Enthalpies of Solution of C-Alkylresorcin[4]arenes in Water and Acetonitrile: Hydrophilic and Hydrophobic Effects.

Enthalpies of solution, Δ H, of six C-alkylresorcin[4]arene ( C-ARA) solids, with alkyl substituents varying systematically from methyl (Me) to hexyl (Hx), have been measured in water, using solubility data, and in acetonitrile, by direct calorimetry. For all of the C-ARAs studied, the values of Δ H were highly favorable (exothermic) for dissolution into water but were strongly unfavorable (endothermic) for dissolution into acetonitrile (ACN). The differences between the two solvents varied systematically with increasing carbon chain length, from about 100 kJ·mol (for Me) to 140 kJ·mol (for Hx). These extraordinary variances can be attributed to the loss of hydrophilic hydration of the eight -OH groups on the rim of the C-ARAs and also by the loss of (enthalpically favorable) hydrophobic hydration of the alkyl chains upon transfer from the highly structured, strongly H-bonding water to the aprotic, relatively weak donor/acceptor ACN. Although Δ H values for the present C-ARAs in HO are strongly favorable, they are more than counteracted by even larger negative changes in the entropy of dissolution, Δ S. This enthalpy/entropy compensation effect is consistent with the low aqueous solubilities (<10 mol·kg) of the C-ARAs and their slight increase with increasing carbon chain length, which is opposite to typical behavior of the homologous series of organic molecules.