Interactions in alkane systems by depolarized Rayleigh scattering and calorimetry. Part 2.—Orientational order in systems containing hexadecane isomers of different structure

Abstract

Depolarized Rayleigh scattering has been used to obtain values of the molecular optical anisotropy (γ²) for the following hexadecane isomers at high dilution in carbon tetrachloride : 2,2,4,4,6,8,8-heptamethylnonane, 6-, 4- and 2-methylpentadecane, 6-pentylundecane and n-hexadecane. The values increase moderately through the series corresponding to increasingly anisotropic molecular shapes. Values of γ² have also been obtained for the hexadecane series in their pure liquid states. These increase rapidly relative to the dilute solution values, indicating the development of short-range orientational order or correlations of molecular orientations. The pure liquid results with 6-, 4- and 2-methylpentadecane show that orientational order depends strongly on the methyl position and length of the straight-chain part of the alkane molecule. Values of γ² for the hexadecane isomers have been found for high dilution in cyclohexane, 2,2-dimethylbutane and n-hexane which indicates that the hexadecanes can correlate their orientations to only a limited extent with these hexane solvents. Heats of mixing have been measured at 25°C through the concentration range for the hexadecane isomers mixed with cyclohexane, 2,2-dimethylbutane and n-hexane. For each hexane, the heats increase through the series of hexadecane isomers. This corresponds to the increase of orientational order in the pure hexadecane isomers and the destruction of order in the hexadecane on mixing with the hexane. The magnitudes and concentration dependences of the heats for the n-C₆+ C₁₆ isomer systems are similar to those for n-C₆+ n-C₁₆ heats at different temperatures. This is explained by changing temperature having the same effect on the order of n-C₁₆ as changing molecular shape on the order of the C₁₆ isomers at 25°C.