Raman spectroscopic studies of surfactant effect on the water structure around hydrate guest molecules

The addition of a small amount of surfactants enhances hydrocarbon enclathration rates without any mechanical agitation, but the role of surfactants has yet to be clarified. This work presents the effect of sodium dodecyl sulfate (SDS) on the water structure around cyclopentane (CP), a sII hydrate former, at the hydrate-water interface. The arrangement of local water molecules was obtained using surface-enhanced Raman spectroscopy (SERS). The Raman spectra reveal that the water structure around CP at the water-hydrate interface is identical to the hexakaidecahedron (5¹²6⁴) cavity of sII hydrates at a SDS concentration higher than 0.087 mM. However, at the CP-water interface, with and without SDS, the water arrangement is clathrate-like. Additionally, we found that the microenvironment of CP in bulk aqueous solutions is close to that in hydrates at a SDS concentration higher than 0.35 mM.