Effects of side chain length and degree of counterion binding on micellization of sodium salts of α-myristic acid alkyl esters in water:: A thermodynamic study

Temperature studies were performed on the micelle formation of a series of anionic surfactants, i.e. Na salts of alpha-sulfonatomyristic acid methyl, ethyl, and propyl esters (abbreviated as alpha-SMy.Me, alpha-SMy.Et and alpha-SMy.Pr, respectively) in water, paying special attention to the effects of the number of carbon atoms in the alkyl chain attached near the hydrophilic group (-SO3) on micellar properties, such as critical micellization concentration (cmc) and the degree of counterion binding (beta) or the degree of electrolytic dissociation of micelles (alpha). The correlation among cmc, beta, and hydrophobicity was discussed in terms of thermodynamic parameters or the changes in the Gibbs free energy, enthalpy, and entropy upon micelle formation (Delta G(m)(o), Delta H-m(o), and Delta S-m(o), respectively). The cmc of the alpha-SMy.series surfactants decreases with increased hydrophobicity, as is expected. The degrees of counterion binding (beta) are ca. 0.7 for alpha-SMy Me, ca. 0.6 for alpha-SMy.Et, and ca. 0.5 for alpha-SMy.Pr in line with increasing hydrophobicity of the side chain, indicating that the larger alkyl group near the headgroup leads to less Coulombic repulsion between headgroups. The absolute value of negative Delta G(m)(o) increases in the order alpha-SMy.Pr < alpha-SMy.Et < alpha-SMy.Me, in parallel with the increasing order of beta.A clear linear relation (the compensation rule) completely holds between Delta H-m(o) and Delta S-m(o) for each surfactant studied, as have been observed for different surfactants.