Surface tensions of aqueous solutions of sodium alkyl sulfates in contact with methane under hydrate-forming conditions

This paper presents experimental data on the surface tensions of aqueous solutions of sodium alkyl sulfates (ionic surfactants) in contact with methane at a pressure of 3.90 MPa and a temperature of 275 K (i.e., a condition in which a clathrate hydrate of methane is thermodynamically stable). These data were obtained in the metastable absence of any hydrate in the experimental system (i.e., every measurement was accomplished during the induction time for hydrate formation). Three sodium alkyl sulfates appreciably different in length of the hydrophobic radicals were used-they were sodium dodecyl sulfate (SDS), sodium tetradecyl sulfate (STS), and sodium hexadecyl sulfate (SHS). The concentration of each of these surfactants was varied over a range including the critical concentration above which the surface tension levels off. On the basis of visual observations of pendant drops of SDS solutions, we identified the critical concentration for SDS as the solubility above which a hydrated solid of SDS forms instead of the critical micelle concentration (CMC) above which micelles of SDS would form. The SDS solubility was thus determined to be (2.2 to 2.3) g center dot kg(-1), which agrees, within mutual uncertainties, with the CMC determined at a higher temperature, 293 K, either in the same pressurized methane ambience or in the air under atmospheric pressure. These results completely conflict with those reported by Sun et al. in their recent paper published in this Journal.