Solvent component, organic

Figure 2. Thermodynamics for AgCl(c) = Ag+(S) + C/"(S) vs. wt % organic solvent component. Equations 5,6,20-22 1 = water-ethanol, 2 = water-acetone, 3 = water-tetrahydrofuran G = 5G, H...

The solvation of chromium(llI) ion in certain mixed-solvent systems has been studied in experiments which are relatively free of ambiguity. The exchange of solvent molecules between the mixed solvent and the solvated species Cr(OH2)w (So)n3+ (So = organic solvent component) is a very slow process. The species with solvation shells having different compositions can be separated from one another by column ion-exchange procedures. Analytical procedures based upon such separations allow evaluation of equilibrium constants for reactions involving replacement of coordinated water by the polar organic component. These equilibrium constants are reviewed in this chapter with attention focused upon the dependence of the equilibrium constants upon solvent composition, and the relationship of relative values of the equilibrium constants to the statistically expected values. 

So = organic solvent component w = number of water molecules coordinated to metal n = number of organic solvent molecules coordinated to metal (g) = gas... 

Z = mole fraction of organic solvent component in mixed solvent... 

G = 1,3-propanedial an20 = activity of water aSo — activity of organic solvent component y(Cr(OH2)63+) = medium activity coefficient for Cr(OH2)63+ y(Cr(OH2)6-nSon3+) = medium activity coefficient for Cr(OH2)6 nSo3+ Kn = equilibrium constant ... 

Let s re-label the two input and two output streams as in the figure at right. In a typical extraction problem, stream L is an aqueous solution containing a solute (component A) we are trying to extract by contact with an organic solvent (component C). So let s assume that the second feed stream (labelled V2) is pure solvent. Let s try to determine the composition of the two product streams. 

With preferential sorption of one component of the binary solvent on the polymer coil, an increase or decrease of the polarity of the polymer microenvironment occurs depending on whether the more polar (water) or less polar (organic solvent) component is sorbed. Preferential sorption occurs for PHEMA in 1-propanol/water, dioxane/water, and acetone/water mixtures (Figures 4 and 5). When the more polar component (water) is preferentially sorbed from mixtures in which its concentration is low, then the apolar contribution of the polymer may be compensated to that extent, since the polarity of the polymer chain microenvironment is even higher than the bulk solvent polarity. As a result, the curves of the dependence of Ej for the polymer on the solvent composition intersect the same dependence for mixed solvents. This phenomenon was observed for PHEMA in 1-propanol/water (Figure 4), dioxane/water, and acetone/water (Figure 5). Preferential sorption is also indicated by the results for PMMA and PBMA in methanol/toluene mixtures. Preferential sorption was previously found in this system by dialysis equilibria. ... 

As discussed in the previous section, the volatile organic solvent component will be retained to a certain extent by the freeze-dried cake. The amount retained will be governed by the solvent used, the formulation which is lyophilized, and the processing conditions selected. Because most residual solvents offer no therapeutic benefit, they should be removed to low levels to meet product specifications, good manufacturing practices, and other quality-based requirements. A major concern which must be addressed is whether the residual solvent present is at an acceptable level... 

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