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Quantifying the statistics of occupancy of solvent molecules in the vicinity of solutes is central to our understanding of solvation phenomena. Number fluctuations in small solvation shells around solutes cannot be described within the macroscopic grand canonical framework using a single chemical potential that represents the solvent bath. In this communication, we hypothesize that molecularsized observation volumes such as solvation shells are best described by coupling the solvation shell with a mixture of particle baths each with its own chemical potential. We confirm our hypotheses by studying the enhanced fluctuations in the occupancy statistics of hard sphere solvent particles around a distinguished hard sphere solute particle. Connections with established theories of solvation are also discussed. [ABSTRACT FROM AUTHOR]

Coordination state probabilities for the [Zn(H2O)n(CH3OH)m]2+ complex in aqueous methanol solutions are calculated as a function of the bulk solution concentration, and the number of methanol ligands, m = 0, 1, ..., 6 with n+m = 6. Zinc ion solvation free energies, which serve to normalize these probabilities, also reproduce the methanol concentration dependence of the experimentally derived free energy of zinc ion transfer from water to aqueous methanol solutions. Coordination state probabilities, p(n, m), are derived by extending quasi-chemical theory of ion hydration to solvent mixtures and mixed ligands. Free energy contributions to p(n, m) include the free energy of forming the mixed-ligand complex in the ideal gas, obtained by quantum chemical calculations, and the solvation free energy of the complex, approximated by a dielectric continuum model. We find that replacing water ligands with methanol ligands preferentially stabilizes methanol-rich complexes in the ideal gas. Conversely, water-rich complexes are stabilized by the solvation free energy contribution, such that the [Zn(H2O)6]2+ complex is the dominant species in solution for all methanol concentrations considered. Stabilization of the methanol-rich complexes is a consequence of the local coordination chemistry, dominated by the delocalization of charge on the zinc ion, while the stabilization of water-rich complexes is a consequence of favorable ion-solvent electrostatic interactions and smaller dielectric cavities for the water-rich complexes at fixed total charge in the dielectric continuum model. Our analysis also highlights an entropic contribution associated with the reversible work required to remove n water and m methanol molecules from bulk solution to form the [Zn(H2O)n(CH3OH)m]2+ complex, which captures the methanol concentration dependence of the solvation free energy of the zinc ion. [ABSTRACT FROM AUTHOR]

Cosolvent hydrophobic preferential interactions withmethane inaqueous methanol solutions are evaluated on the basis of the soluteexcess chemical potential derived from molecular simulations usingthe quasi-chemical (QC) theory generalization of the potential distributiontheorem (PDT). We find that the methane–methanol preferentialinteraction parameter derived from QC theory quantitatively capturesthe favorable solvation of methane in methanol solutions in termsof important local solute–solvent (water and methanol) intermolecularinteractions within a defined inner shell around the solute, and nonlocalsolute interactions with solvent molecules outside this inner shell.Moreover, a unique inner shell can be defined such that the preferentialinteraction parameter is derived exclusively from the free energyof cavity formation in the aqueous cosolvent solution without thesolute, where this cavity corresponds to the specified inner shell,and the mean interaction or binding energy of the solute with solventmolecules outside this inner shell. This inner-shell definition leadsto a description of solute–cosolvent preferential interactionsin which the molecular details of those interactions are derived fromthe effect of cosolvent on cavity statistics in the aqueous cosolventsolution alone. The finding suggests that solution thermodynamic behaviorbeyond steric exclusion (macromolecular crowding) contribute to themolecular mechanisms by which cosolvent preferential interactionsinfluence protein stability and activity. [ABSTRACT FROM AUTHOR]