Thermodynamics of micellization of homologous series of n-alkyl methyl sulphoxides and n-alkyl(dimethyl)phosphine oxides

Abstract

Thermodynamic quantities of micellization have been determined for two series of nonionic surface active agents, the n-alkyl methyl sulphoxides (RMSO) from n-hexyl to n-undecyl, and the n-alkyl(dimethyl)phosphine oxides (RDPO) from n-heptyl to n-dodecyl. Enthalpies of micellization (ΔH_m) for the first three homologues in each series were calculated from direct calorimetry and colligative results using the multiple equilibrium model. ΔH_m values for longer chain homologues were obtained from the temperature dependence of the critical micelle concentration. Although ΔH_m is positive for all the homologues it decreases with increasing chain length. Within experimental error ΔG^°_m, ΔH_m and hence ΔS^°_m are linear functions of chain length indicating the equivalence, with respect to micelle formation, of the methylene groups beyond six carbon atoms from the head groups. For both series the decrease per methylene group in ΔH_m is less than half the decrease in ΔG^°_m and hence there is a significant increase in TΔS^°_m with increasing chain length. The change in all three thermodynamic quantities with increasing alkyl chain length for the RMSO and RDPO series of compounds is in reasonable agreement with the predictions of Nemethy and Scheraga based on the solution properties of n-alkanes.