Dissociation cosolvent mixture

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. 

Aqueous solutions of electrolytes that dissociate into ions have been and are studied so extensively that it is natnral to enquire about the solutions of electrolytes in binary mixtures of water and cosolvents. Mixtures of nonaqueous solvents among themselves, although important for other purposes, have not received much attention when solutions of electrolytes are concerned. Therefore, this section deals only with binary mixtnres of water with cosolvents. Many of the solvents on the list are miscible with water at all proportions, whereas others exhibit limited, even rather low mutual solubility at ambient conditions. For these cases, solubility of water in the solvent, is larger than that of the solvent in water, x, on the mole fraction scale. Conversion to (approximate) values of the solubility on the molar scale is according to ... 

Studies on the nonracemic methylenecyclododecyl ethers (164 equation 37) led to similar conclusions. With HMPA as a cosolvent the secondary allylic ether (164a) afforded mainly (86 14) the ( )-homoallylic alcohol (165a) of 95% ee. The tertiary ether (164b) yielded a 98 2 ( ) (Z) mixture, but the major product (165b) was 40% racemic. A dissociative rearrangement process could account for this result. Without HMPA both ethers rearranged in poor yield (5-30%) to nearly 1 1 mixtures of ( )- and (Z)-cycloalkenes. 

J. Rubino, Electrostatic and non-electrostatic free energy contributions to acid dissociation constants in cosolvent-water mixtures, Int. J. Pharm., 1988, 42,181-191. 

The log-linear model is based on the presumed ideality of the mixtures of water and cosolvent. The log-linear relationship between log (Sn,/SJ and f is exact only if the cosolvent is identical to water, which cannot be the case in reality. Deviation is fortified as any degradation, solvation, dissociation, or solvent mediated polymorphic transitions of the solute occur. The problem is further compounded if the solute dissolves in an amount large enough to exert significant influence on the activity of solvent components. Due to the eom-plexity of the problem, efforts to quantitatively describe the deviations have achieved only limited success. 

Many investigators have studied substitution at iron(II)-diimine complexes in binary aqueous mixed solvents and other investigators in aqueous salt solutions. Some years ago the results of addition of salts and a cosolvent were assessed, for [Fe(5N02phen)3] in water, t-butyl alcohol, acetone, dimethyl sulfoxide, and acetonitrile mixtures containing added potassium bromide or tetra-n-butylammonium bromide. " Now the effects of added chloride, thiocyanate, and perchlorate on dissociation and racemization rates of [Fe(phen)3] in water-methanol mixtures have been established. The main explanation is in terms of increasing formation of ion pairs as the methanol content of the medium increases, but it is somewhat spoiled by the (unnecessary) assumption of a mechanism involving interchange within the ion pairs. Kip values (molar scale) of 11,18, and 25 were estimated for perchlorate, chloride, and thiocyanate in 80% (volume) methanol at 298.2 K. These values may be compared with values of 20, 7, and 4 for association between [Fe(phen)3] and iodide, " [Fe(bipy)3] and iodide, " and [Fe(phen)3] and cyanide " " in aqueous solution (at 298.2,... 

---