Solubilizate structure

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 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. 

Liquid crystal As the name implies, this is an ordered yet fluid phase in which water, surfactant, and solubilizate combine to form anisotropic, organized structures. These are called lyotropic mesomorphic phases, as opposed to thermotropic mesomorphs, which form when certain organic crystals are heated. 

PS-PB-PS 33-80% Dioxane + ethanol THF + allyl alcohol (selective for PS) Solubilization of PB and PS-PB-PS triblock Micellar structure with solubilizate SLS Tuzar et ai (1982)... 

Two models for micelle structure were identiLed in their studies (Xing and Mattice, 1998). In analogy with the structural models for systems involving low molecular weight surfactants, two kinds of aggregates of spherical shape can be pictured, depending on how the solubilizates are located inside the block copolymer micelles. Solubilization takes places in two steps in the Xing and Mattice s simulations (1998). 

As with traditional surfactants, additives may inLuence the onset of micellization of polymeric surfactants and thus affect solubilization. These additives can include inorganic salts and sugars used to adjust isotonicity and even the solubilizate drug itself. In addition to micellization, these additives can inLuence the LCST or CP and even the structure of micelles formed. 

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. 

Since phosphates and sulfates with long chain alkyl substituents form micelles at concentrations above their CMC, the hydrolysis of these esters can be subject to micellar catalysis thereby providing a simplified system in which micelle formation and structure are not alfected by the presence of a foreign solubilizate. The hydrolysis of such surfactants must be considered, however, in investigations of their effects on reaction rates. Fortunately, the rate constants for the neutral hydrolysis of esters such as sodium dodecyl sulfate are extremely slow at 90° = 296 days at pH = 8-63), and the acid-catalyzed hydrolysis of the same ester is some three orders of magnitude faster and thus is still negligible in most cases (Kurz, 1962). 

About two decades ago, most bioabsorbable polymers were natural polymers or derivatives thereof Scientific interest in totally synthetic bioabsorbable polymers has grown considerably since the early seventies because of their relatively low tissue reaction and because of their more predictable in vitro and in vivo properties as compared with the natural materials. Bioabsorbable polymers can be classified into three major groups soluble, solubilizable, and depolymerizable. The structural features and possible mode of bioabsorption of these polymers are outlined as follows. 

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 ... 

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. 

In concentrated aqueous surfactant solutions, although the shape of the micelles may be very different from that in dilute solution, the locus of solubilization for a particular type of solubilizate appears to be analogous to that in dilute solution that is, polar molecules are solubilized mainly in the outer regions of the micellar structures, whereas nonpolar solubilizates are contained in the inner portions. 

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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