Micelle formation, beginning

The CMC is usually determined experimentally by plotting some property as a function of concentration and extrapolating the results at low and high concentrations to an intersection point. It is evident that the value obtained will depend on the type of representation used as well as on the physico-chemical quantity considered. For surfactants with low CMCs, micelle formation begins abruptly and the uncertainties involved are rather small while this is not always the case for surfactants with high CMCs. When the variation of CMC with chemical or physical factors is considered it is often essential to use CMC values obtained in a consistent way. 

Although at low concentrations the surfactant molecules behave independently, at higher concentrations they aggregate to form micelles. The micelles are roughly spherical and typically contain about 50 to 100 molecules. An ionic micelle is shown schematically in Fig. 10.1.3. The polar heads are in contact with the water, and surrounded by a double layer shell, while the central core of the micelle is essentially water free, being made up of the hydrocarbon tails. The concentration at which micelle formation begins is called the critical micelle concentration. Above this critical value the concentration of free surfactant molecules is essentially unchanged and, therefore, so is the surface tension. Any further addition of surfactant molecules would only go into micelle formation (Hiemenz 1986). 

Critical aggregation concentration (cac). A surfactant concentration at which micelle formation begins for a surfactant in the presence of polymer. The cac is an extensive characteristic of the specific surfactant—polymer system. 

We may begin the examination of ionic micelle formation by reviewing the main theories already presented. First of all, the mass action law is extended to ionic micelle formation( 14---16) as... 

Food Product Development Figure 3. Beginning of micelle formation... 

Once a micelle is stung, polymerization proceeds very rapidly. The particle can accommodate more monomer as its polymer content increases and the water-polymer interfacial surface increases concuirently. Tlie new surface adsorbs emulsifier molecules from the aqueous phase. This disturbs the equilibrium between micellar and dissolved soap, and micelles will begin to disintegrate as the concentration of molecularly dissolved emulsifier is restored to its equilibrium value. Thus the formation of one polymer particle leads to the disappearance of many micelles. The initial latex will usually contain about 10 micelles per milliliter water, but there will be only about 10 particles of polymer in the same volume of the final emulsion. When all the micelles have disappeared, the surface tension of the system increases because there is little surfactant left in solution. Any tendency for the mixture to foam while it is being stirred decreases at this time. 

The available data indicate that the hypocholesterolemic and hypolipidemic activity of chitosan is probably due to disruption and/or inhibition of micelle formation. At pH 6.0-6.5 chitosan begins to precipitate and as the linear chains of the polysaccharide start to aggregate, they can entrap the whole micelles. The entrapped cholesterol, fatty acids and monoglycerides thus escape absorption. Such "polar entrapment," shown in Figure 2, can occur in the duodenum. Another mode of action could be the "disintegration" of mixed micelles, which can start before the precipitation of chitosan, and in which the free fatty acids and bile acids are selec-... 

The preparation of microparticles by emulsion polymerization, originally developed in the synthetic rubber industry (Whitby and Katz 1933), allows for the formation of microparticles with a narrow distribution of sizes. In this technique, a monomer is dispersed in a solution of surfactant and water where the surfactant creates micelles in the water. Low solubility of the monomer in water is required, such that the addition of the polymer creates large droplets of the monomer within the water. Small amounts of monomer diffuse through the water into the surfactant micelles. To begin the polymerization process, water-soluble initiator is added to the solution, propagating the monomer to form... 

Equation 6.87 predicts that the time tp until liquid crystal formation begins is proportional to the square of the initial drop radius and inversely proportional to the bulk surfactant concentration. These predictions were in agreement with experiments for systems containing pure nonionic surfactants, n-hexadecane, oleyl alcohol, and water (Lim and Miller, 1991a). Moreover, for a hydrocar-bon alcohol ratio of 3 1 by weight and for solutions of at 30°C, the phase diagram was determined and K calculated as 0.52. When the data were htted to Equation 6.87, D2 was found to be 1.3 x 10" ° m /sec. The Stokes-Einstein equation was then used to estimate micelle radius r. 

It is important to note that at cjyj, which frequently appears as a (second) breakpoint in some physico-chemical properties vs. surfactant concentration plots, and is regarded as the saturation value of polymer with surfactant, the polymer is not necessarily saturated. In the present example the bound amount of surfactant at c is only 11% of the saturation value. As a result of the micelle formation the surfactant activity increases very little above cjyj, consequently IcompLJ cannot further increase due to the constancy of the mean activity of surfactant. The equilibrium constants for the complex formation can be chosen so that at the beginning of the micelle formation the amount of surfactant in complex is close to that for saturation. In this case the mean activity of surfactant as a function of the total surfactant concentration shows an inflexion below cjvi. ... 

Anionic Surfactants onto Kaolinite and lUite. In the investigation of the adsorption of sodium dodecylbenzenesulfonate (SDBS) and sodium dodecyl sulfate (SDS) onto asphalt covered kaolinite and illite surfaces, Siffert et al. [5S] observed Langmuir type I isotherms for SDS adsorption onto Na kaolinite and Na illite while the SDBS exhibited a maximum in adsorption with a decrease beginning near the CMC. Adsorption maxima were observed near the CMC for both surfactants in the Ca kaolinite and Ca illite systems. The adsorption behavior was explained as precipitation of the calcium salt of the surfactants (an idea supported by other studies), and the interaction of the aromatic ring in SDBS with the asphalt. This interaction favors desorption of the asphalt rather than adsorption of the SDBS. The amount of asphalt desorbed by SDBS was twice that desorbed by SDS. Other explanations for adsorption maxima include mixed micelle formation [55] and electrostatic repulsion of micelles from the bdayer covered surface [59]. 

The surface pressure isotherms of surfactant solutions in nonpolar solvents at the water-oil interface are specifically S-shaped (Fig. 1). Unlike the surface pressure isotherms of aqueous solutions of surfactants they do not have sharp bends of curves that correspond to the beginning of micelle formation. Beyond certain concentration the 0(j/0 In C value decreases with the increase in the surfactant concentration. After the beginning of micelle formation in the aqueous solutions, this value tends to zero. 

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