Emulsions continued stability

From the experiments it is clear that poly electrolyte is adsorbed on the surface of the black lipid film. This applies both to the experiments with gelatin and bovine serum albumin, which gave no decrease of film resistance, and to the experiments with bovine erythrocyte ghost protein and polyphosphate. The adsorption of protein on the phospholipid-water interface may be controlled independently by investigating the electrophoretic behavior of emulsion droplets, stabilized by phospholipid, in a protein solution, as a function of pH. In this way Haydon (3) established protein adsorption on the phospholipid-water interface. If the high resistance (107 ohms per sq. cm.) of black lipid films is to be ascribed to the continuous layer of hydrocarbon chains in the interior of the film, as is generally done, an increase in film conductivity is not expected from adsorption without penetration. 

In 1962, Higuchi and Misra examined the quantitative aspects of the rate of growth of the large droplets and the rate of dissolution of the small droplets in emulsion for the case in which the process is diffusion controlled in the continuous phase [4]. It was proposed that unstable emulsions may be stabilized with respect to the Ostwald ripening process by the addition of small amounts of a third component, which must distribute preferentially in the dispersed phase [4]. The obtained stability in miniemulsions is said in the literature to be metastable or fully stable. The stabilization effect by adding a third component was recently theoretically described by Webster and Cates [5]. The authors considered an emulsion whose droplets contain a trapped species, which is insoluble in the continuous phase, and studied the emulsion s stability via the Lifshitz-Slyozov dynamics (Ostwald ripening). 

Polyanhydrides have been chosen for the preparation of microspheres because of their degradation by surface erosion into apparently non-toxic small molecules.f The mixture of polymer and active ingredient is suspended in a miscible solvent, heated 5°C above the melting point of the polymer and stirred continuously. The emulsion is stabilized by cooling below the melting point until the droplets solidify. 

Fig. 2. A schematic representation of the structure of whipped cream, showing the role of fat crystals within the emulsion droplets and partial coalescence of the emulsion in stabilizing the air bubbles and trapping the serum phase into a continuous three-dimensional network.

In the case of highly mobile interface between dispersed phase and dispersion medium (as in foams and emulsions) the condition of zero fluid flow velocity at interface (non-slip condition), determining the validity of Reynolds equation, may not be obeyed. In this case the decrease in the film thickness occurs at a greater rate. However, in foam and emulsion films stabilized by surfactant adsorption layers the conditions of fluid outflow from an interlayer are close to those of outflow from a gap between solid surfaces even in cases when surfactant molecules do not form a continuous solid-like film. This is the case because at surfactant adsorption below Tmax the motion of fluid surface leads to the transfer of some portions of surfactant adsorption layer from central regions of film to peripherical ones, adjacent to the Gibbs-Plateau channels. As a result, the value of adsorption decreases in the center of film, but increases at the periphery, which stipulates the appearance of the surface... 

In 1983, Thingstad and Pengeurd conducted photo-oxidation experiments and found that photo-oxidized oil formed emulsions (11). Nesterova et al. studied emulsion formation and concluded that it was strongly correlated with both the asphaltene and tar content of oil and also the salinity of the water with whieh it was formed (12). Mackay and Nowak studied emulsions and found that stable emulsions had low conductivity and therefore a continuous phase of oil (13,14). Stability was discussed and proposed to be a funetion of oil composition, particularly waxes as asphaltenes. It was proposed that a water droplet could be stabilized by waxes, asphaltenes, or a combination of both. The viscosity of the resulting emulsions was correlated with water content. Later work by the same group reported examination of Russian hypotheses that emulsions are stabilized by colloidal particles which gather at the oil— water interface and may combine to form a near-solid barrier that resists deformation and thus water-water coalescence (15). It was speculated that these particles could be mineral, wax crystals, aggregates of tar and asphaltenes, or mixtures of... 

Nanoparticles, being much smaller than the droplets and mainly wetted by the continuous phase, may also stabilize emulsion droplets [5, 6]. Emulsions thus stabilized are called pickering or particle-stabilized emulsions (PSE) . 

