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

Abstract

Using the Picker flow microcalorimeter, molar excess enthalpies (hE) and molar excess heat capacities (cEP) have been obtained at 25°C for nonane isomers of different degrees of chain branching mixed with n-octane, and for 2,2,5-trimethylhexane and 3,3-diethylpentane (3,3-DEP) mixed with other normal alkanes and with cycloalkanes. As the degree of steric hindrance within the nonane isomer is increased, hE with n-octane changes from positive to negative and cEP from negative to positive. Values of hE are also anomalously negative for 3,3-DEP + n-Cn with n⩽ 14 and for 3,3-DEP + cyclopentane. Correspondingly, cEP is anomalously positive for 3,3-DEP + n-C6 and for 3,3-DEP + cyclopentane. The results are interpreted in terms of two contributions: (1) destruction of orientational order in the pure n-alkane giving hE > 0, cEP < 0, and (2) a “condensation” of the more freely moving molecule, e.g., n-Cn or cyclopentane onto the sterically-hindered 3,3-DEP resulting in a restriction of movement giving hE < 0, cEP > 0, i.e., a creation of order in the solution.