Micelle equilibrium

Moreover, taking into account that reversed micelles coexist with surfactant monomers, in principle, further effects due to the aggregation of polar and amphiphilic so-lubihzates with surfactant monomers and the shift of the monomer/reversed micelle equilibrium must be also considered [25,26],... 

Monomer/Micelle Equilibrium Mixtures of surfactants, like any surfactant species in an aqueous solution, give rise to monomer or micelle aggregates provided that the concentration reaches a minimum value, called the critical micellar concentration (CMC). The micelles thus formed are mixed, i.e. made up of the different surfactant species in solution. 

Size-exclusion chromatography (SEC) has been used to characterize the unimer-micelle distribution. However, SEC is not an absolute method and thus requires calibration. Since it is practically impossible to calibrate a SEC apparatus for the unimers and micelles formed by a block copolymer, only indicative MW values can be obtained. Moreover, several authors have noted a strong perturbation of the unimer-micelle equilibrium during SEC experiments even when interaction of the material with the SEC column was minimized [4,61,62],... 

The dynamics of micellar systems is a very important concern that is relevant at several levels. In this respect, the dynamics required for the establishment of the unimer-micelle equilibrium is one aspect. The so-called problem of micelle hybridization that deals with the exchange rate of unimers between different micelles is closely related. Finally, the chain dynamics of polymer blocks in either the core or the corona is another concern that can, however, be linked to some extent to the first two mentioned concepts. These different aspects have been scarcely studied and reviewed by Tuzar and Kratochvil [6,41], and... 

CMC will change if the additive has an effect on the monomer-micelle equilibrium, and also if the additive changes detergent solubility. The CMC of all ionic surfactants will decrease if coions are added. However, nonionic surfactants show very little change in CMC on the addition of salts, which is to be expected from theoretical considerations. The change of CMC with NaCl for SDS is as follows (Figure 3.10) ... 

Ideal Mixed Micelles. The Critical Micelle Concentration (CMC) is the lowest surfactant concentration at which micelles form the lower the CMC, the greater the tendency of a system to form micelles. When the total surfactant concentration equals the CMC, an infintesimal fraction of surfactant is present as micelles therefore, the CMC is equal to the total monomer concentration in equilibrium with the micellar pseudo—phase. The CMC for monomer—micelle equilibrium is analogous to the dew point in vapor—liquid equilibrium. 

MONOMER MOLE FRACTION OF SURFACTANT A Monomei—micelle equilibrium -for systems in... 

As also seen in Table I, the micellar composition can be a-f-fected substantially by nonideality. In -fact, azeotropic behavior in the monomer—micelle equilibrium is possible -for these nonideal systems i.e., as the monomer composition varies -from pure A to pure B, the micelle can vary -from Xn > y to Xn = y (azeotrope) to Xa < yA. This azeotrope -formation is illustrated -for the cationic/nonionic system in Figure 2, where an azeotrope -forms at Xa = yA = 0.3. The minimum CMC -for a mixture corresponds to the azeotropic composition i-f an azeotrope is present (32.37). For an ideal system, azeotropic behavior is not observed. 

Equations 3 and 4 are derived from Equation 5 (31) which has been Found to be invalid For the systems oF interest. However, Equations 3 and 4 have been shown to accurately describe mixture CMC values and monomer-micelle equilibrium. The resolution is that Equations 3 and 4 should be considered as valuable empirical equations to describe these nonideal systems. The Fact that they were originally derived From regular solution theory is a historical coincidence. 

Solubilization o-f dissolved organic molecules into micelles is important in detergency (2), emulsion polymerization (65). and micellar—enhanced ultra-fiItration (3), Just to name a -few applications. Solubilization also indirectly a-f-fects many other operations because it o-ften a-f-fects monomer—micelle equilibrium, in-fluencing sur-factant adsorption, wetting, etc. when solubi 1 izable, non—sur-factant species are present in solution. 

The equilibrium in these systems above the cloud point then involves monomer-micelle equilibrium in the dilute phase and monomer in the dilute phase in equilibrium with the coacervate phase. Prediction o-f the distribution of surfactant component between phases involves modeling of both of these equilibrium processes (98). It should be kept in mind that the region under discussion here involves only a small fraction of the total phase space in the nonionic surfactant—water system (105). Other compositions may involve more than two equilibrium phases, liquid crystals, or other structures. As the temperature or surfactant composition or concentration is varied, these regions may be encroached upon, something that the surfactant technologist must be wary of when working with nonionic surfactant systems. 

