Size dependent ion hydration, its asymmetry, and convergence to macroscopic behavior

The packing and orientation of water molecules in the vicinity of solutes strongly influence the solute hydration thermodynamics in aqueous solutions. Here we study the charge density dependent hydration of a broad range of spherical monovalent ionic solutes (with solute diameters from approximately 0.4 nm to 1.7 nm) through molecular dynamics simulations in the simple point charge model of water. Consistent with previous experimental and theoretical studies, we observe a distinct asymmetry in the structure and thermodynamics of hydration of ions. In particular, the free energy of hydration of negative ions is more favorable than that of positive ions of the same size. This asymmetry persists over the entire range of solute sizes and cannot be captured by a continuum description of the solvent. The favorable hydration of negative ions arises primarily from the asymmetric charge distribution in the water molecule itself, and is reflected in (i) a small positive electrostatic potential at the center of a neutral solute, and (ii) clear structural (packing and orientation) differences in the hydration shell of positive and negative ions. While the asymmetry arising from the positive potential can be quantified in a straightforward manner, that arising from the structural differences in the fully charged states is difficult to quantify. The structural differences are highest for the small ions and diminish with increasing ion size, converging to hydrophobiclike hydration structure for the largest ions studied here. We discuss semiempirical measures following Latimer, Pitzer, and Slansky [J. Chem. Phys. 7, 108 (1939)] that account for these structural differences through a shift in the ion radius. We find that these two contributions account completely for the asymmetry of hydration of positive and negative ions over the entire range of ion sizes studied here. We also present preliminary calculations of the dependence of ion hydration asymmetry on the choice of water model that demonstrate its sensitivity to the details of ion-water interactions.