Specific ion effects contribute significantly to chemical assembly in aqueous solutions. This talk will discuss a new theoretical approach for computing ion hydration free energies and entropies that partitions the interactions into van der Waals, local electrostatic, and far-field electrostatic contributions.  The partitioning is based on the Local Molecular Field Theory of Weeks and coworkers.  The far-field electrostatic interactions (outside the first hydration shell) display Gaussian behavior, consistent with simple dielectric models.  The local electrostatic contribution requires direct evaluation for accurate free energy estimates.  Hydration entropies computed from the temperature dependence of the free energies display remarkable ion specificity in the local electrostatic contribution, with little or no specificity in the far-field term. These thermodynamic results suggest ion specific effects arise mainly from interactions in the first hydration shell.  Results will also be presented concerning the surface potential of water, and driving forces for ions approaching the water liquid/vapor interface.  Ongoing work involving polarization and charge transfer contributions will also be discussed.

Ion hydration free energies and entropies by energetic partitioning

243rd National Meeting (2012)

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