Structure and Orientation of Water and Choline Chloride Molecules around a Methane Hydrophobe: A Computer Simulation Study.

Recent studies have reported manifold industrial applications of aqueous choline chloride (ChCl) solution as an alternative to deep eutectic solvent. ChCl also serves as a protecting co-solvent for proteins by restricting urea to approach the protein surface and thereby maintaining the water structure around the protein. However, a detailed molecular-level picture of the ChCl and water, even in the absence of urea around a representative hydrophobe is largely lacking. This motivates us to probe the effect of varying wt % of ChCl on the occupancy and orientations of the constituents around a representative solute like methane using computer simulations. Accumulation of water molecules and preferential exclusion of ChCl from the surface of methane perturb the tetrahedral geometry of water around it. We find a tangential alignment of the polar part of the ChCl molecules that interact with water, whereas its hydrophobic part is preferentially facing the methane. With an increase in ChCl wt %, a disruption in the tetrahedrality is evident for water molecules accompanied by a reduction in hydrogen bonds between water pairs in the solution. In short, ChCl induces crowding and modifies the microscopic arrangement and hydrogen bonding structure of the water around the methane and beyond.