EFFECT OF SOLVENT DISCRETENESS ON SOLVATION

Solvent discreteness or ''graininess'' is usually considered to affect solvation energetics by modifying the intermolecular structure of the solvent, which in turn modifies its dielectric constant and the solut e-solvent configurations. In this work, we separate the effect of solv ent discreteness from solvent structure and polarity, as well as the a rrangement of solvent particles around the solute. Because it is rathe r diffficult to do this separation with real solutions and their ''rea listic'' models, we utilized translationally fixed dipole lattices, wh ich allow such a separation. The polarity and the dielectric constant of a dipole lattice can be kept invariant as the number density of the dipoles is varied. The lattice spacing represents the degree of discr eteness or coarseness in a lattice. Dipole lattices that polarize acco rding to the continuum prediction, as well as truly interacting dipole lattices, lead to effective cavity sizes that are significantly small er than the geometrically defined exclusion radius (the radius of a sp here around the center of a solute ion into which solvent particles ca nnot penetrate). The results are similar for lattices of opposite micr oscopic polarization tendencies and opposite ferroelectric divergence properties. Placing the solute in an interstitial or substitutional po sition does not cause a qualitative change in the results; increased s olvent discreteness leads to smaller Born radii. To check whether this interesting result is peculiar to dipole lattice representations, we studied various forms of solute-solvent distribution function g(r). We derived a formula that connects g(r) to an effective cavity radius, w ith the approximation that the microscopic polarization follows the co ntinuum prediction. The effective cavity radius is again found to be s maller than the exclusion radius. In agreement with the dipole-lattice analysis, the effective cavity radius decreases with increasing ''gra ininess'' of the solute-solvent pair distribution function; wavier g(r )'s lead to smaller effective cavity sizes. This result has important conceptual and practical implications in solvation modeling. Solvent d iscreteness becomes an important factor in solvation in its own right, distinct from its indirect effects felt through modified solvent prop erties and solute-solvent configurations. Also, in light of the presen t results, it should be possible to develop better parameterization sc hemes for simplified solvation models.