Molecular dynamics simulation electrolytes

Many molecular dynamics simulations have focused on the electrolyte solution factors and ignored the atomic features of the pore wall. The assumptions of these simulations may match more closely the experiments of inorganic channels, such as track-etched nu-... 

Chapter 15 gives an extensive and detailed review of theoretical and practical aspects of macromolecular transport in nanostructured media. Chapter 16 examines the change in transport properties of electrolytes confmed in nanostructures, such as pores of membranes. The confinment effect is also analyzed by molecular dynamic simulation. 

Calhoun and Voth also utilized molecular dynamic simulations using the Anderson-Newns Hamiltonian to determine the free energy profile for an adiabatic electron transfer involving an Fe /Fe redox couple at an electrolyte/Pt(lll) metal interface. This treatment expands upon their earlier simulation by including, in particular, the influence of the motion of the redox ions and the counterions at the interface. 

What is next Several examples were given of modem experimental electrochemical techniques used to characterize electrode-electrolyte interactions. However, we did not mention theoretical methods used for the same purpose. Computer simulations of the dynamic processes occurring in the double layer are found abundantly in the literature of electrochemistry. Examples of topics explored in this area are investigation of lateral adsorbate-adsorbate interactions by the formulation of lattice-gas models and their solution by analytical and numerical techniques (Monte Carlo simulations) [Fig. 6.107(a)] determination of potential-energy curves for metal-ion and lateral-lateral interaction by quantum-chemical studies [Fig. 6.107(b)] and calculation of the electrostatic field and potential drop across an electric double layer by molecular dynamic simulations [Fig. 6.107(c)]. 

Subject areas for the Series include solutions of electrolytes, liquid mixtures, chemical equilibria in solution, acid-base equilibria, vapour-liquid equilibria, liquid-liquid equilibria, solid-liquid equilibria, equilibria in analytical chemistry, dissolution of gases in liquids, dissolution and precipitation, solubility in cryogenic solvents, molten salt systems, solubility measurement techniques, solid solutions, reactions within the solid phase, ion transport reactions away from the interface (i.e. in homogeneous, bulk systems), liquid crystalline systems, solutions of macrocyclic compounds (including macrocyclic electrolytes), polymer systems, molecular dynamic simulations, structural chemistry of liquids and solutions, predictive techniques for properties of solutions, complex and multi-component solutions applications, of solution chemistry to materials and metallurgy (oxide solutions, alloys, mattes etc.), medical aspects of solubility, and environmental issues involving solution phenomena and homogeneous component phenomena. 

Recent molecular dynamics simulations [28] showed that the commonly held opinion that the interfacial region is depleted of electrolyte ions might not be true, and that some ions might approach the interface. Their accumulation on the interface can be predicted by a suitable negative value for the parameter Bt in the van der Waals interactions of the ions (coupled with an interaction cut-off). However, there are the large negative ions (Cl, Br ) which prefer to accumulate in the vicinity of the interface [28], and not the less-polarizable small and positive ions, as expected from the van der Waals interactions. 

The possible accumulation of negative ions at the air/ water interface was first predicted by Perera and Berkow-itz,8 who found out from molecular dynamics simulations, surprisingly, that the large anions (Cl , Br , and I ) are expelled from water clusters to their interface. Their predictions are supported by the recent large-scale molecular dynamics simulations for the air/water interface of various electrolyte solutions, which reveal that, when the polarization of ions and water molecules is explicitly taken into account, the large anions are accumulated near the interface.9... 

We showed previously that a simple model for the ion-hydration interactions, which separates the ion-hydration forces in a long-range term due to the behavior of water as a continuous dielectric (the screened image force) and a short-range term due to the discreetness of the water molecules (SM/SB), can explain almost quantitatively a number of phenomena related to the electrolyte interfaces.6 In this article, we examined the limitations of the model in predicting the distributions of ions near the air/water interface, by comparison with molecular dynamics simulations. It is clear that the real ion-hydration forces are more complicated than the simple model employed here however, the interfacia] phenomena (including specific ionic effects) can be understood, at least qualitatively, in terms of this simple approach. 

