Lateral Transport Phenomena

Lateral transport phenomena can be observed for example as surface self-diffusion (Vollhardt et al. 1980), from concentration gradients in surface films as the result of compression or expansion of soluble or insoluble monolayers (Dimitrov et al. 1978), from the effect of aggregation or domain formation in such monolayers (Lucassen-Reynders 1987), or from domain movements in monolayers induced by an electric field (Heckl et al. 1988). 

Surface self-diffusion is the two-dimensional analogue of the Brownian motion of molecules in a liquid bulk. Measurements of self-diffusion have to be performed in complete absence of any Marangoni flow caused by surface tension differences. Such experimental conditions are best established in an insoluble monolayer where one part consists of unlabelled and the other of radio-tracer labelled molecules. The movement of molecules within the surface monolayer can be now observed by using a Geiger-Miiller counter. There are possible effects of liquid convective flow in the sublayer which was discussed for example by Vollhardt et al. (1980a). With e special designed apparatus Vollhardt et al. (1980b) studied the self-difihision of different palmitic and stearic acid and stearyl alcohol and obtained self-diffusion coefficients between l-i-4 lO cm /s. 

One of the recent techniques most commonly used to measure diffusion in 2-dimensions is fluorescence recovery after photobleaching (FRAP). This method uses a laser beam focused through a fluorescence microscope to follow the diffusion of fluorescent molecules in a plane perpendicular to the laser beam. The fluorescence intensity from a laser spot of known diameter, typically a few microns, is measured. The laser intensity is then increased by approximately 1000 times. This irreversibly photobleaches any fluorophore in the spot. The intensity is then decreased again and the recovery in fluorescence intensity measured as unbleached molecules diffuse into the spot. The time function of fluorescence intensity is then analysed to give surface self-diffusion coefficient (Clark et al. 1990a, b, Wilde Clark 1993, Ladhaetal. 1994). 

Dynamics in insoluble monolayers were studied for example by Dimitrov et al. (1978). They observed a Marangoni effect during a continuous compression and described it by a mechanism 

Irregularities in dynamic surface tensions of adsorption layers of soluble surfactants were discussed by Lucassen-Reynders (1987) in terms of aggregation phenomena of adsorbed molecules. She gave a theoretical model for the frequency spectrum of surface dilational properties. 

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This book deals mainly with dynamic properties of amphiphiles at liquid/air and liquid/liquid interfaces rather than at solid/liquid interfaces. However static and dynamic contact angles are discussed in Appendix 3B as these phenomena are determined by the kinetics of adsorption of surfactants also at the fluid interface. Some specific aspects of lateral transport phenomena studied by many authors are briefly review in Appendix 3D. 

In considering the dynamic behaviour of amphiphiles at interfaces, we have to include several dynamic processes. There are not only "simple" adsorption/desorption processes but also time-dependent orientations and lateral transport phenomena. Each of these processes is connected to a characteristic time, denoted as relaxation time. Two extreme cases exist. 

Uptake is the process by which chemicals (either dissolved in water or sorbed onto sediment and/or suspended solids) are transferred into and onto an organism. For surfactants, this generally occurs in a series of steps a rapid initial step controlled by sorption, where the surface phenomenon is especially relevant then a diffusion step, when the chemical crosses biological barriers, and later steps when it is transported and distributed among the tissues and organs. 

Sevastianov et al.73,74) have developed a model which considers the effect of surface heterogeniety on the adsorption process. They define centers of irreversible adsorption , labeled P, and centers of irreversible desorption , labeled D. They argue, in agreement with Soderquist and Walton, that desorbed material is conformationally altered and thus cannot readsorb — hence desorption is irreversible. The results of this model are given as Fig. 14, taken from Ref. 7J). The model also includes the case where adsorption may be transport limited. The model fits commonly observed adsorption data, including the overshoot phenomenon (Fig. 14, top) (discussed in Ref. 72)) to be discussed later. 

The starting point of a number of theoretical studies of packed catalytic reactors, where an exothermic reaction is carried out, is an analysis of heat and mass transfer in a single porous catalyst since such system is obviously more conductive to reasonable, analytical or numerical treatment. As can be expected the mutual interaction of transport effects and chemical kinetics may give rise to multiple steady states and oscillatory behavior as well. Research on multiplicity in catalysis has been strongly influenced by the classic paper by Weisz and Hicks (5) predicting occurrence of multiple steady states caused by intrapellet heat and mass intrusions alone. The literature abounds with theoretical analysis of various aspects of this phenomenon however, there is a dearth of reported experiments in this area. Later the possiblity of oscillatory activity has been reported (6). 

