Sodium dodecyl sulfate induced formation of a messy structure methane hydrate: a molecular dynamics simulation study

Molecular dynamics simulation method is used to study the process of methane hydrate formation induced by sodium dodecyl sulfate (SDS) at a concentration of 28.6 mmol L^-1. We demonstrate the transformation of water molecules from disordered arrangement in liquid state to ordered hydrate state using F3 and F4 order parameters, the number of water molecules in different phase states, and the system energy. Besides, not only are the 5¹² and 5¹²6² type water cages formed, but also some uncommon cages, such as 4¹5¹⁰6² cages, proving that the obtained hydrate is not a standard structure I hydrate. The water molecules layer near SDS cannot form four hydrogen bonds with the outer water molecules, and thus do not contribute to the formation of solid hydrate, but rather appears as liquid. So, some methane cannot be confined in the water cage and form bubbles near the SDS hydrophobic tail chain. Thus, SDS contributes to the formation of methane hydrates at 28.6 mmol L^-1, but the final methane hydrate structure is messy.