Critical micelle concentration operational

In order to more closely represent the volatilization environment that would be encountered in an evaporation pond, Triton X-100, a non-ionic emulsifier similar to those used in some pesticide formulations, was added to prepared pesticide solutions at 1000 ppm. The presence of this emulsifier caused a decrease in the percent pesticide volatilized in one day in all cases except for mevinphos (Table VI). Three mechanisms are probably in operation here. First, Triton X-100 micelles will exist in solution because its concentration of 1000 ppm is well above its critical micelle concentration of 194 ppm (30). Pesticide may partition into these micelles, reducing the free concentration in water available for volatilization, which will in turn reduce the Henry s law constant for the chemical (31). Second, the pesticides may exhibit an affinity for the thin film of Triton that exists on the water surface. One can no longer assume that equilibrium exists across the air-water interface, and a Triton X-100 surface film resistance... 

It has been well documented that surfactants self-associate in aqueous solution to minimize the are of contact between their hydrophobictails and the aqueous solution (Mukerjee, 1979 Tanford, 1980). This phenomenon occurs at a critical concentration of surfactant, the critical micelle concentration or CMC (see Figure 12.4) above where the surfactant molecules exist predominantly as monomeric units and above which micelles exist. The CMC can be measured by a variety of techniques, for example, surface tension, light scattering, osmometry, each of which shows a characteristic break point in the plot of the operative property as a function of concentration. Knowing the CMC of the particular surfactant system and understanding the conditions that may raise or lower that critical concentration is important to the design of a formulation based on micellar solubilization. 

An examination of the composition and physical state of sebum suggests that several cleaning mechanisms can operate during its removal from hair. Since sebum is completely molten at body temperature [122], it can be effectively removed by the roll-back mechanism. Also, the presence of approximately 25% free fatty acids in sebum indicates, as discussed in Section IV.C.3, that it is subject to removal by emulsification and mesophase formation. Finally, because the concentration of detergents during shampooing is well above their critical micelle concentrations, sebum can also be cleaned from hair by solubilization. 

Dafniotis, P. (1996) Modelling of Emulsion Copolymerization Reactors Operating below the Critical Micelle Concentration. PhD Thesis. University ofWisconsin-Madison, USA. 

Details are given of a non-steady-state operation for controlling latex particle size distribution by using a continuous emulsion polymerisation of vinyl acetate. The experiment was conducted in a continuously stirred tank reactor under conditions below the critical micelle concentration of the emulsifier. The mean residence time was switched alternately between two values in the nonsteady-state operation to induce oscillations in monomer conversion in time. The effect of the switching operation on particle size distribution is discussed. 13 refs. 

If the interfacial area is small, it can only accommodate a small number of molecules. When, as usual, many more surfactant molecules than this are present, the majority cannot escape from the bulk liquid to the interface and the affinities of the hydrophilic and lipophilic groups must be satisfied by other means if thermodynamic stability is to be achieved. This again occurs by a process of orientation. In an aqueous medium the hydrophobic groups turn towards and associate with one another, forming in effect their own oil phase, surrounded by the hydrophilic groups turned outwards and anchored in the water. This type of internal association and orientation has been termed micelle formation. Micelles are usually spherical in shape. The escape mechanism of micelle formation only becomes operative above a certain minimum surfactant concentration. This concentration has been termed the critical micelle concentration (CMC). CMCs vary from about 5 x 10 mol 1 for the most hydrophilic to about 5 x 10 mol 1 for the most hydrophobic types of surfactant. They are influenced by electrolytes, especially in the case of ionic surfactants, and also by other polar/non-polar chemical compounds such as alcohols, amides and, of course, other surfactants. 

When applied to the precipitation of amphiphilic block copolymers, the nanoprecipitation is often described in terms of self-assembling the precipitated particle is seen as a micelle, which is built by diffusion-limited aggregation of single molecules of the polymer [48]. Different from the usual micellization processes, which take place near the critical micelle concentration and are reversibile, in this case the operation takes place at much larger concentration and the resulting micelles are kinetically frozen... 

The choice of 20 mN/m as a standard value of surface tension lowering for the definition of adsorption efficiency is convenient, but arbitrary. When one discusses the effectiveness of adsorption, as defined as the maximum lowering of surface tension regardless of surfactant concentration, the value of is determined by the system itself and represents a more firmly fixed point of reference. The value of < min for a given surfactant will be determined by one of two factors (1) the solubility limit or Krafft temperature (7k) of the compound or (2) the critical micelle concentration (cmc). In either case, operationally the maximum amount of surfactant adsorbed will be reached at the maximum bulk concentration of free surfactant, assuming one can ignore the slight decreases in a found for some surfactants above the cmc. 

The cac (critical aggregate concentration) values for oleate are in the millimolar range, which means that at the operational concentration of 10-50 mM there will be a signihcant concentration of monomer in equilibrium with the aggregate. This consideration allows us to go back to the question of whether vesicles are chemical equilibrium systems. Oleate vesicles cannot be considered proper chemical equilibrium systems, however they behave in a mixed way, with some features that are typical of micelles in equilibrium (Luisi, 2001). 

Photochemical processes in heterogeneous systems, and across micelle boundaries in particular, clearly has great potential. The photolysis of amphipathic alkylcobaloximes in mixed micelles shows a co-operative effect owing to structure.The photoreduction of anthraquinone in aqueous micellar solution has been compared with that in non-aqueous solution.The dimerization of 3-(n-butyl)cyclopentenone is solvent-dependent and the ratio of isomeric products depends on surfactant concentration. It is suggested that this can be used as a means of critical micellar concentration determination. 

How then is the fall in the conductivity at the critical concentration to be explained According to Hartley there are here two effects which operate against one another. The association of the fatty acid anions into a micelle would — when nothing else happened — undoubtedly lead to an increase of the conductivity (Me Bain effect). Indeed the single fatty acid ions have a fairly large mass in proportion to their charge and thus — in view of Stokes law — a small electrochemical mobility. In the association of these anions into a spherical micelle the charge increases much more rapidly than the radius of the particle. 

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