Micelles are often present in surfactant systems. In some processes, such as solubilization, they are directly involved. Micelles indirectly affect many other processes because monomer concentrations or activities of the surfactant components are dictated by the monomer— micelle equilibrium at total surfactant concentrations above the CMC. Therefore, interest in mixed micelle formation will continue to grow. 

As already discussed in Chapter 1, the relative tendency of a surfactant component to adsorb on a given surface or to form micelles can vary greatly with surfactant structure. The adsorption of each component could be measured below the CMC at various concentrations of each surfactant in a mixture. A matrix could be constructed to tabulate the (hopefully unique) monomer concentration of each component in the mixture corresponding to any combination of adsorption levels for the various components present. For example, for a binary system of surfactants A and B, when adsorption of A is 0.5 mmole/g and that of B is 0.3 mmole/g, there should be only one unique combination of monomer concentrations of surfactant A and of surfactant B which would result in this adsorption (e.g., 1 mM of A and 1.5 mM of B). Uell above the CMC, where most of the surfactant in solution is present as micelles, micellar composition is approximately equal to solution composition and is, therefore, known. If individual surfactant component adsorption is also measured here, it would allow computation of each surfactant monomer concentration (from the aforementioned matrix) in equilibrium with the mixed micelles. Other processes dependent on monomer concentration or surfactant component activities only could also be used in a similar fashion to determine monomer—micelle equilibrium. 

In. my opinion, the study o-f monolayer -formation has less practical importance than the study o-f micelles. Yet, the thermodynamics of monolayer formation has seen substantial study. I think that this is largely due to the fact that the monomer—monolayer equilibrium can be unambiguously and relatively easily measured using the Hutchinson method (25), as exploited by Rosen and Hua ( ), while this cannot be said for monomer—micelle equilibrium. Therefore, mixed monolayer formation will be a more fruitful field for model development in the near future than mixed micelles because of the availability of a method of obtaining experimental data for comparison. 

In aqueous surfactant solutions, either by circumstance or design, non—surface active organic species may be present. Examples are oil recovery, where crude oil is present, or micellar—enhanced ultrafiltration, where micelles are being used to effect a separation of dissolved organic pollutants from water. The ability of mixed micelles to solubilize organic solutes has received relatively little study. In addition, the solubilization of these compounds by micelles may change the monomer—micelle equilibrium compositions. 

As our understanding and ability to model monomer-micelle equilibrium in the absence o-f added solutes evolves, research into the more complex systems will... 

Indirect methods for obtaining information on the kinetics of the associa-tion/dissociation equilibrium include sedimentation velocity and GPC experiments. The application of these techniques is based on comparison of sedimentation or GPC elution curves with model curves based on theories for separation of unimers and micelles during a sedimentation velocity (Gilbert 1955) or GPC (Ackers and Thompson 1965 Coll 1971 Prochazka et at. 1988, 1989) experiment. Experiments have been performed that demonstrate several of the qualitative model predictions (Prochazka et at. 1989). The main conclusions were that GPC curves with two well-separated peaks can only result from a slow dynamic molecule micelle equilibrium, and that no simple interpretation of elution curves in terms of relative concentrations of unimer and micelles is possible (Prochazka et at. 1989). Thus no quantitative information on the kinetics of the molecule micelle equilibrium can be obtained from sedimentation velocity or GPC data. 

Except at concentrations near the CAC, the amount of surfactant adsorbed in the Henry s law region is small in comparison to the amount of surfactant present in the admicelles. This implies that nearly all of the adsorbed surfactant molecules are associated on the mineral surface in the form of admicelles. It is important to keep in mind that Equation 8 is valid only between the CAC and the CMC. Above the CMC, equations could be included to account for the formation of micelles, by including monomer-micelle equilibrium equations (1 1). ... 

CMC = 9 mM) was used. The nonluminescent cationic surfactant seemed to play a noninnocent role. Indeed for concentration of CTAB above the CMC, both 1 and 2 displayed an increase of their emission intensity as well as an elongation of their excited-state lifetimes, suggesting an incorporation of the metallosiufactant in the CTAB-based micelles. In Fig. 8 are depicted the emission spectra of equimolar mixtures of the metallosiufactants 1 and 2, upon variation of the concentration of CTAB. These experiments clearly showed the dependence of the intramicellar energy transfer process occurring between the two amphiphilies due to the micellization equilibrium of CTAB. 

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