Molecular Solvation Theory of Liquid Interfaces and Electric Interfacial Layers in Nanosystems
Authors: Kovalenko Andriy 
Published: February 2020

Solvation is critical for a broad range of chemistry, renewable energy, and bionanomaterials. At present, there is a need of theoretical solvation models that adequately include solvent molecular structure and predict solvation effects, in particular, in local confinement of nanosystems. Such theoretical models are of great demand to extend the capabilities of molecular simulations by far not sufficient to address the whole spectrum of scales from fast to very slow in large nanosystems of interest. Ornstein-Zernike type integral equation theory of liquids which is based on the first principles of statistical mechanics enables predictive microscopic modelling of chemical species, nanoparticles, and biomolecules in solution. In particular, the 3D-RISM-KH molecular theory of solvation1-3 yields 3D maps of correlation functions of solvent molecules site at a solute (supra)molecule or nanoparticle and then solvation free energy (SFE) analytically. With an input of a classical molecular force field of interaction potentials between explicit solute and solvent molecules (same as in molecular simulation), it accurately predicts the solvation structure, SFE, and electrochemistry of chemical and biomolecular species in solution and soft matter.1-5 3D-RISM-KH consistently accounts for effects of chemical specificities on solvation properties in complex nanosystems, problematic for molecular simulations and continuum solvation models.
Analytical route to the solvation free energy allows multiscale coupling of the 3D-RISM-KH molecular theory of solvation with the KS-DFT, CASSCF, and FMO methods of quantum chemistry, MM, MD, and DPD simulations.1-3

The 3D-RISM-KH theory has been applied to evaluate the solvation structure, thermodynamics, and electric interfacial layer of modified cellulose nanocrystals (CNC) in ambient aqueous NaCl solution at concentration 0.0–0.25 mol/kg.4 Analysis of the 3D maps of Na+ density distributions shows the molecular structure and effect of the electric interfacial layer on effective interactions between charged CNC particles. The method readily treats provides structural models and modeling procedure to study effective forces and phase ordering of CNC suspensions in electrolyte solution.
The 3D-RISM-KH theory was employed to study and validate against experiment the molecular recognition between kaolinite clay and a series of heterocyclic aromatic compounds (HAC) representative of N- and S-containing moieties in bitumen asphaltene macromolecules in toluene solvent.5 From the 3D site density distributions, SFE, and 3D-SFE density, 3D-RISM-KH predicts the arrangement and thermodynamics of adsorption of HAC and toluene on the kaolinite surface. The calculated adsorption enthalpy and loading ratio of phenanthridine-acridine were correlated to experimental Langmuir constant and adsorption loading. This provides insights into the specific interactions of clays, bitumen, and solvents components.
Finally, the replica RISM-KH-VM molecular theory of solvation in nanoporous material reveals the mechanism of sorption and electrochemistry of electrolyte solution in nanoporous electrodes.2,3 The latter is drastically different from a planar electrode interface due to substantial distortion electric interfacial layer of electrolyte ions and solvent molecules by nanoporous confinement and surface functionalities. This interplay of nanoporous confinement and chemical specificities results in solvent-specific wetting and water depletion in hydrophobic nanopores, asymmetry in solvation and adsorption of cations and anions, desalination in hydrophobic nanopores and its reversal with external voltage, and specific adsorption in functionalized nanopores. The method reveals that specific capacitance of nanoporous electrodes is determined by a chemical balance in the Nernst-Planck equation comprising 4 electric interfacial layers, rather than 2 layers for a planar electrochemical capacitor. This includes the potential drop across the Stern layer at the surface of nanopores and the Gouy-Chapman layer averaged thermally and statistically over the nanoporous material, the osmotic term due to a difference in the ionic concentration in the electrodes and bulk solution, and the chemical potentials of sorbed solvated ions statistically averaged over the nanoporous material.
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