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 methanemolecules in water, water-sa, water-urea and water-urea-sa mixtures show an enhancement of methane associationon the addition of these osmolytes. These observations are well supported by calculation of equilibriumconstants. Calculation of thermodynamic parameters shows that the association of methane is stabilized byentropy and favored by enthalpy. The hydrophobic solvation of methane is stabilized by enthalpy and destabilizedby entropy. The calculated solvation free energies of methane in these mixtures show that methane isless soluble in the mixtures of urea and sarcosine than in water. The solubility is the least in the water-urea-samixture. Analysis of distributions of solvent and co-solvent around methane suggests that the local densities ofboth urea and sarcosine are diminished around the methane in the mixtures of these osmolytes. The selectivereduction of both urea and sarcosine from methane surface suggests that both urea and sarcosine push methanemolecules towards water and increase the interaction between methane molecules i.e., salting-out of methane.
Volume 134, 2022
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