Hydration of apolar solutes of varying size: a systematic study

We report results of a systematic and extensive study on the orientational structure of water molecules in the vicinity of apolar solutes of varying size in infinitely diluted aqueous solutions at ambient conditions. A wide range of sizes of the solute, modelled as a Lennard–Jones particle, has been considered including also the limiting case of a smooth planar wall (i.e. a solute of an infinite diameter). Both the bivariate and monovariate angle distributions are used to describe the orientation of water molecules. It is demonstrated that the commonly used latter method may lead to misleading or even erroneous conclusions. It is found that all three considered models of water, SPC/E, TIP4P and TIP5P, exhibit, unlike some simplified models, qualitatively the same behaviour, i.e. with increasing size of the solute only the water molecules in closest proximity of the solute adopt orientations which sacrifice a possible hydrogen bond, whereas the preferred orientation of water molecules farther from the surface, but yet belonging to the first solvation shell, is independent of the solute size.