Computer simulation studies on the solvation of aliphatic hydrocarbons in 6.9 M aqueous urea solution

Abstract

We report solvation free energies for six aliphatic hydrocarbons in 6.9 M urea–water mixture obtained by molecular dynamics simulations. Hydrocarbon transfer free energies from water to the urea solution are also presented. Our calculations predict that, except for methane, aliphatic hydrocarbons are more soluble in 6.9 M urea than in water, in satisfactory agreement (deviations smaller than 2 kJ mol⁻¹) with experimental transfer free energies reported in the literature. An analysis of solute–solvent contributions to the solvation enthalpies and entropies indicates that urea enhances the solvation of hydrocarbons compared to pure water due to a favourable van der Waals interaction with the solute whereas the solute–solvent entropy opposes the hydrocarbon transfer. Radial distribution functions between the solute and the solvent are examined and together with an analysis based on Kirkwood–Buff theory indicate a weak preferential urea–hydrocarbon binding. The entropic penalty related with solute urea association is discussed in terms of the molecular interactions in solution.