Calorimetric Study of Micelle Formation of Sodium Alkyl Sulfates in Water

抄録

Calorimetric measurements were conducted on aqueous solutions of sodium alkyl sulfates (SC_nS; H(CH₂)_nOSO₃Na, n = 5–13) at 298.15 K. The solute partial molar enthalpy, H_s, was evaluated referred to infinitely diluted state and compared for the SC_nS homologues. At low concentrations, the H_s 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 H_s decreased with increasing log m: H_s = (17.9 - 1.8 n) - 14/(n-4.6) log m. The H_s observed at the CMC was taken as the enthalpy of micelle formation, ΔH_m, which was expressed as ΔH_m / kJ mol^–1 = 21.3 - 1.7n for SC_nS with n from 7 to 13. The Gibbs energy of micelle formation, ΔG_m, was estimated from the CMC values: ΔG_m / kJ mol^–1 = 0.2 - 3.1 n. The entropy of micelle formation, ΔS_m, was calculated as TΔS_m / 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 H_s for SDS (SC₁₂S) from the concentrations and the apparent molar enthalpies estimated for the sodium ion, dodecyl sulfate ion, and the micelles.