Heats of solution of aliphatic and aromatic, linear, branched, and cyclic alcohols in aqueous sodium dodecyl sulfate micelles at 298°K

Abstract

We have determined the heat of solution ΔH s o of 1-butanol, 1-pentanol, 1-hexanol, 1-heptanol, 1-octanol, 1-nonanol, 2-butanol, 2-pentanol, 3-pentanol, cyclohexanol, benzylalcohol, and phenol in aqueous sodium dodecyl sulfate (SDS) solutions as a function of surfactant concentration. A constant ΔH s o value is obtained in all cases (except 3-pentanol) at higher surfactant concentration corresponding to dilute alcohol molecules in spherical micelles. The standard enthalpy of transfer ΔH t o of the aliphatic alcohols from water to these diluted solutions, corrected for the fraction of solubilized solute, is a decreasing linear function of their molecular surface of contact (MSC). The same linear behavior is found for the corresponding free energy and entropy functions. At the same MSC value ΔH t o is much less positive for the aromatic than for the aliphatic molecules. Comparison of ΔH t o from water to SDS and to 1-octane solutions shows that the specificity observed for the aromatic molecules is three times larger for the transfer to 1-octane than to SDS solutions. These observations support the two-state model of solubilization of aromatic molecules in micelles through surface and interior localization. A parallel situation is pointed out using the magnitude of pairwise thermodynamic coefficients in classical solution chemistry.