Emulsifier adsorbed layer

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. 

The mechanism by which emulsifiers could influence the rate of the thermal initiation reaction is obscure. Most probably the emulsifiers increase the efficiency with which one of the radicals produced in the thermal initiation process escapes into the aqueous phase so that emulsion polymerization may begin. If so those emulsifiers for which exchange between the micelle or the adsorbed layer on a latex particle and true solution in the aqueous phase is most rapid should be most effective in promoting the thermal polymerization. Recently the kinetics of micellization has attracted much attention (29) but the data which is available is inadequate to show whether such a trend exists. 

Similar results to those obtained here by the stability measurements have been reported by Roe and Brass (7.8) They studied polystyrene latex stabilized by potassium palmitate. The analysis supplied by these authors shows that the order of magnitude of the slope of the stability curves can be accounted for as an entropic effect of crowding of adsorbed molecules during an encounter between two particles. They pointed this out as a possible explanation as the amount of emulsifier adsorbed strongly affects the stability without altering the electrophoreti-cally derived double-layer potential. 

The adsorbed layer of emulsifier on the particle surface can affect the stability of latices in three main ways ... 

Bibeau and Matijevic used a fixed value for the Hamaker constant and interpreted the increased stability by addition of emulsifier as being exclusively an effect of increased surface charge. But the various stabilizing mechanisms are not mutually exclusive and may function co-operatively. The effect of adsorbed layers on the energy of attraction between two particles has been considered by Void ( ) and by Vincent (36). 

The presence of emulsifiers (materials that promote emulsion formation) influences the ability to form an emulsion between petroleum and water. Emulsifiers act by lowering the interfacial tension between the phases and creating a strong adsorbed layer around the surface of the internal phase. Emulsifiers that are soluble in water (hydrophilic) promote the creation of oil in water emulsion. Alkaline soaps, starch and so on are such hydrophilic emulsifiers. Hydrophobic emulsifiers (i.e. soluble in petroleum) promote the formation of water in oil emulsions. Hydrophobic emulsifiers include resins dispersed in particle form within soot, clay and other substances. Petroleum emulsions can be characterized using properties such as viscosity, dispersion, density, electrical properties and stability. The viscosity of petroleum emulsion changes within wide ranges and depends on the viscosity of petroleum, temperature, and amounts of petroleum and water. 

A higher degree of emulsion stability depends on the structure of the compounds that constitute the protective layer formed on the surface of the droplet. The emulsifier adsorbs on the surface of the droplet and covers it with a preserving layer thereby stabilizing the droplet. This layer prevents droplets from merging with each other (i.e. promotes emulsion formation and stability). 

Small MW emulsifiers can play an imporfant role in determining the properties of an emulsion. This is because these emulsihers can compete with proteins for the interface. They can co-adsorb with the proteins, which resnlts in a thicker and more viscoelastic adsorbed layer, or a more mobile adsorbed layer, depending on the strength and nature of the interactions (Dickinson, 1993,1994). 

It is apparent that real food emulsions are likely to behave in a more complex way than are simple model systems studied in the laboratory. This may be especially important when lecithins are present in the formulation. Although these molecules are indeed surfactants, they do not behave like other small-molecule emulsifiers. For example, they do not appear to displace proteins efficiently from the interface, even though the lecithins may themselves become adsorbed (123). They certainly have the capability to alter the conformation of adsorbed layers of caseins, although the way in which they do this is not fully clear it is possibly because they can fill in gaps between adsorbed protein molecules (124). In actual food emulsions, the lecithins in many cases contain impurities, and the role of these (which may also be surfactants) may confuse the way that lecithin acts (125). It is possible also for the phospholipids to interact with the protein present to form vesicles composed of protein and lecithin, independently of the oil droplets in the emulsion. The existence of such vesicles has been demonstrated (126), but their functional properties await elucidation. 

Because of the importance of ice cream as a product, much has been written on its stmcture and for mation (171), and the process can only be summarized here. In toppings and ice cream (and indeed simply in whipping cream), it is first necessary to produce a stable emulsion. Ice-cream mix is a complex mixtiue, but the initial emulsion is basically homogenized milk, containing an admixture of small-molecule siufactants as well in whipped toppings, the emulsion is made with oil and a surfactant mixture, which may or may not contain protein and in cream, the natural membrane of phospholipid and protein surrounds the milk fat. In all of these, it is necessary to have some small-molecule emulsifiers so as to exchange with, and weaken the rigidity of, the adsorbed layer of protein (118). The second essential is fliat the fat or oil in the formulation is partly crystalline neiflier completely liquid nor completely solid oil will perform optimally. If the oil is partly crystalline, then the emulsion droplets may not be truly spherical but may have protrusions of crystals on their surfaces. 

