Ion Pairing and Solvent Relaxation
Processes in Aqueous Solutions of Sodium Malonate and Sodium
Succinate
Andrew Tromans, Peter M. May and Glenn Hefter,
* Takaaki Sato and Richard
Buchner*
J. Chem. Phys. B 108 (2004) 13789 - 13795
Abstract
Dielectric relaxation spectra have been measured for aqueous solutions of
sodium malonate and sodium succinate over a wide range of frequencies ( 0.2
<= n/GHz <=89) and solute concentrations (0.025
<= c/M <= 1.0). In addition to the usual processes associated with bulk water,
two further relaxation processes were detected. The faster of these, with a
relaxation time of ~16 ps, is unusual and was attributed to the presence of 'slow'
water in the hydration shells of the anions. The extent of this hydration shell is rather
'fragile' (concentration-dependent) for malonate but not for succinate. The second,
slower, process was attributed to the presence of ion pairs NaX-(aq).
Unambiguous determination of the structure of the ion pair was not possible because of
the internal flexibility of the anions but the data were most consistent with a chelated
solvent-shared ion pair. The kinetics of formation and dissociation of the ion pairs
were evaluated from the effects of solute concentration on their relaxation times.
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