Solubilization site

In addition to the degree of hydrophilicity of the solubilizates, their size and structure, the size of the host microregions, or the occurrence of specific processes must be taken into account in order to rationalize the driving forces of the solubilization process and of the solubilization site within water-containing reversed micelles [25,138,139],... 

An example of recovered distribution is shown in Figure B6.1.2. It concerns the distribution of lifetimes of 2,6-ANS solubilized in the outer core region of sodium dodecylsulfate micelles8 . In fact, the microheterogeneity of solubilized sites results in a distribution of lifetimes. 

The Winsor II microemulsion is the configuration that has attracted most attention in solvent extraction from aqueous feeds, as it does not affect the structure of the aqueous phase the organic extracting phase, on the other hand, is now a W/0 microemulsion instead of a single phase. The main reason for the interest in W/0 microemulsions is that the presence of the aqueous microphase in the extracting phase may enhance the extraction of hydrophilic solutes by solubilizing them in the reverse micellar cores. However, this is not always the case and it seems to vary with the characteristics of the system and the type of solute. Furthermore, in many instances the mechanism of extraction enhancement is not simply solubilization into the reverse micellar cores. Four solubilization sites are possible in a reverse micelle, as illustrated in Fig. 15.6 [19]. An important point is that the term solubilization does not apply only to solute transfer into the reverse micelle cores, but also to insertion into the micellar boundary region called the palisade. The problem faced by researchers is that the exact location of the solute in the microemulsion phase is difficult to determine with most of the available analytical tools, and thus it has to be inferred. 

Although more work is needed to clearly correlate the type of solubilization site occupied by different porphyrins with their reactivity in such sites towards atropisomerization, it is clear that different sites exist and that these sites show quite different reactivity in both thermal and photochemical processes. Preliminary studies have shown that related behavior probably occurs in other organized assemblies formed by dispersion of surfactant molecules in... 

Nazario LMM, Hatton TA, Crespo JPSG (1996) Nonionic cosurfactants in AOT reversed micelles Effect on percolation, size, and solubilization site. Langmuir... 

At low MEGA-n concentration, the II1/1 ratio is low (= 0.84) and almost constant suggesting that pyrene molecules reside in a rather polar environment, which is however not so polar as in an aqueous phase (III/I = 0.63). It may be considered that the very cohesive nature of the vesicular bilayer does not allow pyrene molecules to penetrate deep into the hydrophobic part, and they stay in peripheral solubilization sites. 

Pyrene shown a number of photophysical features that made it an attractive fluorophore to probe the microenvironment in micellar aggregates [19]. For the peaks of pyrene PL, two important peaks at about 373 nm and 390 nm among the five dominant peaks of pyrene fluorescence were numbered as 1 and III, respectively [20]. It has been known that intensity ratio of peak 111 to I (III/I) increased as the polarity at the solubilization site of pyrene decreases. Figure 6 shows fluorescence spectra (A.ex = 310 nm) of pyrene in precursor gel containing TPA and I-IV samples denoted as (a), (b), (c), (d) and (e), respectively. The value of 111/1 of pyrene does not change under silicalite-1 gel due to no formation of micelle. However, in the Fig. 6d (sample II), III/I ratio is rapidly increased, while sample III and IV are decreased slightly again. Previously, Park et al. have reported that 111/1 ratio of pyrene for... 

Figure 57. Stylized representation of the possible solubilization sites for 2- and s-105 in KS and KP gels.

The reactivity of molecules bound to surfaces, located at various kinds of interfaces, solubilized in microheterogeneous media, or incorporated as "guests" in various "hosts" as inclusion complexes has been the subject of much recent study. Indeed the structure of the medium, the nature of "solubilization sites" and reactivity in these environments have all been the focus of independent or interrelated investigations (1-12). Photochemistry has played a major role in these studies both in terms of studies of the media and also in terms of modified or controlled reactivity (1,5,8,9). In the course of these investigations numerous questions have arisen many of these have developed from differing pictures of solute-environment interactions which are furnished by different studies using different molecules as "probes" (5,10-12). Controversies arising... 

