Emulsifier equilibrium concentration

By many properties emulsion aqueous films are analogous to foam films. There are several review articles dedicated to properties of emulsion aqueous films [e.g. 320,503-506]. The properties of microscopic emulsion aqueous films (kinetics of thinning, determination of equilibrium thickness, etc.) are studied employing devices quite similar to those used for foam films [503]. Analogous to foam films, stable (metastable) emulsion films are formed only in the presence of surfactants (emulsifiers) at concentrations higher than the critical concentration of formation of black spots C or the concentration, corresponding to... 

Figure 4. Adsorption of SLS as function of equilibrium concentration of emulsifier in the aqueous phase 1—MA, 2—EA, 3—BA, 4—St...

An emulsion is to prepared of white petroleum oil in 0.001 M KCl using sodium dodecylsulfate as emulsifier. If the surfactant adsorbs at the oil-water interface occupying 0.5 nm at equilibrium concentrations above 0.001 M, how much is needed per liter of emulsion if the average drop size is to be 50 nm in diameter ... 

The synthesis by route C is an alcoholysis reaction. The reaction proceeds as an emulsion due to the immiscibility of triacylglycerol and glycerol. Under this emulsified condition, the equilibrium concentration of the monoacylglycerol is 30% (Yamane et al., 1986). However, when the reaction is run at temperatures slightly below the melting point of the fat used, the concentration is favourably increased to 70-90% (McNeill et al., 1990 McNeill and Yamane, 1991). The reaction rate is affected by moisture content. Relatively high moisture leads to high initial reaction rates (McNeill et al., 1991) but excessive levels result in hydrolysis and production of free fatty acids. Compared to the synthetic pathways of A and B, route C seems a cheaper alternative since the primary reactive components do not include free fatty acids reactants that otherwise would have to be produced by other methods to make them available for the process. 

At low concentrations surfactant molecules adsorbed at the surface are in equilibrium with other molecules in solution. Above a threshold concentration, called the critical micelle concentration (cmc, for short), another equilibrium must be considered. This additional equilibrium is that between individual molecules in solution and clusters of emulsifier molecules known as micelles. 

In order to more closely represent the volatilization environment that would be encountered in an evaporation pond, Triton X-100, a non-ionic emulsifier similar to those used in some pesticide formulations, was added to prepared pesticide solutions at 1000 ppm. The presence of this emulsifier caused a decrease in the percent pesticide volatilized in one day in all cases except for mevinphos (Table VI). Three mechanisms are probably in operation here. First, Triton X-100 micelles will exist in solution because its concentration of 1000 ppm is well above its critical micelle concentration of 194 ppm (30). Pesticide may partition into these micelles, reducing the free concentration in water available for volatilization, which will in turn reduce the Henry s law constant for the chemical (31). Second, the pesticides may exhibit an affinity for the thin film of Triton that exists on the water surface. One can no longer assume that equilibrium exists across the air-water interface, and a Triton X-100 surface film resistance... 

The adsorption and desorption kinetics of surfactants, such as food emulsifiers, can be measured by the stress relaxation method [4]. In this, a "clean" interface, devoid of surfactants, is first formed by rapidly expanding a new drop to the desired size and, then, this size is maintained and the capillary pressure is monitored. Figure 2 shows experimental relaxation data for a dodecane/ aq. Brij 58 surfactant solution interface, at a concentration below the CMC. An initial rapid relaxation process is followed by a slower relaxation prior to achieving the equilibrium IFT. Initially, the IFT is high, - close to the IFT between the pure solvents. Then, the tension decreases because surfactants diffuse to the interface and adsorb, eventually reaching the equilibrium value. The data provide key information about the diffusion and adsorption kinetics of the surfactants, such as emulsifiers or proteins. 

We have tested this hypothesis in some recent o/w thin film experiments [45]. It was not practical to reduce the protein load per unit area of interface to that found in the emulsion experiments, since the very low concentrations required would have been very slow to reach equilibrium adsorption. We circumvented this problem in a unique way. Rather than adsorb emulsifier mixtures from aqueous solution, we formed the oil droplets and the thin film in a preformed emulsion. Therefore, the adsorbed layers on the captive droplets formed by adsorption of surfactant from the continuous phase of the emulsion. The results are shown in Figure 23, where surface diffusion data of FITC-/8-lg in o/w and a/w thin films as a function of added Tween 20 are summarised. 

Very surface-active emulsifiers (nigh HLB value) are capable of forming micelles in water. The latter is in equilibrium with emulsifiers at the air-water interface. At a certain concentration (= critical miceile concentration, CMC) the surface will be saturated with emulsifier and no further reduction in surface tension will be observed. The CMC can be found by surface tension measurements according to Figure 20. 

