Calorimetric Study of Micelle Formation of Sodium Alkyl Sulfates in Water

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, H(s), was evaluated referred to infinitely diluted state and compared for the SCnS 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 Hs decreased with increasing log m: H(s) = (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, ΔH(m), which was expressed as ΔH(m) / kJ mol(-1) = 21.3 - 1.7 n for SCnS 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 Hs for SDS (SC₁₂S) from the concentrations and the apparent molar enthalpies estimated for the sodium ion, dodecyl sulfate ion, and the micelles.