Fumio Endo

The distribution of alkyl paraben in aqueous sodium lauryl sulfate micellar solutions: III. The gel filtration of solubilized systems

Abstract The apparent distribution coefficients, K a , for methyl-, ethyl-, and butylparabens between the sodium lauryl sulfate (SLS) micellar and aqueous phases were measured from the elution curves of SLS and alkyl paraben obtained by the tail analysis on Bio Gel P-4. The results show that the K a was a linear function of the molar fraction of alkyl paraben in the micellar phase ( X s m ). In view of this relationship, the distribution coefficients, K t , were estimated from the K a values on the basis of ideal solution theory. The free energy change of a solubilizate for a transition from the aqueous to micellar phase, ΔG t , was calculated from the K t . The solubilized system was analyzed thermodynamically on the basis of the ΔG t . The results show that an approximately linear relationship exists between the ΔG t and the carbon number of the alkyl paraben. Furthermore, the CMC decreasing effect which describes stabilization of micelle on solubilization was confirmed experimentally to be correlated with ΔG t .

The solution state of nonionic surfactant micelles in aqueous solution

Abstract A gas chromatography procedure has been developed for the analysis of hexaoxyethylene n -lauryl ether (C 12 E 6 ), a nonionic surfactant. The sensitivity is about 0.013 mg of C 12 E 6 in 1 ml of aqueous sample. It is simpler than conventional methods. The gas chromatographic method was used in conjunction with tail analysis (analysis of tail parts of elution curves) from Sephadex G-200 to investigate the solution state of nonionic surfactant micelles in aqueous solution. The tail analysis for the elution curve of C 12 E 6 solution gave the elution volume of C 12 E 6 micelle, V m , and the monomer concentration, C f . The V m values decreased with surfactant concentration at low concentrations and became nearly constant at high concentrations. The decrease of V m suggests that the micellar size increases. The C f values increased toward a limiting concentration, as surfactant concentration was increased. These results suggest that nonionic surfactant systems exhibit a continuous self-association at low concentrations. It was furthermore assumed that the monomer concentration and the micellar size are approximately constant at high concentrations. The micellar weight at high concentrations was estimated to be 270,000 by the application of the Laurent-Killander equation.

Effect of alkylparabens on the solution state of sodium lauryl sulfate micelle. IV. Gel filtration of solubilized systems

Abstract Incorporation of methyl-, ethyl-, and butylparabens into sodium lauryl sulfate (SLS) micelle was examined by measurement of ultraviolet spectra, conductance, degree of counterion binding to the micelle, and elution volume of micelles solubilizing alkylparabens. The results show that alkylparabens penetrated only a little into the palisade layer. This phenomenon was closely related to the diluted charge density of micellar surface and the decrease of micellar volume by the solubilization. The factors for stabilization of the micelle by solubilization of alkylparabens were assumed as reduction of the free energy of a micelle due to the diluted surface charge density on a micelle, the entropy of mixing of the mixed micelle, and the decrease of activity coefficient of SLS molecule in the mixed micelle, γ ∗m , which may be considered as geometrical factors. Among these geometrical factors the hydrophobic interaction between alkylparaben and the hydrocarbon part of SLS micelle will be of importance.