Emulsions mechanical aspects

An important aspect of the stabilization of emulsions by adsorbed films is that of the role played by the film in resisting the coalescence of two droplets of inner phase. Such coalescence involves a local mechanical compression at the point of encounter that would be resisted (much as in the approach of two boundary lubricated surfaces discussed in Section XII-7B) and then, if coalescence is to occur, the discharge from the surface region of some of the surfactant material. 

Liquid emulsions are inherently unstable to a varying degree. It is important to understand, therefore, the mechanisms that contribute to emulsion stability. Before the solidification step, instability of an emulsion can arise due to either phase separation or phase inversion (Mulder and Walstra, 1974). It is evident that the likelihood of phase inversion will increase as the fraction of dispersed phase increases. The vast majority of literature references are concerned with the stability to phase separation as coalescence or creaming in oil-in-water emulsions (Hailing, 1981 Jaynes, 1983). In addition, a method for determining the stability of water-in-oil emulsions to inversion has not been reported. It is usually assumed that certain aspects of oil-in-water emulsion theory apply in reverse to water-in-oil emulsions. 

Besides giving latices of narrow particle size distribution, mixed surfactant systems have shown several other interesting characteristics which lighten some aspects concerning the mechanism of particle nucleation in emulsion polymerization process. 

The object of this study was to clarify some aspects of the mechanism of shear-induced flocculation in colloidal dispersions. Vinyl chloride homopolymer and copolymer latices were prepared by emulsion polymerization using sodium dodecyl sulphate as emulsifier. Agglomeration behavior in these latices was studied by measuring the mechanical stability using a high speed stirring test. The latex particle size was measured by an analytical centrifuge. Molecular areas of emulsifier in the saturated adsorption layer at the surface of homopolymer and copolymer latex particles were estimated from adsorption titration data. 

Emulsion polymerization first gained industrial importance during World War II when a crash research program in the United States resulted in the production of styrene-co-butadiene [SBR] synthetic rubber. The Harkins-Smith-Ewart model [5-6] summarized the results of early research, which focussed on this and similar systems. Current thinking is not entirely in accord with this mechanism. It is still worthwhile to review it very brielly here, however, because it is still widely referenced in the technical literature and because some aspects of the model provide valuable insights into operating procedures. 

So far we have looked at the flow of emulsions in porous media in this section we discuss some aspects of in situ emulsification in porous media that have received little attention. Some evidence suggests strongly that emulsions can be produced in the reservoir rock itself. A discussion on the formation of oil-field emulsions was given by Berkman and Egloff (50). They concluded that emulsions could be formed within the porous rock near the well bore where the velocity gradients (i.e., shear rates) were very high. Emulsions could also be formed as a result of mechanical agitation, for... 

Theoretical aspects of emulsion formation in porous media were addressed by Raghavan and Marsden (51-53). They considered the stability of immiscible liquids in porous media under the action of viscous and surface forces and concluded that interfacial tension and viscosity ratio of the immiscible liquids played a dominant role in the emulsification of these liquids in porous media. A mechanism was proposed whereby the disruption of the bulk interface between the two liquids led to the initial formation of the dispersed phase. The analysis is based on the classical Raleigh-Taylor and Kelvin-Helmholz instabilities. 

It is at present clearly impossible to understand all the aspects of these systems. Nevertheless the mechanism and kinetics of some emulsion systems are reasonably well understood—those in which the monomer is water- insoluble and in which the polymer is soluble in the monomer. An outline is given of this mechanism and the kinetics of polymerization are developed on the basis of this mechanism. This theoretical kinetic behavior is then compared with experimental data, both from the literature and from unpublished results. Whenever possible, the influence of monomer water solubility and monomer solubility of the polymer is commented on. These comments are mostly of a qualitative nature and sometimes even speculative. The present state of our knowledge does not permit going beyond such comments, although recently the literature has given a few attempts at quantitative interpretation of emulsion polymerization of water-soluble monomers. 

Another problem involves the classification of these metal-based heterogeneous systems into suspension, dispersion, and emulsion polymerizations similarly to conventional systems. This is due to not only a lack of detailed analysis of reaction mechanisms and particle sizes but also fundamental differences in several aspects such as the locus of initiation and the molecular weight of polymers in comparison with the conventional counterparts. The terms suspension and emulsion will be used in the following sections for simple classification but are not based on the strict definition for conventional free radical systems. 

