Abstract
Calorimetric measurements were conducted on aqueous solutions of sodium alkyl sulfates (SCnS; H(CH2)nOSO3Na, n = 5–13) at 298.15 K. The solute partial molar enthalpy, Hs, was evaluated referred to infinitely diluted state and compared for the SCnS homologues. At low concentrations, the Hs increased with the molality m until the critical micelle concentration (CMC) was reached. In the concentration range between the CMC and about three times the CMC, the Hs decreased with increasing log m: Hs = (17.9 - 1.8 n) - 14/(n-4.6) log m. The Hs observed at the CMC was taken as the enthalpy of micelle formation, ΔHm, which was expressed as ΔHm / kJ mol–1 = 21.3 - 1.7n for SCnS with n from 7 to 13. The Gibbs energy of micelle formation, ΔGm, was estimated from the CMC values: ΔGm / kJ mol–1 = 0.2 - 3.1 n. The entropy of micelle formation, ΔSm, was calculated as TΔSm / kJ mol–1 = 21.1 + 1.4 n. The large positive entropy term was the driving force for micelle formation. An attempt was also made to calculate the concentration dependence of Hs for SDS (SC12S) from the concentrations and the apparent molar enthalpies estimated for the sodium ion, dodecyl sulfate ion, and the micelles.
