Critical micelle concentration factors affecting

Factors affecting the critical micelle concentration and micellar size... 

We have examined the stmcture of both ionic and nonionic micelles and some of the factors that affect their size and critical micelle concentration. An increase in hydrophobic chain length causes a decrease in the cmc and increase of size of ionic and nonionic micelles an increase of polyoxyethylene chain length has the opposite effect on these properties in nonionic micelles. About 70-80% of the counterions of an ionic surfactant are bound to the micelle and the nature of the counterion can influence the properties of these micelles. Electrolyte addition to micellar solutions of ionic surfactants reduces the cmc and increases the micellar size, sometimes causing a change of shape from spherical to ellipsoidal. Solutions of some nonionic surfactants become cloudy on heating and separate reversibly into two phases at the cloud point. 

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. 

Because there are many factors that have been shown to affect the observed critical micelle concentration strongly, the following discussion has been divided so as to isolate (as much as possible) the various important factors. 

Compared to surfactant micelles, polymeric micelles have a much higher thermodynamic stability. It is well-known that micelles have a critical micelle concentration (CMC) below which only unimers exist but above which both micelles and unimers are present. It was reported that polymeric micelles have a CMC around 10 -10 M [58,59], which is 1000-fold lower than that of surfactant micelles (10 -10 ). The CMC of polymeric micelles is affected by many factors, including the properties of the core-forming blocks, such as hydrophobicity, the glass transition temperature (Tg), the degree of crystallinity, and the length and ratio of the hydrophilic and hydrophobic... 

It is obvious, therefore, that any discussion of cmc data must be tempered with the knowledge that the reported values must not be taken to be absolute rather, they reflect certain variable factors inherent in the procedures employed for their determination. The variations found for nominally the same material under supposedly identical conditions in the literature should be accepted as minor noise that does not significantly affect the overall picture of the system. With those caveats in mind, our attention now turns to some of the trends that have been identified over the years that relate surfactant critical micelle concentrations to molecular structures. 

Studies on the kinetics of LOX reactions have been handicapped by the limited solubility of the substrate, the formation of emulsions, and other physical factors affecting the form of the substrate in aqueous media. Tappel et al. (1952) found that soybean LOX-1 at pH 9 obeyed normal Michaelis-Menten kinetics at low substrate concentrations and that the value for linoleic acid was 2.10 M. This was later confirmed by more extensive work (Galpin and Allen, 1977 Allen, 1968). Galpin and Allen (1977) also showed that the nonmicellar concentration of linoleic acid determined the rate of reaction substrate concentrations above the critical micelle concen-... 

The physical chemistry of micellar structure and formation has been reviewed extensively elsewhere[40,45-47], and is only briefly summarized. The concentration at which micellar aggregation of bile salts molecules occurs (critical micellar concentration, CMC) is affected by bile salt structure, pH, temperature and a variety of other factors. Conjugated bile salts have a higher CMC than the unconjugates, and the CMC for trihydroxycholanates (cholic acid) is higher than for the dihydroxy derivatives. Among the latter, deoxy-cholate forms micelles at a lower CMC than does chenodeoxycholate. 

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