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LIQUID INTEDFACES IN CHEMICAL, BIOLOGICAL, AND PHARMACEUTICAL APPLICATIONS
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 Boundary membranes play a key role in the cells of all contemporary organisms, and simple models of membrane function are therefore of considerable interest. The interface of two immiscible liquids has been widely used for this purpose. For example, the fundamental processes of photosynthesis, biocatalysis, membrane fusion and interactions of cells, ion pumping, and electron transport have all been investigated in such interfacial systems. Processes occurring at the interface between two immiscible liquids present a challenge of great interest because heterogeneous structures of this kind are frequently encountered in Nature. In particular, the electrical double layer at the oil–water interface occurs in a heterogeneous interfacial region that separates different bulk regions of conductive polarized media with a spatial separation of charges. The electrical double layer at the interface between two immiscible liquids determines the kinetics of charge transfer across the phase boundaries, stability and electrokinetic properties of lyophobic colloids, mechanisms of phase transfer or interfacial catalysis, charge separation in biological energy transduction, and heterogeneous enzymatic catalysis. The elucidation of the structure of the interface between two immiscible liquids and the mechanism of charge separation is a fundamental problem of modern chemistry and chemical technology. The interface between two immiscible electrolyte solutions can be either polarizable or nonpolarizable, depending on its permeability to charged particles in the system. If the interface is impermeable to charged particles or the transfer between the phases is difficult, it is called polarizable. A completely nonpolarizable interface containing at least one common ion can pass a high current in either direction without giving rise to a deviation of the interfacial potential difference from the equilibrium value. Although in practice we encounter neither ideally polarizable nor completely nonpolarizable interfaces, under certain conditions the properties of some interfaces are close to ideal. Since any interface is permeable to ions to some extent, only an approximation of the limiting version of a polarizable interface can be realized experimentally. Michael Faraday first studied electron transfer reactions at oil–water interfaces to prepare colloidal metals by reducing metal salts at the ether–water or carbon disulfide– water interfaces. As the field progressed after Faraday’s pioneering observations, it became clear that vectorial charge transfer at the interface between two dielectric media is an important stage in many bioelectrochemical processes such as those mediated by energy-transducing membranes. Studies have been made on redox and hydrolysis reactions catalyzed by enzymes, photosynthetic pigments, metal complexes of porphyrins, bacteria, and submitochondrial particles, as well as in systems with an extended surface— in microemulsions, vesicles, and reversed micelles. Naturally immobilized enzymes and pigments embedded in a hydrophilic-hydrophobic interface have properties similar to their functional state in a membrane. For instance, certain enzymes can be highly active at the interface but virtually inactive in a homogeneous medium. The interface between two immiscible liquids with immobilized photosynthetic pigments can serve as a convenient model for investigating photoprocesses that are accompanied by spatial separation of charges. The efficiency of charge separation defines the quantum yield of any photochemical reaction. Heterogeneous sytems will be most effective in this regard, where the oxidants and the reductants are either in different phases or sterically separated. Different solubilities of the substrates and reaction products in the two phases of heterogeneous systems can alter the redox potential of reactants, making it possible to carry out reactions that cannot be performed in a homogeneous phase. Elucidation of photosynthetic mechanisms is also significant in designing artificial systems for solar energy utilization. The quantum yield of the photocatalytic reaction depends first of all on the efficiency of the photochemical charge separation. The most effective system should be a heterogeneous system in which the oxidant and the reductant are either in different phases or sterically separated. The difference in solubilities of the substrates and reaction products in both phases of such a heterogeneous structure as the octane–water interface can shift the reaction equilibrium. The redox potential scale is thereby altered, making it possible to carry out reactions that cannot be performed in a homogeneous phase. Extraction of the reaction products and adsorption of the reaction components determine high catalytic properties of the interface between two immiscible liquids, recognized recently as interfacial catalysis. Chemical models of photosynthesis have been used to investigate two types of reactions: photosynthesis and photocatalysis. In photosynthetic processes the standard Gibbs free energy of the reaction is positive, and solar energy is utilized to perform work. In photocatalytic processes the free energy is negative and solar energy is used to overcome the activation barrier. The processes of life have been found to generate electric fields in every organism that has been examined with suitable and sufficiently sensitive measuring techniques. The electrochemical conduction of electrochemical excitation over specialized structures must be regarded as one of the most universal properties of living organisms. It arose at the early stages of evolution in connection with the need for transmission of a signal about an external influence from one part of a biological system to another. Clearly, then, the chemical and physical properties of liquid interfaces represent a significant interdisciplinary research area for a broad range of investigators, such as those who have contributed to this book. The chapters are organized into three parts. The first deals with the chemical and physical structure of oil–water interfaces and membrane surfaces. Eighteen chapters present discussion of interfacial potentials, ion solvation, electrostatic instabilities in double layers, theory of adsorption, nonlinear optics, interfacial kinetics, microstructure effects, ultramicroelectrode techniques, catalysis, and extraction. The second part, on biological applications of interfacial phenomena, has ten chapters that deal successively with protein encapsulation, membranes, effects of phospholiiv pids, biocatalysis, oscillation of membrane potentials, and fundamentals of interfacial electrochemical phenomena in green plants. The final part, on pharmaceutical applications, consists of five chapters and includes topics such as drugs at liquid interfaces, NMR studies, and drugs and gene delivery. We thank the authors for the time they spent on this project and for teaching us about their work. Alexander G. Volkov |
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