Critical micelle concentration depression

Several variations in chemical constitution, which lead to a depression of the Krafft-Point (for example, branching of the hydrophobic part of the molecule), frequently result in diminished hydrophobicity of the molecule. At constant molecular weight, the critical micelle concentration (Cj.) is shifted with increased branching to higher concentrations, the surface activity diminishes, the tendency to adsorb at hydrophobic interfaces decreases, etc. (j, 14, 15). Therefore, the nature of the oxyethylene groups in aTkyl ether sulfates is of major importance. 

The up to now most frequently used techniques as, for example, vapour pressure osmometry (VPO) or freezing point depression (with its limitation regarding the solvent dependent measuring temperature) are based upon the colligative properties of the system the classical absolute light-scattering and ultracentrifugation techniques are only occasionally and approximately applicable with respect to the determination of CMC values. Evaluation of critical micelle concentrations which are based on these latter methods suffer considerably from the insensitivity of these techniques if measurements below the CMC, i.e., below about 10-3 mol dm-3, are carried out. More sensitive methods will be discussed below. 

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... 

In sufficiently dilute aqueous solutions surfactants are present as monomeric particles or ions. Above critical micellization concentration CMC, monomers are in equilibrium with micelles. In this chapter the term micelle is used to denote spherical aggregates, each containing a few dozens of monomeric units, whose structure is illustrated in Fig. 4.64. The CMC of common surfactants are on the order of 10 " -10 mol dm . The CMC is not sharply defined and different methods (e.g. breakpoints in the curves expressing the conductivity, surface tension, viscosity and turbidity of surfactant solutions as the function of concentration) lead to somewhat different values. Moreover, CMC depends on the experimental conditions (temperature, presence of other solutes), thus the CMC relevant for the expierimental system of interest is not necessarily readily available from the literature. For example, the CMC is depressed in the presence of inert electrolytes and in the presence of apolar solutes, and it increases when the temperature increases. These shifts in the CMC reflect the effect of cosolutes on the activity of monomer species in surfactant solution, and consequently the factors affecting the CMC (e.g. salinity) affect also the surfactant adsorption. 

Extensively studied nonionic surfactants are PDMS / polyether block copolymers [31] and poly(ethylene oxide) substituted trisiloxanes [32-35]. They are able to achieve a maximum surface tension depression to about 21 mN/m and a low critical micelle concentration (CMC) [36], These siloxane surfactants can self-assemble into a variety of... 

N-Lauroyl ED3A acid, below pH 4, is a white, insoluble crystalline powder. Above pH 5, whereupon two of the acetate groups become neutralized, crystal-clear aqueous solutions form. Since the molecule is amphiphilic in nature, it was expected that the product should exhibit surface activity. Two primary characteristics of a surfactant are depression of surface tension of aqueous solutions and the presence of a critical micelle concentration (CMC). Acetate salts of N-acyl ED3 A were found to exhibit both of these properties. 

The critical micelle concentration is affected by the presence of electrolytes and other components of the surfactant solution [142,168,192,193]. Electrolytes depress the cmc of ionic surfactants and increase the micellar size by decreasing the thickness of the ionic cloud around the ionic groups and reducing the electrostatic... 

In fluorosurfactants, the hydrophobic group is a per-fluoroalkyl unit in which all of the hydrogen atoms are replaced by fluorine. Anionic fluorosurfactants can be prepared from all classes of anionics. Their critical micelle (forming) concentration is remarkably low and therefore they depress the surface tension of water to a very low value, even at small concentration levels. Furthermore, they are stable against thermal and chemical influences. Hence, fluorosurfactants can be used in aggressive media and at high temperatures. 

A well-known feature of surfactant solutions is their ability to dissolve a variety of oil-soluble materials, e.g., hydrocarbons, esters, perfumes, dyes, and so on. This property is utilized in the compounding of many formulations, the process involving the dissolution of the solubihzate in the surfactant micelles. Formation of polymer/surfactant complexes is currently regarded as a depression of the critical aggregation concentration of the surfactant. According to this picture, superior solubilization in such systems can be anticipated... 

The surface tension measurement is a kind of effective approach for the study of IL aggregation state. The critical micelle (or aggregation) concentration (CMC) can be determined by surface tension measurement. Surface tension measurement revealed that the dissolution of Triton X-100 in IL ([bmim][BFJ) depressed the surface tension in a manner analogous to aqueous solutions, and a relatively higher CMC was obtained compared to that of water as shown in Figure 18.8 [46]. 

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