Interfacial reactions probe

The high rate of mass transfer in SECM enables the study of fast reactions under steady-state conditions and allows the mechanism and physical localization of the interfacial reaction to be probed. It combines the usefid... 

To ensure that the detector electrode used in MEMED is a noninvasive probe of the concentration boundary layer that develops adjacent to the droplet, it is usually necessary to employ a small-sized UME (less than 2 /rm diameter). This is essential for amperometric detection protocols, although larger electrodes, up to 50/rm across, can be employed in potentiometric detection mode [73]. A key strength of the technique is that the electrode measures directly the concentration profile of a target species involved in the reaction at the interface, i.e., the spatial distribution of a product or reactant, on the receptor phase side. The shape of this concentration profile is sensitive to the mass transport characteristics for the growing drop, and to the interfacial reaction kinetics. A schematic of the apparatus for MEMED is shown in Fig. 14. 

Typical theoretical concentration profiles, observed at a probe electrode, for the consumption of a receptor phase species in a first-order interfacial reaction are shown in Fig. 16. The simulation involved solving Eq. (30) with appropriate boundary conditions. 

Mathias, L.J. and Vaidya, R.A. (1986) Inverse phase transfer catalysis. First report of a new class of interfacial reactions./. Am. Chem. Soc., 108, 1093. Fife, W.K. and Xin, Y. (1987) Inverse phase-transfer catalysis probing its mechanism with competitive transacylation. J. Am. Chem. Soc., 109, 1278. 

The specific features of the interfacial reactions, notably the complexation of metal ions, were reviewed. The dynamic nano-properties of the interface were also discussed from the experimental results of single molecule probing and other dynamic microscopic... 

Monolayer and multilayer thin films are technologically important materials that potentially provide well-defined molecular architectures for the detailed study of interfacial electron transfer. Perhaps the most important attribute of these heterogeneous systems is the ease with which their molecular architecture can be synthetically varied to tailor the properties of the ensemble. Assemblies incorporating specifically designed structures can, in principle, meet the needs of a variety of technological applications and be used as models for understanding fundamental interfacial reaction mechanisms. In fact, molecular assemblies are nearly ideal laboratories for the fundamental study of electron-transfer reactions at interfaces. In this chapter, the use of monolayer and multilayer assemblies to probe fundamental questions regarding electron transfer in surface-confined molecular assemblies will be addressed. 

FIG. 22 Schematic diagram of the SECM apparatus employed for probing interfacial reactions at the ITIES. An UME tip is used to measure the local concentration of a reactant or product in the near-interface region of an expanding droplet. (From Ref. 65.)... 

All these results are consistent with the hypothesis that aryl cations react in aqueous media at diffusion-controlled rates with all nucleophiles that are available in the immediate neighbourhood of the diazonium ion. On this basis Romsted and coworkers (Chaudhuri et al., 1993a, 1993b) used dediazoniation reactions as probes of the interfacial composition of association colloids. These authors determined product yields from dediazoniation of two arenediazonium tetrafluoroborates containing ft-hexadecyl residues (8.15 and 8.16) and the corresponding diazonium salts with methyl groups instead of Ci6H33 chains. ... 

The reaction was followed by the local measurement of chloride ions, at a potentio-metric Ag/AgCl microelectrode probe, positioned in the aqueous receptor phase, as DCE droplets containing TPMCl were grown (feeder phase). The reaction was shown unambiguously to occur interfacially, and was first-order in TPMCl with a hydrolysis rate constant of 6.5 x 10 cms. A typical concentration-distance profile determined in these experiments is shown in Fig. 18. 

Alternatively, a higher rate of mass transport in steady-state measurements with a larger UME can be obtained by using it as a tip in the scanning electrochemical microscope (SECM). The SECM has typically been employed for probing interfacial ET reactions [29]. Recently, micropipettes have been used as SECM probes (see Section IV.B below) [8b,30]. Although the possibility of probing simple and assisted IT at ITIES by this technique was demonstrated, no actual kinetic measurements have yet been reported. 

The tip current depends on the rate of the interfacial IT reaction, which can be extracted from the tip current vs. distance curves. One should notice that the interface between the top and the bottom layers is nonpolarizable, and the potential drop is determined by the ratio of concentrations of the common ion (i.e., M ) in two phases. Probing kinetics of IT at a nonpolarized ITIES under steady-state conditions should minimize resistive potential drop and double-layer charging effects, which greatly complicate vol-tammetric studies of IT kinetics. 

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. 

It is for this reason that spectroscopy offers the only experimental method for characterizing the interfacial region that is not automatically destined to run into basic conceptual difficulties. This is not to say that difficulties of a technical nature will not arise (40-48), nor that the conceptual difficulty of differing time scales among spectroscopic techniques will cause no problems (50). Nonetheless, it is to be hoped that future investigations of sorption reactions will focus more on probing the molecular structure of the mineral/water interface than on attempting simply to divine what the structure may be. 

Given the existence of interphases and the multiplicity of components and reactions that interact to form it, a predictive model for a priori prediction of composition, size, structure or behavior is not possible at this time except for the simplest of systems. An in-situ probe that can interogate the interphase and provide spatial chemical and morphological information does not exist. Interfacial static mechanical properties, fracture properties and environmental resistance have been shown to be grealy affected by the interphase. Careful analytical interfacial investigations will be required to quantify the interphase structure. With the proper amount of information, progress may be made to advance the ability to design composite materials in which the interphase can be considered as a material variable so that the proper relationship between composite components will be modified to include the interphase as well as the fiber and matrix (Fig. 26). 

Figure 11 [76-79]. In this system, Pe or DPA in the droplet acts as a fluorescence probe for the interfacial ET. Namely, fluorescence of Pe or DPA in the droplet is quenched by FeCp-X, but not by FeCp-X+ produced by the ET reaction. Therefore, the time course of the FeCp-X concentration in the oil droplet ([FeCp-X]0) during electrolysis of Fe(II) in water can be determined by that of the fluorescence intensity of the fluorescer (/F). Although fluorescence quenching by Fe(II) or Fe(III) is also expected to take place at the droplet/water interface, such a contribution is neglected compared to the quenching by FeCp-X in the droplet interior, owing to the short diffusion length of the excited singlet state Pe or DPA.

Our interest in SERS stemmed from our research activities concerned with establishing connections between the molecular structure of electrode interfaces and electrochemical reactivity. A current objective of our group is to employ SERS as a molecular probe of adsorbate-surface interactions to systems of relevance to electrochemical processes, and to examine the interfacial molecular changes brought about by electrochemical reactions. The combination of SERS and conventional electrochemical techniques can in principle yield a detailed picture of interfacial processes since the latter provides a sensitive monitor of the electron transfer and electronic redistributions associated with the surface molecular changes probed by the former. Although few such applications of SERS have been reported so far the approaches appear to have considerable promise. 

The past decade has seen a dramatic improvement in the strategies and instrumentation available to characterize the structures of interfacial supramolecular assemblies. Current thrusts are towards in situ techniques that probe the structure of the interfacial supramolecular assembly with increasingly fine spatial and time resolution. The objective of this field is to assemble reaction centers around which the environment is purposefully structured at the molecular level, but extends over supramolecular domains. The properties of the assembly are controlled not only by the properties of the molecular building blocks but especially by the interface. Therefore, the focus is on both the interfacial and bulk properties of monolayers and thin films. Issues that need to be addressed include the film thickness, structural homogeneity and long-range order, as well as the electrochemical and... 

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