Films model studies

Films and membranes include a formidable array of materials that are widely used in a range of industrial applications. Films find application as coatings and packaging materials such as food wraps, and membranes are used for separations, controlled release, coating and packaging barriers and contact lenses. Structural studies of films fall into two major categories model film studies and the study of commercial films. Model studies are generally. conducted in university research laboratories where the thin, flat structure of a melt cast or drawn film provides an ideal speci-... 

Semiconductor photocatalysts in a form of colloids, powders, porous granules, thin films or bulk solids including single crystals (used in model studies) provide both liquid phase and gas phase transformations. Comprehensive reviews in this field can be found in monographs [4] (Chapters by N.S.Lewis and M.L.Rosenbluth M.Gratzel M.Schiavello and A.Sclafani P.Pichat and J.-M.Herrmann G.A.Somorjai T.Sakata H.Tributsch M.A.Fox H.Al-Ekabi and N.Serpone D.F.Ollis, E.Pelizzetti and N.Serpone) [8] (Chapter by Yu.A.Gruzdkov, E.N.Savinov and V.N.Parmon) and [3]. 

Lacy, C. E., A. E. Dukler, 1994, Flooding in Vertical Tubes—I Experimental Studies of the Entrance Region, Int. J. Multiphase Flow 20 219-233 Flooding in Vertical Tubes—II A Film Model for Entrance Region Flooding, Int. J. Multiphase Flow 20 235-247. (3)... 

The mass and heat transport model should be able to predict mass and energy fluxes through a gas/vapour-liquid interface in case a chemical reaction occurs in the liquid phase. In this study the film model will be adopted which postulates the existence of a well-mixed bulk and a stagnant transfer zone near the interface (see Fig. 1). The equations describing the mass and heat fluxes play an important role in our model and will be presented subsequently. 

In this paper, the volatilization of five organophosphorus pesticides from model soil pits and evaporation ponds is measured and predicted. A simple environmental chamber is used to obtain volatilization measurements. The use of the two-film model for predicting volatilization rates of organics from water is illustrated, and agreement between experimental and predicted rate constants is evaluated. Comparative volatilization studies are described using model water, soil-water, and soil disposal systems, and the results are compared to predictions of EXAMS, a popular computer code for predicting the fate of organics in aquatic systems. Finally, the experimental effect of Triton X-100, an emulsifier, on pesticide volatilization from water is presented. 

In the seventies, the growing interest in global geochemical cycles and in the fate of man-made pollutants in the environment triggered numerous studies of air-water exchange in natural systems, especially between the ocean and the atmosphere. In micrometeorology the study of heat and momentum transfer at water surfaces led to the development of detailed models of the structure of turbulence and momentum transfer close to the interface. The best-known outcome of these efforts, Deacon s (1977) boundary layer model, is similar to Whitman s film model. Yet, Deacon replaced the step-like drop in diffusivity (see Fig. 19.8a) by a continuous profile as shown in Fig. 19.8 b. As a result the transfer velocity loses the simple form of Eq. 19-4. Since the turbulence structure close to the interface also depends on the viscosity of the fluid, the model becomes more complex but also more powerful (see below). 

Cince the introduction by Mueller et al. (11) of a method for producing black lipid films, the study of black lipid films has expanded rapidly. Numerous publications have appeared in which these films are used to study physicochemical properties, others in which they are used as a model for biological membranes. Survey articles have been published by Mueller et al. (12), by Thompson (18), and more recently by Tien and Dawidowicz (19). As a model for a cellular membrane the black lipid film is bound to be a poor model since it lacks an essential part of even the simplest membrane conception, the polyelectrolyte coating. One of the few properties of black lipid membranes within biological ranges is its water permeability (4, 5, 6, 20). 

Film penetration studies show unequivocally that lecithin-cholesterol mixtures containing from 0 to 50 mole % cholesterol and lecithin—lactoside mixtures containing from 0 to 80 mole % Ci6-dihydroceramide lactoside have the same effect as pure lecithin. This suggests the presence of a lipid complex in which lecithin prevents the interaction of the cholesterol or ceramide lactoside with globulin. Over these ranges of composition the lipid film would consist of a mixture of the lecithin-cholesterol or the lecithin-lactoside complex with excess lecithin. One may picture two models in which the protein contact is restricted to molecules of lecithin. In one, individual polar groups of the protein interact with the excess lecithin molecules as well as with the lecithin portions of the complex. In the other model, the protein as a whole interacts with the lecithin sites of polymeric lipid structures. The latter, which could be referred to as surface micelles (I), are visualized also through the term "mono-... 

Depending on the properties of these films, which can be described by insulating or semiconductor models, ionic migration assisted by the high electrical fields set up across the surface layers and ionic diffusion are possible during the processes of formation—dissolution of the films. The study of the electrokinetics of most of these systems is at present the subject of considerable research. 

Lynn (L14), 1960 Theoretical consideration of acceleration of film near inlet slots of various types. Model studies suggested that acceleration should be complete in a very short distance. 

The two-film model representation can serve as a basis for more complicated models used to describe heterogeneously catalyzed RSPs or systems containing suspended solids. In these processes a third solid phase is present, and thus the two-film model is combined with the description of this third phase. This can be done using different levels of model complexity, from quasi-homogeneous description up to the four-film presentations that provide a very detailed description of both vapor/gas/liquid-liquid and solid/liquid interfaces (see, e.g., Refs. 62, 68 and 91). A comparative study of the modeling complexity is given in Ref. 64 for fuel ether synthesis of MTBE and TAME by CD. 

Nearly all thin film photolysis studies involving triazine herbicides have utilized model surfaces such as filter paper (Jordan et al., 1964 Morita et al., 1988), aluminum (Jordan et al., 1965), glass (Pape and Zabik, 1972 Chen et al., 1984 Hubbs and Lavy, 1990), and silica gel (Lotz et al., 1983). A shortcoming of the use of model surfaces is that herbicide dissipation due to volatility losses is often not accounted for (Hubbs and Lavy, 1990). Konstantinou et al. (2001) studied the sunlight photolysis of atrazine, propazine, and prometryn on soil (sandy clay loam, clay loam, and... 

The sea-to-air flux of DMS in the study area was calculated to be about 4.4 /imol m 2d 1 over the open ocean (Drake Passage) and about 1.2 /imol m d 1 from the inshore waters of Gerlache and Bransfield Strait (121. The calculations were based on a simple parameterization of the stagnant film model (22). The results are estimated to be uncertain by a factor of 2 (1). The difference between the open ocean and inshore area values can be attributed mainly to differences in wind velocities rather than sea surface temperatures or aqueous DMS concentrations between both regions. 

We studied these phenomena experimentally in a wetted wall column and two stirred cell reactors and evaluated the results with both a penetration and a film model description of simultaneous mass transfer accompanied by complex liquid-phase reactions [5,6], The experimental results agree well with the calculations and the existence of the third regime with its desorption against overall driving force is demonstrated in practice (forced desorption or negative enhancement factor). 

Corresponding model studies in which DAA-saturated laminate films as well as DAA-saturated mono films were heated under the cooking conditions confirmed the suspected dehydration reaction of DAA by the ethylene ionomer. This gave an logical explanation for the mechanism of formation for the 4-methyl-4-mercaptopentane-2-one off-odor (Fig. 13-5). 

---