Evidence for immobile transitional state of water in methane clathrate hydrates grown from surfactant solutions

We report studies of the kinetics of growth of methane hydrate from liquid water containing small amounts of surfactant (<500 ppm of sodium dodecyl sulfate, SDS). The kinetics are monitored using simultaneous measurements of the uptake of methane detected by a pressure drop in the gas phase, and either visual observations of the amount of liquid water and solid phase in the reaction vessel, or in situ micro-Raman measurements or in situ NMR measurements. These diagnostics show that the uptake of methane and the conversion of liquid water to a solid phase do not occur simultaneously; the uptake of gas always lags the visual and spectroscopic signatures of the disappearance of liquid water and the formation of solid. The evidence suggests that the SDS causes water to form an intermediate immobile solid-like state before combining with the methane to form hydrate. This growth mechanism is related to the surfactant and disappears for low SDS concentrations (< 25 ppm). (C) 2015 Elsevier Ltd. All rights reserved.