Surface-active components

An important industrial example of W/O emulsions arises in water-in-crude-oil emulsions that form during production. These emulsions must be broken to aid transportation and refining [43]. These suspensions have been extensively studied by Sjoblom and co-workers [10, 13, 14] and Wasan and co-workers [44]. Stabilization arises from combinations of surface-active components, asphaltenes, polymers, and particles the composition depends on the source of the crude oil. Certain copolymers can mimic the emulsion stabilizing fractions of crude oil and have been studied in terms of their pressure-area behavior [45]. 

Film Rupture. Another general mechanism by which foams evolve is the coalescence of neighboring bubbles via film mpture. This occurs if the nature of the surface-active components is such that the repulsive interactions and Marangoni flows are not sufficient to keep neighboring bubbles apart. Bubble coalescence can become more frequent as the foam drains and there is less Hquid to separate neighbors. Long-Hved foams can be easHy... 

Adhesives and Sealants. Most industrial adhesives contain surface active components and additives, and air entrainment during their mechanical appHcation can significantly reduce joint strength. Defoamers are usually formulated into adhesives to protect users against such difficulties. Additional benefits, such as improved uniformity of products, increased throughput and reduced labor costs can also result from the use of defoamers during adhesive appHcation. The footwear and nonwoven fabric industries are extensive users of defoamers in this way. 

The first studies of the electrical double-layer structure at Sn + Pb and Sn + Cd solid drop electrodes in aqueous surface-inactive electrolyte solutions were carried out by Kukk and Piittsepp.808 Alloys with various contents of Pb (from 0.2 to 98%) were investigated by impedance.615,643,667,816 Small amounts of Pb caused dramatic shifts of toward more negative values. For alloys with Pb bulk content 0.2%, was the same as for pc-Pb. The was independent of Crf and frequency. C xt Cjl plots were linear, with/pz very close to unity. Thus the surface of Sn + Pb alloys behaves as if it were geometrically smooth, and Pb appears to be the surface-active component. 

Anodically polished and then cathodically reduced Cd + Pb alloys have been studied by impedance in aqueous electrolyte solutions (NaF, KF, NaC104, NaN02, NaN03).827 For an alloy with 2% Pb at cNap 0.03 M, Emfo = -0.88 V (SCE) and depends on cNaF, which has been explained by weak specific adsorption of F" anions. Surface activity increases in the sequence F" < CIO4 < N02. The Parsons-Zobel plot at E is linear, with /pz = 1.33 and CT° = 0.31 F m"2. Since the electrical double-layer parameters are closer to those for pc-Pb than for pc-Cd, it has been concluded that Pb is the surface-active component in Cd + Pb alloys827 (Pb has a lower interfacial tension in the liquid state). 

Various pc electrode models have been tested.827 Using the independent diffuse layer electrode model74,262 the value of E n = -0.88 V (SCE) can be simulated for Cd + Pb alloys with 63% Pb if bulk and surface compositions coincide. However, large deviations of calculated and experimental C,E curves are observed at a 0. Better correspondence between experimental and calculated C,E curves was obtained with the common diffuse-layer electrode model,262 if the Pb percentage in the solid phase is taken as 20%. However, the calculated C, at a Ois noticeably lower than the experimental one. It has been concluded that Pb is the surface-active component in Cd + Pb alloys, but there are noticeable deviations from electrical double-layer models for composite electrodes.827... 

To mitigate the effects of corrosion resulting from the presence of salts, it is advantageous to reduce the salt concentration to the range of 3 to 5 ppm. Typically, brine droplets in crude oil are stabilized by a mixture of surface-active components such as waxes, asphaltenes, resins, and naphthenic acids that are electrostatically bound to the droplets surface. Such components provide an interfacial film over the brine droplet, resulting in a diminished droplet coalescence. Adding water to the crude oil can decrease the concentration of the surface-active components on the surface of each droplet, because the number of droplets is increased without increasing component concentration. 

The amount of added water required for desalting may be minimized by adding a chemical emulsion breaker to the crude that is capable of displacing the surface-active components from the brine droplets. Quatemized carboxylic-sulfonic acid salts, shown in Figure 22-9, are useful for desalting [1791]. Preferably, the chemical emulsion breaker is used in combination with a delivery solvent, such as diethylene glycol monobutyl ether. 

