Solubilization in micellar systems

Gao, Z., Wasylishen, R.E., and Kwak, J.C.T., An NMR paramagnetic relaxation method to determine distribution coefficients of solubilization in micellar systems, J. Phys. Chem., 93, 2190, 1989. Treiner, C., The partitioning of neutral solutes between micelles and water as deduced from critical micelle concentration determinations, in Solubilization in Surfactant Aggregates, Christian, S.D. and Scamehorn, J.R, Eds., Marcel Dekker, New York, 1995, chap. 12. 

Although much of this book is concerned with solubilization in micellar systems, there is a need to discuss the phenomenon of hydrotropy, as there is now a considerable body of literature on the pharmaceutical aspects of the subject. As has been discussed, hydrotropy is the term reserved for the action of increasing the solubility of a solute by a third substance which is not highly surface active - at least one which does not form micelles at low concentrations. The mechanism of... 

More hydrophobic additives such as free fatty acids and their esters and amides, long-chain monohydric alcohols, and nitriles may have an even more dramatic effect on the phase behavior of a surfactant due to solubilization phenomena. The general subject of solubilization in micellar systems is discussed in Chapter 6. For now, we will focus on the effects that the presence of solubilized materials may have on liquid crystal phases. 

Fluorescence quenching studies in micellar systems provide quantitative information not only on the aggregation number but also on counterion binding and on the effect of additives on the micellization process. The solubilizing process (partition coefficients between the aqueous phase and the micellar pseudo-phase, entry and exit rates of solutes) can also be characterized by fluorescence quenching. 

Romsted LS (1977) A general kinetic theory of rate enhancements for reactions between organic substrates and hydrophUic ions in micellar systems. In Mittal KL (ed) Micellization, Solubilization, Microemulsions. Plenum Press, New York... 

A two-state model of solubilization may be used to describe the location of solutes in micellar systems. This model involves a distribution between a dissolved state, which is associated with the core, and an adsorbed state, associated with the micellar water interface. The molecules in thi dissolved state remain in the micelle because of the solvent properties of the core. Molecules in the adsorbed state are due to the surface activity of the dissolved species, similar to a surface exces (Mukerjee, 1979). 

In micellar systems, nonpolar molecules are solubilized within the internal micelle hydro-phobic core, polar molecules are adsorbed on the micelle surface and substances with intermediate polarity are distributed along surfactant molecules in intermediate positions. 

Equilibrium Solubilization of Benzene in Micellar Systems and Micellar-Enhanced Ultrafiltration of Aqueous Solutions of Benzene... 

N.m.r. and e.s.r. techniques similar to those used for the determination of the location and environment of solubilizates in micellar systems have also been employed in investigations of solubilization by lipid micelles and of protein-substrate interactions (McDonald and Phillips, 1967 Meadows et al., 1967 Spotswood et al., 1967 Chapman, 1968 Penkett et al., 1968 Mildvan and Weiner, 1969 Raftery et al., 1969 Roberts et al., 1969a, b Rosenberg et al., 1969 Small et al., 1969). 

Double Layer Interactions and Interfacial Charge. Schulman et al (42) have proposed that the phase continuity can be controlled readily by interfacial charge. If the concentration of the counterions for the ionic surfactant is higher and the diffuse electrical double layer at the interface is compressed, water-in-oil microemulsions are formed. If the concentration of the counterions is sufficiently decreased to produce a charge at the oil-water interface, the system presumably inverts to an oil-in-water type microemulsion. It was also proposed that for the droplets of spherical shape, the resulting microemulsions are isotropic and exhibit Newtonian flow behavior with one diffused band in X-ray diffraction pattern. Moreover, for droplets of cylindrical shape, the resulting microemulsions are optically anisotropic and non-Newtonian flow behavior with two di-fused bands in X-ray diffraction (9). The concept of molecular interactions at the oil-water interface for the formation of microemulsions was further extended by Prince (49). Prince (50) also discussed the differences in solubilization in micellar and microemulsion systems. 

The concept of organometallic catalysis in micellar systems is compared by several writers [15, 33, 34] with the concept of heterogeneous catalysis on solid surfaces since the solubilization of the reactants in the core of the micelle containing catalytically active sites can be compared with adsorption on surfaces and the number of micelles to surface area (cf. Section 4.5). 

An additional argument for a distinction between micelles and microemulsions is that in all the literature on the solubilization of hydrocarbons, dyes, and other substances in micellar solutions, the ratio of solubilized molecules to surfactant molecules very rarely exceeds, or even approaches, 2. Many microemulsion systems, on the other hand, have been described in which the dispersed phase surfactant (and cosurfactant) ratio exceeds 100 Because of the relatively low ratios of additive to surfactant obtainable in micellar systems, it is clear that there can exist no oil phase that can be considered separate from the body of the micelle. In many microemulsions, however, the size of the droplet and the high additive surfactant ratio requires that there be a core of dispersed material that will be essentially equivalent to a separate phase of that material. The seemingly obvious conclusion is that microemulsion systems (in the latter case, at least) possess an interfacial region composed primarily of surfactant (and cosurfactant), analogous to that encountered in macroemulsions. 

Kabalnov, A. and Weers, J., Kinetics of mass transfer in micellar systems surfactant adsorption, solubilization kinetics, and ripening, Langmuir, 12, 3442, 1996. Kanniah, N., Gnanam, F.D., and Ramasamy, F, Revert and direct Liesegang phenomenon of silver iodide Factors influencing the transition point, J. Colloid Interface Set, 94, 412, 1983. 

