Solubilizate

Conventional cosmetic emulsions (macroemulsions) normally contain about 70% or more of the external phase, which may be a mixture of components. The internal phase is routiaely iatroduced iato the external phase at an elevated temperature with vigorous agitation. The emulsifiers are distributed according to their solubility between the two phases. The level of emulsifiers (rarely more than about 10%) is kept low siace excessive amounts may destabilize emulsions or form a clear solubilizate. Auxiliary emulsifiers and other components are iacluded ia the phases ia which they are soluble. 

Ionic, polar and amphiphilic solubilizates are forced to reside for relatively long times in very small compartments within the micelle (intramicellar confinement, compart-mentalization) involving low translational diffusion coefficients and enhancement of correlation times. 

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. 

Generally, solubilization occurs spontaneously when the pure solubilizate contacts the solution of reversed micelles. Often, vigorous stirring consistently reduces the time necessary to obtain complete solubihzation and thermodynamically stable systems. 

The contemporaneous presence of different solubilizates sometimes involves competition for the micellar binding sites [31], For instance, from an analysis of the heats of solution of benzene and water in solutions of reversed micelles of tehaethylene glycol dodecyl ether in decane, a competition between water and benzene for the surfactant hydrophilic groups was shown [32],... 

Obviously, water, aqueous solutions of salts, and mixtures of highly hydrophilic solvents have also been found to be solubilized in the micellar core [13,44]. The maximum amount of such solubilizates that can be dissolved in reversed micelles varies widely, strongly depending on the nature of the surfactant and the apolar solvent, on the concentrations of surfactant and of additives, and on temperature [24,45-47]. 

The importance of the material exchange process can hardly be overemphasized since it is the mechanism whereby the equUibrium miceUar size and polydispersity are reached and maintained, the reversed micelles of ionic surfactants become charged, polar and amphiphilic solubilizates are transported, and hydrophilic reactants can come in... 

Since very often the physicochemical properties of solubilizates are modified when they are entrapped in reversed micelles, almost all the experimental techniques can been used, and have been used, to study solubilization [28,31,118-122]. 

Moreover, as a general behavior and indirect effect of solubilizate/micelle interactions, incorporation of additives influences the stability and the phase diagram of the system [48,119,124]. 

The different location of polar and amphiphilic molecules within water-containing reversed micelles is depicted in Figure 6. Polar solutes, by increasing the micellar core matter of spherical micelles, induce an increase in the micellar radius, while amphiphilic molecules, being preferentially solubihzed in the water/surfactant interface and consequently increasing the interfacial surface, lead to a decrease in the miceUar radius [49,136,137], These effects can easily be embodied in Eqs. (3) and (4), aUowing a quantitative evaluation of the mean micellar radius and number density of reversed miceUes in the presence of polar and amphiphilic solubilizates. Moreover it must be pointed out that, as a function of the specific distribution law of the solubihzate molecules and on a time scale shorter than that of the material exchange process, the system appears polydisperse and composed of empty and differently occupied reversed miceUes [136],... 

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],... 

In the case of Kryptofix 221D, a cryptand able to complex the alkali metal cations [141-143], it has been observed that it is solubilized mainly in the palisade layer of the AOT-reversed micelles. And from an analysis of the enthalpy of transfer of this solubilizate from the organic to the micellar phase it has been established that the driving force of the solubilization is the complexation of the sodium counterion. In addition, the enthalpy... 

Nanoparticles solubilized in w/o microemulsions have been obtained by performing in situ suitable reactions [196], by dispersion of particles [219,220], or by controlled nanoprecipitation of a solubilizate [221,222]. 

Another method is based on the evaporation of a w/o microemulsion carrying a water-soluble solubilizate inside the micellar core [221,222], The contemporaneous evaporation of the volatile components (water and organic solvent) leads to an increase in the concentration of micelles and of the solubilizate in the micellar core. Above a threshold value of the solubilizate concentration, it starts to crystallize in confined space. Nanoparticle coalescence could be hindered by surfactant adsorption and nanoparticle dispersion within the surfactant matrix. 

Solutions of surfactant-stabilized nanogels share both the advantage of gels (drastic reduction of molecular diffusion and of internal dynamics of solubilizates entrapped in the micellar aggregates) and of nonviscous liquids (nanogel-containing reversed micelles diffuse and are dispersed in a macroscopicaUy nonviscous medium). Effects on the lifetime of excited species and on the catalytic activity and stability of immobilized enzymes can be expected. 

Hydrophobic solubilizates such as styrene (S) decrease the saponification rate of the EUP. Accordingly, the EUP-molecules in micelles containing S are more resistant against hydrolytic degradation than molecularly dissolved EUP-mole-cules. Obviously, the access of the base to the hydrophobic interior of these micelles and microemulsion droplets is more difficult. 

Contrary to hydrotropes, micelle-forming surfactants spontaneously self-aggregate cooperatively above the critical micelle concentration (cmc) even in the absence of solubilizate. Typical examples of micelle-forming surfactants include sodium dode-cylsulfate (SDS), dodecyltrimethylammonium bromide (DTAB), cetyltrimethyl-ammonium bromide (CTAB), and heptaoxyethylene dodecyl ether (C12E7) (Scheme 2). 

In summary, our bioavailability study provided for fhe firsf time data for fhe shorf-ferm bioavailability of ot-tocopherol solubilizate in comparison to regular fat-soluble preparations. Our results pointed to a higher short-term bioavailability of vitamin E in micelles versus fat-soluble forms of fhis vifamin in healthy adult volunteers both with regard to AUCs and with regard to maximum increases in plasma vitamin concentrations. 

Solubilizate 1 Solubilizate 2 1 Oily dispersion 1 Oily dispersion 2 3 Crystalline... 

FIGURE 5.8 Bioavailability of 60-mg CoQio single dosing (early absorption). AUCo 4 h (pmol/mmol x h) after a single dose of 60-mg CoQiq. Data are expressed as mean SD. Differences between formulations were tested with ANOVA and post hoc test Student-Newman-Keuls test. p < 0.05 (both solubilizates vs oily dispersion 1 and oily dispersion 2 vs crystalline) p < 0.01 (both solubilizates vs crystalline). 

Schulz, C., Obermiiller-Jevic, U. C., Hasselwander, O., Bernhardt, J., and Biesalski, H. K. (2006). Comparison of the relative bioavailabiUty of different coenzyme QIO formulations with a novel solubilizate (Solutrade mark QIO). Int. J. Food Sci. Nutr. 57, 46-55. 

Microemulsions are dynamic systems in which droplets continually collide, coalesce, and reform in the nanosecond to millisecond time scale. These droplet interactions result in a continuous exchange of solubilizates. The composition of the microemulsion phase determines the exchange rate through its effect on the elasticity of the surfactant film surrounding the aqueous microdomains. Compared with nonionic surfactant-based microemulsions, AOT reverse micelles have a more rigid... 

Surfactantvesicles 3D 300-100000 A diameter Shaking, sonication, or alcohol injection Days to weeks Solubilizates and particles could be in situ generated or placed into the bilayer or aqueous interior a high level of organization was possible 55, 72, 101... 

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