Mechanism of emulsion

The basic constituents of all commercial emulsion polymerization recipes are monomers, emulsifiers, and polymerization initiators. Other common components are modifiers, inorganic salts and free alkaH, and shortstops. The function of these different components and the mechanism of emulsion polymerization have been described (43,44). 

The kinetic mechanism of emulsion polymerization was developed by Smith and Ewart [10]. The quantitative treatment of this mechanism was made by using Har-kin s Micellar Theory [18,19]. By means of quantitative treatment, the researchers obtained an expression in which the particle number was expressed as a function of emulsifier concentration, initiation, and polymerization rates. This expression was derived for the systems including the monomers with low water solubility and partly solubilized within the micelles formed by emulsifiers having low critical micelle concentration (CMC) values [10]. 

Therefore, the polymerization progresses within the micelle structure by following the traditional mechanism of emulsion polymerization. 

Gardon, J.L., "Mechanism of Emulsion Polymerisation", AIChE Symp., May 1969... 

Features 2 to 4 are attributed to the aqueous medium. Emulsion polymerization forms submicrometer-sized particles, so-called latex particles. The particles are stabilized with ionic and/or noionic emulsifiers. The process to form submicrometer particles is very complicate because of the contribution of two phases, aqueous and oil, to particle. The mechanism of emulsion polymerization is described in the next section. 

Harkins proposed tile mechanism of emulsion polymerization in 1943 (2). His qualitative theory needs a few corrections but its basis has been and will be accepted. The theory is explained using Figure 11.1.2. 

The mechanism of emulsion polymerisation is complex. The basic theory is that originally proposed by Harkins21. Monomer is distributed throughout the emulsion system (a) as stabilised emulsion droplets, (b) dissolved to a small extent in the aqueous phase and (c) solubilised in soap micelles (see page 89). The micellar environment appears to be the most favourable for the initiation of polymerisation. The emulsion droplets of monomer appear to act mainly as reservoirs to supply material to the polymerisation sites by diffusion through the aqueous phase. As the micelles grow, they adsorb free emulsifier from solution, and eventually from the surface of the emulsion droplets. The emulsifier thus serves to stabilise the polymer particles. This theory accounts for the observation that the rate of polymerisation and the number of polymer particles finally produced depend largely on the emulsifier concentration, and that the number of polymer particles may far exceed the number of monomer droplets initially present. 

Figure 3.27 Breakdown mechanisms of emulsions (from top to bottom creaming, coalescence, flocculation and Ostwald ripening).

Hassander, H., Johansson, B., Tomell, B. 1989. The mechanism of emulsion stabilization by small silica (Ludox) particles. Coll. Surf. 40, 93105. 

The mechanism of emulsion polymerization ensures that the polymer concentration at the polymerization locus is semidilute or concentrated, which results in a greater probability of branch chain formation [299, 300]. This effect produces unique average branching densities and unique distributions of branching densities, that are significantly different from corresponding bulk polymerization [266,301]. 

Figure 22. The two types of capture mechanisms of emulsion flow in porous media. (Reproduced with permission from reference 12. Copyright 1986 Pergamon Press PLC.)...

Fat crystallization has been extensively studied in bulk fats and, to a lesser extent, in emulsified fats. It has been shown that the crystallization behavior of a fat will proceed quite differently, depending on whether it is in bulk or emulsified form (4,5). Authors have examined the effect of the state of dispersion on the crystallization mechanisms (nucleation, crystallization rate) and polymorphic behavior (6-11) of partial- and triglycerides in bulk and emulsified form. Understanding the mechanisms of emulsion nucleation and crystallization is one of the first steps in understanding the destabilization of emulsions and partial coalescence, e.g., stabilization of liquid fat emulsions by solid particles (fat) or control of the polymorphic form of crystals during the process of partial coalescence to control the size of aggregates and textural properties. 