Margarine, which is produced worldwide in amounts exceeding 7 million t/a, is a water in oil emulsion. Its stability is achieved by an increase in viscosity of the continuous fat phase due to partial crystallization and through emulsifiers. The fat crystals form a three dimensional network. They should be present in the P -modification a conversion P —> P is undesirable because the P-modification causes a sandy texture defect. Hydrogenated fats, which are frequently used as... 

Conduchvity measurements are more specific in their application being of particular use in the case of aqueous dispersions of particles, stabilized by some type of ionic stabilizer. The information obtained with this technique is the conductivity of the continuous phase. In the case of emulsion polymerizations stabilized with ionic surfactants, this is related to the concentration of free surfactant, which, when combined with absorption isotherms, for example, or an empirical model, can be used to follow the evolution of the surface area of a latex. This is a promising method, but given its complexity and the need to develop more robust means of linking the conductivity to properties of interest, it has not found widespread use in commercial production at the current time. 

In principle, it should then be possible to predict the stability of an emulsion system from the interfacial rheology of the continuous phase. Figure 1.5 shows the relative stability to coalescence of an emulsion system stabilized by a protein (beta-lactoglobulin) with increased concentrations of non-ionic surfactant (Tween 20). In this case the presence of surfactants has entirely destabilized the protein emulsion. 

Fat crystals infiuence emulsion (in)stability, depending primarily on how they are wetted by the continuous or dispersed phases [37], based on a concept... 

Emulsion Process. The emulsion polymerization process utilizes water as a continuous phase with the reactants suspended as microscopic particles. This low viscosity system allows facile mixing and heat transfer for control purposes. An emulsifier is generally employed to stabilize the water insoluble monomers and other reactants, and to prevent reactor fouling. With SAN the system is composed of water, monomers, chain-transfer agents for molecular weight control, emulsifiers, and initiators. Both batch and semibatch processes are employed. Copolymerization is normally carried out at 60 to 100°C to conversions of - 97%. Lower temperature polymerization can be achieved with redox-initiator systems (51). 

The inverse emulsion form is made by emulsifying an aqueous monomer solution in a light hydrocarbon oil to form an oil-continuous emulsion stabilized by a surfactant system (21). This is polymerized to form an emulsion of aqueous polymer particle ranging in size from 1.0 to about 10 pm dispersed in oil. By addition of appropriate surfactants, the emulsion is made self-inverting, which means that when it is added to water with agitation, the oil is emulsified and the polymer goes into solution in a few minutes. Alternatively, a surfactant can be added to the water before addition of the inverse polymer emulsion (see Emulsions). 

Emulsion Polymerization. When the U.S. supply of natural mbber from the Far East was cut off in World War II, the emulsion polymerization process was developed to produce synthetic mbber. In this complex process, the organic monomer is emulsified with soap in an aqueous continuous phase. Because of the much smaller (<0.1 jira) dispersed particles than in suspension polymerization and the stabilizing action of the soap, a proper emulsion is stable, so agitation is not as critical. In classical emulsion polymerization, a water-soluble initiator is used. This, together with the small particle size, gives rise to very different kinetics (6,21—23). 

Because nitrile rubber is an unsaturated copolymer it is sensitive to oxidative attack and addition of an antioxidant is necessary. The most common practice is to add an emulsion or dispersion of antioxidant or stabilizer to the latex before coagulation. This is sometimes done batchwise to the latex in the blend tank, and sometimes is added continuously to the latex as it is pumped toward further processing. PhenoHc, amine, and organic phosphite materials are used. Examples are di-Z fZ-butylcatechol, octylated diphenylamine, and tris(nonylphenyl) phosphite [26523-78-4]. All are meant to protect the product from oxidation during drying at elevated temperature and during storage until final use. Most mbber processors add additional antioxidant to their compounds when the NBR is mixed with fillers and curatives in order to extend the life of the final mbber part. 

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