True micellar systems

True micellar systems have low capacity for dissolving non-polar reactants, however. They are therefore of limited preparative value. Microemulsions, which contain not only surfactant and water but also an oil component, can dissolve appreciable amounts of both a polar and a non-polar reactant and are therefore much more practically useful as media for organic synthesis. There has been considerable interest in the use of microemulsions as media for organic reactions in recent years [7—11]. Not only can such a formulation be a way to overcome compatibility problems, the capability of microemulsions to compartmentalise and concentrate reactants can also lead to considerable rate enhancement compared to one-phase systems. A third aspect of interest for preparative organic synthesis is that the large oil-water interface of the system can be used as a template to induce regioselectivity. These aspects will be dealt with in this chapter. 

Above we used the words continuous phase and dispersed phase to refer to the medium and to the particles, respectively, in the colloidal size range. It should be understood that these are solvent and solute in lyophilic systems. In micellar systems, the micelles are dispersed in an aqueous continuous phase. Furthermore, the system as a whole is generally called a dispersion when we wish to emphasize the colloidal nature of the dispersed particles. This terminology is by no means universal. Lyophilic dispersions are true solutions and may be called such, although this term ignores the colloidal size of the solute molecules. 

The micellar phenomenon cannot be discussed without considering a surfactant property which is intrinsically related to the very existence of micelles the so-called detergency, i.e., the ability of surfactant molecules to take up (= solubilize) polar material, for example, water in the polar core of the inverted micelles. Thus, micelli-zation and solubilization are competitive processes. It is obvious that the tendency to solubilize minute amounts of polar impurities, particularly, water is quite pronounced. In principle it must appear, therefore, doubtful whether it is reasonable at all to discuss true binary systems, i.e., surfactant plus solvent, except by way of... 

Measurement of self-diffusion coefficients by means of PGSE techniques has evolved to become one of the most important tools in the characterization of surfactant systems. In particular, this is true of those surfactant systems that are isotropic liquid solutions such as micellar systems and microemulsions. The technique has been described in a number of review articles [9,11-13]. An account of the most recent developments of the method can be found in Ref 9. We do not dwell on the technical aspects here but merely note that the technique requires no isotopic labeling (avoiding possible disturbances due to addition of probes) furthermore, it gives component-resolved diffusion coefficients with great precision in a minimum of measuring time. 

Microemulsions or swollen micellar systems represent an intermediate state between micellar solutions and true emulsions as shown in the hypothetical phase diagram in Fig. 8.31. They are readily distinguished from emulsions by their transparency and more fundamentally by the fact that they represent single thermodynamically stable solution phases. The term microemulsion was introduced to describe systems first identified by Hoar and Schulman [166]. Interest in these fluid translucent isotropic dispersions of oil or water has grown rapidly a book devoted to the theory and practice of microemulsions was published in 1977 [167]. 

When these criteria are applied to micellar systems, it is clear that micellar systems do indeed represent a true equilibrium state. The situation, however, is more complex with vesicles. One reason is that, in the case of phospholipids, changes could take a long time, e.g., weeks/months, as the monomer concentration external to the vesicle bilayer can be very low (of the order of nanomolar in the case of phospholipids) and this implies that any adjustment to a new equilibrium state will be very slow and hard to monitor. As a result, few systems would appear to conform to all the criteria indicated above. It seems that vesicle systems that have a larger monomer concentration or critical vesicle concentration (cvc) are often unstable in the direction of forming lamellar phases, a process that may take place over a period of hours. 

The aggregation of surfactants into clusters or micelles in dilute solutions, as we will see, is a direct consequence of the thermodynamic requirements of the particular surfactant-solvent system under consideration. It has been suggested that phases occurring between the simplest micelles and true crystals are natural consequences of the removal of water from the micellar system, but do not constitute thermodynamically distinct states. In other words, the factors determining the structures of the mesophases are identical to those that control the formation of micelles in the first place. The same would be true of aggregates other than micelles, which do not fall under the classification of mesophases. 

The anionic nucleophiles such as hydioxamate, oximate, and diiolate posses very high nucleophilicity toward phen esteR. This is also true in polymeric systems. Furdier improvements in r tctivity were sadueved in die form of the zwitterionic nucleophile and in micellar envirrHimaits. Since decomposition of the inter-... 

