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 hE 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 hE while excess heat capacities (except for cis-decalin) are positive, whereas non-polar mixtures almost invariably give negative cpE. For c-C5+trans-decalin, cpE is found to decrease with increasing temperature. It is suggested that the anomalous results in these systems, hE–, cpE+, dcpE/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.