Molar volumes of ethylcyclohexane and butyronitrile glasses resulting from vapor deposition: dependence on deposition temperature and comparison to alkylbenzenes.

Molar volumes (Vm) of vapor-deposited ethylcyclohexane (ECH) and butyronitrile (BN, sometimes called butanenitrile) glasses were studied as a function of deposition temperature (Td). ECH glasses deposited at Td sufficiently below their glass-transition temperature (Tg) exhibited changes in Vm on heating similarly to alkylbenzenes. At Td close to Tg, ECH formed dense glasses as alkylbenzenes do, although these glasses were only slightly more dense than its supercooled liquid (SCL) states at the same temperatures. For BN, no indication of the formation of dense glasses was observed even at Td close to Tg, and the variations in Vm with the temperature elevation were different from those of alkylbenzenes. Analysis of the initial Vm of the deposited glasses of different compounds demonstrated that its Td-dependence was well correlated with the steepness index (m) of the corresponding SCL. Quantum-chemical calculations concerning dimer formation by the studied compounds showed that the hydrogen bond between a C-H bond in the alkyl group and π-electrons in the phenyl ring stabilizes the alkylbenzene dimers, suggesting the possibility of the dense glass formation and large m of these compounds. The small m value of BN was also discussed on the basis of the calculation results.