Chemical reaction at liquid

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. 

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. 

Pipe Lines The principal interest here will be for flow in which one hquid is dispersed in another as they flow cocurrently through a pipe (stratified flow produces too little interfacial area for use in hquid extraction or chemical reaction between liquids). Drop size of dispersed phase, if initially very fine at high concentrations, increases as the distance downstream increases, owing to coalescence [see Holland, loc. cit. Ward and Knudsen, Am. In.st. Chem. Eng. J., 13, 356 (1967)] or if initially large, decreases by breakup in regions of high shear [Sleicher, ibid., 8, 471 (1962) Chem. Eng. ScL, 20, 57 (1965)]. The maximum drop size is given by (Sleicher, loc. cit.)... 

A worker was told to control the temperature of a chemical reaction at 60° C he adjusted the setpoint of the leiiijier.ilure controller to be 60. The scale indicated 0-100% of a temperature range of 0-200 C. so tlie sci point was is. iliv 1 -O C A runaway reaction resulted which overpressured the vessel. Discharged liquid iirul injured il e worker. 

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... 

The variation of reaction rate with temperature follows the Arrhenius equation, which we have used to study the rate of chemical reactions in the interstellar medium ISM (Section 5.4, Equation 5.9), and can be applied to the liquid phase or reactions occurring on surfaces. Even the smallest increases in temperature can have a marked effect on the rate constants, as can be seen in the increased rate of chemical reactions at body temperature over room temperature. Considering a reaction activation energy that is of the order of a bond energy, namely 100 kJ mol-1, the ratio of the rate constants at 310 K and 298 K is given by ... 

Chemistry within the body is approximately five times faster than in a test tube at room temperature. The reverse is true, of course, with chemical reactions in liquid methane at 100 K some 1.2 x 1035 times slower than at 298 K. Neutral chemical reactions remain slow in solution at 100 K if they have a significant activation barrier. As with the ISM, chemistry involving breaking of chemical bonds is frozen out at 100 K and has direct implications for chemistry on the surface of Titan, for example. 

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

Chemists exploit the temperature dependence of reaction rates by carrying out chemical reactions at elevated temperatures to speed them up. In organic chemistry, especially, reactions are commonly performed under reflux that is, while boiling the reactants. To prevent reactants and products from escaping as gases, a water-cooled condenser tube is fitted to the reaction vessel. The tube condenses the vapours to liquids and returns them to the reaction vessel. Figure 6.15 shows an experiment performed under reflux. 

Figure 25. Evolution with time of the chemical reaction in liquid butadiene at 0.6 GPa and 300 K. Upper panel Purely pressure-induced reaction, the formation of vinylcyclohexene is revealed by the growth of the bands of the dimer in the 650- to 750-cm frequency range. Lower panel In this case the reaction is assisted by the irradiation with few milliwatts of the 488-nm line of an Ar+ laser. The fast increase of the characteristic polymer band at 980 cm indicates the selective formation of polybutadiene.

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. 

S.S. Cherry et al, Identification of Important Chemical Reactions in Liquid Propellant Rocket Engines , Pyrodynamics 6 (3—4), 275—96 (1969) CA 70,. 98394 (1969) [The authors state that the kinetics of nonequilibrium expansion of the propint system N204/A-50 (UDMH 49 plus hydrazine 51 wt%) can be described by the following gas phase reactions with an accuracy such that not more than 0.5 lb force-sec/lb mass variation in specific impulse (at a nozzle expansion rate of 40) is produced, as compared to the results of a full kinetic analysis ... 

However, a question arises - could similar approach be applied to chemical reactions At the first stage the general principles of the system s description in terms of the fundamental kinetic equation should be formulated, which incorporates not only macroscopic variables - particle densities, but also their fluctuational characteristics - the correlation functions. A simplified treatment of the fluctuation spectrum, done at the second stage and restricted to the joint correlation functions, leads to the closed set of non-linear integro-differential equations for the order parameter n and the set of joint functions x(r, t). To a full extent such an approach has been realized for the first time by the authors of this book starting from [28], Following an analogy with the gas-liquid systems, we would like to stress that treatment of chemical reactions do not copy that for the condensed state in statistics. The basic equations of these two theories differ considerably in their form and particular techniques used for simplified treatment of the fluctuation spectrum as a rule could not be transferred from one theory to another. 

These reactions have a characteristic free energy which implies the minimal voltage required. As discussed in section 4.1, an excited molecule is at the same time more easily oxidized and reduced than the ground state species. Reactions of excited molecules at electrodes are however practically unknown because their short lifetimes preclude the contact with the electrode when irradiation takes place in the bulk of the liquid. In practice the photoelectro-chemical reactions at non-excited electrodes are simply the thermal reactions of photoproducts. We shall give here two examples of such reactions. 

Ray Kapral came to Toronto from the United States in 1969. His research interests center on theories of rate processes both in systems close to equilibrium, where the goal is the development of a microscopic theory of condensed phase reaction rates,89 and in systems far from chemical equilibrium, where descriptions of the complex spatial and temporal reactive dynamics that these systems exhibit have been developed.90 He and his collaborators have carried out research on the dynamics of phase transitions and critical phenomena, the dynamics of colloidal suspensions, the kinetic theory of chemical reactions in liquids, nonequilibrium statistical mechanics of liquids and mode coupling theory, mechanisms for the onset of chaos in nonlinear dynamical systems, the stochastic theory of chemical rate processes, studies of pattern formation in chemically reacting systems, and the development of molecular dynamics simulation methods for activated chemical rate processes. His recent research activities center on the theory of quantum and classical rate processes in the condensed phase91 and in clusters, and studies of chemical waves and patterns in reacting systems at both the macroscopic and mesoscopic levels. 

---