Micelles solubilization sites

The complex changes of molecular weight of TTAB micelles in H2O-O.IM KBr upon addition of pentanol may be explained in terms of a distribution of alcohol between different micelle solubilization sites. In H2O, the alcohol may essentially be dissolved in the palissade layer, thereby decreasing the molecular weight by the effects discussed above. In H2O-O.IM KBr, pentanol is salted out of water and may preferentially dissolve into the micelle hydrophobic core, thereby increasing the micelle molecular weight. 

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],... 

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. 

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

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... 

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. 

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. 

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. 

Working with spectroscopic probes, one needs to know the solubilization site of the probe, which should be determined independently on the spectroscopic effect to be exploited. However, when micelles of a homologous series of surfactants are investigated, information on variation of solubilization sites may be obtained. Roelants et al. (177) concluded from activation energies of quenching processes that the quencher molecule iV-methyl-A7-decylaniline resides a little deeper in TTAC micelles than in CTAC micelles. 

The preferred location of aromatic solutes in micelles cannot be learned unequivocally from the literature. Evidence has been presented for solubilization in the micelle core, at the surface, or at both these sites, depending on the concentration (43,199-204). Therefore the solubilization site of aromatic compounds seems to depend on details of the respective systems. In the case of the probe acridine, site information can be derived from relative quantum yields... 

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 micelles . In fact, the microheterogeneity of solubilized sites results in a distribution of lifetimes. 

The PPIE model fits the kinetic data representing the effects of vesicles on thiolysis and hydrolysis. However, several of the parameters used in the fitting procedure were derived from the effect of micelles on the same reaction or depended on assumptions regarding solubilization sites or rates of substrate permeation. In addition, use of a single rate constant to represent reactivity in the vesicle ignores the fact that a solution containing vesicles has, in principle, several potential reaction sites, and no a priori theory predicts that reactivity at all the sites is equal or comparable. 

Several groups of investigators have looked at the effect of reverse micelles on reactions involving small molecules or ions a rationalization of the kinetic data obtained requires some knowledge about a) the solubilization sites for the small molecules within the micelles b) the nature of the water pool and c) the dynamics of solubilizate exchange among micelles. From a practical point of view, carrying out... 

In this chapter I have attempted to present a panoramic view of the contributions of calorimetry to the study of solutions of reversed micelles. In particular, it has been shown that it is possible with calorimetry to obtain information on the energetic state of water and that of other solubilizates within reversed micelles, the complete set of thermodynamic parameters for the solubilization process, and the preferential solubilization site as well as information on the intermicellar interactions and the energetic state of solid nanoparticles entrapped in the micellar core. All these data together with those obtained by other techniques help to better and better define the structural and dynamical picture of solutions of reversed micelles and to exploit their potential technological applications. 

A SAIL [Cj jmim][Cl] is found to self-aggregate to form micelles in basic aqueous solution in a more efficient manner than its more conventional surfactant counterpart DeTACl. Interaction and behavior of common calixarenes with [Cj mim][Cl] depend on the fimctionahties present on the molecular architecture of the calixarene. Calixarene with -NOj (electron-withdrawing functionalities) appears to strongly interact with both unaggregated and micellar [Cj mim][Clj. A common calixarene with -NO functionality locates itself in such a manner to have its -NO groups oriented toward the palisade layer, whereas other calixarenes have their organic frameworks oriented in the palisade layer. A tetradansylated calixarene readily partitions into the hydrophobic microenvironment of [Cj mim][Cl] micelles. SAIL appears to effectively control the solubilization sites and thus the properties of calixarenes in the solution. 

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