Molecular Structure and Orientational Order Effects in Enthalpies and Heat Capacities of Solute Transfer into n-C16 Part 1.—Normal and Branched Alkane Solutes

Abstract

Enthalpies ΔH(b → n) and heat capacities ΔCp(b → n) have been obtained at 25°C for the transfer of solute molecules into normal-C16(n) from the highly-branched-C16, 2,2,4,4,6,8,8,-heptamethylnonane (b). Solutes include normal alkanes from C5 to C20 and the series of highly branched alkanes from 2,2-dimethylbutane to heptamethylnonane and polyisobutylene, as well as siloxanes and nonanes of different degrees of internal steric hindrance. With the Prigogine–Flory theory as background, results are interpreted through enthalpy and heat capacity changes associated with the elimination in b and formation in n of a cavity and of solute–cavity interactions. Interpretation is simplified by the virtual identity of the b and n liquids excepts for the presence in n of short-range orientational order. For a solute incapable of correlating its molecular orientations with those of n, e.g. a branched alkane or a lower n-alkane, ΔH(b → n) and ΔCp(b → n) are large and, respectively, positive and negative. The values are attributed to the destruction of order in forming the cavity in n. For solutes capable of correlating their molecular orientations with those of n, e.g. a longer n-alkane, interaction between the solute and the cavity in n becomes important and both ΔH(b → n) and ΔCp(b → n) change sign to become negative and positive, respectively. It is demonstrated that the orientational order can be described as a pair-wise interaction of short-range character. The order in the pure n-C16 increases intermolecular cohesion, but only slightly compared with the usual dispersion force interaction, contributing ca. 3% of the energy of vaporization at 25°C. However, due to the sensitivity of the order to temperature, the order contribution to the configurational heat capacity of pure n-C16 is ca. 20% of the total.