Abstract:
Heats of mixing at 25°C of SnR4(R = CH3, C2H5, C4H10) with seven n-alkanes from C5 to C16, three very branched alkanes 2,2,4-trimethylpentane, 2,2,4,6,6-pentamethylheptane, 2,2,4,4,6,8,8-heptamethylnonane and two lightly branched alkanes 2-methylpentane and 2-methylundecane are reported. Except for SnBu4 with n-C5 and n-C6, and SnR4 with the n-Cn series gives positive Δhm values which increase rapidly with n. In contrast, any SnR4 with the very branched alkanes give small or negative ΔhM values which are independent of the number of carbon atoms in the alkane. Intermediate results are obtained with the lightly branched alkanes. The heats indicate that only the alkane exterior of the SnR4 molecules interacts with the alkane component. We associate the positive contribution in the normal alkane systems with destruction, by the spherical SnR4 molecules, of the orientational order existing in the n-Cn components. The order being absent in the globular branched alkanes, the ΔhM are small and negative as required by the difference in free volumes of the components. A quantitative treatment is given in terms of recent free volume theories coupled with an estimate of the orientational order obtained from depolarized Rayleigh scattering results. It is possible that part of the highly negative heats found with tetrabutyltin and the highly branched alkanes, which are unexplained by free volume differences, reflects preferential packing, an effect observed with spherical molecules of different size.
