Emulsifiers s. Surface-active

Two-component waterborne urethane dispersions are similar to the one-component PUD s in that a polyurethane dispersion comprises one of the two components. The second component is usually a crosslinker from the following classes of materials (a) polyisocyanates, (b) aziridines, (c) polycarbodiimides, and (d) epoxies. Many of the crosslinkers are not inherently water-soluble or water-dispersible. Therefore, they must be modified with surface active agents themselves, so as to become emulsifiable in water. 

MAIs may also be formed free radically when all azo sites are identical and have, therefore, the same reactivity. In this case the reaction with monomer A will be interrupted prior to the complete decomposition of all azo groups. So, Dicke and Heitz [49] partially decomposed poly(azoester)s in the presence of acrylamide. The reaction time was adjusted to a 37% decomposition of the azo groups. Surface active MAIs (M, > 10 ) consisting of hydrophobic poly(azoester) and hydrophilic poly(acrylamide) blocks were obtained (see Scheme 22) These were used for emulsion polymerization of vinyl acetate—in the polymerization they act simultaneously as emulsifiers (surface activity) and initiators (azo groups). Thus, a ternary block copolymer was synthesized fairly elegantly. 

Kato S, Sato K, Maeda D, Nomura M. A kinetic investigation of styrene emulsion polymerization with surface active polyelectrolytes as the emulsifier. II Effects of molecular weight and composition. Colloids Surf 1999 A153 127-131. 

Due to the relatively weak adsorption of homopolymers at the L/L interface, and in some cases at the S/L interface, homopolymers are seldom used as emulsifiers or dispersants. For this purpose, the molecule is modified to include some specific units that have strong adsorption to the surface. A good example is partially hydrolysed poly (vinyl acetate), which is commercially referred to as poly(vinyl alcohol) (PVA). The polymer contains 4-12% acetate groups (i.e. 96-88% hydrolysed) and these groups impart an amphipathic character to the chain. The polymer becomes surface-active at the L/L interface and hence it can be used as an emulsifier. In addition, on a hydrophobic surface such as polystyrene, the acetate groups become preferentially adsorbed on the surface of the particles, thus leaving the PVA units dangling in solution as loops and tails . The latter provide the required steric stabilization. 

In the interfacial synthesis, toluene and aqueous s ium hydroxide are used as the biphasic reaction medium, palladium-(4-dimethylaminophenyl) diphenylphospine complex as the surface-active catalyst complex, and DSS as die emulsifier. The nature of the phosphine ligand is important. Little or no reaction occurs with triethoxylphenylphosphine and triphenylphosphine as the ligand. Presumably, (4-dimethylaminophenyl)diphenylphosphine is effective in promoting the reaction because it is surface-active. A cationic surfactant, e.g., cetyltrimethylammonium bromide, may be used in place of DSS as the emulsifier. In that event, however, the surfactant also functions as a phase-transfer agent, and the overall reaction has a phase-transfer component in addition to the interfadal component. 

The Function of Detergency. The cleaning action of detergents is based on their ability to emulsify or disperse different types of soil and hold it in suspension in water. The workhorse involved in this job is the surfactant, a compound used in all soaps and determents. This abUity comes from the surfactant s molecular structiu"e and surface activity. When a soap or determent product is added to water that contains insoluble materials like dirt, oil, or grease, surfactant molecules adsorb onto the substrate (clothes) and form clusters called micelles, which surround the immiscible droplets. The micelle itself is water soluble and allows the trapped oil droplets to be dispersed throughout the water and rinsed away. While this is a simplified explanation, detergency is a complex set of interrelated functions that rehes on the diverse properties of surfactants, their interactions in solution, and their unique ability to disrupt the surface tension of water. 

A wide variety of surface-active agents can be used to prepare emulsion formulations. This variety is tremendously narrowed in the case of parenteral formulations. One of the major obstacles is the hemolytic effect of the majority of emulsifiers, which excludes their use for intravenous application (49) as seen in Fig. 9. The emulsion droplet surface can be charged negatively or positively by the incorporation of charged emulsifiers or enriched with reactive group.s to which ligands can be covalently linked. 

At relatively low concentrations, gum arabic yields solutions that are essentially Newtonian in behavior and have very low viscosities compared to other polysaccharides of similar molecular mass. This behavior is similar to that of globular proteins. The intrinsic viscosities at pH 5.5 of gum arabic solutions are similar to those of P-lactoglobulin. Randall et al. [158] and Williams et al. [148] concluded that the AGP fraction is responsible for the gum s emulsifying ability. Consequently, relatively high concentrations of gum arabic are required to produce stable emulsions of relatively small droplet size. At a lower gum arabic concentration, there is insufficient surface-active material to fully coat all the droplets. Therefore, it has been concluded that although gum arabic is basically a polysaccharide, its interfacial and emulsifying properties are derived from its proteinaceous nature. 

FIGURE 7.7 (See color insert) Adoptively transferred D011.10 transgenicT cells can be identified by expression of CD4+ and KJ-126 in spleen cell suspension from Balb/c mice after ovalbumin (OVA) immunization. Balb/c mice were injected iv with D011.10 spleen cells containing 3-5 x 1 06CD4+KJ-126+ cells and immunized by intraperitoneal injection of 2 mg OVA emulsified in complete Freund s adjuvant 2 days later. OVA immunization increases the frequency of KJ+T cells and alters the expression of various surface molecules consistent with T cell (Tc) activation. 

Figure 1 Relationships of S with interfacial tension and emulsifying activity of proteins. I, bovine serum albumin 2, /3-lactoglobulin 3. trypsin 4, ovalbumin 5, conalbuntin 6, lysozyme 7, K-casein 8, 9, I0, II, and 12, denatured ovalbumin by heating at 85°C for l, 2, 3, 4, and 5 min respectively 13, 14, 15, 16. 17, and 18. denatured lysozyme by heating at 85"C for l, 2, 3, 4, 5, and 6 min respectively 19, 20, 21, 22, and 23, ovalbumin bound with 0.2, 0.3, 1.7, 5.7, and 7.9 mol of sodium dodecyl sulfate/mol of protein respectively 24, 25, 26, 27, and 28, ovalbumin bound with 0.3, 0.9, 3.1,4.8, and 8.2 mol of linoleate/mol of protein respectively. Interfacial tension measured at corn oil/0.20c protein interface with a Fisher Surface Tensiontat Model 21. Emulsifying activity index calculated from the absorbance at 500 nm of the supernatant after centrifuging blended mixtures of 2 ml of corn oil and 6 ml of 0.5% protein in 0.01 M phosphate buffer, pH 7.4 S initial slope of fluorescence intensity (FI) vs. percent protein plot. 10 /al of 3.6 mM m-parinaric acid solution was added to 2 ml of 0.002 to 0.1% protein in 0.01 M phosphate buffer, pH 7.4, containing 0.002% SDS. FI was measured at 420 nm by exciting at 325 nm. (From Ref. 2. Reprinted by permission.)...

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