Moment Analysis of Solute Permeation Kinetics at an Interface of Mixed Micelles of Anionic and Nonionic Surfactants

Solubilization phenomena of neutral solutes into mixed micelles of anionic and nonionic surfactants were studied by means of micellar electrokinetic chromatography (MEKC). MEKC experiments were conducted by using mixed micelles consisting of sodium dodecylsulfate (SDS) and polyoxyethylene sorbitan monolaurate (Tween 20) or monostearate (Tween 60) as pseudo-stationary phases while changing the concentration of SDS-Tween mixed micelles and the molar fraction of Tween 20 or 60 in mixed micelles. Elution peak profiles measured by MEKC were analyzed on the basis of the moment theory to determine the solute partition equilibrium constant (K-p) and the rate constants of solute permeation at the interface of mixed micelles (k(in) and k(out)). It seems that the values of K-p, k(in), and k(out) are increased by adding Tween 20 or 60 and that the influence of the addition of Tween 60 on their values is more remarkable than that of Tween 20. It was demonstrated that moment analysis of MEKC data is effective for studying solubilization phenomena of surfactant micelles under the conditions that both immobilization and chemical modification (i.e., fluorescence labeling) of solute and micelles are not required.