Calorimetric distinction between the inter- and intra-molecular rearrangement processes in alkylcyclohexane liquids by means of spatial confinement

Abstract

We report an investigation, by means of adiabatic calorimetry, on the thermal properties of ethylcyclohexane (ECH) confined to within silica-gel mesoscopic pores with an average pore size of 6 nm. As a result of confinement, crystallization was circumvented within the entire temperature range. The devitrification behavior of the confined glass was observed to mainly proceed in a two-step manner and these steps were attributed to the existence of distinct relaxation processes involving molecules in the inner-pore and pore-wall-interface regions. Additionally, two secondary relaxations, of smaller magnitudes, were identified: (1) one occurring within the glassy state, below the primary devitrification process; and (2) another occurring within the supercooled liquid state. Revisit of the data of similar investigations on crystallization-circumvented methylcyclohexane (MCH) indicated the existence of a relaxation process analogous to this latter one (2). The characteristic temperatures for the latter process in both ECH and MCH were found to be independent of the degree of confinement. On the other hand, the former secondary relaxation (1) occurring within the glassy state of ECH displayed pore-size dependent behavior. We interpreted the behavioral difference, between the two processes, as associated with the distinct origins of either intermolecular or intramolecular processes; namely, the relaxation (1) as ascribed to the so-called β-relaxation commonly observed in glasses and the relaxation (2) to the molecular rearrangement modes involving the positional conversion, of the substituted alkyl-group, between the axial and equatorial configurations. The energy difference between the axial and equatorial states inferred from the magnitude of heat-capacity jump at the glass transition temperature was rationalized to be (7.7 ± 0.3) kJ mol−1.