Emulsion critical micelle concentration

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]. 

Like other emulsifiers, an EUP forms micelles at a critical micelle concentration (CMC). For comonomer-free EUP-emulsions of the (MA+HD)- type the CMC is about 5 X 10"4 g/ml [115,118]. The CMC depends on the composition and chain length of the polyester, the presence of an electrolyte [118] and the temperature. 

Fluid colloidal system of two or more components. (Gold Book online, 1972 entry [2].) Note Examples of colloidal sols are protein sols, gold sols, emulsions and surfactant solutions above their critical micelle concentrations. 

The effect of surfactant concentration on critical osmotic pressure was also studied [97], Below a critical surfactant concentration, emulsions are always unstable due to incomplete coverage of the oil-water interfaces. Above this, Jt increases with increasing surfactant concentration until the critical micelle concentration (CMC) is reached, above which it remains more or less constant. 

Changes in surface pressure with time or concentration can be used to measure various fundamental properties. Changes in surface pressure versus protein concentration curves can be used to determine the excess surface concentration (critical micelle concentration), defined as the amount of protein at the surface divided by the surface area. Indicates minimum amount of protein needed to form an emulsion. 

Block or graft copolymers in a selective solvent can form structures due to their amphiphilic nature. Above the critical micelle concentration (CMC), the free energy of the system is lower if the block copolymers associate into micelles rather than remain dispersed as single chains. Often the micelles are spherical, with a compact core of insoluble polymer chains surrounded by a corona of soluble chains (blocks) [56]. Addition of a solvent compatible with the insoluble blocks (chains) and immiscible with the continuous phase leads to the formation of swollen micelles or polymeric micro emulsion. The presence of insoluble polymer can be responsible for anomalous micelles. 

Monodispersed sols containing spherical polymer particles (e.g. polystyrene latexes22"24, 135) can be prepared by emulsion polymerisation, and are particularly useful as model systems for studying various aspects of colloidal behaviour. The seed sol is prepared with the emulsifier concentration well above the critical micelle concentration then, with the emulsifier concentration below the critical micelle concentration, subsequent growth of the seed particles is achieved without the formation of further new particles. 

Additives are usually amphiphilic in nature, and thus are either ionic or neutral surfactants or even polymers. The role of surfactants in solvent extraction is ambiguous. Usually, they should be avoided as they lower the interfacial tension, which may lead to emulsion formation in an agitated extractor. However, every metal-loaded ion exchanger is amphiphilic, and can adsorb at the interface or aggregate in the bulk phase. This occurrence is well known with sodium or other metals [17], and above a critical surfactant concentration (cmc, critical micelle concentration) micellar aggregates are formed. A dimensionless geometric parameter is decisive for the structure of the associates, according to Fig. 10.6 ... 

The key feature of Inisurfs is their surfactant behavior. They form micelles and are adsorbed at interfaces, and as such they are characterized by a critical micelle concentration (CMC) and an area/molecule in the adsorbed state. This influences both the decomposition behavior and the radical efficiency, which are much lower than those for conventional, low molecular weight initiators. Tauer and Kosmella [4] have observed that in the emulsion polymerization of styrene, using an Inisurf concentration above the CMC resulted in an increase in the rate constant of the production of free radicals. This was attributed to micellar catalysis effects as described, for example, by Rieger [5]. Conversely, if the Inisurf concentration was below the CMC the rate constant of the production of free radicals decreased with an increase in the Inisurf concentration, which was attributed to enhanced radical recombination. Also note that a similar effect of the dependence of initiator efficiency on concentration was reported by Van Hook and Tobolsky for azobisisobutyronitrile (AIBN) [6]. 

Measurement of characteristics of the emulsion droplets in concentrated media is indeed a difficult task. Some indirect methods have been used. The interfacial area and therefore the droplet size were determined by measuring the critical micelle concentration of miniemulsions [43]. Erdem et al. determined droplet sizes of concentrated miniemulsions via soap titration, which could be confirmed by CHDF measurements [44]. Droplet sizes without diluting the system can much better be estimated by small angle neutron scattering (SANS) measurements [23]. 

Breltenbach et al. (12) established that in the case of vinyl acetate emulsion polymerization the dls-pertion rnedlim of the resultant latex contains the amoimt of emulsifier exceeding by a factor of two the critical micelle concentration (CMC), i.e. the reaction system is not depleted of the emulsifier. We obtained similar results in the case of methylacrylate polymerizing with alkyl- and al larylsulphonates. 

The surface activity of these compounds was not studied in detail. As mentioned in the Experimental Section, all were about equivalent in nucleating particles during emulsion polymerization. The resulting latexes when dialyzed to remove excess salt were stable against settling even at 10% solids over many months. Data on samples where both latex particle size and critical micelle concentration were measured is shown in Table II. 

At the right concentration (the critical micelle concentration). the molecules concentrate at the interface between immiscible solvents, such as water and lipid, and walcr-in-oil or oil-in-waier emulsions may be formed wilb ibe ammonium head group in the water layer and the nonpolar hydrocarbon chain associated with the oil phase. The synlhc.s and antimicrobial actions of the members of this class oi compounds were first reported in 1908. but it was not until the pioneering work of Gerhard Domagk in 1935 that attention was directed to their usefulness as antiseptics, disinfectants. and preservatives. 

Surfactants or emulsifiers help stabilize the emulsion and are classified into four broad categories anionic, cationic, nonionic, and zwitterionic. Surfactants are dissolved in water at low concentrations, where they form aggregates or micelles. At a concentration greater than their critical micelle concentration (CMC), all excess molecules form micelles. 

In practice, the emulsions are formed in the presence of surfactants. At concentrations above the critical micellization concentration (CMC) the swollen micelles can serve as carriers of oil between the emulsion droplets of different size. In other words, surfactant micelles can play the role of mediators of the Ostwald ripening. Micelle-mediated Ostwald ripening has been observed in solutions of nonionic surfactants. In contrast, it was found that the micelles do not mediate the Ostwald ripening in undecane-in-water emulsions at the presence of an ionic surfactant (SDS). It seems that the surface charge due to the adsorption of ionic surfactant (and the resulting double layer repulsion) prevents the contact of micelles with the oil drops, which is a necessary condition for micelle-mediated Ostwald ripening. 

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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