Effects of molecular shape on the excess enthalpies and heat capacities of cycloalkane systems

Abstract

Using the Picker flow microcalorimeter, excess enthalpies have been obtained at 25°C for cycloalkanes from c-C₅ to c-C₈ mixed with cyclic alkanes of increasingly anisotropic molecular shape from methylcyclohexane through 1,2-, 1,3- and 1,4-cis- and trans-dimethylcyclohexane isomers to cis- and trans-decalin. The h^E values are positive and small for the globular c-C₆, c-C₇ and c-C₈ mixed with any of the anisotropic cyclics. However, the plate-like c-C₅ gives unexpected negative h^E while excess heat capacities (except for cis-decalin) are positive, whereas non-polar mixtures almost invariably give negative c_p^E. For c-C₅+trans-decalin, c_p^E is found to decrease with increasing temperature. It is suggested that the anomalous results in these systems, h^E–, c_p^E+, dc_p^E/dT–, are due to a hindering of the rotation of the plate-like cyclopentane molecules by the anisotropic cyclics, the resulting order in the solution decreasing on raising the temperature.