There are some special cases in FFF related to the two extreme limits of the cross-field driving forces. In the first case, the cross-field force is zero, and no transverse solute migration is caused by outer fields. However, because of the shear forces, transverse movements may occur even under conditions of laminar flow. This phenomenon is called the tubular pinch effect . In this case, these shear forces lead to axial separation of various solutes. Small [63] made use of this phenomenon and named it hydrodynamic chromatography (HC). If thin capillaries are used for flow transport, this technique is also called capillary hydrodynamic fractionation (CHDF). A simple interpretation of the ability to separate is that the centers of the solute particles cannot approach the channel walls closer than their lateral dimensions. This means that just by their size larger particles are located in streamlines of higher flow velocities than smaller ones and are eluted first (opposite to the solution sequence in the classical FFF mode). For details on hydrodynamic chromatography,see [64-66]. 

Some theories are now available in which tangential ion transport in the stagnant layer Is accounted for. or which may be modified to include this phenomenon. The quantity K° is system-specific, whereas K is generic therefore evaluation of the former requires insight into such properties as the distribution of charges and lateral mobilities. On the other hand, when K may... 

Although the phenomenon of multidrug resistance of bacteria was observed more than fifty years ago, it took 20 years until the first drug transporter, P-glycoprotein, was discovered as the responsible cellular factor for the outward transport of xenobiotics of different chemical structure. Another ten years later, experimental results on different tumor cell lines indicated that P-glycoprotein also occurs in advanced cancers and plays a major role in contributing to the non-response to chemotherapy. 

Blaedel and Haupert (28) demonstrated the feasibility of using this phenomenon as a preconcentration technique using isotope tracer studies on the ions Na4", Cs4", Zn24", and later Blaedel and Christensen (29) extended the work to include the anions 1 and HP01 . They found anion transport to be much slower than the previously reported cation transport. Coion transport in the anion exchange membranes was much higher and apparently dependent on the anionic charge of the bulk electrolyte. Further studies with more recently available membranes (1) seem to be needed. 

In most formulas for potassium nitrate-based flitter effects, the sulfur content of the stars will be found to be below the stoichiometric requirements for the formation of sulfide from all of the potassium nitrate. In most of the formulas of this type there is insufficient carbon to perform the total reduction of the potassium nitrate to form sulfides. Potassium nitrate can react with sulfur to produce sulfate directly and this is common in flitter effects. In all cases flitter effects will be found to have insufficient molten sulfide melts to protect the aluminum from direct reaction with oxygen from air. A thin layer of potassium sulfide at the melting point is quickly oxidized and thus there is rapid loss of the sulfur content. A thin layer of potassium sulfide on aluminum is insufficient to cause delay. The oxidation of the aluminum takes place first through a rate moderating oxygen transport system liquid layer covering the aluminum and then must later take place within the solid jacket of potassium aluminate that forms over the aluminum. This explains the observation that most flitter sparks lose incandescence in a smooth decent of temperatures at the end of their burn. This can also explain why some formulas appear to produce sparks at more than one temperature. Adjustment of flitter effects is easily made with an understanding of the phenomenon involved. 

In EMST, the first step consists of miniaturization of the products to the microrange or even in nanoscale, which is known as scahng down. In microrange, the electrochemical reactions are similar to that in the macroscopic domain such as charge transfer reactions and nucleation phenomenon at the solid-liquid interface as well as reaction and transport mechanism in liquid, i.e., electrolyte solution. However, in scaling down, the dimensions of the product become similar to the dimension range of diffusion layer. In this microscopic domain of electrochemistry, the linear dimensions, i.e., lateral x-y and vertical z of the product, become smaller and tend to spherical symmetry and are represented only... 

Prior to the consolidation of fouling, which represents the penetration of the solute molecules of the feed fluid in the pores of the membrane, blocking them, there is an increased concentration of solutes on the membrane surface due to the concentration of solutes in solution, resulting from transport by convection, which is known as concentration polarization. This polarization leads to the precipitation of solute molecules on the surface of the membrane, a phenomenon known as the formation of gel layer. Later, the adsorption of small molecules on the inner wall of pores, and a complete occlusion by the molecules of solute leads to consolidated fouling. These phenomena determine a rapid reduction in the permeate flux. 

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