Heats of mixing of tetra-alkyltin compounds with normal and branched alkanes. Order in long alkanes

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-C_n series gives positive Δh_m values which increase rapidly with n. In contrast, any SnR₄ with the very branched alkanes give small or negative Δh_M 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-C_n components. The order being absent in the globular branched alkanes, the Δh_M 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.