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 (h^E) and molar excess heat capacities (c^E_P) 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, h^E with n-octane changes from positive to negative and c^E_P from negative to positive. Values of h^E are also anomalously negative for 3,3-DEP + n-C_n with n⩽ 14 and for 3,3-DEP + cyclopentane. Correspondingly, c^E_P 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 h^E > 0, c^E_P < 0, and (2) a “condensation” of the more freely moving molecule, e.g., n-C_n or cyclopentane onto the sterically-hindered 3,3-DEP resulting in a restriction of movement giving h^E < 0, c^E_P > 0, i.e., a creation of order in the solution.