Interface between two liquid solvents — Two liquid solvents can be miscible (e.g., water and ethanol) partially miscible (e.g., water and propylene carbonate), or immiscible (e.g., water and nitrobenzene). Mutual miscibility of the two solvents is connected with the energy of interaction between the solvent molecules, which also determines the width of the phase boundary where the composition varies (Figure) [i]. Molecular dynamic simulation [ii], neutron reflection [iii], vibrational sum frequency spectroscopy [iv], and synchrotron X-ray reflectivity [v] studies have demonstrated that the width of the boundary between two immiscible solvents comprises a contribution from thermally excited capillary waves and intrinsic interfacial structure. Computer calculations and experimental data support the view that the interface between two solvents of very low miscibility is molecularly sharp but with rough protrusions of one solvent into the other (capillary waves), while increasing solvent miscibility leads to the formation of a mixed solvent layer (Figure). In the presence of an electrolyte in both solvent phases, an electrical potential difference can be established at the interface. In the case of two electrolytes with different but constant composition and dissolved in the same solvent, a liquid junction potential is temporarily formed. Equilibrium partition of ions at the - interface between two immiscible electrolyte solutions gives rise to the ion transfer potential, or to the distribution potential, which can be described by the equivalent two-phase Nernst relationship. See also - ion transfer at liquid-liquid interfaces. 

Describe the basic principles of the Monte Carlo and molecular dynamic simulation methods applied to electrolytes. How are the adjustable parameters determined ... 

Geiger A. Molecular dynamics simulation study of the negative hydration effect in aqueous electrolyte solutions. Ber. Bunsenges. Phys. Chem. 1981 85 52-63. 

P.B. Balbuena, K.P. Johnston and P.J. Rossky, Molecular dynamics simulation of electrolyte solutions in ambient and supercritical water. 1. 

P. B. Balbuena, K. P. Johnston, and P. J. Rossky, Molecular Dynamics Simulation of Electrolyte Solutions in Ambient and Supercritical Water I. Ion Solvation, J. Phys. Chem. 100,2706-2715 (1996). 

In the present paper, the method which the authors employed previously to derive an expression for the solubility of various proteins in aqueous solutions, has been extended to the solubility of gases in mixtures of water + strong electrolytes. One parameter equation for the solubility of gases has been derived, which was used to represent the solubilities of oxygen, carbon dioxide and methane in water -i- sodium chloride. In additions, the developed theory could be used to examine the local composition of the solvent around a gas molecule. The results revealed that the oxygen, carbon dioxide and methane molecules are preferentially hydrated in water-i-sodium chloride mixtures. A similar result was obtained for the water -i- methane -i- sodium chloride by molecular dynamics simulations [72]. 

Typical systems contain between several hundred and a few thousand water molecules. The number of ions is much smaller. In practice, it is determined by two contradicting requirements, namely (i) the need to achieve small concentrations, as they are typical for most experiments, and (ii) the statistical efficiency of the simulation, which requires the average over as many ions as possible. As a compromise, electrolyte concentrations are usually in the range between 0.5 and 3mol/l. As more powerful computers become available and thus the simulation of larger systems becomes possible, systems at lower concentrations can be investigated. Typical time scales for Molecular Dynamics simulations reach up to about 2 nanoseconds at present. 

E. Spohr, P. Commer, and A. A. Kornyshev, Enhancing Proton Mobility in Polymer Electrolyte Membranes Lessons from Molecular Dynamics Simulations, Journal of Physical Chemistry B, 106,10560 (2002). 

Adamson AW (1990) Physical chemistry of surfaces, 5th ed. Wiley, New York, p 101 ff Alejandro 1, Tddesley DJ, Chapela GA (1995) Molecular dynamics simulation of the orthobaric densities and surface tension of water. J Chem Phys 102 4574—4583 Allen HC, Gregson DE, Richmond DL (1999) Molecular structure and adsorption of dimethyl sulfoxide at the surface of aqueous solutions. J Phys Chem B 103 660-666 Aveyard R, Saleem SM (1976) Interfacial tensions at alkane-aqueous electrolyte interfaces. J Chem Soc, Faraday Trans 1(72) 1609-1617... 

Vatamanu J, Borodin O, Smith GD (2010) Molecular dynamics simulations of atomically flat and nanoporous electrodes with a molten salt electrolyte. Phys Chem ChemPhys 12 170-182... 

The use of molecular dynamics to study the electric double-layer structure started a little over a decade ago, with the hope of determining more accurate structures because the classical description of an electric double layer based on the Poisson-Boltzmann equation is accurate only for low surface potential and dilute electrolytes. The Poisson-Boltzmann equation only considers the electrostatic interactions between the charged surface and ions in the solution, but not the ion-ion interactions in the solution and the finite molecule size, which can be taken into account in molecular dynamics simulations. It was shown [6, 7] that the ion distribution in the near-wall region could be significantly different from the prediction of classical theory. Typical molecular dynamics simulation results of counterion and co-ion concentrations in a nanochannel are shown in Fig. 2a. The ion distribution obtained... 

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