Kinetics of interfacial processes

The success of SECM methodologies in providing quantitative information on the kinetics of interfacial processes relies on the availability of accurate theoretical models for mass transport and coupled kinetics, to allow the analysis of experimental data. The geometry of SECM is not conducive to exact analytical solution and hence a number of semiana-lytical [40,41], and numerical [8,10,42 46], methods have been introduced for a variety of problems. 

To appreciate the impact of SECM on the study of phase transfer kinetics, it is useful to briefly review the basic steps in reactions at solid/liquid interfaces. Processes of dissolution (growth) or desorption (adsorption), which are of interest herein, may be described in terms of some, or all, of the series of events shown in Figure 1. Although somewhat simplistic, this schematic identifies the essential elements in addressing the kinetics of interfacial processes. In one limit, when any of the surface processes in Figure 1 (e.g., the detachment of ions or molecules from an active site, surface diffusion of a species across the surface, or desorption) are slow compared to the mass transport step between the bulk solution and the interface, the reaction is kinetically surface-controlled. In the other limit, if the surface events are fast compared to mass transport, the overall process is in a mass transport-controlled regime. 

The RDE consists of a disc (e.g. of Pt, Ni, Cu, Au, Fe, Si, CdS, GaAs, glassy carbon and graphite) set into an insulating (PTFE) surround. The electrode is rotated about its vertical axis (Figure 11.13), typically between 400 and 10,000 rpm. The theory for the hydrodynamics at the RDE (40-42) assumes that the electrode is uniformly accessible and affords a precise and reproducible control of the convection and diffusion of reactant to the electrode. Hence, the RDE can be used to study the kinetics of interfacial processes. 

Chemical dynamics and modeling were identified as important research frontiers in Chapter 4. They are critically important to the materials discussed in this chapter as well. At the molecular scale, important areas of investigation include studies of statistical mechaiucs, molecular and particle dynamics, dependence of molecular motion on intermolecular and interfacial forces, and kinetics of chemical processes and phase changes. 

The theoretical results described have implications for the design of experimental approaches for the study of transfer processes across the interface between two immiscible phases. The current response in SECMIT is clearly sensitive to the relative diffusion coefficients and concentrations of a solute in the two phases and the kinetics of interfacial transfer over a wide range of values of these parameters. 

The ITIES with an adsorbed monolayer of surfactant has been studied as a model system of the interface between microphases in a bicontinuous microemulsion [39]. This latter system has important applications in electrochemical synthesis and catalysis [88-92]. Quantitative measurements of the kinetics of electrochemical processes in microemulsions are difficult to perform directly, due to uncertainties in the area over which the organic and aqueous reactants contact. The SECM feedback mode allowed the rate of catalytic reduction of tra 5-l,2-dibromocyclohexane in benzonitrile by the Co(I) form of vitamin B12, generated electrochemically in an aqueous phase to be measured as a function of interfacial potential drop and adsorbed surfactants [39]. It was found that the reaction at the ITIES could not be interpreted as a simple second-order process. In the absence of surfactant at the ITIES the overall rate of the interfacial reaction was virtually independent of the potential drop across the interface and a similar rate constant was obtained when a cationic surfactant (didodecyldimethylammonium bromide) was adsorbed at the ITIES. In contrast a threefold decrease in the rate constant was observed when an anionic surfactant (dihexadecyl phosphate) was used. 

In voltammetric experiments, electroactive species in solution are transported to the surface of the electrodes where they undergo charge transfer processes. In the most simple of cases, electron-transfer processes behave reversibly, and diffusion in solution acts as a rate-determining step. However, in most cases, the voltammetric pattern becomes more complicated. The main reasons for causing deviations from reversible behavior include (i) a slow kinetics of interfacial electron transfer, (ii) the presence of parallel chemical reactions in the solution phase, (iii) and the occurrence of surface effects such as gas evolution and/or adsorption/desorption and/or formation/dissolution of solid deposits. Further, voltammetric curves can be distorted by uncompensated ohmic drops and capacitive effects in the cell [81-83]. 

