Effect of NaCl, NaOH, and poly(ethylene oxide) on methane solubilization in sodium dodecyl sulfate solutions

Solubility of methane in aqueous solutions of sodium dodecyl sulfate (SDS), SDS + 0.1 M NaCl, SDS + 0.1 M NaOH, SDS + 0.1 wt % poly(ethylene oxide) (PEO), SDS + 0.1 wt % PEO + 0.1 M NaCl, and SDS + 0.1 wt % PEO + 0.1 M NaOH have been determined at 298.15, 308.15, and 318.15 K. The molality of SDS (m(SDS)) is up to 0.050 mol/kg. The solubility decreases with increasing temperature and increases linearly with concentrations of SDS above the critical micelle concentration (CMC) or critical aggregation concentration(CAC) of the surfactant, indicating that micelles in the solutions solubilize the gas molecules. It was found that the solubilization ability of the micelles bound to PEO and the free micelles is the same. The solubilization property of SDS is changed by the addition of PEG, although the solubilizing effect of the polymer alone is not considerable. NaCl and NaOH also affect the solubilization noticeably. The standard Gibbs energies for the transfer of methane from bulk solutions to the micelles are large negative values, indicating that the hydrophobic gas prefers to exist in the hydrophobic interior of the micelles.