Due to micelle formation the total surfactant concentration undergoes an abrupt increase. Since true (molecular) solubility of surfactants, determined by the CMC, remains essentially constant, an increased surfactant concentration in solution is caused by an increase in a number of formed micelles. Micellar solubility increases with increase in temperature, and thus a continuous transition from pure solvent and true solution to micellar solution, and further to different liquid crystalline systems and swollen surfactant crystals (see below), may take place in the vicinity of the Krafft point. 

In contrast to aqueous systems, micelle formation in non-polar media is driven by the benefit in energy rather than by an increase in entropy. The replacement of polar group - hydrocarbon interaction (as in the case of dissolution) with the interaction between polar groups upon their association into micellar core is thermodynamically beneficial. The benefit in energy upon association of polar groups is so large, that even at low concentrations true surfactant solutions contain small pre-micellar associates rather than individual surfactant molecules. 

If this reaction were to occur only in the aqueous bulk, the film has no role to play, the entire system is pseudohomogeneous, and a true kinetic analysis as outlined earlier is possible. If mass transfer effects are present between the pseudophases, the analysis outlined before for such systems would apply. However, if reaction occurs in the film, two situations can arise (1) reaction occurs only in the micelles present in the film and not in the rest of the film, and (2) reaction occurs in both the micellar and aqueous phases in the film. The analysis of both the situations is very similar to that for microphase action described in Chapter 23. 

For both O/W and W/O systems, the amount of monomer is usually restricted to 5-10 wt% with respect to the overall mass, and that of surfactant(s) lies within the same range or even above. Nevertheless, there have been a few studies in which the formulation deviated from these conditions. For instance, surfactant concentrations of 2 wt V() were reported [56-58,69,124,125]. However, in this case the amount of monomer was also very low (< 2 wt%) so that the systems must be considered as micellar solutions rather than true microemulsions. Conversely, a 1994 study of Gan et al. [82] reported the polymerization of styrene up to 15 wt% using only about 1 wt V(> dodecyltrimethylammonium bromide surfactant (DTAB) in a Winsor I-like system. This system consists of a microemulsion (lower) phase topped off with pure styrene. The polymerization takes place in the microemulsion phase, while the styrene phase acts as a monomer reservoir. Such a polymerization process is novel, but it yields latices of large particle size ( 100 nm) that can be more easily obtained by conventional emulsion polymerization. 

Fig. 1. The fatty acid soap-water phase diagram of McBain (58) modified (1) to show the molecular arrangement in relation to aqueous concentration (abscissa) and temperature (ordinate). Ideal solution, i.e., true molecular solution, is to the left of the vertical dashed line, indicating the critical micellar concentration (CMC), which varies little with temperature. At concentrations above the CMC, provided that the temperature is above the critical micellar temperature (CMT), a micellar phase is present. At high concentrations, the soap exists in a liquid crystalline arrangement, provided that the solution is above the transition temperature of the system, i.e., the temperature at which a crystalline phase becomes liquid crystalline. The Krafft point is best defined (D. M. Small, personal communication) as the triple point, i.e., the concentration and temperature at which the three phases (true solution, micelles, and solid crystals) coexist, but in the past the Krafft point has been equated with the CMT. The diagram emphasizes the requirement for micelle formation (a) a concentration above the CMC, (b) temperature above the CMT, and (c) a concentration below that at which the transition from micelles to liquid crystals occurs. Modified from Hofmann and Small (1).

Depending on the relative rates of the chemical and diffusion steps, the reaction can proceed in the kinetic, diffusion, or mixed regime, the entire process being controlled by the rate of the chemical step, a diffusion process, or by both kinetics and diffusion. Thus, under very good hydrodynamic conditions, e.g., upon vigorous agitation, the influence of the diffusion can be substantially eliminated and the kinetic results can be used to discuss the reaction mechanism. This conclusion is not always true, and the use of typical surfactant micellar aqueous solutions with extractants dissolved (solubilized) in micellar pseudophase (micelles) and inorganic species dissolved in aqueous pseudophase mimic the extraction systems effectively and the diffusion processes are totally eliminated. 

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