Surface behavior, between phases

Surface Behavior. Most extraction processes deal with several phases. At the boundaries between these phases, an interface exists which can be populated with or depopulated of polymer. Situations in which the polymer should accumulate at the surface of one phase are 1. the flocculation of clays and fines or 2. the formation of foams, while situations in which the polymer should depopulate the surface of the phase boundary are 3 minimizing adsorption in mineral acid leaching or 4. minimizing surface tension with surfactants in oil recovery by miscible flooding.,... 

Our discussion of two-dimensional phases has drawn heavily on the analogy between bulk and surface behavior. This analogous behavior is not restricted to thermodynamic observations, but extends to other areas also. The viscosity of surface monolayers is an excellent example of this. To illustrate the parallel between bulk and surface viscosity, let us retrace some of the introductory notions of Chapter 4, restricting the flow to the surface region. 

Figure 8.8. Schematic illustrating the analogy between colloid flocculation behavior and phase behavior of the stabilizer in bulk solution. As density is lowered, separation of solvent from chains in bulk solution resembles separation of solvent from chains on surfaces, which produces flocculation.

In the field of surface and colloid science, characterization of surfaces has become an important utility in accounting for the observed behavior of any two-phase system. Many factors can affect the state of an interfacial region including van der Waals, electrostatic, acid-base, and covalent interactions between phase components. A complete account of the forces operating at an interface does much to predict and clarify the behavior of a system. In this context surface characterization is essential. 

The use of inverse gas chromatography (IGC) to study the properties of polymers has greatly increased in recent years (1,2). The shape and position of the elution peak contain information about all processes that occur in the column diffusion of the probe in the gas and the polymer phases, partitioning between phases, and adsorption on the surface of the polymer and the support. Traditional IGC experiments aim at obtaining symmetrical peaks, which can be analyzed using the van Deemter (3j or moments method (4). However, the behavior of the polymer-probe system is also reflected in the asymmetry of the peak and its tail. A method that could be used to analyze a peak of any shape, allowing elucidation of all the processes on the column, would be of great use. 

In addition to high filtration rates, asymmetric membranes are most fouling resistant. Conventional symmetric structures act as depth filters and retain particles within their internal structure. These trapped particles plug the membrane and the flux declines during use. Asymmetric membranes are surface filters and retain all rejected materials at the surface where they can be removed by shear forces applied by the feed solution moving parallel to the membrane surface. The difference in the filtration behavior between a symmetric and an asymmetric membrane is shown schematically in Figure 1.10. Two techniques are used to prepare asymmetric membranes one utilizes the phase inversion process and the other leads to a composite structure by depositing an extremely thin polymer film on a microporous substructure. 

Oscillatory measurements using the cone-and-plate viscometer are sometimes carried out to demonstrate the elastic behavior of a viscoelastic fluid [10]. The fluid in the viscometer is subjected to an oscillatory strain imposed on the bottom surface while the response of the shearing stress is measured on the top surface. If the phase shift between the input strain and the output stress is 90°, the sample is purely viscous if it is 0°, the sample is completely elastic. A measured phase shift between 0° and 90° demonstrates that the fluid is viscoelastic. 

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