Emulsion stabilization mechanisms

Theoretical and experimental investigations dealing with the polyelectrolyte adsorption and the structure of polyelectrolyte liquid films have been carried out for more than twenty years in order to give a better understanding of emulsion stabilization mechanisms by polyelectrolytes on a microscopic scale. In this section, we describe the structure of liquid films composed of three kinds of polyelectrolytes the diblock amphiphilic polyelectrolytes, which lead to the formation of brushes, the homopolyelectrolytes, which form a semicrystalline structure within the films, and finally the amphiphilic random polyelectrolytes. Special attention is given to the charged monomer layer thickness at interfaces. 

DT Wasan, AD Nikolov. Emulsion Stability Mechanisms. Proceedings of the First World Congress on Emulsions, Paris, 1993, pp 93—112. 

DT Wasan. Emulsion stability mechanisms. First World Congress on Emulsion, Paris, 1993. Proceedings, Vol 4, pp 93—112. 

Often, in bench studies aimed at understanding emulsion-stabilization mechanisms, a hypothesis is devised. Most often the components of crudes are first separated, and a model emulsion is prepared from various combinations of the components in a model oil and in water of quality similar to that of formation or process water. The stability or instability is traced either by water resolution or by observing flie interfacial film properties under some form of externally applied stress over time. The stress may include temperature increases or solvent changes. The system may then be modified by the demulsifier and the changes in behavior are compared to that without the demulsifier. Deductions are then made about the film mechanics of the system in response to the variables. 

Many different combinations of surfactant and protective coUoid are used in emulsion polymerizations of vinyl acetate as stabilizers. The properties of the emulsion and the polymeric film depend to a large extent on the identity and quantity of the stabilizers. The choice of stabilizer affects the mean and distribution of particle size which affects the rheology and film formation. The stabilizer system also impacts the stabiUty of the emulsion to mechanical shear, temperature change, and compounding. Characteristics of the coalesced resin affected by the stabilizer include tack, smoothness, opacity, water resistance, and film strength (41,42). 

In the case of emulsions with three liquids the presence of the third phase results in a reduction of the energy input for the emulsification process, whereas the emulsion with a Hquid crystal as the third phase shows interesting stabilization mechanisms. Finally, the emulsion with added particles illustrates the importance of Hquid—solid wetting for stabiHty. 

Phase Separation. An approximate estimation of phase separation may be obtained visually. In general, creaming, flocculation, and coalescence have occurred before phase separation is visible, thus sometimes making quantitative evaluations more difficult. Accelerating the separation by centrifugation followed by appropriate analysis of the specimens may be useful to quantitatively determine the phase separation. Details on mechanisms of creaming and phase separation as well as some advances in the monitoring techniques of emulsion stability have been reviewed by Robins [146]. 

The reverse emulsion stabilized by sodium dodecylsulfate (SDS, R0S03 Na+) retards the autoxidation of dodecane [24] and ethylbenzene [21,26,27]. The basis for this influence lies in the catalytic decomposition of hydroperoxides via the heterolytic mechanism. The decay of hydroperoxides under the action of SDS reverse micelles produces olefins with a yield of 24% (T=413 K, 0.02mol L 1 SDS, dodecane, [ROOH]0 = 0.08 mol L 1) [27], The thermal decay gives olefins in negligible amounts. The decay of hydroperoxides apparently occurs in the ionic layer of a micelle. Probably, it proceeds via the reaction of nucleophilic substitution in the polar layer of a micelle. 

Emulsifier is not a necessary component for emulsion polymerization if ihe following conditions are satisfied The particles are formed by homogeneous nucleation mechanism, and the particles are stabilized by factor(s) olher than emulsifier. As to the latter, the sulfate end group that is the residue of persulfate initiator serves for stabilization of dispersion via interparticle electrorepulsive force (20). When the stabilization mechanism works well, a small number of particles grow during polymerization without aggregation, keeping the size distribution narrow. Finally stable, monodisperse, anionic particles are obtained. 

In a previous paper 8), was inferred for com oil-in-water as well as toluene-in-water emulsions stabilize by bovine serum albumin (BSA). The effects of pH, ionic strength and BSA concentration on Hmax were investigated. Comparison of experimental maximum disjoining pressure witii predicted Ilmax indicated that steric interaction is the predominant mechanism of stabilization in such systems. 

The mechanical properties are also affected by the surfactant concentration in the emulsion precursor. Maxima in both crush strength and Young s modulus were shown at the surfactant concentration for optimum emulsion stability. Foams prepared from 100% styrene were found to have much lower compressive moduli than those containing DVB [130], This was attributed to plasticisa-tion of the polymer by the surfactant. 

Stabilization Mechanisms. The stabilization of an emulsion involves slowing the destabilization, primarily the flocculation process. This may be achieved in two principal manners by reducing the mobility of droplets through enhanced viscosity or by inserting an energy harrier between them. 

There is a fundamental problem which must be solved when dealing with whippable emulsions. Before use the emulsion must be sufficiently stable. On the other hand, it must be possible to destabilize the emulsion by mechanical treatment combined with air incorporation (whipping, air pressure, cooling, freezing). The partly destabilized fat globules in the whipped emulsion are important for the stability of the foam structure. There is a... 

In whippable emulsions with a high fat content, the air-water interface of the foam after whipping is dominated by adsorbed deproteinated fat globules. In whippable emulsions with a low fat content other foam stabilizing mechanisms come into play, such as protein-hydrocolloid and protein-emulsifier interactions. The former subject may be studied by... 

An example of the importance of the wettability of fine particles is provided by what are called Pickering emulsions, that is, emulsions stabilized by a fdm of fine particles. The particles can be quite close-packed and the stabilizing fdm between droplets can be quite rigid, providing a strong mechanical barrier to coalescence. See Section 5.4.1. 

There is no simple, direct relationship between elasticity and emulsion or foam stability because additional factors, such as film thickness and adsorption behaviour, are also important [204]. Nevertheless, several researchers have found useful correlations between EM and emulsion or foam stability [131,201,203], The existence of surface elasticity explains why some substances that lower surface tension do not stabilize foams [25]. That is, they do not have the required rate of approach to equilibrium after a surface expansion or contraction as they do not have the necessary surface elasticity. Although greater surface elasticity tends to produce more stable bubbles, if the restoring force contributed by surface elasticity is not of sufficient magnitude, then persistent foams may not be formed due to the overwhelming effects of the gravitational and capillary forces. More stable foams may require additional stabilizing mechanisms. 

Other drug-delivery systems may include double emulsions, usually W/O/W, for transporting hydrophilic dmgs such as vaccines, vitamins, enzymes, hormones [441], The multiple emulsion also allows for slow release of the delivered drug and the time-release mechanism can be varied by adjusting the emulsion stability. Conversely, in detoxification (overdose) treatments, the active substance migrates from the outside to the inner phase. 

An emulsion stabilized by fine particles. The particles form a close-packed structure at the oil-water interface, with significant mechanical strength, which provides a barrier to coalescence. 

Proteins, which are also surface active, can be used to prepare food emulsions. The protein molecules adsorb at the O/W interface and they may remain in their native state (forming a rigid layer of unfolded molecules) or undergo unfolding, forming loops, tails, and trains. These protein molecules stabilize the emulsion droplets, either by a steric stabilization mechanism or by producing a mechanical barrier at the O/W interface. 

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