Emulsions are dispersions of one liquid in another liquid, most commonly water-in-oil or oil-in-water. The total interfacial area in an emulsion is very large, and sinee flie interfacial area is associated with a positive free energy (the interfacial tension), the emulsion system is thermodjmam-ically unstable. Nevertheless, it is possible to make emulsions with an excellent long-term stability. This requires the use of emulsifiers that accumulate at the oil/water interface and create an energy barrier towards flocculation and coalescence. The emulsifiers can be ionic, zwitterionic, or nonionic surfactants, proteins, amphiphilic polymers, or combinations of polymers and surfactants. The structure of the adsorbed layer at the water/oil interface may be rather complex, involving several species adsorbed directly to the interface as well as other species adsorbing on top of the first layer. 

Emulsions are created by applying mechanical agitation to an oil phase, thus dispersing it into droplets. In order to stabilize the droplets, they need to be protected by an adsorbed layer of emulsifiers or protecting colloids. The emulsifying material creates repulsive and attracting interaction patterns, which determines the properties of the final food system. 

In nature as well as in technology, polymeric emulsifiers and stabiUzers play a major role in the preparation and stabiUzation of emulsions. Natural materials such as proteins, starches, gums, cellulosics, and their modifications, as well as synthetic materials such as polyvinyl alcohol, polyacryhc add, and polyvinylpyrrolidone, have several characteristics that make them extremely useful in emulsion technology. By the proper choice of chemical composition, such materials can be made to adsorb strongly at the interface between the continuous and dispersed phases. By their presence, they can reduce interfacial tension and/or form a barrier (electrostatic and/or steric) between drops. In addition, their solvation properties serve to increase the effective adsorbed layer thickness, increase interfacial viscosity, and introduce other factors that tend to favor the stabilization of the system. 

Boieshan [91,9S] considers that the superficial layers of the emulsifier in the particle surface are important in the emulsion polymerization of vinyl chloride. The author suggests that the polymerization of vinyl chloride is a homogeneous process in which the number of the latex particles is only a secondary factor. The adsorbed layer zone of emulsifier saturated with monomer and oligomer radicals governs the overall rate of polymerization. The monomer saturated zone favors the growth events as well as the decomposition of... 

The degree of stability of many dispersions cannot be explained solely on the basis of Fa and Fr. Elworthy and Florence [56] have treated the stability of emulsions of chlorobenzene and anisole stabilized with a series of synthetic polyoxyethylene ethers in the light of colloid theory and have shown that electrical stabilization alone cannot explain the stability observed. The nature of this other force which is invoked to explain discrepancies between theory and experiment is not fully worked out. Nevertheless, much interest has been shown in this alternative mechanism of stabilization, which for non-ionic emulsions appears to play the major role [64]. Results have indicated that the thickness and degree of solvation of adsorbed layers is critical [65]. Thus, the particular conformation and length of the polyoxyethylene chains of non-ionic surfactants at interfaces is likely to be an important factor in the stabilization of emulsified droplets. 

The only way significant amounts of immiscible fluids can be mixed together is if the interfacial layer surrounding the dispersed droplets is occupied by an adsorbed layer of molecules that keep the droplets from coalescing. Figure 1.1 shows the importance of the interfacial layer in emulsion systems for the two main classes of surface-active molecules, surfactants and proteins, that stabilize them. Low molecular weight surfactants, lipids, and emulsifiers self-assemble at interfaces with the appropriate part of the molecule associating with the appropriate hydrophilic or hydrophobic phases. Proteins, on the other hand. 

Suspensions of oil in water (32), such as lanolin in wool (qv) scouring effluents, are stabilized with emulsifiers to prevent the oil phase from adsorbing onto the membrane. Polymer latices and electrophoretic paint dispersions are stabilized using surface-active agents to reduce particle agglomeration in the gel-polarization layer. 

The final factor influencing the stabiHty of these three-phase emulsions is probably the most important one. Small changes in emulsifier concentration lead to drastic changes in the amounts of the three phases. As an example, consider the points A to C in Figure 16. At point A, with 2% emulsifier, 49% water, and 49% aqueous phase, 50% oil and 50% aqueous phase are the only phases present. At point B the emulsifier concentration has been increased to 4%. Now the oil phase constitutes 47% of the total and the aqueous phase is reduced to 29% the remaining 24% is a Hquid crystalline phase. The importance of these numbers is best perceived by a calculation of thickness of the protective layer of the emulsifier (point A) and of the Hquid crystal (point B). The added surfactant, which at 2% would add a protective film of only 0.07 p.m to emulsion droplets of 5 p.m if all of it were adsorbed, has now been transformed to 24% of a viscous phase. This phase would form a very viscous film 0.85 p.m thick. The protective coating is more than 10 times thicker than one from the surfactant alone because the thick viscous film contains only 7% emulsifier the rest is 75% water and 18% oil. At point C, the aqueous phase has now disappeared, and the entire emulsion consists of 42.3% oil and 57.5% Hquid crystalline phase. The stabilizing phase is now the principal part of the emulsion. 

---