Chemical reaction at liquid interface

Kinetics of chemical reactions at liquid interfaces has often proven difficult to study because they include processes that occur on a variety of time scales [1]. The reactions depend on diffusion of reactants to the interface prior to reaction and diffusion of products away from the interface after the reaction. As a result, relatively little information about the interface dependent kinetic step can be gleaned because this step is usually faster than diffusion. This often leads to diffusion controlled interfacial rates. While often not the rate-determining step in interfacial chemical reactions, the dynamics at the interface still play an important and interesting role in interfacial chemical processes. Chemists interested in interfacial kinetics have devised a variety of complex reaction vessels to eliminate diffusion effects systematically and access the interfacial kinetics. However, deconvolution of two slow bulk diffusion processes to access the desired the fast interfacial kinetics, especially ultrafast processes, is generally not an effective way to measure the fast interfacial dynamics. Thus, methodology to probe the interface specifically has been developed. 

I. Benjamin, Chemical reactions and solvation at liquid interfaces a microscopic perspective, Chem. Rev. (Washington, D. C.), 96 (1996) 1449-75 I. Benjamin, Theory and computer simulations of solvation and chemical reactions at liquid interfaces, Acc. Chem. Res., 28 (1995) 233-9 L. R. Martins, M. S. Skaf and B. M. Ladanyi, Solvation dynamics at the water/zirconia interface molecular dynamics simulations, J. Phys. Chem. B, 108 (2004) 19687-97 J. Faeder and B. M. Ladanyi, Solvation dynamics in reverse micelles the role of headgroup-solute interactions, J. Phys. Chem. B, 109 (2005) 6732 10 W. H. Thompson, Simulations of time-dependent fluorescence in nano-confined solvents, J. Chem. Phys., 120 (2004) 8125-33. 

Molecular dynamics simulations of chemical reactions at liquid interfaces... 

Although our focus in this chapter is on chemical reactions at liquid interfaces, it is important to discuss the unique properties of the liquid interfacial region that are relevant to the goal of understanding chemical reactivity. 

Chemical reactions at liquid interfaces occur between solvated species. The solvated species may be adsorbed at the interface, or their presence Aere may be a relatively rare event. Similarly, the products of the reaction may be adsorbed at the interface, or they may diffuse to the bulk of one or both liquids (in the case of... 

Chemical reactions at liquid interfaces exhibit remarkable patterns. Chemical reactions are necessary for structure formation, and hydrodynamic and diffusion effects in the absence of reaction could not generate these patterns. However, different types of reactions led qualitatively to the same result (Avnir et al., 1984). Additionally, surface-driven convection might have a crucial role for the onset of convection patterns in chemically active medium (Muller et al., 1985). 

Spatial Structures Formed by Chemical Reactions at Liquid Interfaces Phenomenology, Model Simulations, and Pattern Analysis... 

INTRODUCTION. A remarkably wide-scope phenomenon has recently been revealed. Chemical reactions at liquid interfaces proceed in a patterned way spectacular structures form and grow while matter or energy influx are maintained [1]. 

I. Benjamin, Arc. Chem. Res., 28,233 (1995). Theory and Computer Simulations of Solvation and Chemical Reactions at Liquid Interfaces. 

The diffusivity in gases is about 4 orders of magnitude higher than that in liquids, and in gas-liquid reactions the mass transfer resistance is almost exclusively on the liquid side. High solubility of the gas-phase component in the liquid or very fast chemical reaction at the interface can change that somewhat. The Sh-number does not change very much with reactor design, and the gas-liquid contact area determines the mass transfer rate, that is, bubble size and gas holdup will determine reactor efficiency. 

The liquid-liquid interface is not only a boundary plane dividing two immiscible liquid phases, but also a nanoscaled, very thin liquid layer where properties such as cohesive energy, density, electrical potential, dielectric constant, and viscosity are drastically changed along with the axis from one phase to another. The interfacial region was anticipated to cause various specific chemical phenomena not found in bulk liquid phases. The chemical reactions at liquid-liquid interfaces have traditionally been less understood than those at liquid-solid or gas-liquid interfaces, much less than the bulk phases. These circumstances were mainly due to the lack of experimental methods which could measure the amount of adsorbed chemical species and the rate of chemical reaction at the interface [1,2]. Several experimental methods have recently been invented in the field of solvent extraction [3], which have made a significant breakthrough in the study of interfacial reactions. 

Solid-liquid reactions are much more complex than solid-gas reactions and include a variety of technically important processes such as corrosion and electrodeposition. When a solid reacts with a liquid, the products may form a layer on the solid surface or dissolve into the liquid phase. Where the product forms a layer covering the surface completely, the reaction is analogous to solid-gas reactions if the reaction products are partly or wholly soluble in the liquid phase, the liquid has access to the reacting solid, and chemical reaction at the interface therefore becomes important in determining the kinetics. 

