Interfacial adsorption of an inhalation anesthetic onto ionic surfactant micelles and its desorption by high pressure

Abstract

The effects of pressure and temperature on the critical micelle concentration (CMC) of sodium dodecylsulfate (SDS) were measured in the presence of various concentrations of an inhalation anesthetic, methoxyflurane. The change in the partial molal volume of SDS on micellization, Δ V m , increased with the increase in the concentration of methoxyflurane. The CMC-decreasing power, which is defined as the slope of the linear plot between ln(CMC) vs. mole fraction of anesthetic, was determined as a function of pressure and temperature. Since the CMC-decreasing power is correlated to the micelle/water partition coefficient of anesthetic, the volume change of the transfer ( ΔV p o) of methoxyflurane from water to the micelle can be determined from the pressure dependence of the CMC-decreasing power. The value of ΔV p o amounts 6.5±1.8 cm 3·mol −1, which is in reasonable agreement with the volume change determined directly from the density data, 5.5±0.6 cm 3 · mol −1. Under the convention of thermodynamics, this indicates that the application of pressure squeezes out anesthetic molecules from the micelle. The transfer enthalpy of anesthetic from water to the micelle is slightly endothermic. The partial molal volume of methoxyflurane in the micelle (112.0 cm 3·mol −1) is smaller than that in decane (120.5 cm 3·mol −1) and is larger than that in water (108.0 cm 3·mol −1). This indicates that the anesthetic molecules are incorporated into the micellar surface region, i.e., the palisade layer of the micelle in contact with water molecules, rather than into the micelle core.