Reactivity at Liquid Interfaces

TheoreticaE - and experimentaE studies of chemical reaction dynamics and thermodynamics in bulk liquids have demonstrated in recent years that one must take into account the molecular structure of the liquid to fully understand solvation and reactivity. The solvent is not to be viewed as simply a static medium but as playing an active role at the microscopic level. Our discussion thus far underscores the unique molecular character of the interface region asymmetry in the intermolecular interactions, nonrandom molecular orientation, modifications in the hydrogen-bonding network, and other such structural features. We expect these unique molecular structure and dynamics to influence the rate and equilibrium of interfacial chemical reactions. One can also approach solvent effects on interfacial reactions at a continuum macroscopic level where the interface region is characterized by gradually changing properties such as density, viscosity, dielectric response, and other properties that are known to influence reactivity. 

Computational studies of neat liquid surfaces are becoming a mature area of study, but investigation of chemical reaction thermodynamics and dynamics is much more limited. This is due, in part, to the scarcity of molecular-level experimental data. While some computational work focused on reactions that were also studied experimentally, most of the published computational work relied on simple model reactions to address these two important general questions  

Methods used to study reactivity in bulk liquids are relatively well developed and generally can be used without major modification to study reactions at interfaces. The computational approach typically involves these steps  

Define a reaction coordinate X(r), which in general is a function of some (or even many) of the atomic positions in the systems. Examples include a torsional angle in a molecule for a conformational transition, a bond distance for a simple dissociation reaction, or, a function of many solvent atomic positions in the case of an electron transfer reaction. Keep in mind that in many cases the simple classical force fields described earlier in this chapter are inadequate for describing the proper dynamics of the system along the coordinate X. A quantum description at some level is likely needed 

Perform nonequilibrium trajectory calculations to explore possible dynami- 

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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. 

While the pair approximation of Eq. [2] is efficient for computer simulations, a better agreement with experimental data can sometimes be achieved by utilizing more general force fields. n-Body potentials, which depend on the simultaneous positions of n particles with n> 2, provide a more refined description of condensed phase systems, but only in a few cases have they been used for liquid surfaces.obvious case where three-(or higher)-body potentials are necessary is when classical MD is used to model a chemical reaction. The simple A -I- BC atom exchange reaction, for example, has been modeled with the three-body LEPS potential.The topic of potentials used to model chemical reactions will be further discussed in the section on reactivity at liquid interfaces. 

The present review intends to cover the work reported on nonlinear optics at liquid-liquid interface since the first report of S. G. Grubb et al. [18]. The theoretical aspects of nonlinear optics are first introduced in Section II. The experimental results covering the molecular structure of liquid interfaces are presented in Section III, followed by a section devoted to the dynamics and the reactivity at these interfaces. Section V focuses on new aspects where spherical interfaces with radii of curvature of the order of the wavelength of light are investigated. Section VI presents the field of SFG. 

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. 

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. 

Slevin CJ, Macpherson IV, Unwin PR (1997) Measurement of local reactivity at liquid/solid, liquid/ liquid, and liquid/gas interfaces with the scanning electrochemical microscope principles, theory, and applications of the double potential step chronoamperometric mode. J Phys Chem B 101(50) 10851-10859. doi 10.1021/jp972587i... 

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