Critical micelle concentration, definition

A structure formed by the reversible association of am-phiphiles in apolar solvents. In inverted micelles, the polar portion of the amphiphile is concentrated in the interior of the macrostructure. Such association usually occurs with aggregation and is not typically characterized by a definite nucleation stage. Thus, inverted micelles (also referred to as inverse or reverse micelles) often fail to exhibit critical micelle concentration behavior. See Micelle... 

In solutions of surfactants, aggregates are formed at definite concentrations. These micelles, formed at the critical micelle concentration (cmc), are very characteristic and of different shapes... 

Solubilization can be defined as the preparation of a thermodynamically stable isotropic solution of a substance normally insoluble or very slightly soluble in a given solvent by the introduction of an additional amphiphilic component or components. The amphiphilic components (surfactants) must be introduced at a concentration at or above their critical micelle concentrations. Simple micellar systems (and reverse micellar) as well as liquid crystalline phases and vesicles referred to above are all capable of solubilization. In liquid crystalline phases and vesicles, a ternary system is formed on incorporation of the solubilizate and thus these anisotropic systems are not strictly in accordance with the definition given above. 

Surfactants by definition self-organise in water giving rise to micelles of varying size and shape. The core of micelles is non-polar and can solubilise reactants that are insoluble in water. Thus, a simple surfactant-water system at a surfactant concentration well above the critical micelle concentration can be used to overcome the problem of reactant incompatibility the polar reagent will be situated in the bulk aqueous domain, the non-polar reagent will be present in the micelles, and the reaction will occur at the micelle boundary. Organic reactions in micellar systems have been reported more than 40 years ago [1,2]. 

Single molecules or micelles associate spontaneously in a thermodynamic equihbrium at a definite critical micelle concentration within a biocoUoidal system [47]. Analogously to micelle formation in liquid systems, aggregation of surfactants at a surface depends on a critical hemi-micellar concentration [48, 49]. The removal of the hydrophobic molecular region from the hydrophihc interface... 

Whenever a system has a composition that lies in the polyphasic region, it will generally separate (at equilibrium) into two phases, the representative points of which are located at the two extremes of the tie-line (see Fig. 3). In most cases the tie-hne is inchned i.e., one of the phases is rich in surfactant because it is located relatively near A or far from the OW side. If it is also located far from the AW and AO sides, then it contains both W and O in sizable amounts and fits the definition of a microemulsion (shaded region). Near the upper end of the tie-line in Fig. 3, it is an O/W type microemulsion. The other extreme of the tie-line is located near the OW side and near one of the component vertices (O in Rg. 3) and thus contains essentially one of the components. It is called an excess phase, in this case an oil excess phase. In most cases, particularly with ionic surfactant, the excess phase does contain a very small concentration of amphiphile, about the critical micelle concentration (cmc). In other words, the excess phase does not contain micelles, and as a consequence no micellar solubilization of the other phase can occur in the excess phase, an important feature when the mass balance is to be discussed. 

The choice of 20 mN m" as a standard value of surface tension lowering for the definition of adsorption efficiency is convenient, but, as mentioned, somewhat arbitrary. When one discusses the effectiveness of adsorption, as defined as the maximum lowering of surface tension regardless of surfactant concentration, the value of o-min is determined only by the system itself and represents a more firmly fixed point of reference. The value of oinm for a given surfactant will be determined by one of two factors (1) the solubility limit or Krafft temperature (Tk) of the compound, or (2) the critical micelle concentration (cmc). In either case, the maximum amount of surfactant adsorbed is reached, for all practical purposes, at the maximum bulk concentration of free surfactant. 

There is some disagreement within the surfactant literature as to the exact definition of solubilization, particularly as the ratio of surfactant to additive decreases, and one approaches the nebulous frontier between swollen micellar systems and the micro- and macroemulsion regions. For present purposes, solubilization will be defined as the preparation of a thermodynamically stable, isotropic solution of a substance (the additive ) normally insoluble or only slightly soluble in a given solvent by the addition of one or more amphiphilic compounds at or above their critical micelle concentration. By the use of such a definition, a broad area can be covered that includes both dilute and concentrated surfactant solutions, aqueous and nonaqueous solvents, all classes of surfactants and additives, and the effects of complex interactions such as mixed micelle formation and hydrotropes. It does not, however, limit the phenomenon to any single mechanism of action. 

It is the view of the present authors that a physiological role for these inhibitors cannot be disregarded on the basis of these arguments rather, the question must remain open until more definitive experiments are performed. It seems reasonable that, in the intracellular environment, the critical micelle concentration for the fatty acyl-CoA derivatives may never be achieved and that the largest fraction of these inhibitory substances is bound at hydrophobic sites of intracellular proteins and lipid structures such as membranes. In this case, the degree of inhibition would depend on the relative affinity of hydrophobic sites on the carboxylase, the concentration of free fatty acyl-CoA, and that bound at other intracellular hydrophobic sites. 

The summation in Equation 4.1 is carried out over aU components. UsnaUy an equimolecular dividing surface with respect to the solvent is introduced for which the adsorption of the solvent is set zero by definition [4,5]. Then the snmmation is carried ont over aU other components. Note that F, is an excess surface concentration with respect to the bulk F is positive for surfactants, which decreases o in accordance with Equation 4.1. On the contrary, F is negative for aqneous solutions of electrolytes, whose ions are repelled from the surface by the electrostatic image forces [5] consequently, the addition of electrolytes increases the surface tension of water [6]. For surfactant concentrations above the critical micellization concentration (CMC) = constant and, consequently, a = constant (see Equation 4.1). 

In the previous relationship, the solubilization parameters SP refer to the Vo/Vs and Vw/l s ratios of the volume of oil and water solubilized in the microemulsion phase (the surfactant-rich phase) per volume of surfactant. If the surfactant is a solid, and as a more general definition, the solubilization parameters may be taken as Vo/nts and V ulfn, where ms is the mass of surfactant. The volumes can be deduced firom the measurement of the microemulsion-phase volume at equilibrium mid the amount of surfactant, oil, and water that were added to the system in the first place. It is helpful to note that in two- or three-phase systems, the excess phase(s) contain(s) at most the critical micelle concentration of the surfactant, which is, in general, a negligible amount in the mass balance. Thus, all the surfactants can be assumed to be in the microemulsion phase, and the excess phase can be assumed to be a pure component. 

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