The solubility of gases in solutions containing sodium alkylsulfates of various chain lengths

Abstract

Measurements have been made to determine the solubilities of oxygen, methane, ethane, and propane at elevated pressures in aqueous solutions of various sodium alkylsulfates having alkyl chains ranging from 6 to 12 carbons in length. The solubility of each gas follows Henry's law at all surfactant concentrations. Below the critical micelle concentration, the solubility of each gas is nearly independent of surfactant concentration and approximates the corresponding solubility in water. Above the critical micelle concentration, the solubility of each gas increases linearly with surfactant concentration indicating that micelles act to solubilize gas molecules. Intramicellar solubilities and associated free energy changes are calculated from these latter data. The intramicellar solubility of each gas is found to be considerably less than the solubility of that same gas in paraffinic hydrocarbon solvents. This discrepancy becomes greater as the alkyl chain of surfactant anion shortens and is more pronounced for gases having large partial molar volumes. These trends are interpreted using a simple pseudophase model in which the hydrocarbon-like interior of a micelle is considered to be subject to a Laplace pressure acting across the micelle surface.