Interfacial influence

Fig. 6.3-3. They do not relax proportionally to the fourth root of time, which would be significant if the interfacial influence were important. Thus only the first term in the brackets of Equation 6.3-1 is required to explain diffusion near a consolute point.

The first term in square brackets is the effect of diffusion the second is the interfacial influence. 

While we don t know the interfacial influence y, we do know the characteristic length Amax in the original solution it is nothing more than the size of the molecules or clusters. Thus we can combine these last relations to obtain... 

One fascinating feature of the physical chemistry of surfaces is the direct influence of intermolecular forces on interfacial phenomena. The calculation of surface tension in section III-2B, for example, is based on the Lennard-Jones potential function illustrated in Fig. III-6. The wide use of this model potential is based in physical analysis of intermolecular forces that we summarize in this chapter. In this chapter, we briefly discuss the fundamental electromagnetic forces. The electrostatic forces between charged species are covered in Chapter V. 

The influence of an applied magnetic field, as introduced in section Bl.5.2.2. is quite different from that of an applied electric field. A magnetic field may perturb the interfacial nonlinear response (and that of the weak bulk tenns), but it does not lead to any dipole-allowed bulk nonlmear response. Thus, in the presence of magnetic fields and magnetization, SHG remains a probe that is highly specific to surfaces and interfaces. It... 

Other Interaction Processes. The selectivity of flotation reagents in a pulp and their functions depend on their interactions with the mineral phases to be separated, but other physicochemical and hydrodynamic processes also play roles. AH adsorption—desorption phenomena occur at the sohd—hquid interfacial region. Surface processes that influence such adsorptions include activation and depression. Activators and depressants are auxiUary reagents. 

The chemical composition, physical stmcture, and key physical properties of a foam, namely its stabiHty and theology, are all closely interrelated. Since there is a large interfacial area of contact between Hquid and vapor inside a foam, the physical chemistry of Hquid—vapor interfaces and their modification by surface-active molecules plays a primary role underlying these interrelationships. Thus the behavior of individual surface-active molecules in solution and near a vapor interface and their influence on interfacial forces is considered here first. 

Surface Tension. Interfacial surface tension between fluid and filter media is considered to play a role in the adhesion of blood cells to synthetic fibers. Interfacial tension is a result of the interaction between the surface tension of the fluid and the filter media. Direct experimental evidence has shown that varying this interfacial tension influences the adhesion of blood cells to biomaterials. The viscosity of the blood product is important in the shear forces of the fluid to the attached cells viscosity of a red cell concentrate is at least 500 times that of a platelet concentrate. This has a considerable effect on the shear and flow rates through the filter. The surface stickiness plays a role in the critical shear force for detachment of adhered blood cells. 

A key feature of encapsulation processes (Figs. 4a and 5) is that the reagents for the interfacial polymerisation reaction responsible for shell formation are present in two mutually immiscible Hquids. They must diffuse to the interface in order to react. Once reaction is initiated, the capsule shell that forms becomes a barrier to diffusion and ultimately begins to limit the rate of the interfacial polymerisation reaction. This, in turn, influences morphology and uniformity of thickness of the capsule shell. Kinetic analyses of the process have been pubHshed (12). A drawback to the technology for some apphcations is that aggressive or highly reactive molecules must be dissolved in the core material in order to produce microcapsules. Such molecules can react with sensitive core materials. 

Any conclusion that a low interfacial tension per se is an indication of enhanced emulsion stabiUty is not rehable. In fact (8), very low interfacial tensions lead to instabiUty. The stabiUty of an emulsion is influenced by the charge at the interface and by the packing of the emulsifier molecules, but the interfacial tension at the levels found in the common emulsion has no influence on stabiUty. 

For modest changes in temperature the influence of temperature upon the interfacial area a may be neglected. For example, in experiments on the absorption of SO9 in water, Whitney and Vivian [Chem. Eng, Pi og., 45, 323 (1949)] found no appreciable effect of temperature upon kcCi over the range from 10 to 50°C. 

It shoiild be noted that the influence of substituting solvents of widely differing viscosities upon the interfacial area a can be very large. One therefore should be cautious about extrapolating data to account for viscosity effects between different solvent systems. 

Rizzuti et al. [Chem. Eng. Sci, 36, 973 (1981)] examined the influence of solvent viscosity upon the effective interfacial area in packed columns and concluded that for the systems studied the effective interfacial area a was proportional to the kinematic viscosity raised to the 0.7 power. Thus, the hydrodynamic behavior of a packed absorber is strongly affected by viscosity effects. Surface-tension effects also are important, as expressed in the work of Onda et al. (see Table 5-28-D). 

Properties of Component Phases As discussed in the preceding subsection, dispersions of gases in liquids are affected by the viscosity of the hquid, the density of the liquid and of the gas, and the interfacial tension between the two phases. They also may be affected directly by the composition of the hquid phase. Both the formation of bubbles and their behavior during their lifetime are influenced by these quantities as weh as by the me(manical aspects of their environment. 

One cannot quantitatively predict the effect of the various interfacial phenomena thus, these phenomena will not be covered in detail here. The following literature gives a good general review of the effects of interfacial phenomena on mass transfer Goodridge and Robb, Ind. Eng. Chem. Fund., 4, 49 (1965) Calderbank, Chem. Eng. (London), CE 205 (1967) Gal-Or et al., Ind. Eng. Chem., 61(2), 22 (1969) Kintner, Adv. Chem. Eng., 4 (1963) Resnick and Gal-Or, op. cit., p. 295 Valentin, loc. cit. and Elenkov, loc. cit., and Ind. Eng. Chem. Ann. Rev. Mass Transfer, 60(1), 67 (1968) 60(12), 53 (1968) 62(2), 41 (1970). In the following outhne, the effects of the various interfacial phenomena on the factors that influence overall mass transfer are given. Possible effects of interfacial phenomena are tabulated below ... 

Information on the coefficients is relatively undeveloped. They are evidently strongly influenced by rate of drop coalescence and breakup, presence of surface-active agents, interfacial turbulence (Marangoni effect), drop-size distribution, and the like, none of which can be effectively evaluated at this time. 

The molecules of liquids are separated by relatively small distances so the attractive forces between molecules tend to hold firm within a definite volume at fixed temperature. Molecular forces also result in tlie phenomenon of interfacial tension. The repulsive forces between molecules exert a sufficiently powerful influence that volume changes caused by pressure changes can be neglected i.e. liquids are incompressible. 

Deruelle, M., Leger, L. and Tirrell, M., Adhesion at the solid-elastomer interface influence of the interfacial chains. Macromolecules, 28(22), 7419-7428 (1995). 

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