Calorimetric Study of Micelle Formation of Alkylammonium Chlorides in Water

Calorimetric measurements were conducted on aqueous solutions of n-alkylammonium chlorides (C_nAC; H(CH₂)_nNH₃Cl, n = 1 - 12) at 298.15 K. The solute partial molar enthalpy, H_s, was evaluated with reference to the infinitely diluted state. At low concentrations, the H_s increased with the molality, m_t, 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 linearly with increasing log m_t. 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 = 20.1 - 1.4n for C_nAC with n from 7 to 12. The Gibbs energy of micelle formation, ΔG_m, was estimated from the CMC values: ΔG_m / kJ mol^-1 = 4.1 - 3.3 n with n from 5 to 12. The entropy of micelle formation, ΔS_m, was calculated as TΔS_m / kJ mol^-1 = 16.4 + 1.9 n with n from 7 to 12. The large positive entropy term was determined to be the driving force for micelle formation. It was inferred that the difference of the CMC of C_nAC and sodium n-alkyl sulfate (SC_nS;H(CH₂)_nOSO₃Na, n = 5 - 13) homologs seemed to be caused by the difference in the number of possible hydrogen bonds to the head group of the micelle state. Based on an attempt to calculate the H_s for C₁₂AC from the partial molar enthalpy and the concentrations of the constituent individual ions above the CMC, the increase in concentration of the micellar ions seemed to be the largest contribution to the concentration dependence of the H_s.