Critical micelle concentration influencing factors

R.W. Egan, M.A. Jones and A.L. Lehninger, Hydrophile-lipophile balance and critical micelle concentration as key factors influencing surfactant disruption of mitochondrial membranes, J. Biol. Chem. 251 (1976) 4442-4447. 

Figure 3.8 shows the dynamic surface tension of a pure anionic and a non-ionic surfactant dependent on the absorption time after the creation of new surface for different concentrations [9]. For both surfactants, the time dependence of the surface tension is greatly reduced when the concentration increases and this effect is especially pronounced when the critical micelle concentration is reached. The reason for this dependence is the diffusion of surfactant molecules and micellar aggregates to the surface which influences the surface tension on newly generated surfaces. This dynamic effect of surface tension can probably be attributed to the observation that an optimum of the washing efficiency usually occurs well above the critical micelle concentration. The effect is an important factor for cleaning and institutional washing where short process times are common. 

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. 

Another important factor which is not directly related to the hydrophilicity/lipophilicity ratio of various pluronics but which strongly influences the efflux pump inhibitoiy activity of pluronics is micelle formation. Above a certain concentration, the so called critical micelle concentration (CMC), amphiphilic block copolymers self assemble into micelles. It has been demonstrated, that the efflux pump inhibitory activity of pluronics increases with increasing pluronie concentrations, but only until the CMC is reached. Above the CMC, substrate accumulation in cancer cells could not be further increased or was found to even decrease [37]. Therefore, the occurrence of pluronie unimers can be regarded as the crucial prerequisite for the efflux pump inhibitory activity of pluronics [37]. As one mechanism for efflux pump inhibition has been identified to be ATP depletion, it seems necessary that the pluronie unimers are transported into the cells in order to exert this action. 

Micelle formation and critical micelle concentration of different surfactants in aqueous solvents have been extensively studied from different angles and discussed at length [41,3]. Some discussion on the factors influencing the CMC in aqueous media has been made in Section 2.4. We therefore move now to the case of CMC in non-polar solvents. 

FACTORS THAT INFLUENCE THE CRITICAL MICELLE CONCENTRATION... 

The influence of surfactants and plant species on the retention of spray solution has been examined by de Ruiter and Uffing. They reported a linear relationship between retention and the logarithm of surfactant concentrations. It would appear that there is a need to add a surfactant concentration far in excess of the critical micelle concentration (CMC) and that the diffusion of surfactant from the bulk to the surface of the flattening drop is a retention-determining factor. The dynamic surface tension is regarded as a useful parameter determining surfactant type and concentration with respect to efficient surface retention. 

In this chapter we will see how the surface activity of a molecule is related to its molecular structure and look at the properties of some surfactants which are commonly used in pharmacy. We will examine the nature and properties of films formed when water-soluble surfactants accumulate spontaneously at liquid/air interfaces and when insoluble surfactants are spread over the surface of a liquid to form a monolayer. We will look at some of the factors that influence adsorption onto solid surfaces and how experimental data from adsorption experiments may be analysed to gain information on the process of adsorption. An interesting and useful property of surfactants is that they may form aggregates or micelles in aqueous solutions when their concentration exceeds a critical concentration. We will examine why this should be so and some of the factors that influence micelle formation. The ability of micelles to solubilise water-insoluble drugs has obvious pharmaceutical importance and the process of solubilisation and its applications will be examined in some detail. 

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