Thermodynamic effects of orientation order in chain-molecule mixtures. Part 3.—Heats of mixing of dimethylsiloxanes with normal and branched alkanes

Heats of mixing at 25°C have been obtained calorimetrically for dimethylsiloxane + alkane mixtures, where component 1 is hexamethyldisiloxane, decamethyltetrasiloxane, polydimethylsiloxane or octamethylcyclotetrasiloxane. Component 2 is a normal-Cn n= 6, 8, 12 or 16, or a corresponding branched-Cn,: 2,2-dimethylbutane, 2,2,4-trimethylpentane, 2,2,4,6,6-pentamethylheptane or 2,2,4,4,5,6,8,8- heptamethylnonane. For the four dimethylsiloxane + n-Cn series, the molar heats and heats per unit volume are positive and increase rapidly with n. With the four branched-Cn series, the heats are much smaller and, expressed per unit volume, vary less with n. Values of the X12 and X12/s1 parameters are derived, and have a similar dependence on n. The results are explained through an energetic effect associated with a correlation of molecular orientations in the n-Cn, but which is absent in the branched-Cn and the dimethylsiloxanes. This is consistent with depolarized Rayleigh scattering results of Bothorel and Clement and with previous heats of mixing for systems containing n-Cn and branched-Cn.