Critical micelle concentration pressure effect

For solutions of AEg with different distributions of hydrocarbon chain lengths, the Y log C curves appear to be different than mono-component system. The surface pressure at critical micelle concentration (iTcjic) AEg with a long hydrocarbon chain (C gEg) is Increased by adding the short AEg, but the effect is not significant if the hydrocarbon chain is in a wide distribution (i.g. coconut fatty radical) (Figure 2,3,4). As for the efficiency of surface tension reduction there is a synergestic effect for the mixed... 

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. 

Bioaccumulation All classes of surfactant are active surface tension depressants. At the critical micelle concentration (CMC) abrupt changes occur in the characteristic properties of surfactants such that surface and interfacial tensions in an aqueous system are at their minimum while osmotic pressure and surface detergent properties are significantly increased. The CMC for most surfactants is reached around 0.01% (18, 19). These effects have an impact on the potential for bioaccumulation of the pesticide, and in the organisms monitored the presence of Dowanol and nonylphenol increased the accumulation of fenitrothion and aminocarb at least 20-300% respectively, over the accumulation obtained in their absence (20). In effect, these adjuvants... 

If surfactant is added to a suspension polymerization system, a number of phenomena may occur. If the surfactant is added in small amounts (below the critical micelle concentration or CMC), the reduction in interfacial tension between the organic and aqueous phases will result in smaller monomer droplets, but it has hardly any other effect. If surfactant is added above the CMC, and an oil-soluble initiator is used, the process is commonly termed a microsuspension polymerization. Due to the reduced interfacial tension, the droplet diameter (and hence bead diameter) is reduced to approximately 10-40 pm. Little polymerization takes place in the aqueous phase or in particles generated from surfactant micelles because of the hydrophobic nature of the initiator. However, some smaller particles initiated from surfactant micelles may be found. The kinetics are still essentially those of a bulk free radical polymerization. Microsuspension polymerization is used to produce pressure-sensitive adhesives for repositionable notes. 

The efficiency of a surfactant in reducing surface tension can be measured by the same quantity that is used to measure the efficiency of adsorption at the liquid-gas interface (Chapter 2, Section HIE), pC20, the negative log of the bulk phase concentration necessary to reduce the surface tension by 20 dyn/cm (mN m-1). The effectiveness of a surfactant in reducing surface tension can be measured by the amount of reduction, or surface pressure, IIcmc, (= To Ycmc) attained at the critical micelle concentration, since reduction of the tension beyond the CMC is relatively insignificant (Figure 5-3). 

We have seen in Figure 2-15 that the surface tension of a solution of an individual surfactant decreases steadily as the bulk concentration of surfactant is increased until the concentration reaches a value known as the critical micelle concentration (CMC), above which the tension remains virtually unchanged. The surface tension at the CMC is therefore very close to the minimum tension (or maximum surface pressure) that the system can achieve. The surface pressure at this point, I ICmc, is therefore a suitable measure of the effectiveness of a surfactant in reducing surface tension (Figure 5-3). 

FIGURE 2.8 The effect of micelle formation on some solution properties, (a) Schematic picture of micelle formation, (b) Osmotic pressure, surface tension, and turbidity of solutions of sodium dodecyl sulfate (SDS) as a function of concentration (approximate). CMC = critical micellization concentration. 

In aqueous solution micelles are generally thought to be spherical as long as the surfactant concentration remains close to the critical micelle concentration. Rod-like micelles may form at higher surfactant concentrations [1, 2]. Addition of a third component such as neutral salt or non-electrolytes may favour longer micellar structures, for instance rod-like micelles [3-6]. An increase in temperature, on the other hand, seems to favour spherical micelles [7, 8]. The effect of pressure on the shape transition point is not known, though it appears that the aggregation number of micelles decrease with pressure at least up to about 160 MPa [9-12]. 

Micellar media are formed from tensioactive molecules in aqueous solution. Mi-cellization is a manifestation of the strong self-association of water and water-like solvents [95]. Micelles are known to increase the solubilization of weakly polar substances in water and, as a consequence, their presence determines the magnitude of hydrophobic interactions. Micelles aggregate spontaneously in aqueous solution beyond a critical concentration which is a function of pressure [96]. As a result, pressure may induce an extra kinetic effect on the rate of organic reactions carried out in aqueous micellar systems. Representative ionic micelles are sodium dodecyl sulfate (SDS) and tetradecyltrimethylammonium bromide (TTAB). Recent examples demonstrate the beneficial effect of the presence of surfactants in Lewis acid-catalyzed reactions, a kind of biactivation [97]. 

---