Sulfonated Castor Oil as an Efficient Biosurfactant for Improving Methane Storage in Clathrate Hydrates

High toxicity and huge foaming are two severe challenges for gas storage strategies based on promoting the gas hydrate formation using surfactants. The present study used castor oil as an eco-friendly resource to develop novel biosurfactants for methane storage. Transmission and scanning electron microscopy, dynamic light scattering, and interfacial tension measurements revealed the surfactant properties of sulfonated castor oil (SCO). In addition, a high-pressure autoclave and a microdifferential scanning calorimeter test unveiled SCO as an effective kinetic hydrate promoter. The results showed that SCO significantly enhanced the rate of methane hydrate formation. A maximum of 76% water-to-hydrate conversion was observed in 0.1 wt % SCO solution under stirring conditions. Pure water, 0.1 wt % SCO, and 0.1 wt % sodium dodecyl sulfate (SDS) solutions allowed 50% conversion to be achieved for 329, 39, and 27 min, respectively. This made the castor oil-based reagent as effective as the well-known kinetic hydrate promoter SDS. Furthermore, the SCO solution's foam ratio and stability were 8.25 and 2.75 times lower than those of SDS. Additionally, SCO showed a more favorable safety profile for humans and the environment as it is toxic to animals in higher concentrations than SDS according to in vivo studies. In addition, a combination of high-pressure DSC, low-temperature powder X-ray diffractometry, and visual analysis of hydrate samples depending on the temperature mode, promoter type, and its concentration revealed that SCO and SDS enhanced hydrate growth by different mechanisms. The loose hydrate mass was squeezed out toward the gas phase in both cases. However, in the case of SDS, the hydrate traditionally climbed the cell walls while SCO seemed to change the wall wettability, which led to the transfer of water into the reaction zone along with the forming hydrate crystals and the domed shape of the hydrate. These findings provide reliable evidence to synthesize efficient and environmentally friendly reagents based on castor oil to improve methane storage in the clathrate hydrate.