Normal Micelles - Solubilizate Probes. The addition of a probe molecule, usually bearing a C=0 group, to a micelle has been used to asses die solubilization site of the probe (67) and to infer the extent of penetration of water into micelles (68,69). The basis of such studies is the well known decrease in the 0=0 band frequency upon hydrogen bond formation (70 -73). Two important concepts must be addressed, however, when using probes in studies of micelles the solubilization site of the probe (micelle core or palisade layer) and the possibility of probe-induced changes in the micelle. 

A solute (additive) can be located in reverse micelles in different solubilization sites in the water core, in the interfacial region or in the bulk solvent. Solubilization into the water cores increases the inner volume at constant interfacial area, resulting in radial growth. If the micelle is too small to receive a solute molecule without deformation, e.g., at low water content, a segregation occurs between small free molecules and the large objects which are covered with surfactant (Chatenay et al., 1987 Encinas and Lissi, 1986 Pileni et al., 1985). 

Figure 3. Simplified cross section of an aqueous normal micelle showing possible solubilization sites. A charged solute (A) would be electrostatically repelled from the micelle surface if it were of the same charge-type as the ionic micelle while an oppositely charged solute (B) would be electrostatically attracted to the micellar surface. Nonpolar solutes (C) would partition to the outer part of the more hydrophobic core region. Amphiphilic solutes (D) would attempt to align themselves so as to maximize the electrostatic and hydrophobic interactions possible between itself and the surfactant molecules. "Reproduced with permission from Ref. 49. Copyright 1984, Elsevier. "...

In aqueous solutions the micellar assembly structure allows sparingly soluble or water-insoluble chemical species to be solubilized, because they can associate and bind to the micelles. The interaction between surfactant and analyte can be electrostatic, hydrophobic, or a combination of both [76]. The solubilization site varies with the nature of the solubilized species and surfactant [77]. Micelles of nonionic surfactants demonstrate the greatest ability for solubilization of a wide group of various compounds for example, it is possible to solubilize hydrocarbons or metal complexes in aqueous solutions or polar compounds in nonpolar organic solutions. As the temperature of an aqueous nonionic surfactant solution is increased, the solution turns cloudy and phase separation occurs to give a surfactant-rich phase (SRP) of small volume containing the analyte trapped in micelle structures and a bulk diluted aqueous phase. The temperature at which phase separation occurs is known as the cloud point. Both CMC and cloud point depend on the structure of the surfactant and the presence of additives. Table 6.10 gives the values of CMC and cloud point for the surfactants most frequently applied in the CPE process. 

The microenvironment generates a spatially inhomogeneous distribution of reagents within the reaction volume. The local properties of a solubilization site can strongly affect the energetics of solvent-solute interactions and therefore the intrinsic reac-... 

In this chapter the main aim is to present partition coefficients for solutes in aqueous surfactant systems, having carried out a critical evaluation of the data. We concentrate on polar solutes. Energetics of solubilization and solubilization sites in the micelles are also discussed. 

Insight into the mode of anaesthetic action has been achieved by the examination of solubilization sites and acid-base forms of dibuccaine hydrochloride in neutral, anionic, and cationic micellar environments. The latter offer some approximation to the structure of different possible sites for action of an anaesthedc in a living organism. 

The following will focus on studies exploring the nature of the solubilization sites. Studies concerning dynamics in polymeric micelles , the overall shape and aggregation numbers are included in Sects. 3.5 and 4.2. 

In more elaborate studies with pyrene it was shown that the hydrophobicity of the solubilization sites increases with the length of the alkyl tails incorporated [103, 128, 185,186, 196, 292], Similar results were obtained with various dansyl derivatives [152] and with methyl orange [200]. Such behaviour agrees well with that of low molecular weight surfactants [185]. Additionally, the polarity of the sites seems to depend on the rigidity of the hydrophobic tails. In copolymer soaps of head geometry based on polyacrylamide, the polarity around the... 

Alternatively, the geometrical effects on solubilization sites can be explained by a simple excluded volume effect, forcing the probe closer to the micellar surface in tail end geometry. This explanation is supported by the findings that rather high polarities were observed for a number of vinylic polysoaps of the tail-end type in comparison to their monomers [64, 65, 74, 164, 245]. In contrast, polysoaps of the head type may show similar or even less polar environments than analogous monomers [152, 167, 187]. 

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