Transfer of anesthetics and alcohols into ionic surfactant micelles in relation to depression of krafft point and critical micelle concentration, and interfacial action of anesthetics

Abstract

The Krafft points of sodium dodecyl sulfate (SDS) and sodium tetradecyl sulfate (STS) decreased linearly with the increase of the concentration of added alcohols (1-butanol, 1-pentanol, 1-hexanol, and 1-heptanol). On the basis of a theory which treats the Krafft point as the melting point of the hydrated solid surfactant, the partition coefficients, K, of alcohols between the aqueous and the micellar phases were calculated from the Krafft-point depressions by the aid of thermodynamics. The values of K obtained by the present method were in good agreement with those measured by other methods. The transfer free energies of alcohols from the bulk solution to the SDS and STS micelles, ΔG p o = − RT In K, decreased linearly with the increase of the carbon number of the alcohol. ΔG p o per methylene group was almost the same for both SDS and STS micelles: −2.45 kJ mol −1 for the SDS micelle and −2.51 kJ mol −1 for the STS micelle. However, the values of ΔG p o of the alcohols into the STS micelle were more negative compared with those into the SDS micelle. This means that the more hydrophobic micelles are favored for the transfer of alcohol. The method presented here is generally applicable to determine the micelle/water partition coefficients of solutes which form mixed micelle. The partition coefficients of inhalation anesthetics between the aqueous and SDS micellar phases were methoxyflurane 1320, halothane 1140, and enflurane 990. These values are in the same order of their clinical potencies. The critical micelle concentration (CMC) of SDS was measured by the conductivity method as a function of the added anesthetics. The CMC decreased linearly with the increase of the anesthetic concentration. The decrease of the CMC by anesthetics was correlated to the micelle/water partition coefficient of these anesthetics by means of the relation proposed by Shirahama and Kashiwabara ( J. Colloid Interface Sci. 36, 65 (1971)) , and was ascribed mainly to the increase in entropy of mixing in the micelle due to the solubilization of the inhalation anesthetics. The decrease of CMC by anesthetics was accompanied by a small but unequivocal release of the counterions from the micellar surfaces which indicates the tendency of anesthetics to dehydrate the interface.