Photochemical processes in monolayers at the air-water interface can be controlled externally by variation of the various parameters like matrix composition, subphase composition, temperature and surface pressure. When the product of the reactions has a different area per molecule, the surface pressure may change at constant monolayer area. An interfacial shock wave has been generated in this way. This technique permits the investigation of the kinetics of reorganization processes and the transmission of mechanical signals in monolayers. 

IC reactions can be of two types. The nucleation of the new phase (e.g. Ni produced by the reduction of NiO) may be rate determining. In that case, separate nuclei of the new phase can be detected (Fig. 1(a)). These are called nucleation-controlled interface or NCI reactions. In other cases, all the surface of the initial solid reacts, and a continuous interface entirely covers the solid reactant. With respect to kinetics, the interfacial process entirely controls the reaction in this last case. This is an ICI reaction (Fig. 1(b)). 

To access the potential influence of spillover on catalysis and interfacial transport, more qualitative studies are required. Further, it is, for instance, necessary to isolate the individual steps in the phenomena and account for the reaction kinetics of the process. As an example, what is the difference between inter- and intraparticle transport on the support ... 

The net interfacial electron flux is the result of many discrete tunneling events between electron levels in phases A and B. Usually, electron tunneling is an elastic process it occurs between an occupied and an empty electron level of the same energy. Hence, the probability of electron tunneling depends on the distribution of the electron levels at both sides of the interface, and on their occupancy. The electronic structure of solids is thus of primary importance for the kinetics of interfacial electron transfer. In analogy, electrochemical electron transfer can be regarded as the result of discrete tunneling events between electron levels in a solid (the elec-... 

Prokopuk N. and Lewis N. S. (2004), Energetics and kinetics of interfacial electron-transfer processes at chemically modified InP/hquid junctions , J. Phys. Chem. B 108, 4449-4456. 

An alternative electrochemical approach to the measurement of fast interfacial kinetics exploits the use of the scanning electrochemical microscope (SECM). A schematic of this device is shown in Fig. 14 the principle of the method rests on the perturbation of the intrinsic diffusive flux to the microelectrode, described by Eq. (34) above. A number of reviews of the technique exist [109,110]. In the case of the L-L interface, the microelectrode probe is moved toward the interface once the probe-interface separation falls within the diffusion layer, a perturbation of the current-distance response is seen, which can be used to determine the rate of interfacial processes, generally by numerical solution of the mass-transport equations with appropriate interfacial boundary conditions. The method has been... 

In the absence of surface recombination, all minority carriers that are collected by diffusion and migration in the semiconductor/electrolyte junction will eventually either transfer to redox species in the solution or react with the semiconductor itself leading to anodic or cathodic photodecomposition. Slow interfacial kinetics will result in the build up of photogenerated carriers at the interface, but unless photocurrent multiplication occurs, the saturation photocurrent will simply be determined by the light intensity, and the quantum efficiency will be unity. This means that the photocurrent contains no information about interfacial kinetics. In reality, most semiconductor/electrolyte interfaces are non-ideal, and a substantial fraction of the photogenerated electrons or holes do not take part in interfacial redox reactions because they recombine via surface states (see section 2.3.3). It is this competition between interfacial electron transfer and surface recombination that opens the way to obtain information about the rates of interfacial processes. 

A complete screening of an EOR surfactant must include determination of the kinetics of interfacial tension changes in addition to their equilibrium values. Considerable work remains to be done to characterize dynamic processes such as oil droplet mobilization, entrapment and oil bank formation. 

When we have chosen the reactor configuration, we can consider the quantitative aspects of reactor design. In order to convert a certain amount of matter per unit time, the reactor should have a sufficient volume. This volume is either determined by the necessary volume of the reaction phase or phases, or by the necessary interfacial area which requires a certain volume to support it. If we have data on the kinetics of the process, i.e., both the kinetics of the chemical reactions and of the relevant transport processes, it is generally possible to estimate the reactor volume needed for a desired production rate. 

Various types of spontaneous processes take place in microemulsions. The surfactant and cosurfactant exchange between the interfacial film separating water and oil domains and the bulk phases. Also collisions between droplets with temporary merging of the collided droplets ( sticky collisions) have been evidenced. The kinetics of these processes (and of other ones) is reviewed in Chapter 5, Sections VI.F and VIII, and Chapter 10. 

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