Water-phase controlled

At first sight, there seems to be a basic difference between the two regimes with respect to the influence of Kia/Vl. In the water-phase-controlled regime, the overall exchange velocity, via/w, is independent of Kia/v/, whereas in the air-phase controlled regime v(a/w is linearly related to Ga/w. Yet, this asymmetry is just a consequence of our decision to relate all concentrations to the water phase. In fact, for substances with small Kia/v/ values, the aqueous phase is not the ideal reference system to describe air-water exchange. This can be best demonstrated for the case of exchange of water itself (Kia/V1 = 2.3 x 10 5 at 25°C), that is, for the evaporation of water. 

Figure 20.7 Overall air-water transfer velocity vla/w as a function of Henry s Law coefficient for two very different wind conditions, 10 = 1 m s l (calm overland condition) and Kl0 = 20 m s 1 (rough ocean conditions). The solid lines are calculated for average compound properties Diz = 0.1 cm2 s 1 and Sc,w = 600. The dashed line indicates the boundary between air-phase- and water-phase-controlled transfer velocities. See Table 20.5 for definitions of parameters and substances.

Figure 5.15. Plot of log air saturation concentration = p°/RT versus water solubility C°ater for selected chemicals (25 °C). Chemicals of equal H lie on the same 45° diagonal. Water phase control and air phase control refer to the transfer kinetics (rate-controlling steps) in the two-film theory. (Adapted from Mackay, 1991.)...

Emulsion paints are characterized by the fact that the binder (polymer) is in a water-dispersed form, whereas in a solvent paint it is in solution form. In emulsion systems the external water phase controls the viscosity, and the molecular weight of the polymer in the internal phase does not affect it, so polymers of high molecular weight are readily utilized in these systems. This is an advantage of emulsion paints. 

In general, water-phase controlled means that IOOKb/Kb is greater than 90%, and air-phase controlled means that 100 Kb/Kb is less than 10%. Values between 10% and 90% are considered to be controlled by both phases. 

A user roadmap is given in Eigure 9.13. For the air-water transfer of a given chemical, one needs to first determine the Henry s law coefficient. Then choose the appropriate ratio of Kl /Kq, and determine whether the compound is water-phase controlled, gas-phase controlled, or controlled by both phases. Then, one simply needs to choose the correct equation(s) for the application. 

Water-Based Muds. About 85% of all drilling fluids are water-based systems. The types depend on the composition of the water phase (pH, ionic content, etc), viscosity builders (clays or polymers), and rheological control agents (deflocculants or dispersants (qv)). 

Ethylbenzene Hydroperoxide Process. Figure 4 shows the process flow sheet for production of propylene oxide and styrene via the use of ethylbenzene hydroperoxide (EBHP). Liquid-phase oxidation of ethylbenzene with air or oxygen occurs at 206—275 kPa (30—40 psia) and 140—150°C, and 2—2.5 h are required for a 10—15% conversion to the hydroperoxide. Recycle of an inert gas, such as nitrogen, is used to control reactor temperature. Impurities ia the ethylbenzene, such as water, are controlled to minimize decomposition of the hydroperoxide product and are sometimes added to enhance product formation. Selectivity to by-products include 8—10% acetophenone, 5—7% 1-phenylethanol, and <1% organic acids. EBHP is concentrated to 30—35% by distillation. The overhead ethylbenzene is recycled back to the oxidation reactor (170—172). 

Total Salinity. The salinity control of oil-base mud is very important for stabilizing water-sensitive shales and clays. Depending upon the ionic concentration of the shale waters and of the mud water phase, an osmotic flow of pure water from the weaker salt concentration (in shale) to the stronger salt concentration (in mud) will occur. This may cause a dehydration of the shale and, consequently, affect its stabilization. 

By covalently attaching reactive groups to a polyelectrolyte main chain the uncertainty as to the location of the associated reactive groups can be eliminated. The location at which the reactive groups experience the macromolecular environment critically controls the reaction rate. If a reactive group is covalently bonded to a macromolecular surface, its reactivity would be markedly influenced by interfacial effects at the boundary between the polymer skeleton and the water phase. Those effects may vary with such factors as local electrostatic potential, local polarity, local hydrophobicity, and local viscosity. The values of these local parameters should be different from those in the bulk phase. 

The reactivity modification or the reaction rate control of functional groups covalently bound to a polyelectrolyte is critically dependent on the strength of the electrostatic potential at the boundary between the polymer skeleton and the water phase ( molecular surface ). This dependence is due to the covalent bonding of the functional groups which fixes the reaction sites to the molecular surface of the polyelectrolyte. Thus, the surface potential of the polyion plays a decisive role in the quantitative interpretation of the reactivity modification on the molecular surface. 

Metal carbonate decompositions proceed to completion in one or more stages which are generally both endothermic and reversible. Kinetic behaviour is sensitive to the pressure and composition of the prevailing atmosphere and, in particular, to the availability and ease of removal of C02. The structure and porosity of the solid product and its relationship with the reactant phase controls the rate of escape of volatile product by inter-and/or intragranular diffusion, so that rapid and effectively complete withdrawal of C02 from the interface may be difficult to achieve experimentally. Similar features have been described for the removal of water from crystalline hydrates and attention has been drawn to comparable aspects of reactions of both types in Garner s review [ 64 ]. 

Emulsifiers assist the stabilizing hydrocolloids in controlling crystal structure. They accentuate the function of the homogenizer in reducing the size of the fat globules. They also reduce the interfacial tension between the fat and water phases of the mix. The result is smaller ice particles and air cells when the mix is frozen and a smoother and creamier finished product. 

Y Sela, S Magdassi, N Garti. Release of markers from the inner water phase of W/O/W emulsions stabilized by silicone based polymeric surfactants. J Control Release 33(1) 1-12, 1995. 

Ten layers of dimyristoylphosphatidylethanolamine (DMPE) LB films were deposited on the QCM. The QCM was immersed into temperature-controlled water phase, and the frequency was followed with time. Typical time-courses of frequency changes are shown in Figure 8. The phase transition temperature from solid to liquid crystalline states of... 

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