The nucleation and growth of methane hydrates in the presence of phenylalanine and tryptophan: a comparative molecular dynamics simulation study

Abstract

A series of nanoscale molecular dynamics simulations are carried out at 270 K and 500 bar to explore the nucleation of hydrates using a mixture of water, methane and amino acids. Different concentrations of phenylalanine and tryptophan are used to study the nucleation and growth of methane hydrates. It is found that both amino acids promote the nucleation and growth of methane hydrates at lower concentrations. The formation of different types of rings, partially and fully formed cages formed between water molecules are analyzed. Hydrogen bond analysis revealed a decrease in the number of hydrogen bonds formed between water molecules in the presence of amino acids. Each molecule of phenylalanine and tryptophan forms five to six hydrogen bonds with water molecules. The formation of hydrate cages is analyzed using the F₄ structural order parameter, which indicated the co-existence of the hydrate cages as well as the liquid phase. The analysis of the radial distribution function (RDF) obtained for different concentrations of phenylalanine and tryptophan confirmed the encapsulation of methane molecules inside various water cages. The studies revealed that, at high concentrations of phenylalanine and tryptophan, the amino acids act as an inhibitor delaying both the nucleation and the growth of the hydrates. The formation of the cages and the encapsulation of methane molecules are further supported using the mean square displacement of methane molecules.