Interfacial reactions monolayers

Tjandra et al. (1998) have proposed an interfacial reaction model for the kinetics of the reaction between 1-bromo octane and sodium phenoxide to give 1-phenoxyoctane in a nonionic microemulsion. In this model the microemulsion is assumed to consist of the aqueous phase and the interface is covered by a monolayer of surfactant molecules. It is thus possible to assess the interfacial area from the concentration of the surfactant in the microemulsion medium. 

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 this chapter, some useful measurement methods of the interfacial adsorption and interfacial reaction, which take place at the monolayer region of the liquid-liquid interface, are reviewed along with some essential examples. 

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. 

Interfacial reactions on surfaces are becoming increasingly important due to their widespread applications. Self assembled monolayers (SAMs) provide excellent platforms to study these reactions. This review focuses on the various organic reactions carried out on these SAMs. Challenges and limitations of organic reactions at surfaces are discussed. Emerging biocatalytic techniques for carrying out surface reactions are reviewed. 

Self-assembled monolayers (SAMs) are perhaps the best model for studying these interfacial reactions. Figure 1 shows an idealized view and stmctural model of a SAM on a metal substrate. SAMs are defined as monomolecular films of a surfactant formed spontaneously on a substrate upon exposure to a surfactant solution. Virtually any functional group can be introduced in these monolayers... 

The physical chemist is very interested in kinetics—in the mechanisms of chemical reactions, the rates of adsorption, dissolution or evaporation, and generally, in time as a variable. As may be imagined, there is a wide spectrum of rate phenomena and in the sophistication achieved in dealing wifli them. In some cases changes in area or in amounts of phases are involved, as in rates of evaporation, condensation, dissolution, precipitation, flocculation, and adsorption and desorption. In other cases surface composition is changing as with reaction in monolayers. The field of catalysis is focused largely on the study of surface reaction mechanisms. Thus, throughout this book, the kinetic aspects of interfacial phenomena are discussed in concert with the associated thermodynamic properties. 

At the air-water interface, water molecules are constantly evaporating and condensing in a closed container. In an open container, water molecules at the surface will desorb and diffuse into the gas phase. It is therefore important to determine the effect of a monomolecular film of amphiphiles at the interface. The measurement of the evaporation of water through monolayer films was found to be of considerable interest in the study of methods for controlling evaporation from great lakes. Many important atmospheric reactions involve interfacial interactions of gas molecules (oxygen and different pollutants) with aqueous droplets of clouds and fog as well as ocean surfaces. The presence of monolayer films would thus have an appreciable effect on such mass transfer reactions. 

For SDS, the reaction proceeded to a reproducible end point rapidly —viz., 1 to 2 minutes—when nonionic surface active impurities such as parent dodecyl alcohol, DOH, were removed by ethyl ether extractions. This impurity effect was verified by adding traces of alkyl alcohol—viz., 1 X 10 9 mole per liter—to purified SDS, whereupon the penetration reaction rate was halved. A possible explanation for this behavior is that formation of an SDS-DOH interfacial complex reduced the SDS activity in the interface and consequently its rate of reaction with the protein monolayer. The reasons for the somewhat slower rate of reaction of Cetab with the protein film are more obscure. The reaction rate did not increase after extracting the detergent repeatedly. Two possible reasons for the time dependence in this case may have been that (1) the ether extraction method was not effective in removing surface active impurities, or (2) because of the greater bulk of the Cetab hydrocarbon chain, Ci6 vs. Ci2 for SDS, more time was required for diffusion and appropriate orientation before complex formation. 

Photochemical Generation of an Interfacial Shock Wave. Both the an-thocyanidine and the thioindigo monolayers showed a decrease in surface pressure at constant area during the photoisomerization reaction. A different behavior is observed with mixed monolayers of the surface active spiropyran SP and octadecanol (OD), molar ratio SP 0D = 1 5, on illumination with UV radiation. The isomerization of the spiropyran to the merocyanine MC causes an increase in surface pressure at constant area (5, 14). This is shown in Figure 4, where the sudden rise in surface pressure it upon repeated 0.5 s exposures (as indicated by the arrows) can be seen to occur in a wide surface pressure range (15). The kinetics of the relaxation process following the surface pressure increase depends on the surface pressure. 

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. 

---