Interfacial processes manipulation

Derivatized semiconductor photoelectrodes offer a way to design photosensitive interfaces for effecting virtually any redox process. Manipulation of interfacial charge transfer processes has been demonstrated using hydrolytically unstable redox... 

The above example shows how the interfacial processes occurring at thin polymer films can be manipulated by a change in the contacting electrolyte. This section... 

Understanding of the structure of the adsorbed surfactant and polymer layers at a molecular level is helpful for improving various interfacial processes by manipulating the adsorbed layers for optimum configurational characteristics. Until recently, methods of surface characterization were limited to the measurement of macroscopic properties like adsorption density, zeta-potential and wettability. Such studies, while being helpful to provide an insight into the mechanisms, could not yield any direct information on the nanoscopic characteristics of the adsorbed species. Recently, a number of spectroscopic techniques such as fluorescence, electron spin resonance, infrared and Raman have been successfully applied to probe the microstructure of the adsorbed layers of surfactants and polymers at mineral-solution interfaces. 

Tools shape how we think when the only tool you have is an axe, everything resembles a tree or a log. The rapid advances in instrumentation in the last decade, which allow us to measure and manipulate individual molecules and structures on the nanoscale, have caused a paradigm shift in the way we view molecular behavior and surfaces. The microscopic details underlying interfacial phenomena have customarily been inferred from in situ measurements of macroscopic quantities. Now we can see and fmgeT physical and chemical processes at interfaces. 

Surface tension and contact angle phenomena play a major role in many practical things in life. Whether a liquid will spread on a surface or will break up into small droplets depends on the above properties of interfaces and determines well-known operations such as detergency and coating processes and others that are, perhaps, not so well known, for example, preparation of thin films for resist lithography in microelectronic applications. The challenge for the colloid scientist is to relate the macroscopic effects to the interfacial properties of the materials involved and to learn how to manipulate the latter to achieve the desired effects. Vignette VI provides an example. 

In the following, factors external to the actual interfacial supramolecular assembly, which are capable of modifying the photoinduced electron injection process, are considered. This discussion will concentrate on how the rate of charge injection can be manipulated by changing the composition of the electrolyte and by changing the external potential applied to the semiconductor film. 

In typical investigations on the behavior of modified TiC>2 surfaces upon irradiation, measurements are carried out in acetonitrile containing 0.1 M UCIO4. Under these conditions, very fast electron injection into the semiconductor surface is observed. However, it has been noted that this injection process depends strongly on the lithium concentration of the contacting acetonitrile solution [10,11]. In the absence of lithium, no injection is observed. This is an important observation since this opens up the possibility of modulating the photophysical behavior of the interfacial supramolecular assembly by external manipulation of the conditions. In this section, this observation is discussed in more detail, and, in addition, the possibility to use the surface potential of the semiconductor surface as a driving force will be considered. 

Microemulsions have the ability to partition polar species into the aqueous core or nonpolar solutes into the continuous phase (See Fig. 1). They can therefore greatly increase the solvation of polar species in essentially a nonpolar medium. The surfactant interfacial region provides a dramatic transition from the highly polar aqueous core to the nonpolar continuous-phase solvent. This region represents a third type of solvent environment where amphiphilic solutes can reside. Such amphiphilic species will be strongly oriented in the interfacial film so that their polar ends are in the core of the microemulsion droplet and the nonpolar end is pointed towards or dissolved in the continuous phase solvent. When the continuous phase is a near-critical liquid (7)j = r/7 > 0.75) or supercritical fluid, additional parameters such as transport properties, and pressure (or density) manipulation become important aids in applying this technology to chemical processes. 

The way in which a particular system behaves depends on the interfacial energies between the solid substrate and any contacting liquid, and between the liquid and the second fluid (air). By manipulating these factors, the wetting process can be controlled. This may be achieved by the use of surfactants. 

Interactions between proteins and polysaccharides give rise to various textures in food. Protein-stabilized emulsions can be made more stable by the addition of a polysaccharide. A complex of whey protein isolate and carboxymethylcellulose was found to possess superior emulsifying properties compared to those of the protein alone (Girard et al., 2002). The structure of emulsion interfaces formed by complexes of proteins and carbohydrates can be manipulated by the conditions of the preparation. The sequence of the addition of the biopolymers can alter the interfacial composition of emulsions. The ability to alter interfacial structure of emulsions is a lever which can be used to tailor the delivery of food components and nutrients (Dickinson, 2008). Polysaccharides can be used to control protein adsorption at an air-water interface (Ganzevles et al., 2006). The interface of simultaneously adsorbed films (from mixtures of proteins and polysaccharides) and sequentially adsorbed films (where the protein layer is adsorbed prior to addition of the polysaccharide) are different. The presence of the polysaccharide at the start of the adsorption process hinders the formation of a dense primary interfacial layer (Ganzelves et al., 2008). These observations demonstrate how the order of addition of components can influence interfacial structure. This has implications for foaming and emulsifying applications. 

Table I. lists a few possible advantages for conducting chemical synthesis in a supercritical environment. For example, supercritical fluids might provide a means to manipulate reaction environments by altering density and temperature to influence both reaction rate and selectivity. Futhermore, in a homogeneous supercritical phase, one can, in principle, eliminate interfacial transport limitations. Effectively, temperature and pressure are used to alter density in a way to influence both solvation dynamics and equilibrium solubility. With supercritical fluid solvents, there is the possibility of integrating both reaction and separation processes, which could lead to economic advantages over conventional synthetic processes carried out in liquid solvents.

Therefore, for the optimisation of the design of hquid-liquid contacting equipment and the improvement of mass transfer processes the interfacial hydrodynamics needs to be well understood, so that it can be taken into account at a design stage and if necessary (and possible ), enable the manipulation of the interfacial behaviour to enhance the mass transfer rates. 

In the next paper Surfactants Effects on Mass Transfer in Liquid-Liquid Systems Dr Alcina Mendes (Imperial College, UK) reviews the work done by herself and co-workers on the effect of surfactants on mass transfer in binary and ternary liquid-liquid systems. The selected organic-aqueous interfaces has been visualised during the mass transfer process in the presence of ionic and non-ionic surfactants. Results obtained in laboratory and under microgravity conditions are reported. The most significant finding is that surfactants in some cases can induce or increase convection. The latter enhance the mass transfer rate as compared to the Pick s law. The latter means that surfactants can be used to manipulate interfacial stability and particularly in space applications. 

In developing an understanding of the likely magnitude of heat release, the influence of reaction can be initially ignored, so that r(c, T) = o, and the processes taking place now simply involve physical dissolution and diffusion of the dissolved gas and the accompanying conduction of the heat of solution into the semi-infinite liquid phase. By manipulation of the two simultaneous unsteady diffusion/conduction equations, which obey the interfacial boundeiry condition... 

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