Simulation techniques, liquid-solid interfaces

Molecular modeling is another attractive approach that can provide necessary insight to the phenomena near surfaces. In this chapter, we illustrate methods that are commonly used for the study of wettability on solid surfaces. We begin with the thermodynamics of liquid-solid interface in the next section followed by simulation techniques and some illustrative examples. 

One of the fundamental problems in chemistry is understanding at the molecular level the effect of the medium on the rate and the equilibrium of chemical reactions which occur in bulk liquids and at surfaces. Recent advances in experimental techniques[l], such as frequency and time-resolved spectroscopy, and in theoretical methods[2,3], such as statistical mechanics of the liquid state and computer simulations, have contributed significantly to our understanding of chemical reactivity in bulk liquids[4] and at solid interfaces. These techniques are also beginning to be applied to the study of equilibrium and dynamics at liquid interfaces[5]. The purpose of this chapter is to review the progress in the application of molecular dynamics computer simulations to understanding chemical reactions at the interface between two immiscible liquids and at the liquid/vapor interface. 

Many of the important chemical applications of ILs will occur at solid surfaces, including electrochemical processes at IL-electrode interfaces, lubrication of ILs, fabrication of IL solid electrolytes and IL solid catalysts, etc. When a solid interface is present, molecules near the interface are subject to diflferent interactions than in the bulk phase, and the free energy of a surface can often be reduced by local changes in molecular orientation, aggregation, density, or composition. Familiar examples include surface adsorption, wetting and the electrochemical double-layer structure, where dipole moments usually lie at the interface. The surfaces of ionic liquids at the solid surface show dramatic changes in local structure, which can be demonstrated using simulations and probed by a number of experimental techniques. Due to a wide variety of experimental, theoretical, and... 

The availability of thermodynamically reliable quantities at liquid interfaces is advantageous as a reference in examining data obtained by other surface specific techniques. The model-independent solid information about thermodynamics of adsorption can be used as a norm in microscopic interpretation and understanding of currently available surface specific experimental techniques and theoretical approaches such as molecular dynamics simulations. This chapter will focus on the adsorption at the polarized liquid-liquid interfaces, which enable us to externally control the phase-boundary potential, providing an additional degree of freedom in studying the adsorption of electrified interfaces. A main emphasis will be on some aspects that have not been fully dealt with in previous reviews and monographs [8-21]. 

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. 

The following five chapters deal with problems associated with solid phases, in some cases involving surface and interfacial problems. In Chapter 14, Steele presents a review of physical adsorption investigated by MD techniques. Jiang and Belak describe in Chapter 15 the simulated behavior of thin films confined between walls under the effect of shear. Chapter 16 contains a review by Benjamin of the MD equilibrium and non-equilibrium simulations applied to the study of chemical reactions at interfaces. Chapter 17 by Alper and Politzer presents simulations of solid copper, and methodological differences of these simulations compared to those in the liquid phase are presented. In Chapter 18 Gelten, van Santen, and Jansen discuss the application of a dynamic Monte Carlo method for the treatment of chemical reactions on surfaces with emphasis on catalysis problems. Khakhar in... 

In their efforts to improve and characterize materials, specialists have made use of a large variety of physical techniques that yield precise information about not only bulk properties but also solid surfaces. Such information coupled with the use of computer graphics and simulation can lead to new classes of materials, with novel structures as well as chemical, electrical, magnetic, or mechanical properties. Techniques of materials characterization have undergone a dramatic change in the last few years. Present-day electron microscopes have atomic resolution enabling the study of materials at the atomic level under real conditions (in air, with a liquid interface, in a vacuum, etc.). These are reasons to reconsider the structure morphology-properties relationships. 

The CFD model simulates particle-droplet interaction in the surrounding gas by solving the incompressible Navier—Stokes (N—S) equations coupled with the Volume of Fluid (VOF) method, six Degrees of Freedom (6-DOF) method, and dynamic mesh technique [42]. A solid particle is represented as a rigid body with 6-DoF motion, and the gas—liquid interface is described by the VOF method. A body-attached mesh, which follows the body motion, is used for rigid body motion simulation. The CFD model has been developed in the open-source CFD code OpenFOAM. 

The interface between a solid electrode and a liquid electrolyte is a complicated many-particle system, in which the electrode ions and electrons interact with solute ions and solvent ions or molecules through several chatmels of interaction, including forces due to quantum-mechanical exchange, electrostatics, hydrodynamics, and elastic deformation of the substrate. Over the last few decades, surface electrochemistry has been revolutionized by new techniques that enable atomic-scale observation and manipulation of solid-liquid interfaces, yielding novel methods for materials analysis, synthesis, and modification. This development has been paralleled by equally revolutionary developments in computer hardware and algorithms that by now enable simulations with millions of individual particles, so there is now significant overlap between system sizes that can be treated computationally and experimentally. 

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