Immiscible, surface

Emulsifiers. The chemical stmctures of emulsifiers, or surfactants (qv), enable these materials to reduce the surface tension at the interface of two immiscible surfaces, thus allowing the surfaces to mix and form an emulsion (33). An emulsifier consists of a polar group, which is attracted to aqueous substances, and a hydrocarbon chain, which is attracted to Hpids. 

Haller, W., Blackburn, D. H., Wagstaff, F. E. Charles, R. J. 1970. Metastable immiscibility surface in the system Na20—B2O3—Si()2. Journal of the American Ceramic Society, 53, 34-39. 

Equation (23) obviously gives the two-dimensional ideal gas law when a > a2 and with the o2 term included represents part of the correction included in Equation (15). This model for surfaces is, of course, no more successful than the one-component gas model used in the kinetic approach however, it does call attention to the role of the substrate as part of the entire picture of monolayers. We saw in Chapter 3 that solution nonideality may also be considered in osmotic equilibrium. Pursuing this approach still further results in the concept of phase separation to form two immiscible surface solutions, which returns us to the phase transitions described above. 

Incompatible commingling molecules separate soon after the commingling stimulus is withdrawn such systems have short lifetimes and are therefore said to be unstable. For longer life, surface active compounds (surfactants), efficacious in small quantities, are added to decrease the contact angle between the immiscible surfaces, lower a, and permit the interfusion of the immiscible surfaces. 

The molecular models may predict at least two types of phase transitions Transitions leading to (i) two immiscible surface solutions, and (ii) surface precipitation. 

By analogy to phase transitions in liquid mixtures, the transitions we discuss here are first-order transitions leading to two immiscible surface solutions. One of the main features of these transitions is that the capacitance peaks become needle-like or disappear creating a capacitance pit. However, there are experimental systems where the creation of a capacitance pit is not necessarily associated with the disappearance of both peaks. ° This feature may be predicted by the molecular models as described in the subsection below. 

Figure 14 depicts the shape of the capacitance plots when such a surface transformation takes place. It is seen that the most notable differentiation from the transitions leading to two immiscible surface solutions is not the orthogonal shape of the pits, which may also be predicted by the previous type of transitions at great values, but the possibility of existence of capacitance peaks outside the transition region, a feature which has been detected experimentally. 

Flotation. Flotation is a gravity separation process which exploits differences in the surface properties of particles. Gas bubbles are generated in a liquid and become attached to solid particles or immiscible liquid droplets, causing the particles or droplets to rise to the surface. This is used to separate mixtures of solid-solid particles and liquid-liquid mixtures of finely divided immiscible droplets. It is an important technique in mineral processing, where it is used to separate different types of ore. 

The principal point of interest to be discussed in this section is the manner in which the surface tension of a binary system varies with composition. The effects of other variables such as pressure and temperature are similar to those for pure substances, and the more elaborate treatment for two-component systems is not considered here. Also, the case of immiscible liquids is taken up in Section IV-2. 

Fig. IV-21. Surface pressure versus area for monolayers of immiscible components a monolayer of pure cadmium arachidate (curve 1) and monolayers of mixed merocyanine dye, MC2, and cadmium arachidate of molar ratio r = 1 10 (curve 2) 1 5 (curve 3), 1 2 (curve 4), and pure MC2 (curve 5). The subphase is 2.5 x 0 M CdC, pH = 5.5 at 20°C. Curve 3a (O) was calculated from curves 1 and 5 using Eq. IV-44. (From Ref. [116].)...

Figure 5.7 Sketch of an interfacial polymerization with the collapsed polymer film being withdrawn from the surface between the immiscible phases. [Redrawn with permission from P. W, Morgan and S. L. Kwolek, J. Chem. Educ. 36 182 (1959) copyright by the American Chemical Society.]...

Emulsives are solutions of toxicant in water-immiscible organic solvents, commonly at 15 ndash 50%, with a few percent of surface-active agent to promote emulsification, wetting, and spreading. The choice of solvent is predicated upon solvency, safety to plants and animals, volatility, flammabiUty, compatibihty, odor, and cost. The most commonly used solvents are kerosene, xylenes and related petroleum fractions, methyl isobutyl ketone, and amyl acetate. Water emulsion sprays from such emulsive concentrates are widely used in plant protection and for household insect control. 