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. 

Understanding chemical reactivity at liquid interfaces is important because in many systems the interesting and relevant chemistry occurs at the interface between two immiscible liquids, at the liquid/solid interface and at the free liquid (liquid/vapor) interface. Examples are reactions of atmospheric pollutants at the surface of water droplets[6], phase transfer catalysis[7] at the organic liquid/water interface, electrochemical electron and ion transfer reactions at liquidAiquid interfaces[8] and liquid/metal and liquid/semiconductor Interfaces. Interfacial chemical reactions give rise to changes in the concentration of surface species, but so do adsorption and desorption. Thus, understanding the dynamics and thermodynamics of adsorption and desorption is an important subject as well. 

Another motivation for studying reactions at liquid interfaces is that the unique nature of the interfacial region provides an interesting opportunity to examine the fundamental aspects of medium effects on chemical reactions. The interfacial region is characterized by anisotropic intermolecular forces[9] which give rise to specific molecular orientations. Thus, averaging procedures that are at the core... 

If we consider the system as a binary one with a surface-active material and bulk liquid, it is physically instructive to write the individual material balance relation for the surface excess concentration F (mol m ). The procedure for this is exactly as was carried out for the bulk binary system treated in Section 3.3. No chemical reaction at the interface is assumed, the system is considered to be dilute, the multicomponent mass flux is assumed to follow Pick s law, and the diffusion coefficients are taken to be constant. The expression for the surface concentration then becomes... 

Most substances acquire a surface charge when brought into contact with a polar medium because of chemical reaction at the interface. The charging mechanisms include ionization, ion adsorption, and ion dissolution [1]. Charge separation occurs at the solid/liquid interface when a particle is suspended in a liquid solution. As shown in Fig. 1, a negatively charged particle is surrounded by a diffuse layer... 

The study of chemical properties and chemical reactions of molecules adsorbed at aqueous interfaces is still in its infancy. The first results obtained with the QM/MM MD approach show that interface solvation phenomena cannot be understood (even qualitatively) from the usual concepts of bulk solutions. Water interfaces (and more generally liquid interfaces) have to be considered as a medium on their own, characterized by a complex dynamics behavior that involves large fluctuations of the physical and chemical properties of the solute. These large fluctuations arise from at least two main factors (1) the translational movements of the solute with respect to the formal average interface, and (2) the orientational motions of the solute, the dynamics of which is strongly controlled by the nature of specific solute-solvent interactions. Therefore, obtaining reliable theoretical results of chemical properties at liquid interfaces is a difiicult task that in general requires the use of explicit solvation models and the achievement of statistical simulations. 

Heterogeneous electron reactions at liquid liquid interfaces occur in many chemical and biological systems. The interfaces between two immiscible solutions in water-nitrobenzene and water 1,2-dichloroethane are broadly used for modeling studies of kinetics of electron transfer between redox couples present in both media. The basic scheme of such a reaction is... 

In bulk solution dynamics of fast chemical reactions, such as electron transfer, have been shown to depend on the dynamical properties of the solvent [2,3]. Specifically, the rate at which the solvent can relax is directly correlated with the fast electron transfer dynamics. As such, there has been considerable attention paid to the dynamics of polar solvation in a wide range of systems [2,4-6]. The focus of this chapter is the dynamics of polar solvation at liquid interfaces. 

The dissolution process, in general, consists of the following chemical reaction at the solid-liquid interface ... 

Inspired by these Surface Science studies at the gas-solid interface, the field of electrochemical Surface Science ( Surface Electrochemistry ) has developed similar conceptual and experimental approaches to characterize electrochemical surface processes on the molecular level. Single-crystal electrode surfaces inside liquid electrolytes provide electrochemical interfaces of well-controlled structure and composition [2-9]. In addition, novel in situ surface characterization techniques, such as optical spectroscopies, X-ray scattering, and local probe imaging techniques, have become available and helped to understand electrochemical interfaces at the atomic or molecular level [10-18]. Today, Surface electrochemistry represents an important field of research that has recognized the study of chemical bonding at electrochemical interfaces as the basis for an understanding of structure-reactivity relationships and mechanistic reaction pathways. 

Benjamin, I. 1996. Chemical reactions and salvation at liquid interfaces A microscopic perspective. Chem. Rev. 96 (4) 1449-1475. 

The measurement of fast chemical reactions at the liquid-liquid interface is very difficult, since the reaction which starts just after the contact of two phases has to be measured. The dead time of the HSS method was several seconds and that of the two-phase stopped flow method was from few tenth millisecond to several hundred milliseconds. The micro-two-phase sheath flow method is the only method that can measure such fast interfacial reactions, which finish within 100 ps. An inner organic microflow was generated in an outer aqueous phase... 

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