Energetic Analysis of Adsorption and Absorption of Small Molecule to Nanodroplet of Water.

Adsorption and absorption were analyzed for nonpolar and polar solutes to a water droplet of nanometer size and to a planar slab. All-atom molecular dynamics simulation was performed, and the free energy change for bringing the solute to the water aggregate was computed over a wide range of temperature. It was seen in both the droplet and slab systems that the solute is preferably located at the surface, and the propensity of the nonpolar solute at the surface relative to the bulk was found to be larger in the droplet than in the slab. A molecular-sized curvature thus enhances the surface propensity of a nonpolar solute, whereas the curvature effect is weaker for polar one. The attractive and repulsive interactions of the solute with water were further analyzed, and the role of the repulsive interaction is discussed with respect to the stability of the surface-bound state.