This is very similar to the flotation procedure described under basic physical treatments. In the case of foam fractionation, not only are the pollutants raised to the surface where they can be skimmed off, but a froth, like beer foam, is produced in which the pollutants become concentrated. The key to the process is the adsorption of the pollutants onto the surface-active agents that cause the froth to form. Sometimes a surfactant is added so that non-surface-active components can be removed. 

During the past few years, the determination of the interfacial properties of binary mixtures of surfactants has been an area in which there has been considerable activity on the part of a number of investigators, both in industry and in academia. The Interest in this area stems from the fact that mixtures of two different types of surfactants often have interfacial properties that are better than those of the individual surfactants by themselves. For example, mixtures of two different surface-active components sometimes reduce the interfacial tension at the hydrocarbon/water interface to values far lower than that obtained with the individual surfactants, and certain mixtures of surfactants are better foaming agents than the individual components. For the purpose of this discussion we define synergism as existing in a system when a given property of the mixture can reach a more desirable value than that attainable by either surface-active component of the mixture by itself. 

We are interested in the behavior of surfactant molecules in the mixed adsorbed film. The nonideal behavior of a mixed adsorbed film is correlated to activity coefficients of surface-active components with reference to the pure adsorbed film of each component. In the same manner as the previous paper ( ), we can express the chemical potentials of 1-octadecanol and dodecylammonium chloride in the mixed adsorbed film as follows ... 

The oil itself is a complex mixture containing surface-active components. 

In a relatively new process for production and fractionation of fine particles by the use of compressible media - the PGSS process (Particles from Gas-Saturated Solutions) - the compressible medium is solubilized in the substance which has to be micronized [58-61]. Then the gas-containing solution is rapidly expanded in an expansion unit (e.g., a nozzle) and the gas is evaporated. Owing to the Joule-Thomson effect and/or the evaporation and the volume-expansion of the gas, the solution cools down below the solidification temperature of the solute, and fine particles are formed. The solute is separated and fractionated from the gas stream by a cyclone and electro-filter. The PGSS process was tested in the pilot- and technical size on various classes of substances (polymers, resins, waxes, surface-active components, and pharmaceuticals). The powders produced show narrow particle-size distributions, and have improved properties compared to the conventional produced powders. 

In milk, the important interfaces are those between the liquid product and air and between the milk plasma and the fat globules contained therein. Studies of the surface tension (liquid/air) have been made to ascertain the relative effectiveness of the milk components as depressants to follow changes in surface-active components as a result of processing to follow the release of free fatty acids during lipolysis and to attempt to explain the foaming phenomenon so characteristic of milk. Interfacial tensions between milk fat and solutions of milk components have been measured in studies of the stabilization of fat globules in natural and processed milks. 

The surface tension of milk is on the order of 50 dynes cm-1 at 20°C, compared to that of water, which is 72.75 dynes cm-1 at the same temperature. The milk proteins, milk fat, phospholipids, and free fatty acids are the principal surface-active components determining the surface properties of milk. 

When two emulsion drops or foam bubbles approach each other, they hydrodynamically interact which generally results in the formation of a dimple [10,11]. After the dimple moves out, a thick lamella with parallel interfaces forms. If the continuous phase (i.e., the film phase) contains only surface active components at relatively low concentrations (not more than a few times their critical micellar concentration), the thick lamella thins on continually (see Fig. 6, left side). During continuous thinning, the film generally reaches a critical thickness where it either ruptures or black spots appear in it and then, by the expansion of these black spots, it transforms into a very thin film, which is either a common black (10-30 nm) or a Newton black film (5-10 nm). The thickness of the common black film depends on the capillary pressure and salt concentration [8]. This film drainage mechanism has been studied by several researchers [8,10-12] and it has been found that the classical DLVO theory of dispersion stability [13,14] can be qualitatively applied to it by taking into account the electrostatic, van der Waals and steric interactions between the film interfaces [8]. 

