Salting-out of methane in the aqueous solutions of urea and sarcosine

Abstract

Hydrophobic association and solvation of methane molecules in aqueous solutions of urea and sarcosine (sa) have been studied using MD simulations. The potentials of mean force (PMFs) between methane molecules in water, water-sa, water-urea and water-urea-sa mixtures show an enhancement of methane association on the addition of these osmolytes. These observations are well supported by calculation of equilibrium constants. Calculation of thermodynamic parameters shows that the association of methane is stabilized by entropy and favored by enthalpy. The hydrophobic solvation of methane is stabilized by enthalpy and destabilized by entropy. The calculated solvation free energies of methane in these mixtures show that methane is less soluble in the mixtures of urea and sarcosine than in water. The solubility is the least in the water-urea-sa mixture. Analysis of distributions of solvent and co-solvent around methane suggests that the local densities of both urea and sarcosine are diminished around the methane in the mixtures of these osmolytes. The selective reduction of both urea and sarcosine from methane surface suggests that both urea and sarcosine push methane molecules towards water and increase the interaction between methane molecules i.e., salting-out of methane. Solvation free energies of methane molecules and the potentials of mean force between methanes were computed using molecular dynamics simulations in mixtures of osmolytes. Both urea and sarcosine enhance the association of methane molecules (saltingout) because of the exclusion of urea and sarcosine from the methane surface.