A wide variety of capsules loaded with water-immiscible or water-iasoluble materials have been prepared by complex coacervation. Capsule size typically ranges from 20—1000 p.m, but capsules outside this range can be prepared. Core contents usually are 80—95 wt %. Complex coacervation processes are adversely affected by active agents that have finite water solubiUty, are surface-active, or are unstable at pH values of 4.0—5.0. The shell of dry complex coacervate capsules is sensitive to variations ia atmospheric moisture content and becomes plasticized at elevated humidities. 

Static mixing of immiscible Hquids can provide exceUent enhancement of the interphase area for increasing mass-transfer rate. The drop size distribution is relatively narrow compared to agitated tanks. Three forces are known to influence the formation of drops in a static mixer shear stress, surface tension, and viscous stress in the dispersed phase. Dimensional analysis shows that the drop size of the dispersed phase is controUed by the Weber number. The average drop size, in a Kenics mixer is a function of Weber number We = df /a, and the ratio of dispersed to continuous-phase viscosities (Eig. 32). 

Wettabihty is defined as the tendency of one fluid to spread on or adhere to a soHd surface (rock) in the presence of other immiscible fluids (5). As many as 50% of all sandstone reservoirs and 80% of all carbonate reservoirs are oil-wet (10). Strongly water-wet reservoirs are quite rare (11). Rock wettabihty can affect fluid injection rates, flow patterns of fluids within the reservoir, and oil displacement efficiency (11). Rock wettabihty can strongly affect its relative permeabihty to water and oil (5,12). When rock is water-wet, water occupies most of the small flow channels and is in contact with most of the rock surfaces as a film. Cmde oil does the same in oil-wet rock. Alteration of rock wettabihty by adsorption of polar materials, such as surfactants and corrosion inhibitors, or by the deposition of polar cmde oil components (13), can strongly alter the behavior of the rock (12). 

Immiscible Blends. When two polymers are blended, the most common result is a two-phase composite. The most interesting blends have good adhesion between the phases, either naturally or with the help of an additive. The barrier properties of an immiscible blend depend on the permeabihties of the polymers, the volume fraction of each, phase continuity, and the aspect ratio of the discontinuous phase. Phase continuity refers to which phase is continuous in the composite. Continuous for barrier appHcations means that a phase connects the two surfaces of the composite. Typically, only one of the two polymer phases is continuous, with the other polymer phase existing as islands. It is possible to have both polymers be continuous. 

The use of a water-immiscible Hquid to separate coal from impurities is based on the principle that the coal surface is hydrophobic and preferentially wetted by the nonaqueous medium whereas the minerals, being hydrophilic, remain suspended in water. Hence, separation of two phases produces a clean coal containing a small amount of a nonaqueous Hquid, eg, oil, and an aqueous suspension of the refuse. This process is generally referred to as selective agglomeration. 

The interfacial area between gases and hquids, immiscible liquids, and solids and hquids may be enlarged or reduced by these viscous and inertia forces when interacting with interfacial forces such as surface tension. 

The prediction of drop sizes in liquid-liquid systems is difficult. Most of the studies have used very pure fluids as two of the immiscible liquids, and in industrial practice there almost always are other chemicals that are surface-active to some degree and make the pre-dic tion of absolute drop sizes veiy difficult. In addition, techniques to measure drop sizes in experimental studies have all types of experimental and interpretation variations and difficulties so that many of the equations and correlations in the literature give contradictoiy results under similar conditions. Experimental difficulties include dispersion and coalescence effects, difficulty of measuring ac tual drop size, the effect of visual or photographic studies on where in the tank you can make these obseiwations, and the difficulty of using probes that measure bubble size or bubble area by hght or other sample transmission techniques which are veiy sensitive to the concentration of the dispersed phase and often are used in veiy dilute solutions. 

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