Micellization phenomena

A slight increase in the turbidity upon heating of aqueous solutions of the s-fractions of the NVCl/NVIAz-copolymers obtained from the feeds with initial comonomer molar ratios of 75 25 (Tcp 65 °C) and 80 20 (Tcp 66 °C) could be due to the micellization phenomena, although the absence of DSC peaks over the same temperature range testified to the non-cooperative character of the process. This could indicate that the chains of these s-type copolymers had, nevertheless, a certain amount of oligoNVCl blocks non-buried by the hydrophilic microenvironment sufficiently well and thus capable of participating in the hydrophobically-induced associative intermolecular processes at elevated temperatures. At the same time, the sequence of monomer units in the s-copolymers obtained from the feeds with the initial comonomer ratios of 85 15 and 90 10 (mole/mole) corresponded to the block-copolymers of another type. The basis for such a conclusion is the lack of macroscopic heat-induced phase separation at elevated temperatures (Fig. 3 a and b) and, simultaneously, the transi-... 

Perfluorocarbons bearing a polar hydrophilic head are very active surfactants. Indeed, the presence of fluorine atoms strongly lowers the critical micelle concentration (CMC) of an amphiphilic compound. Moreover, fluorination generally has important effects on micellization phenomena, especially on the size and shape of formed micelles. 

Asta eva, I., X. Zhong, and F. A. Eisenberg. 1993. Critical micellization phenomena in block polyelectrolyte solutions.Macromolecule 6 7339-7352. 

In view of this phase concept which is confirmed by the micellization phenomena in many nonpolar detergent solutions, it has been suggested by Eicke and Christen40 that in line with this reasoning a nucleation step is to be expected (in the approximation of the phase separation model). In order to explain the origin of the energy necessary to overcome the potential barrier associated with the postulated... 

The use of substances that form micelles as mobile phase additives continues to serve as an area of academic and practical interest. Often touted as a new form of chromatography, micelle chromatography should perhaps be considered as a fascinating example of the incorporation of secondary equilibria for the enhancement of selectivity and the adjustment of retention. in terms of practical chromatography, it is not yet clear that micelle chromatography solves any problems that cannot be solved by conventional means. Hhat is more clear is that micelle chromatography may provide a new route to the study of micelle phenomena."... 

Both HA and FA have been shown to form micelles, but not at concentrations that are environmentally relevant. The inability of FA to solubilize pyrene at concentrations above its CMC, and the lower solubility enhance of DDT in FA micelles compared to HA suggests that the smaller size of the molecules which comprise FA, and consequently the micelles that form from it, affects the solubilization phenomena. The SAXS analysis of HA does not show an abrupt change in size or fractal dimension as the solution concentration increases beyond the CMC which suggests that only a portion of the molecules which comprise HA are involved in the micellization phenomena. 

These include studies of adsorption and micellization phenomena (Chapters III, VI), the formulation of detergents, investigation of the effect of surfactants on the mechanical properties of various materials (see Chapter IX), control over the biodegradability of surfactants into the environment, etc. Although qualitative and structural analyses of surfactants are certainly important, we will only briefly mention them and will devote most of our attention to the discussion of methods used for the quantitative analysis of surfactants, i.e. methods for determining surfactant concentration in solutions. The subjects of qualitative and structural analysis are extensively covered in the literature and those interested may refer to the relevant monographs and references therein [31-35]. 

The study of vesicle dynamics is now a mature area of research. The main processes involved in bilayer mobility and solute transport are quite well vmderstood, but this is not so with the spontaneous formation and breakdown of vesicles. There is a significant difference between the d5mamics of micelle forma-tion/breakdown and vesicle formation/breakdown. Micelle phenomena occur on a very short time scale, with processes for most micellar systems taking place in time scales less than 1 sec. For vesicles of synthetic surfactants like the alkylbenzene-sulfonates, the relevant processes are in the second to minute time range, although surfactant monomer exchange between vesicles and aqueous solution may well take place in the mil-... 

As suggested above, the main recovery mechanism of surfactants retained in the rock can be interpreted as a micellization phenomenon inside the pores. Upon contact with micelles from the desorbent agent, the adsorbed surfactants are solubilized in the form of mixed micelles. This also explains the effectiveness of the desorbent still observed at low concentration (0.27% in Test 3 in Table in, concentration much higher than the CMC of NP 30 EO equal to 0.016%). 

For any aqueous surfactant solution, there is a relatively small range of concentrations below which virtually all surfactant is present as monomers, and above which virtually all additional surfactant is present in micellar form. This micellization phenomenon causes significant changes in the bulk physical properties of the solution. The cmc is an important parameter which deserves careful attention in order to minimize uncertainties in K m measurement. 

FIG. 18 DSC thermogram of 10% triblock copolymer EO28PO48EO28 in water showing the micellization phenomenon. (Erom Ref. 157.)... 

Two main approaches to the thermodynamic analysis of the micellization process have gained wide acceptance. In the phase separation approach the micelles are considered to form a separate phase at the CMC, whilst in the mass-action approach micelles and unassociated monomers are considered to be in association-dissociation equilibrium. In both of these treatments the micellization phenomenon is described in terms o.f the classical system of thermodynamics. Theories of micelle formation based on statistical mechanics have also been proposed [16Q-162] but will not be considered further. The application of the mass-action and phase-separation models to both ionic and non-ionic micellar systems will be briefly outlined and their limitations discussed. More recent developments in this field will be presented. 

