Solubilization mechanism

Fig. 2.11. Solubilization mechanism of zinc dialkyldithiophosphate molecules (ZDDP) by soft-core reverse micelles (RMs)...

An evaluation of the effect of pH on the aqueous solubility of a drug substance is an essential component of preformulation research, and such work is usually conducted along with determinations of ionization constants, solubilization mechanisms, and dissolution rates. ° Methods for the determination of the solubility... 

Delaney [13] describes the solubilization mechanism as controlled by a double phenomenon the affinity of the compound for itself and the affinity of the compound for the solvent. The latter effect is simply described either by the log P property or by very sophisticated methods such as statistical thermodynamic or quantum mechanical techniques. These very intensive calculation methods have not yet proved their superiority over the simpler and faster methods that tend to mimic the successful log P fragment calculator. 

PPS doped with AsFs dissolves readily in AsFs, but cast films are no longer soluble. Frommer has suggested that the solubilization mechanism involves solvation of both reactive radical intermediates and dopant counterions.f In addition, dialkyl esters of phosphoric acid dope PANi and render it soluble in certain solvents such as decalin. The resulting solutions can be mixed with conventional polymers and used to prepare films and fibers. 

One of the important properties of surfactants that is directly related to micelle formation is solubilization. Solubilization may be defined as the spontaneous dissolving of a substance (solid, liquid, or gas) by reversible interaction with the micelles of a surfactant in a solvent to form a thermodynamically stable isotropic solution with reduced thermodynamic activity of the solubilized material. Although both solvent-soluble and solvent-insoluble materials may be dissolved by the solubilization mechanism, the importance of the phenomenon from the practical point of view is that it makes possible the dissolving of substances in solvents in which they are normally insoluble. For example, although ethylbenzene is normally insoluble in water, almost 5 g of it may be dissolved in 100 mL of a 0.3 M aqueous solution of potassium hexadecanoate to yield a clear solution. 

There are five important criteria in prodrug design (l)adequate stability to the variable pH environment of the GI tract, (2) adequate solubility or solubilization mechanisms,... 

Solubilization Mechanism. All our experimental results support the proposal that the correlation between degree of preoxidation in the polyethylene and solubility is caused by exchange reactions ... 

An important issue in this type of behavior is the nature of the phase boundaries in the light solvents. In the case of the pentane and butane systems, the upper temperature phase boundary is marked by a gradual fogging of the solution due to precipitation of water, which is consistent with a natural curvature (solubilization) mechanism. The lower temperature boundary is marked by a sudden precipitation of both water and surfactant, which indicates a haze point (micelle-micelle interaction) transition. In the propane system it is difficult to determine the phase transition mechanism because of the low solubilization of water [12]. Nonetheless, it appears that the relative location of haze point and solubilization boundaries are the same in both light and heavy alkanes. 

Solubilization as a surface reaction This is the major solubilization mechanism for oils that are practically insoluble in water [156,158,160,162-170]. The uptake of such oils cannot happen in the bnlk of the aqueous phase. The solubilization can be realized only at the oil-water interface. The mechanism may include (1) micelle adsorption, (2) uptake of oil, and (3) desorption of the swollen micelles [168-170]. Correspondingly, the theoretical description of the process involves the rate constants of the three consecutive steps. 

The fraction solubilized, by sodium perfluorooctanoate and the partition coefficient, increase with increasing chain length of the alcohol (Table 6.16) [211]. The increase in solubilization was attributed to the increase in hydrophobicity of the alcohols with increasing chain length. The partition ratio of benzene was found to approximate that of butanol. This unexpected result suggested that the solubilization mechanism, and possibly the site of benzene in the micelle, is different from that of alcohols [211]. 

The partition coefficients in the sodium perfluorooctanoate-sodium decanoate system (Table 6.16) are similar to those of the pure perfluorooctanoate system. For benzene, however, the partition coefficient was higher in the mixed system, suggesting again a different solubilization mechanism. 

Figure 6.4. A schematic representation of the generally accepted extraction/solubilization mechanism of phase transfer catalysis.

The exact mechanisms by which various additives affect the effective HLBs of smfactants are not fully understood. For nonionic POE surfactants, in which hydration of the POE chain is the primary solubilizing mechanism in aqueous solution, the extent of chain hydration has seldom been found to be increased by the addition of materials that salt in the surfactants. That conclusion is based on the observation that the viscosity of the solutions is not significantly affected, indicating that the hydrodynamic radius of the molecules is not increased by increased hydration. In fact, the actions of such additives are in all probability related to their effects on the stmcture of the solvent, altering the thermodynamics of solvent-solute interactions. 

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