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. 

In micelles composed of some nonionic surfactants like polyoxyethylene derivatives with polar head group, the additives are preferentially located deep within the palisades layer (Figure 3c), whereas in case of ionics, the materials solubilized in this system may be intended to situated on the micellar surface (Figm-e 3d). 

Solutions of water-containing reversed micelles are systems characterized by a multiplicity of domains apolar bulk solvent, oriented alkyl chains of the surfactant, hydrated surfactant headgroup region at the water/surfactant interface, and bulk water in the micellar core. Many polar, apolar, and amphiphilic substances, which are preferentially solubilized in the micellar core, in the bulk organic solvent, and in the domain comprising the alkyl chains and the hydrated surfactant polar heads, henceforth referred to as the palisade layer, respectively, may be solubilized in these systems at the same time. Moreover, it is possible that (1) local concentrations of solubilizate are very different from the overall concentration, (2) molecules solubilized in the palisade layer are forced to assume a certain orientation, (3) solubilizates are forced to reside for long times in a very small compartment (compartmentalization, quantum size effects), (4) the structure and dynamics of the reversed micelle hosting the solubilizate as well as those of the solubilizate itself are modified (personalization). 

Solubilization of bioactive components in micellar systems controlled drug release... 

A typical example of graft copolymers is that recently published by Winnik and co-workers [309]. These authors have studied the solubilization of cyclosporine A, as a model drug, in micellar systems based on hydroxypropylcellulose-g-polyoxyethylene alkyl ether. They could demonstrate that the drug loading in the micelles, with diameters ranging from 78 to 90 nm, increases with the number of grafted chains. 

A wide range of chemical reactions in micellar systems depend mainly upon the difference of properties of the micellar phase and the bulk phase. One should only distinguish the solubilized and non-solubilized reactant molecules, the micelles altogether being considered as a pseudophase. For second-order reactions, the intermicellar distribution of reactant molecules should be taken into consideration as discussed above. 

Thus, for the effective charge separation in micellar systems, the most convenient location of the reactants is at the interface, and after the reaction, one of the products should exit into the bulk phase. This product should have a very small solubilization coefficient. The other charge carrier produced should move towards the micelle nucleus. If one aims to obtain very long relaxation times, it is possible to overcome the difficulties of obtaining a very strong change of the solubilization coefficient due to... 

This Gibb s energy, which is associated with transfer in micelles, is tabulated in the literature (18, 23) and is often compared to the Gibb s energy of transfer between octanol and water. The octanol/water scale has been used to correlate the solubility data in micellar systems for compounds belonging to the same series (18). As illustrated in Figure 9.2 (24) for some solutes solubilized... 

Figure 9.18. Variation of the reduced average curvature (//) versus the interfacial molar composition X, with C representing the chain length of the surfactant A H)l = H(X 0)) — jH(X = 0)). The dashed line is calculated with the wedge model , experimental points obtained by phase diagram determination and titration in the Winsor III domain , maximum of solubilization obtained in micellar system A, obtained in a Winsor I microemulsion...

Since 1964 there have been several comprehensive reviews of solubilization in surfactant systems, notably those by Swarbrick [2], Mulley [3], Sjoblom [4], Droseler and Voight [5], Elworthy et al [6], and Florence [1]. These reviews together cite over a thousand sources primarily concerned with pharmaceutical applications. Other major publications which deal with micellar systems implicating solubilized species include Cordes [7], Fendler and Fendler [8] and the collections of papers edited by Mittal contain several contributions on the topic [9]. 

In this chapter we wish to explore not only the influence of micelles on reaction rates and the course of reactions, both chemical and photochemical, but also the stability of surfactants themselves and how aggregation can affect their stability. The chemical modification of surface-active agents and attempts to polymerize surfactant micelles will also be covered. The literature on reactivity in micellar systems has grown enormously since 1968 when an account of the pharmaceutical aspects was given in the first edition of this book [1], to the extent that a book has been devoted to the subject reviewing and collating the data in the literature prior to mid-1974 [2]. Here we can probably only hope to extract some of the salient features of the subject, and could certainly not claim to be comprehensive. The reference list, however, contains several reviews which should be consulted for more detailed treatments. The analytical consequences of solubilization of chromophoric species and change in the apparent dissociation constants of compounds in the presence of surfactants is also discussed at the end of the chapter. 

Photochemistry in micellar systems is a type of micellar catalysis in the sense that the photochemical process takes place in the micellar domain. The outstanding recent progress in micellar photochemistry is well laid out in a number of review articles and papers on photochemical and photophysical processes in micellar assemblies. " As described in previous chapters, hydrophobic organic solutes solubilize well in the micellar core, whereas the micellar surface controls the concentration of hydrophilic solutes. The electrostatic potential of up to a few hundred millivolts at the surface of ionic micelles is especially effective in attracting or repelling ionic species. Thus, micelles are microscopically heterogeneous and well suited as surfaces for reactions of appropriate reactants. 

The effects of added electrolytes on a micellar system were discussed in Chapter 4. For the case of ionic micelles, the effect of such addition is to decrease the cmc and increase the aggregation number. Such changes are predictable in micellar systems and might be expected to produce parallel effects on solubilization. In fact, however, the results are not always so easily analyzed. At surfactant concentrations near the cmc, it is usually found that the solubilizing power of a system will increase with the addition of electrolyte, as a result of the greater number of micelles available in the system. At surfactant concentrations well above the... 

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