Early discussions of the micellization phenomenon emphasized that the dislike of the hydrophobic portion of a surfactant molecule for water was not a repulsive interaction, but rather an attractive preference of water for water and hydrocarbon for hydrocarbon. It was not suggested that there existed a particularly strong attraction among the hydrophobic chains in the molecules, since their interactions are nonpolar and, therefore, relatively small. That idea was reflected in the low melting and boiling points of hydrocarbons relative to polar materials of similar or lower molecular weight. Because of its chemical nature, however, water possesses a very strong cohesive force, which results in many of its unusual properties. 

This localization phenomenon has also been shown to be important in a case of catalysis by premicellar aggregates. In such a case [ ] premicellar aggregates of cetylpyridinium chloride (CPC) were shown to enhance tire rate of tire Fe(III) catalysed oxidation of sulphanilic acid by potassium periodate in tire presence of 1,10-phenantliroline as activator. This chemistry provides a lowering of tire detection limit for Fe(III) by seven orders of magnitude. It must also be appreciated, however, tliat such premicellar aggregates of CPC actually constitute mixed micelles of CPC and 1,10-phenantliroline tliat are smaller tlian conventional CPC micelles. 

Figures 5 and 6 show that the concentration of the two surfactants in the effluents increases simultaneously with the production of the desorbent, which confirms the mixed micellization mechanism described above. Figure 5, where the three additives are produced lately, illustrates the phenomenon particularly well. At the lower pH corresponding to strong adsorption conditions for sulfonate (test 4), the one pore-volume micellar slug would have been entirely consumed by the medium in the absence of any desorbent.

The isotherm characterizing the binding phenomenon of small molecules with surfactant micelles or polymers has been largely studied by means of PFG-NMR. 

Haapakka and Kankare have studied this phenomenon and used it to determine various analytes that are active at the electrode surface [44-46], Some metal ions have been shown to catalyze ECL at oxide-covered aluminum electrodes during the reduction of hydrogen peroxide in particular. These include mercu-ry(I), mercury(II), copper(II), silver , and thallium , the latter determined to a detection limit of <10 10 M. The emission is enhanced by organic compounds that are themselves fluorescent or that form fluorescent chelates with the aluminum ion. Both salicylic acid and micelle solubilized polyaromatic hydrocarbons have been determined in this way to a limit of detection in the order of 10 8M. 

A similar multiphase complication that should be kept in mind when discussing solutions at finite concentrations is possible micelle formation. It is well known that for many organic solutes in water, when the concentration exceeds a certain solute-dependent value, called the critical micelle concentration (cmc), the solute molecules are not distributed in a random uncorrelated way but rather aggregate into units (micelles) in which their distances of separation and orientations with respect to each other and to solvent molecules have strong correlations. Micelle formation, if it occurs, will clearly have a major effect on the apparent activity coefficient but the observation of the phenomenon requires more sophisticated analytical techniques than observation of, say, liquid-liquid phase separation. 

PEO-based copolymers have received much attention. In this respect, PEO-PPO and PEO-PPO-PEO Pluronic copolymers were investigated in organic solvents such as formamides, as illustrated by the works of Lindmann and coworkers and Alexandridis et al. [92-94], However, the formation of reverse micelles in organic solvents from PEO-based block copolymers has been shown to be a complex phenomenon due to the ability of PEO to crystallize. 

The apparent rate constant for polymerization increased with decreasing monomer concentrations of [16 n = 3 and 6], whereas [16 n = 2] did not show this effect. Thus, the phenomenon must be caused by a diminished termination rate because of an increased electrostatic repulsion of the ionic polymer radicals. Unfortunately, however, the properties of these polymers are similar to those of ordinary polymer micelles. Therefore, immobilization appears to be inefficient for systems in dynamic equilibrium. 

When binding of a substrate molecule at an enzyme active site promotes substrate binding at other sites, this is called positive homotropic behavior (one of the allosteric interactions). When this co-operative phenomenon is caused by a compound other than the substrate, the behavior is designated as a positive heterotropic response. Equation (6) explains some of the profile of rate constant vs. detergent concentration. Thus, Piszkiewicz claims that micelle-catalyzed reactions can be conceived as models of allosteric enzymes. A major factor which causes the different kinetic behavior [i.e. (4) vs. (5)] will be the hydrophobic nature of substrate. If a substrate molecule does not perturb the micellar structure extensively, the classical formulation of (4) is derived. On the other hand, the allosteric kinetics of (5) will be found if a hydrophobic substrate molecule can induce micellization. 

DR. GUILLERMO FERRAUDI (University of Notre Dame) From your talk it appears that an important aspect of micelles is the modified reactivity imparted to excited states or chemical intermediates. If micelles are to be used to exploit this phenomenon, the structure of the micelle should be carefully defined. Can you tell us something more about the structural properties of micelles For example, if triton X-100 is used to form micelles, a variation in conditions yields micelles with different shapes, different dimensions, etc. 

This phenomenon can be exploited for separation and concentration of solutes. If one solute has certain affinity for the micellar entity in solution then, by altering the conditions of the solution to ensure separation of the micellar solution into two phases, it is possible to separate and concentrate the solute in the surfactant-rich phase. This technique is known as cloud point extraction (CPE) or micelle-mediated extraction (ME). The ratio of the concentrations of the solute in the surfactant-rich phase to that in the dilute phase can exceed 500 with phase volume ratios exceeding 20, which indicates the high efficiency of this technique. Moreover, the surfactant-rich phase is compatible with the micellar and aqueous-organic mobile phases in liquid chromatography and thus facilitates the determination of chemical species by different analytical methods [104]. 

---