Schematic representation of the quasichemical approach to solvation.
Free energy cost for creating an oxygen unoccupied cavity of size , , in NPT simulations of AMOEBA or SPC/E water models.
Ion-water(hydrogen) and ion-water(oxygen) radial distribution functions for the , , , and ions modeled with the AMOEBA force field.
Effect of polarizability on the energy of collapsing the inner-shell partition. For ions of all sizes (first index: , , ), the curve shifts downward with increasing polarizability (second index: , , , ) indicating that increased polarization causes water to pack more closely.
Contributions to the OS-LR conditional free energy average at . As explained in the text, the components are averages over the coupled and uncoupled cases. The corresponding contributions from a nonpolarizable system are shown as single points. In the upper left-hand figure, the upper curves for each ion are the total hydration free energies obtained by adding the inner-shell and outer-shell packing contributions.
Electrostatic potential at the center of a test solute in NPT simulations of AMOEBA or SPC/E water models: (lower section) hard spheres of size , (upper section) and ions (left and right of plot, respectively).
Effect of size and polarizability on the total electrostatic contribution to the OS-LR free energy component. Decreases in size and, to a lesser extent, increases in polarizability lead to more favorable interactions. Interestingly, variations in the ion sizes produce about a 5 kcal/mol separation when compared at equal nominal radii, contradicting the Born model prediction. Labels are as in Fig. 4.
Effect of size and polarizability on the solvation shell organization. Plotting the projection of the average center of mass of the closest n solvent waters onto a vector in the direction of the first three shows how quickly subsequent layers of solvent relax to an isotropic distribution. Labels are by polarizability parameter with the exception of the lower right panel, which shows uncharged, nonpolarizable solutes with the same vdW parameters as the corresponding ions.
Summary of the systems modeled in the present study. The labels are defined in the text.
Partial molar hydration quantities for whole salts at infinite dilution, in kcal/mol and . Numbers in parentheses are one standard deviation errors.
Comparison of local solvation environment indicators: , the number of waters required for Fig. 8 to reach zero, average distance to the closest water oxygen (Å), average ion dipole moment (Debye), average first-shell water dipole moment (Debye), and single-ion total hydration free energy (kcal/mol). Numbers in parentheses indicate numerical uncertainty in the last digit. The label indicates an uncharged solute.
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