Monoglycerides and mono-diglycerides have low HLB values and cannot form micelles. They build up a multi-layer at the surface, resulting in a constantly decreasing surface tension as their concentration increases. However, in systems with proteins such as fat-free ice cream mixes, these emulsifiers behave as if they have a CMC. A possible explanation for this observation is that the unbound emulsifier in the fat-free mix is in equilibrium with the protein-bound emulsifier. Above a certain concentration of emulsifier in the mix, any surplus of emulsifier will adhere to the protein in the water phase after the surface has been saturated. The unadsorbed emulsifier is seen as very small crystals less than 200 nm by electron microscopy analysis4. ... 

Fitch-Roe approach. (At lower than critical emulsifier concentrations, micelles are not generated. With increasing amount of emulsifier, the physical properties of aqueous medium discontinuously change in the vicinity of its critical concentration. The critical emulsifier concentration is an important material constant.) Roe [140] proved that the two theories can be described by very similar quantitative relations. This latter theory stress the importance of dissolution equilibria for the equilibrium monomer concentration in the aqueous phase. 

Surface tension measurement. Adsorption titration, also called soap titration, (2.3) was carried out by the drop volume method at different polymer concentrations. The equivalent concentration of salt was held constant. The amount of emulsifier necessary to reach the critical micelle concentration (CMC) in the latex was determined by each titration. The total weight of emulsifier present in the latex is the weight of emulsifier in the water plus the weight of emulsifier adsorbed. The linear plot of emulsifier concentration (total amount of emulsifier corresponding to the end-point of each titration) versus polymer concentration gives the CMC as the intercept and the slope determines the amount of emulsifier adsorbed on the polymer surface in equilibrium with emulsifier in solution at the CMC (E ). 

Once a micelle is stung, polymerization proceeds very rapidly. The particle can accommodate more monomer as its polymer content increases and the water-polymer interfacial surface increases concuirently. Tlie new surface adsorbs emulsifier molecules from the aqueous phase. This disturbs the equilibrium between micellar and dissolved soap, and micelles will begin to disintegrate as the concentration of molecularly dissolved emulsifier is restored to its equilibrium value. Thus the formation of one polymer particle leads to the disappearance of many micelles. The initial latex will usually contain about 10 micelles per milliliter water, but there will be only about 10 particles of polymer in the same volume of the final emulsion. When all the micelles have disappeared, the surface tension of the system increases because there is little surfactant left in solution. Any tendency for the mixture to foam while it is being stirred decreases at this time. 

A consideration of the adsorption kinetics is very important m an estimation of the effectiveness of surfactants under the dynamic conditions of emulsion polymerization. In a stalagmometric study of dynamic and static adsorption of emulsifiers of various structure at the air-water interface, it was established that adsorption values of micelle-forming sui c-tants differ significantly in the period of drop formation (Nikitina et ai, 1961). This was explained by the considerable period needed for establishment of adsorption equilibrium connected with the kinetics of adsorption layer formation. The authors concluded tirat for usual concentrations of surfactant solutions the period of estaUishm t of adsorption equilibrium can be taken as equal to 2 min. Figure 2 shows the adsorption isotherms of... 

When an emulsifier or soap is dissolved in water, the solute molecules associate to form small clusters called micelles. The hydrocarbon parts of the emulsifier molecules constitute the interior of the micelles, the surface of which is formed by the ionic groups of the emulsifier. A small fraction of the soap is molecularly dissolved in the water and there is a dynamic equilibrium between the micelles and these single molecules in the aqueous phase. Micelles are of colloidal size, consisting of a relatively small number of soap molecules of the order of 100 molecules. This corresponds to a diameter of about 50 A., if one assumes the cluster to be spherical. At the concentrations usually employed in emulsion polymerization, there are some 10 micelles per milliliter of water. 

When surface active agents are considered, a further complication may be encountered. Because of their surface active nature, the surfactants not only emich at the surfaces, but also form extended structures themselves. At low concentrations, the surfactants remain as dissolved monomers or asssociate to oligomers. However, when the critical micellization concentration (cmc) is surpassed, a cooperative association is activated to micelles (1 to 10 nm) consisting typically of some 50 to 100 monomers. At stiU higher concentrations, or in the presence of cosurfactants (alcohols, amines, fatty acids, etc.), liquid crystalline phases may separate. These phases have an infinite order on the x-ray scale, but may remain as powders on the NMR (nuclear magnetic resonance) scale. When the lamellar liquid crystalline phase is in equilibrium with the liquid micellar phase the conditions are optimal for emulsions to form. The interface of the emulsion droplets (1 to 100 pm) are stabilized by the lamellar liquid crystal. Both the micelles and the emulsions may be of the oil in water (o/w) or water in oil (w/o) type. Obviously, substances that otherwise are insoluble in the dispersion medium may be solubilized in the micelles or emulsified in the emulsions. For a more thorough analysis, the reader is directed to pertinent references in the literature. ... 

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