Role of hydration forces in the properties of electrolyte solutions in the bulk and at interfaces
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Role of hydration forces in the properties of electrolyte solutions in the bulk and at interfaces Maria L. Sushko* and Kevin M. Rosso Physical Sciences Division, Pacific Northwest National Laboratory, Richland, WA 99352, U.S.A. *Email: [email protected] ABSTRACT We present a theoretical approach for modeling electrolyte solutions at interfaces that reaches into the mesoscale while retaining molecular detail. The total Hamiltonian of the system includes interactions arising from density and charge density (ion correlation) fluctuations, direct Coulomb interactions between ions, and at interfaces the image interactions, ion-solid and ionwater dispersion interactions. The model was validated against its ability to reproduce ion activity in 1:1 and 2:1 electrolyte solutions in the 0-2 M concentration range, its ability to capture the ion-specific effect in 1:1 electrolytes at the air-water interface, and solvent structure in a confined environment between hydrophobic surfaces, revealing the central role of ion hydration interactions in specific ion thermodynamic properties in the bulk solutions and at interfaces. The model is readily extensible to treat electrolyte interactions and forces across charged solid-water interfaces. INTRODUCTION Microscopic interactions in electrolyte solutions and electrolyte/solid interfaces play central role in many chemical and biological processes including nucleation, particle mediated crystal growth, self-assembly, and freezing transitions. The challenge for theoretical treatment of microscopic interactions in heterogeneous condensed phase systems is in accurate description of many-body interactions that is valid for a wide concentration range from dilute solutions to the solid state. Empirical potential force-fields that use the concept of pair-wise atom-atom interactions are usually fitted to reproduce either the properties of dilute solutions or the properties of solid state, but are not suitable for the intermediate concentration range or for the description of freezing transitions, except in some very rare cases.1 Mesoscopic theories of electrolyte solutions, on the other hand, treat many-body interactions from first principles, but lack the essential microscopic detail. One breakthrough in the theory of heterogeneous condensed matter systems came with the development of the Fundamental Measure Theory2 (FMT), which linked the theory of many-body interactions and molecular detail. Coupled with a first principles treatment of first and second order electrostatics, FMT became the basis for classical Density Functional Theory (cDFT).3 Here we present a minimum cDFT model for electrolyte solutions in the bulk and at model interfaces that does not rely on ad hoc parameters, such as fitted “hydrated” ion radii, but encompasses all relevant physically meaningful interactions. We focus particularly on revealing the role of hydration forces in modifying ion thermodynamic properties in bulk electrolyte and at the air-water interface. THEORY
We employ classical Density Functional Theory
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