Structure of the Solubilizate

For polar solubilizates, the situation is complicated by the possibility of variation in the depth of penetration into the micelle as the structure of the solubilizate is 

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The location of a solubilized molecule in a micelle is determined primarily by the chemical structure of the solubilizate. Solubilization can occur at a number of different sites in a micelle ... 

Temperature has an effect on the extent of micellar solubilization which is dependent on the structure of the solubilizate and of the surfactant. In most cases the amount of solubilization increases with temperature. This effect has been considered to be due to ... 

A molecule solubilized by an ionic surfactant may be in the core of the micelle, between the hydrophobic chains, in the layer formed by the counterions, or on the surface of the micelle. Because the micelle is in a dynamic state, the loci of the solubilizate in a micelle cannot be sharply defined. Like surfactant molecules or ions in the micelle, molecules of the solubilizate are mobile and a rapid interchange between different sites may be possible. It is believed, however, that the preferred location of the solubilizate in a given micelle depends on the structure of the solubilizate as well. 

The structure and dynamics of the reversed micelle hosting the solubilizate, as well as the physicochemical properties (structure, dynamics, and reactivity) of the solubilizate, are modified. 

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],... 

There is considerable interest in establishing the location within a micelle of the solubilized component. As we have seen, the environment changes from polar water to nonpolar hydrocarbon as we move radially toward the center of a micelle. While the detailed structure of the various zones is disputed, there is no doubt that this gradient of polarity exists. Accordingly, any experimental property that is sensitive to the molecular environment can be used to monitor the whereabouts of the solubilizate in the micelle. Spectroscopic measurements are ideally suited for determining the microenvironment of solubilizate molecules. This is the same principle used in Section 8.3, in which the ultraviolet spectrum of solubilized benzene was used to explore the solvation of micelles. Here we take the hydration for granted and use similar methods to locate the solubilizate. 

The very complex variation of the amount solubilized, as well as physico-chemical properties, with chemical structure of solubilizate and surfactant as well as with surfactant concentration cannot be adequately discussed solely in terms of the energetical conditions of the solubilizate in the micelles. Thus one should also consider the conditions in the phase which separates out at the solubilization limit this is in most cases a liquid crystalline phase. A fundamental basis for a proper understanding of solubilization in surfactant systems is, therefore, a detailed information on phase equilibria in three-component systems surfactant-solubilizate-water. Due in particular... 

The rate constant for the reentry is of the magnitude expected for a diffusion-controlled reaction as in Eq. (5.6). This means that the exit rate is determined by the partition coefficient of the solubilizate in its triplet state between the micelle and the aqueous solution. Table 5.2 shows the exit rate constants k for several systems. The water solubilities of the probes are also given to show the correlation between kt and the solubility in water. These studies give further support to the view that the micelle has a very dynamic structure, which makes it easy for the solubilizate to enter and leave the aggregate. 

In aqueous systems, non-polar additives such as hydrocarbons tend to be intimately associated with the hydrocarbon core of the micelle. Polar and semi-polar materials, such as fatty acids and alcohols are usually located in the palisades layer, the depth of penetration depending on the ratio of polar to non-polar structures in the solubilizate molecule. 

A preferred location of the solubilizate molecule within the micelle is largely dictated by chemical structure. However, solubilized systems are dynamic and the location of molecules within the micelle changes rapidly with time. Solubilization in surfactant aqueous systems above the critical micelle concentration offers one pathway for the formulation of poorly soluble drugs. From a quantitative point of view, the solubilization process above the CMC may be considered to involve a simple partition phenomenon between an aqueous and a micellar phase. Thus the relationship between surfactant concentration Cm and drug solubility Ctot is given by Eq. (3). 

In the present contribution, a detailed description of the composition dependence of the solubilizate order is given and the effect of solubilizate chain-length variation studied in order to characterize the structure and dynamics within the bilayer interior containing these surprisingly thick oil layers. 

Two aspects of solubilization which remain to be investigated, are the variation of the apparent distribution coefficient with so-lubilizate concentration in the micellar ph.ase, and the mechanism of the incorporation of solubilizate into micelle. Since a micelle is assumed to consist of a hydrocarbon core (in liquid state), and surrounded by a palisade layer of hydrophilic group. Fig. 1, the following possible ways have been suggested for the incorporation of a solubilizate in a micelle [5,74] (a) adsorption on the surface of the micelle, (b) deep or short penetration into the palisade layer, and (c) dissolution into the (liquid like) hydrocarbon core. These mechanisms of incorporation are closely related to the structure of the micelle. However, the effect of solubilizate on the structure of the micelle has not been satisfactori 1y studied in the current literature [75,76,77]. 

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 is the process of preparation of thermodynamically stable isotropic solution of a substance (normally insoluble or sparingly soluble in a given solvent) by incorporation of an additional amphiphiUc component(s) [27]. It is the incorporation of the compound (referred to as solubilizate or substrate) within micellar (L phase) or reverse micellar (L2 phase) system. Lipophilic (water insoluble) substances become incorporated in the (normal micelle) phase. Hydrophilic (water soluble) substances are incorporated in the L2 phase. The site of incorporation of the solubilizate is closely related to its structure, as illustrated in Fig. 2.22 nonpolar solubilizate in the hydrocarbon core semipolar or polar solubilizate oriented within the micelle (short or deep) [28]. 

Solubilizate structure. Generalizations about the manner in which structure affects solubilization are complicated by the existence of different solubilization sites. The main parameters that may be considered when investigating solubi-Uzates are Polarity, polarizability, chain length and branching, molecular size and shape. The most significant effect is perhaps the polarity of the solubilizate and sometimes they are classified into polar and apolar however, difficulty exist with intermediate compounds. 

The amount of vibrational fine structure shown in the ultraviolet absorption spectrum of a compound in solution is a function of the interaction between solvent and solute. Moreover, the extent of interaction between the solvent and solute increases with increasing solvent polarity, thereby decreasing the fine structure. As the micelle is characterized by regions of different polarity, ultraviolet spectra have been used as a means of obtaining information on the environment of the solubilizate in the micelle [18, 79,93-101] Bjaastad and Hall [102] utilized an empirical method of determining the polarity of the microenvironment of the solubilizate molecule based on the so-called Z-value method of Kosower [103,104]. 

The amount of a solubilizate which can be solubilized depends on several factors. The dominant variables are the structures of the surfactant and the solubilizate. Both the structure of the hydrophobic chain and the nature of the counterion can affect solubilization. Although the relation between solubilization and surfactant structure is complex, it is clear that the interactions between a solubilizate molecule and the lipophobic hydrophobe of a fluorinated surfactant must be different from interactions between the solubilizate and the lipophilic hydrophobe of a hydrocarbon surfactant. Solubilization by fluorinated surfactants is therefore of great theoretical as well as practical interest. The published information on solubilization by fluorinated surfactants is, however, sparse. 

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