On Micelle Formation and Degree of Counterion Binding of α-Sulfonatomyristic Acid Ethyl Ester in Water : a Study Performed by Comparing with α-Sulfonatomyristic Acid Methyl Ester

Sodium salts of α-sulfonatomyristic acid ethyl ester (α-SMy.Et) and methyl ester (α-SMy.Me) in aqueous medium were investigated in terms of critical micellization concentration (CMC) and degree of counterion binding to micelles (β) as functions of temperature and added salt concen­tration. For the determination of CMC, the differential conductance (derived from the specific con­ductance, κ as a function of the molarity, (C), ∂κ/∂C being plotted against √C) was applied to the systems with and without added NaCl. The β values were determined from the Corrin-Harkins plot By using the data of CMC and β as a function of temperature, the standard Gibbs energy change (ΔG⁰_m, ) was evaluated on the basis of phase separation model, and from the van’t Hoff plot the standard enthalpy and entropy changes (ΔH⁰_m, and ΔS⁰_m) were evaluated. The enthalpy-entropy com­pensation relation, empirically expressed as ΔS⁰_m = (1/300) ΔH⁰_m + σ, was found to completely hold for both the surfactants and also for other surfactants. The degree of micellar dissociation (α) was examined in terms of the conductivity data and compared with the empirically estimated values (α_EX) assuming α_EX = 1-β.