Crystallization of lard and PSCO emulsions with similar initial droplet size distributions led to partial coalescence under perikinetic and orthokinetic conditions. The application of shear accelerated the destabilization of the emulsions after the achievement of a critical SFC. The SFC was found not to be the sole contributing factor to emulsion destabilization. Crystal morphology and distribution within the droplet are important factors in the destabilization of these emulsified fats. The emulsions are relatively stable in the short term when crystals are small and form quickly in a consolidated mass in the bulk of the droplet. Polymorphic transitions were not detectable as a source of destabilization in this experiment. The observation of the microstructure of bulk and emulsified fats gave insight into the mechanisms of emulsion destabilization. 

Gardon, J.L. Mechanism of emulsion polymerization. Rubber Chem. Technol. 1970, 43, 74. 

Several mechanisms have been proposed to explain the mechanism of emulsion polymerisation, but no single mechanism can explain aU of the happenings. Barrett and Thomas [11] suggested that particles are formed in emulsion polymerisation by two main steps ... 

The mechanism of emulsion formation is not yet fully understood, but it probably starts with sea energy forcing the entry of small water droplets, about 10 to 25 pin (or 0.010 to 0.025 mm) in size, into the oil. If the oil is only slightly viscous, these small droplets will not leave the oil quickly. On the other hand, if the oil is too viscous, droplets will not enter the oil to any significant extent. Once in the oil, the droplets slowly gravitate to the bottom of the oil layer. Any asphaltenes and resins in the oil will interact with the water droplets to stabilize them. Depending on the quantity of asphaltenes and resins, as well as aromatic compounds which stabilize asphaltenes and resins in solution, an emulsion may be formed. The conditions required for emulsions of any stability to form may only be reached after a period of evaporation. Evaporation lowers the amount of low-molecular weight aromatics and increases the viscosity to the critical value. 

Direct and inverse emulsions are found in everyday-life products. For instance, both types of emulsion are common in food oil-in-water emulsions include milk, cream, and mayonnaise butter and margarine are examples of water-in-oil emulsions. Furthermore, recent studies were carried out, with the help of professional sensory panelists, to determine the influence of emulsion type on the perception of taste [36], In relation to the breakdown mechanisms of emulsions (creaming/sedimentation, flocculation, coalescence, and Ostwald ripening), controlling emulsion type can be regarded as a key parameter to design stabilization/destabilization processes. In cosmetic... 

Fig. 1.14. Mechanism of emulsion stabilisation a) electric repulsion, b) steric hindrance According to Hamaker (1937) the Free Energy of the attraction forces is approximately...

The core of a micelle has been shown to have properties akin to a liquid hydrocarbon. One piece of evidence for this is that a surfactant solution above the c.m.c. is capable of taking up substantial quantities of non-polar organic (lipophilic) substances. These enter the core of the micelle, which can now swell because the added material has no hydrophilic moiety which needs to he on the surface (Figure 11.8). Solubilisation plays an important role in detergency and in the detailed mechanism of emulsion polymerisation. 

In connection with crude oil research, the properties and mechanisms of emulsion stability in terms of thin liquid films formed at water-crude oil interface have been extensively discussed. We will summarize and review the literature in the next section. 

The behavior of thin liquid films formed between coalescing drops and bubbles has attracted considerable attention in an attempt to understand the stabilizing mechanisms of emulsions and foams. It is now generally recognized that the drainage of this film plays a crucial role in determining the stability of the dispersion. 

The mechanisms of emulsion polymerization using classical emulsifiers are complex. The use of macromolecular emulsifiers complicates even more the picture of phenomena occurring in emulsion polymerization for the following reasons ... 

Polymerizations performed with an emulsifier above its critical micelle concentration with all the monomer solubilized within the micelles and without any monomer present as emulsion droplets may be described as micellar polymerizations [62]. Although such systems can never produce a high yield of polymer per unit volume they are advantageous if it is desired to use photochemical initiation, these being transparent whereas emulsions are opaque. Micellar polymerizations can help to elucidate the mechanism of emulsion polymerizations. They are useful practically when it is desired to copolymerize hydrophilic and hydrophobic monomers to synthesize associative thickeners [63,64]. 

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