Calorimetric Study of Micelle Formation of Alkylammonium Chlorides in Water

Abstract

Calorimetric measurements were conducted on aqueous solutions of n-alkylammonium chlorides (CnAC; H(CH2)nNH3Cl, n = 1 - 12) at 298.15 K. The solute partial molar enthalpy, Hs, was evaluated with reference to the infinitely diluted state. At low concentrations, the Hs increased with the molality, mt, until the critical micelle concentration (CMC) was reached. In the concentration range between the CMC and about three times the CMC, the Hs decreased linearly with increasing log mt. The Hs observed at the CMC was taken as the enthalpy of micelle formation, ΔHm, which was expressed as ΔHm / kJ mol-1 = 20.1 - 1.4n for CnAC with n from 7 to 12. The Gibbs energy of micelle formation, ΔGm, was estimated from the CMC values: ΔGm / kJ mol-1 = 4.1 - 3.3 n with n from 5 to 12. The entropy of micelle formation, ΔSm, was calculated as TΔSm / 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 CnAC and sodium n-alkyl sulfate (SCnS;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 Hs 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 Hs.