Micellar Effects on the Hydrolysis Reaction of an Anionic Surfactant in Aqueous Solution

The hydrolysis mechanisms of 2-sulfoalkanoic acid methyl ester salts (methyl ester sulfonate, MES) were studied. Under acidic, neutral, and alkaline conditions, the hydrolysis rates of MES were strongly affected by the state of aggregation and the hydrolysis rate changed at the critical micelle concentration (CMC). Under acidic conditions, the hydrolysis rates of MES were enhanced by micellar formation and correlated quantitatively with the amount of protons bound on the micellar surfaces being measured by the electric conductivity along with fluorescence and hydrogen-ion electrode measurements. MES having longer acyl chain lengths showed higher hydrolysis rates above the CMC because of the many bound protons and the higher degree of counterion binding to the micelle, β. The rates were affected by the type of counterion and followed the order of sodium > potassium > calcium salts of MES. In alkaline conditions, the rates of MES hydrolysis were suppressed to lower values by micelle formation and MES with longer acyl chain lengths showed lower rates above the CMC. Additionally, the hydrolysis rates of MES under neutral conditions were suppressed to a very low value by micelle formation. These mechanisms of inhibitory effect of hydrolysis by the aggregation should be dominated by the electrical repulsive interactions between hydroxyl anions and anionic micellar surfaces, as well as by interference with the penetration of the hydroxyl anion to the ester group in the micelles for alkaline hydrolysis and by interference from water molecules for neutral hydrolysis.