Reactions at Liquid Interfaces

Molecular dynamics calculations of equilibrium and nonequilibrium solvent effects on the rate of a model isomerization reaction at the liquid-vapor interface of a Leonard—Jones liquid have recently heen reported by Ben-jamin.i In general, the liquid—vapor interface is characterized hy sharp variations in density and dielectric properties over a distance of a few molecular diameters, and therefore surface effects on the rate of reaction are expected to 

Experimentally, several applications of fast spectroscopic techniques have been reported for reactions at liquid interfaces.i Most relevant to Benjamin s molecular dynamics calculations has been the study by Sitzmann and EisenthaU on the picosecond isomerization dynamics of the electronically excited state of DODCl using surface second harmonic generation techniques. In this experiment, a rate enhancement of a factor of 2.5 at the surface was suggested to be due to the reduced friction at the surface. 

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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... 

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... 

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... 

The rapidly increasing practical importance of reactions at liquid interfaces is seen from the now common industrial use of reactions in emulsions, including the processes of soap manufacture and polymerization of synthetic rubber. 

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]. 

Despite the significant computational and experimental progress made in recent years, much more research is needed at both fronts to gain a molecular-level understanding of structure, thermodynamics, and dynamics at liquid interfaces. In particular, molecular-level, time-resolved experimental studies of solvation, relaxation, and reactions at liquid interfaces are needed. 

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. 

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... 

Electron-transfer reactions at liquid-liquid interfaces have the form ... 

So far, the lattice gas has been the only model that makes predictions both about structure and reactions at liquid-liquid interfaces. There are however, various other models for particular features of these interfaces. Some of them give similar, others give contradictory results. In the following, we briefly review a few of these models. 

Dynamic Aspects of Heterogeneous Electron-Transfer Reactions at Liquid-Liquid Interfaces... 

Extending the formalism for ET in homogeneous phase, reactions at liquid-liquid interfaces can be described in terms of a series of elementary steps initiated by the approach of reactants to the interfacial region and the formation of the ET precursor complex [1,5,60],... 

The general criteria for an experimental investigation of the kinetics of reactions at liquid-liquid interfaces may be summarized as follows known interfacial area and well-defined interfacial contact are essential controlled, variable, and calculable mass transport rates are required to allow the transport and interfacial kinetic contributions to the overall rate to be quantified direct interfacial contact is preferred, since the use of a membrane to support the interface adds further resistances to the overall rate of the reaction [14,15] a renewable interface is useful, as the accumulation of products at the interface is possible. Finally, direct measurements of reactive fluxes at the interface of interest are desirable. 

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. 

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. 

Fig. 16 Two different types of reaction at liquid/liquid interfaces, the solvent extraction of copper and the transfer of NOJ.

Electrodes with liquid ion-exchange membranes are typified by a calcium-sensitive electrode (Figure 6.4). The membrane-liquid consists of the calcium form of a di-alkyl phosphoric acid, [(RO)2POO ] 2Ca2+, which is prepared by repeated treatment of the acid with a calcium salt. The internal solution is calcium chloride and the membrane potential, which is determined by the extent of ion-exchange reactions at the interfaces between the membrane and the internal and sample solutions, is given by... 

Electron-transfer reactions at liquid-liquid interfaces involve redox couples on each side of the interface. The basic scheme is (see Fig. 12.5) ... 

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