Nonaqueous solutions desolvation

With regard to counterion binding, some similarities are also observed between ionomer nonaqueous solution and polyelectrolyte aqueous solution. This is the short-range effect due to partial desolvation associated with coun-... 

The double well potential in the gas phase is characterized by two equivalent ion-dipole minima separated by a symmetric transition state. However, the ion-dipole interactions are nullified in water by partial desolvation processes. Subsequent calculations lead to the result that in nonaqueous solution the reaction may also proceed with the presence of intermediates (Ref.63b). 

The physical properties of the anhydrate form and two polymorphic monohydrates of niclosamide have been reported [61], The anhydrate form exhibited the highest solubility in water and the fastest intrinsic dissolution rate, while the two monohydrates exhibited significantly lower aqueous solubilities. In a subsequent study, the 1 1 solvates of niclosamide with methanol, diethyl ether, dimethyl sulfoxide, N,/V -dimethyl formamide, and tetrahydrofuran, and the 2 1 solvate with tetraethylene glycol, were studied [62], The relative stability of the different solvatomorphs was established using desolvation activation energies, solution calorimetry, and aqueous solubilities. It was found that although the nonaqueous solvates exhibited higher solubilities and dissolution rates, they were unstable in aqueous media and rapidly transformed to one of the monohydrates. 

The unimolecular rate constants,, for water release from the hydration shells of cations [12] are expected to correspond with the values of deduced from Equation 4.46. These rate constants, obtained from ultrasound absorption [97,98], and NMR line widths [99-102] depend on the competition between water molecules and anions for sites in the coordination shell of the cations. These rate constants at 25°C span nearly 17 orders of magnitude (from K, the fastest to Rh, the slowest), and hence the logarithms log(fe/s ) are shown in Table 5.4. Considerably less experimental information is known for the rate of desolvation pertaining to the first solvation shell of cations in nonaqueous [12] solvents. Hardly any experimental rate constants regarding the rates of dehydration of anions are available. Computer simulations fill this gap concerning ions of both signs in aqueous solutions (Section 5.2.2). 

Even though less numerous, Raman studies of nitrate solutions in nonaqueous solvents have covered a range of metal ions (Li, Na, Ag, and Cu " ) in both protic and aprotic solvents [260-264]. Wooldridge et al. [260] carried out a vibrational study of alkali metal (lithium and sodium) nitrates dissolved in dimethylsulfoxide (0.5-2 M) at different temperatures (298-400 K). They found spectral evidence of an increase in the ionic pairing process when the temperature increases. Interestingly, in LiNOg solutions, the contact ionic pair formation promoted by the thermal increase is accompanied by a change in the ion-pair structure from monodentate to bidentate and by the partial desolvation of the alkaline ion ... 

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