The difference between well-known SCF antisolvent techniques such as GAS, PCA, and SEDS usually can be attributed to the specific nozzle mixing (or dispersing) technique involved. Enhanced mass and heat transfer can also be achieved by using mechanical and ultrasonic mixers and ultrafast jet expansion techniques. There are new developments for particle formation by means of dispersed systems such as emulsions, micelles, colloids, and polymer matrixes. It should be emphasized that all these processes involve the same fundamental aspects of mass and heat transfer phenomena between an SCF and a subcritical phase. Clearly the ultimate goal of all SCF particle technologies is to achieve predictable, consistent, and economical production of fine pharmaceuticals or chemicals. This is possible only on the basis of comprehensive mechanistic understanding and well-developed scale-up principles. 

Contamination in residual fuel oil may be indicated by the presence of excessive amounts of water, emulsions, and inorganic material such as sand and rust. Appreciable amounts of sediment in a residual fuel oil can foul the handling facilities and give problems in burner mechanisms. Blockage of fuel hlters (ASTM D-2068, ASTM D-6426) due to the presence of fuel degradation products may also result. This aspect of fuel quality control may be dealt with by placing restrictions on the water (ASTM D-95, IP 74), sediment by extraction (ASTM D-473, IP 53), or water and sediment (ASTM D-96, IP 75) values obtained for the fuel. 

The reaction mechanism is still obscure, and the authors just suppose that the acidity necessary for getting the condensation of the OPUF nearby the anode, could arise from the acidic hydrolysis of Fe3+ cations originated by dissolution of the stainless steel anode. The applicative aspects of this system are noteworthy and further knowledge is necessary. Tidswell and Train110 ni>112) studied in great detail the homo- and copolymerization of vinyl acetate in aqueous emulsions. The possible initiation mechanisms are compared and the hypothesis of the formation of a vinyl acetate-hydrogen radical acting as true chain initiator is discussed. 

FIG. 11 Pseudophase diagram for 30 wt% cyclohexane in water stabilized by PAA (Carbopol 980). The c values are shown as the curve drawn in the bottom left-hand corner of the diagram. (Reprinted from Colloids and Surfaces A Physicochem Eng Aspects, 88, Lockhead RY, Rulinson CJ, An investigation of the mechanism by which hydrophobically modified hydrophilic polymers act as primary emulsifiers for oil in water emulsions. 1. Poly(acrylic acids) and hydroxyethyl celluloses. 27-32, Copyright (1994), with permission from Elsevier Science.)... 

Lochhead RY, Rulinson CJ. An investigation of the mechanism by which hydrophobically modified hydrophilic polymers act as primary emulsifiers for oil in water emulsions. 1. Poly(acrylic acids) and hydroxyethyl celluloses. Colloids Surfaces A Physicochem Eng Aspects 1994 88 27-32. 

His research interests have included many aspects of colloid and interface science applied to the petroleum industry, including research into mechanisms of processes for the improved recovery of light, heavy, or bituminous crude oils, such as in situ foam, polymer or surfactant flooding, and surface hot water flotation from oil sands. These mostly experimental investigations have involved the formation and stability of dispersions (foams, emulsions, and suspensions) and their flow properties, elec-trokinetic properties, interfacial properties, phase attachments, and the reactions and interactions of surfactants in solution. 

As explained before, when surfactant, water, and monomer(s) are mixed, the colloidal system obtained consists of monomer-swollen micelles (if the surfactant concentration exceeds its CMC) and monomer droplets dispersed in an aqueous phase that contains dissolved molecules of surfactant and a small amount of the sparingly water-soluble monomer(s). When free radicals are generated in the aqueous phase by action of an initiator system, then the emulsion polymerization takes place. Its evolution is such that the colloidal entities initially present tend to disappear and new colloidal entities (polymer latex particles) are bom by a process called nucleation. For convenience, we first focus on the particle nucleation mechanisms, a very important aspect of emulsion polymerization. 

The interfacial aspects of emulsification, including thermodynamics of emulsion formation and breakdown, have been reviewed and described recently by Tadros. The role of emulsifiers is discussed in detail and the mechanisms outlined, although complex, are related to the particle size if for no other reason than that the number density is proportional to l/(dd). 

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