a thermotropic liquid crystal. In order to check the methodology, we measure the pressure ($P$) versus temperature ($T$) relations of 4′-n-octylbiphenyl-4-carbonitrile (8CB) sealed in a metal tube at various densities using the Berthelot method. Two results are reported: (i) the $P$–$T$ phase diagram of 8CB is drawn in a triangular region bounded by three points, namely, (0.5 °C, 0 bar), (55 °C, +730 bar), and (75 °C, 0 bar) and (ii) the polymorphism of 8CB’s crystalline phase is detected only by cooling paths in the negative-pressure range; two melting points immediately below 10.5 and 15.8 °C, are measured for crystals formed upon cavitation in “super-expanded” smectic phase between $-50$ and $-110$ bar, while the reported melting point, 21 °C, is measured for crystals formed in the supercooled smectic phase under positive pressures. The thermobarometry of thermotropic liquid crystals is feasible by the metal tube Berthelot method, and its extension in the negative-pressure range will be interesting for the study of organic compounds.
2008-07-25