Enthalpies of transfer of -CH 2- between water and nonaqueous or mixed aqueous solvents. Comparison of values from n-alcohols and tetraakylammonium halides

Abstract

The transfer enthalpies of - CH 2- moieties, Δ t H θ ( CH 2), have been calculated from the transfer enthalpies of two types of solute, the n-alcohols where there is a single alkyl chain and the tetraalkylammonium ions where four alkyl chains are attached to the central nitrogen atom. It is found that the Δ t H θ ( CH 2) values, between organic solvents, are both independent of the type of solute used in the calculation and the length of the alky chain. In contrast Δ t H θ ( CH 2) to water are chain length dependent, becoming about 0.5 kJ mol −1 more endothermic for each - CH 2- in the alkyl chain. Moreover the Δ t H θ ( CH 2) calculated from the tetraalkylammonium ion data are around 1.8 kJ mol −1 more exothermic than those calculated from the n-alcohol data. The differences found between the Δ t H θ ( CH 2) values into water persist into the mixed aqueous - organic mixed solvents up to relatively high concentrations of the organic cosolvent. In aqueous N,N-dimethylformamide mixtures both sets of Δ t H θ ( CH 2) values have similar composition dependences and pass through maxima at water rich compositions while in the aqueous propan-l-ol system those calculated form the n-alcohol data pass through sharp maxima while those from the tetraalkylammonium ion data have a much broader maximum, or a plateau, in the water rich composition range. These data are interpreted in terms of intramolecular chain - chain interactions in the case of the tetraalkylammonium ions, which is not possible for the n-alcohols.