BJ: Do molecules as small as neopentane induce a hydrophobic response similar to that of large hydrophobic surfaces

This is the first of two papers aimed at understanding, from an atomistic perspective, hydrophobicity at different length scales and how properties such as local densities and angular profiles change for hydrophobic solutes of different sizes. In a subsequent publication we will describe the hydrophobic hydration and hydrophobic interaction of platelike molecules of nanoscale size. Molecular dynamics is used to compute radial and orientational distribution functions of water around three different molecules: argon, methane, and neopentane. In addition, the potential of mean force between two neopentane molecules is computed. The results for the full OPLS/AA1 force field are compared with the solute-solvent WCA truncated OPLS/AA force field for these systems. This work addresses the question of whether a molecule of the size of neopentane is large enough to induce a hydrophobic response similar to that of large hydrophobic molecules or paraffin walls. We answer this question in the affirmative. The orientational distribution of water molecules in the first shell neighboring the neopentane molecule is very similar to that near a paraffin wall, in contrast to argon and methane. In addition, the potential of mean force between two neopentane molecules, with the WCA truncated OPLS/AA potential, displays a dewetting-like transition much like that found between two macroscopic hydrophobic objects. We conclude that neopentane defines a length scale for the observation of large-scale hydrophobicity. Smaller molecules fit into a water clathrate, whereas larger molecules force the water to reorganize such that there are dangling OH bonds pointing toward the hydrophobic surface. Large-scale hydrophobicity arises in solute molecules as small as neopentane with diameter (d 5.2 Å). 1