Microenvironment of Cyclopentane At Water-Hydrate Interfaces and In Bulk Aqueous Solutions

Gas hydrates are non-stoichiometric crystalline compounds in which hydrogen-bonded water cavities encage small molecules such as hydrogen, methane, and cyclopentane (CP). Clathrate hydrates have attracted a lot of attention, because they are not only a potential cause of pipeline blockage in oil and gas industries, but also a promising storage medium for natural gas and a possible option for carbon capture and sequestration. However, applications of the hydrate-based technology depend on a solid understanding of the hydrate formation kinetic including both hydrate nucleation and crystal growth. The formation is usually carried out with a small amount of surfactants, e.g., sodium dodecyl sulfate (SDS), to enhance enclathration kinetics in the absence of mechanical agitation. The promoting role of surfactants is still poorly understood, although a lot of effort has been made to clarify the mechanism. This work aims at elucidating the involvement of SDS for hydrate formations semi-directly using surface enhanced Raman spectroscopy (SERS). SERS can identify the changes in molecular vibrations when hydrate formers are blue-shifted (confined) or red-shifted (relaxed). A blue-shift in wavenumber indicates a possible reorganization of water structure around the hydrate former, CP, indicating a hydrate-like state. It was observed that above 0.087 mM SDS, the symmetric vibration of the ring is matched close to hydrate-like frequencies (~890cm-1) in the hydrate-water interface. Even for CP dissolved in bulk SDS solutions, the blue-shift can be found as the SDS concentration increases above 0.35 mM. This blue-shift could be a possible indication for the fast formation and reduced induction time of gas hydrate formations in surfactant solutions.