Micellar Solvation Solubilization

Special conditions are found in solutions of large cations and anions possessing a long unbranched hydrocarbon chain, e.g. CH3—(CH2) —CO M , CH3—(CH2) — SOf M , or CH3—(CH2) —N(CH3) X (with n 7). Such compounds are known as amphiphiles, reflecting the presence of distinct polar and nonpolar regions in the 

The hydrophobic part of the aggregate molecules forms the core of the micelle while the polar head groups are located at the micelle-water interface in contact with the water molecules. Such micelles usually have average radii of 2... 4 nm and contain 50... 100 monomers in water. Their geometric structure is usually roughly spherical or ellipsoidal. In non-aqueous nonpolar solvents, the micellar structures are generally the inverse of those formed in water. In these solvents, the polar head groups form the interior of the micelle while the hydrocarbon chains of the ions are in contact with the nonpolar solvent. 

Cationic 1-Hexadecyl (=Cetyl)-trimethylammonium bromide (CTAB) CH3-(CH2)i5-N(CH3)3Br 0.0013 78 

Typical surfactants are listed in Table 2-10 along with their respective cmc values 

The existence of micelles in solutions of large ions with hydrocarbon chains is responsible for the observation that certain substances, normally insoluble or only slightly soluble in a given solvent, dissolve very well on addition of a surfactant (detergent or tenside). This phenomenon is called solubilization and implies the formation of a thermodynamically stable isotropic solution of a normally slightly soluble substrate (the solubilizate) on the addition of a surfactant (the solubilizer) [128, 133], Non-ionic, nonpolar solubilizates such as hydrocarbons can be trapped in the hydrocarbon core of the micelle. Other amphiphilic solutes are incorporated alongside the principal amphiphile and oriented radially, and small ionic species can be adsorbed on the surface of the micelle. Two modes of solubilizate incorporation are illustrated in Fig. 2-13. 

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Sometimes, the physicochemical properties of ionic species solubilized in the aqueous core of reversed micelles are different from those in bulk water. Changes in the electronic absorption spectra of ionic species (1 , Co ", Cu " ) entrapped in AOT-reversed micelles have been observed, attributed to changes in the amount of water available for solvation [2,92,134], In particular, it has been observed that at low water concentrations cobalt ions are solubihzed in the micellar core as a tetrahedral complex, whereas with increasing water concentration there is a gradual conversion to an octahedral complex [135],... 

In a multiphase formulation, such as an oil-in-water emulsion, preservative molecules will distribute themselves in an unstable equilibrium between the bulk aqueous phase and (i) the oil phase by partition, (ii) the surfactant micelles by solubilization, (iii) polymeric suspending agents and other solutes by competitive displacement of water of solvation, (iv) particulate and container surfaces by adsorption and, (v) any microorganisms present. Generally, the overall preservative efficiency can be related to the small proportion of preservative molecules remaining unbound in the bulk aqueous phase, although as this becomes depleted some slow re-equilibration between the components can be anticipated. The loss of neutral molecules into oil and micellar phases may be favoured over ionized species, although considerable variation in distribution is found between different systems. 

The structure and properties of water soluble dendrimers, such as 46, is, in itself, a very promising area of research due to their similarity with natural micellar systems. As can be seen from the two-dimensional representation of 46 the structure contains a hydrophobic inner core surrounded by a hydrophilic layer of carboxylate groups (Fig. 12). However these dendritic micelles differ from traditional micelles in that they are static, covalently bound structures instead of dynamic associations of individual molecules. A number of studies have exploited this unique feature of dendritic micelles in the design of novel recyclable solubilization and extraction systems that may find great application in the recovery of organic materials from aqueous solutions [84,86-88]. These studies have also shown that dendritic micelles can solubilize hydrophobic molecules in aqueous solution to the same, if not greater, extent than traditional SDS micelles. The advantages of these dendritic micelles are that they do not suffer from a critical micelle concentration and therefore display solvation ability at nanomolar... 

The water in the RMs is considered to be a composite of two different types the "bound water" region, and the remaining "free water" region. On the basis of the IR data up to a Wo = 4, the water solvates the AOT ion-pair, further increasing in the water concentration up to a Wo = 10, probably giving rise to a hydration shell around the new-separated ions of AOT. Further increasing water concentration gives rise to the so called "free water". It has been shown by various physico-chemical techniques that the water of the reverse micelle behaves differently from normal water, especially at low concentrations (Wo < 10). Solubilization of water by such micelles promotes dissociation of ion pairs in the micelle to form micellar free ions. 

The reverse micelle phase behavior in supercritical fluids is markedly different than in liquids. By increasing fluid pressure, the maximum amount of solubilized water increases, indicating that these higher molecular weight structures are better solvated by the denser fluid phase. The phase behavior of these systems is in part due to packing constraints of the surfactant molecules and the solubility of large micellar aggregates in the supercritical fluid phase. 

On the other hand, when optically active 2-octyl triflate (197) was hydrolyzed in the solubilized state with surfactant concentrations greater than their CMC, the stereochemical course changed to net retention (ca. 48% for CTAB, and ca. 27% for NaLS)193 The rate constants, very much lower than those in the absence of micelles, indicate that the reaction proceeded in the micelles. The different behavior of (196) and (197), particularly in the presence of cationic micelles (CTAB), may be due to the greater efficiency of triflates in producing carbocations. The comparison of (196) and (197) then suggests that the solvation model of micellar stereochemical control applies to unassisted solvolyses whereas the double inversion mechanism operates with leaving groups which invite participation of a nucleophile. 

Subsequently, another MA uptakes these chains from the solvent phase and thus becomes larger. The rate of the chain uptake process is evidently proportional to the critical micellar concentration (cmc) or the concentration of unimolecular chains that are solubilized under a given set of solvation conditions. If a solvent is sufficiently poor for the insoluble block(s), the cmc can be very low. Thus, the chain-exchange kinetics can be slow for block copolymers. Fusion or fission of micelles would not occur to a significant extent for MAs whose cores are protected from contacting one another by thick repulsive coronas. 

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