Interactions in alkane systems by depolarized Rayleigh scattering and calorimetry. Part 1.—Orientational order and condensation effects in n-hexadecane + hexane and nonane isomers

Abstract

Molecular optical anisotropies (γ²) have been obtained from depolarized Rayleigh scattering experiments on the 5 hexane isomers and 15 nonane isomers of widely varying degrees of branching. Values of γ² have also been obtained for these molecules at high dilution in carbon tetrachloride. The latter values reflect the anisotropies of the molecular shapes of the isomers while the pure state values also contain a contribution due to correlation of the orientations of neighbouring molecules. Thus, for highly branched, isotropic isomers, the pure and dilute solution γ² values are the same and are small. With increasing molecular anisotropy, the pure state value increases more rapidly than the dilute, the divergence reflecting the increasing correlation of molecular orientations. Values of γ² have been found for n-hexadecane at high dilution in the hexanes and nonanes. They increase with the anisotropy of the solvent molecules and yield increasing values of the J₁₂ parameter, which characterizes the correlation of orientations of the n-hexadecane and solvent molecules. The J₁₂ values are given approximately by the geometric mean, (J₁₁J₂₂)^½, of the order parameters of the pure liquids, indicating that the mixing of two liquids results in a net destruction of orientational order, i.e., ΔJ₁₂=½(J₁₁+J₂₂)–J₁₂ is positive. This change explains the variation in the heats of mixing of n-C₁₆+ hexane systems. Molar heats of mixing have been measured at 25°C through the concentration range for n-C₁₆ with 21 nonane isomers. With increasing nonane branching, Δh_M increases proportionally to ΔJ₁₂. However, when nonane isomers have high degrees of steric hindrance, e.g., 3,3-diethylpentane or 2,3,3,4-tetramethylpentane, a new and large negative contribution is encountered in Δh_M. For example, Δh_M for n-C₁₆+ 3,3-diethylpentane is S-shaped and negative at high nonane concentrations. Values of Δv_M and Δs_M also contain negative contributions for the few systems studied. The effect resembles a “condensation” of the less hindered alkane onto the large sterically hindered nonane, similar to the condensation effect found in cholesterol + lipid systems.