Changes in water structure induced by a hydrophobic solute probed by simulation of the water hydrogen bond angle and radial distribution functions

Abstract

In order to better characterize changes in water structure induced by a hydrophobic solute the oxygen–oxygen and hydrogen–hydrogen radial distribution functions ( g oo ( r), g hh ( r)) and the hydrogen bond angle distribution function p(θ) for water molecules in the first hydration shell of the tetramethyl ammonium (TMA) cation were computed using Monte Carlo simulations. g oo ( r) and g hh ( r) were corrected for the effect of solute volume exclusion on the local solvent density so that intrinsic structural changes independent of local solvent density variations could be detected. Comparison of g hh ( r) of TMA's first hydration shell water with g hh ( r) for bulk water shows subtle but clear evidence of structure formation induced by the ion. These changes in g hh ( r) are very similar to those seen experimentally for larger tetra-alkyl ammonium ions in previous neutron diffraction experiments. Larger changes in p(θ) in the first hydration shell of TMA were seen. Comparison of changes in p(θ) with changes in g oo ( r) and g hh ( r) show that the angle distribution function provides the most sensitive way to analyze water structure changes associated with hydrophobic solvation.