Infrared spectroscopy on the role of surfactants during methane hydrate formation

Abstract

Gas hydrates are ice-like compounds consisting of a rigid water framework hosting small molecules inside crystal cavities. In the present study, a gas hydrate autoclave that enables precise control and observation of temperature and pressure was modified for facilitating in situ mid-infrared spectroscopic studies on the formation of bulk gas hydrates via a polycrystalline silver halide fiber fitted through the vessel serving as active evanescent field sensing element. Methane hydrates were grown inside the autoclave with addition of three different surfactants, i.e., sodium dodecyl sulfate (SDS), dioctyl sodium sulfosuccinate (Aerosol-OT/AOT), and cetylpyridinium chloride (CPC). The accelerating effect of surface-active molecules on the formation of gas hydrates was studied via fiberoptic evanescent field infrared spectroscopy. Thereby, detailed molecular information on the mechanisms of gas hydrate formation and the role of detergents in that process was collected indicating that remaining free guest molecules are in fact trapped within the interstitial water of gas hydrate crystals. Furthermore, the mechanism of gas hydrate formation proposed earlier by our research team for propane could also be confirmed for methane, and for additional detergents thereby leading to a generic mechanism.