In cosolvent mixture

Usually, the solubility of the salt is determined from separate, more concentrated solutions. To conserve on sample, the titration of the salt may be performed with an excess of the counterion concentration [479]. Also, some amount of sample salt may be conserved by titrating in cosolvent mixtures, where salts are often less soluble. 

A plot of the Yasuda-Shedlovsky equation generally is a straight line. The fitted coefficients A and B are then used to estimate the pK value in a 100% aqueous solution, for which [H2O] = 55.5 molal and e = 78.3. Successful use of this approach in cosolvent mixtures requires a complex pH electrode calibration procedure [20]. 

K.V. Peinemann, J.F. Maggioni, S.P. Nunes, Poly (ether imide) membranes obtained from solution in cosolvent mixtures. Polymer 39 (1998) 3411. 

In cosolvent mixtures containing a low fraction of water (for example, ethanol/water. Figure 6) selective sorption of water on P m takes place and the polarity of the microenvironment of PVIm-II is higher than the polarity of the solvent used. On the other hand, selective sorption of alcohol diminishes the polarity of the microenvironment of PVIm at a lower content of alcohol in the binary mixture a higher difference between the polarity of the microenvironment of the polymer and the polarity of the solvent was observed. 

Mixed-solvent solutions of various cosolvent-water proportions are titrated and psKa (the apparent pKa) is measured in each mixture. The aqueous pKa is deduced by extrapolation of the psKa values to zero cosolvent. This technique was first used by Mizutani in 1925 [181-183]. Many examples may be cited of pKa estimated by extrapolation in mixtures of methanol [119,161,162,191,192,196,200], ethanol [184,188-190,193], propanol [209], DMSO [212,215], dimethylformamide [222], acetone [221], and dioxane [216]. Plots of psKa versus weight percent organic solvent, Rw = 0 — 60 wt%, at times show either a hockey-stick or a bow shape [119]. For Rw > 60 wt%, S-shaped curves are sometimes observed. (Generally, psKa values from titrations with Rw > 60 wt% are not suitable for extrapolation to zero cosolvent because KC1 and other ion pairing interferes significantly in the reduced dielectric medium [223].)... 

Much effort has been expended on models that can be used to predict the solubility behavior of solutes, with good success being attained using a semi-empirical, group contribution approach [75]. In this system, the contributions made by individual functional groups are summed to yield a composite for the molecule, which implies a summation of free energy contributions from constituents. This method has proven to be useful in the prediction of solubility in water and in water-cosolvent mixtures. In addition to the simplest methodology, a variety of more sophisticated approaches to the prediction of compound solubility have been advanced [68]. 

Yalkowsky has shown that the solubility of a compound in a cosolvent mixture (S ) can be estimated through a log-linear solubility relation ... 

More recent work with cosolvency in dilute systems seems to indicate that the magnitude of the solubility enhancement is linear up to some 10-20% cosolvent fraction [55,172,184,250-262]. At very low concentrations of cosolvent, the assumption of non-interaction between the cosolvent and water cannot hold. In dilute solutions the individual cosolvent molecules will be fully hydrated and, as a result, will disrupt the water network structure. If the total volume disrupted is regarded as the extended hydration shell, and if Sc is the average solubility within this shell, then the overall solubility Sm in the water-cosolvent mixture will be approximated by... 

Addition of a cosolvent is an alternative mechanism to increase contaminant solubility in an aqueous solution. When a contaminant with low solubility enters an aqueous solution containing a cosolvent (e.g., acetone), the logarithm of its solubility is nearly a linear function of the mole fraction composition of the cosolvent (Hartley and Graham-Bryce 1980). The amount of contaminant that can dissolve in a mixture of two equal amounts of different solvents, within an aqueous phase, is much smaller than the amount that can dissolve solely by the more powerful solvent. In the case of a powerful organic solvent miscible with water, a more nearly linear slope for the log solubility versus solvent composition relationship is obtained if the composition is plotted as volume fraction rather than mole fraction. 

Crystalline salts of many organic acids and bases often have a maximum solubility in a mixture of water and water-miscible solvents. The ionic part of snch a molecule requires a strongly polar solvent, snch as water, to initiate dissociation. A mixture of water-miscible solvents hydrates and dissociates the ionic fraction of pollutants at a higher concentration than wonld either solvent alone. Therefore, from a practical point of view, the deliberate nse of a water-soluble solvent as a cosolvent in the formnlation of toxic organic chemicals can lead to an increased solnbility of hydrophobic organic contaminants in the aqueous phase and, conse-qnently, to a potential increase in their transport from land surface to groundwater. 

Current work with supercritical fluids can also illustrate the importance of cosolvents. Cosolvent effects in supercritical fluids can be considerable for systems where the cosolvent interacts strongly with the solute. A correlation suggests that both physical and chemical forces are important in the solvation process in polar cosolvent supercritical CO2 mixtures. The model coupled with the correlation represents a step toward predicting solubilities in cosolvent-modified supercritical fluids using nonthermody-namic data. This method of modeling cosolvent effects allows a more intuitive interpretation of the data than either a purely physical equation of state or ideal chemical theory can provide (Ting et al., 1993). 

Jouyban, A., Soltanpour, S., Soltani, S., Chan, H. K., Acree, W. E. (2007) Solubility prediction of drugs in water-cosolvent mixtures using Abraham solvation parameters. J Plmrm Sci 10, 263-277. 

Iodoxvbenzoic acid (IBX) is also a useful oxidizing reagent. Insoluble in most solvents (except DMSO) it can be used with other cosolvent mixtures. 

As a consequence of the model employed, values of Hb(H20) and N ought to be independent of the choice of the cosolvent as long as specific structural effects are absent. Therefore, we applied Equation 3 to the enthalpies of solution of n-Bu4NBr in DMSO-water mixtures (iO), since DMSO is a dipolar aprotic solvent like DMF. The best fit of the AHE values in this mixture yields Hb(H20) = —49.2 kj mol-1 and N/4 = 6.4, in excellent agreement with our values at 25°C given in Table III. 

The reaction mixture contains a dilute solution of benzophenone solute, a few mole percent isopropanol and the remainder supercritical carbon dioxide solvent. This is analogous to a solute dissolved in a supercritical fluid/cosolvent mixture. These types of systems are important because in many applications researchers have found that the addition of a small amount of cosolvent (such as acetone or an alcohol) of volatility intermediate between that of the solute and the SCF can greatly enhance the solubility of the solute (Van Alsten, 1986). 

Jouyban et al. (2004) applied ANN to calculate the solubility of drugs in water-cosolvent mixtures, using 35 experimental datasets. The networks employed were feedforward back-propagation errors with one hidden layer. The topology of neural network was optimized in a 6-5-1 architecture. All data points in each set were used to train the ANN and the solubilities were back-calculated employing the trained networks. The difference between calculated solubilities and experimental... 

Jouyban, A., M. R. Majidi, H. Jalilzadeh, and K. Asadpour-Zeynali. 2004. Modeling drug solubility in water-cosolvent mixtures using an arti cial neural netwEdfmaco59 505-512. 

S/v is the solubility in water, and f is the fraction of organic solvent in the cosolvent mixture. If the cosolvent mixture contains more than two organic solvents (i.e., a ternary or higher cosolvent mixture), the total drug solubility can be approximated by a summation of solubilization potentials as... 

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