Uniform submicron particles cannot be obtained by hydrolysis of concentrated Ti(OR)4 solutions, as the increase of concentration results in agglomeration of the hydrolysis product, these agglomerates cannot be redispersed. Agglomeration is a result ofthe sticking together ofthe particles in the course of Brown movement, which can be overcome electrostatically (it assumes introduction in the solution of electrolyte ions) or sterically by introduction of surface active components, which should be able to diffuse from solution and bind with the surface of the particle by physical adsorption. To prevent ag-... 

The situation is somewhat different for a solution, particularly if a surface-active component is present. The measured surface tension is strongly influenced by adsorption at the liquid-vapor surface (16). Yet adsorption does not influence the values of the Hamaker coefficients that must be used in Equations 3 and 10, which are related to those of the pure substances by the volume fraction weighted averages. Thus, for solution /... 

The square-root dependence of the rate of polymerization on the initiator concentration and the first-order dependence on the macromonomer concentration strongly deviate from the micellar model. The linear dependence of Rp on the macromonomer concentration was attributed to the linear dependence of the number of micelles on the macromonomer concentration. The deviation results from the fact that the macromonomer acts as monomer and emulsifier and/or surface active component is formed during polymerization, i.e., it takes part in the growth and nucleation events. The increase in the reaction order above 0.4 was also discussed in terms of the variation of the surface activity of graft copolymer with its molecular weight. 

It is very doubtful, however, if these solutions can be treated as two component systems they contain water and at least two ions, of which the adsorption is not the same and soaps, unless rather strongly alkaline, contain free fatty acid which is much more strongly adsorbed than the anion of the fatty acid. The course of a surface tension-concentration curve in a solution with several components may be complicated it seems possible that a rise of tension with increasing concentration may be due to a moderately surface active ion, present in large amount, displacing a more surface active component, present in smaller amount, from the surface. 

For multicomponent liquid mixtures in a slurry bubble column, the gas-phase characteristics appear to be greatly altered if small concentrations (less than 0.1 percent) of a surface active species are present (24). The degree to which the gas-phase characteristics are changed appears to be related to the type and concentration of the surface-active component and the turbulence in the bubble column. 

The origin of the extremes in the accumulation ratio (//) versus surfactant concentration curves is different for the individual compounds (in particular for proteins [74]) and for mixtures of two surface active components [78], For individual substances a maximum is reached at saturation of the adsorption layer and, respectively, CF after which further increase in concentration Cl leads to decrease in Ef. 

In producing dairy-based foams two principal options exist to make use of surface active components, proteins or fat, the latter one being discussed in Section 19.3. Observations comparable to pouring beer into glasses are made in filling skim milk... 

Determinations of the adsorption isotherms for a number of organic solvent-water systems in contact with hydrocarbonaceous stationary phases have shown that a layer of solvent molecules forms on the bonded-phase surface and that the extent of the layer increases with the concentration of the solvent in the mobile phase. For example, methanol shows a Langmuir-type isotherm when distributed between water and Partisil ODS (56). This effect can be exploited to enhance the resolution and the recoveries of hydrophobic peptides by the use of low concentrations, i.e., <5% v/v, of medium-chain alkyl alcohols such as tm-butanol or tert-pentanol or other polar, but nonionic solvents added to aquo-methanol or acetonitrile eluents. It also highlights the cautionary requirement that adequate equilibration of a reversed-phase system is mandatory if reproducible chromatography is to be obtained with surface-active components in the mobile phase. 

Leo Vroman and co-workers, who present additional data in this volume, have made major contributions to our understanding of the fundamentals of this adsorption process. They have shown that events at the substrate/blood interface are not static after the first layer of protein is adsorbed, but that the protein layer is continuously remodeled, reacted with, or converted by other surface-active components in intact plasma (37). 

Confection science is extremely complicated because of the great diversity of tria-cylglycerol molecules and the presence of several surface-active components that play a major role in defining the quality of the final products. 

The dissolution of surface-active components in the spreading liquid greatly affects its spreading behaviour. For Instance, an amphiphilic substance (e.g. oleic acid) will adsorb strongly at the water-benzene and benzene-air Interface, thereby reducing and considerably. As a result, a solution of oleic acid in benzene spreads readily over the water surface. 

---