Hydrophobic hydration of methane

The method of neutron diffraction and isotopic substitution (NDIS) with H/D substitution on both the solute and the solvent was used to determine the hydration structure of a methane molecule (CM4, M = H or D) in terms of partial pair correlation functions g(HM)(r) and g(OM)(r). Analysis of these two functions shows that a methane molecule is surrounded by a shell containing 19(2) water molecules, and provides direct experimental evidence that water molecules in the first coordination shell of apolar molecules are tangentially orientated. There is generally good agreement between these observations and those obtained from the theoretical and computer-based model calculations. However, it is clear that the model calculations over-emphasize the longer range structure around the hydrophobic molecule.