Study of water-sodium dodecyl sulfate micellar solubilization thermodynamics for several solute homolog series by headspace gas chromatography

We employed headspace gas chromatography to measure water-to-sodium dodecyl sulfate (SDS) micellar partition coefficients for homologous series of n-alkylbenzenes, 1-nitroalkanes, 1-alkanols, and 2-ketones. The free energy of transfer per methylene unit is -612 cal/mol for polar solutes and -534 cal/mol for nonpolar solutes. Henry's law was also explored for the n-alkylbenzenes and 2-ketones and found to hold to total solute mole fractions of at least 7 × 10^-3 and 3.4 × 10^-2 in the SDS micellar phase, respectively. Additionally, we compared water-to-SDS partition coefficients to water-to-bulk organic solvent partition coefficients for methylene units and whole solutes. From these comparisons we conclude that short, polar organic solvents such as methanol and nitromethane make the best models of the solutes' micellar environment and that the solutes and their methylene units are located in the relatively polar palisade and/or Stern layers of the SDS micelles.