Ion Pairing and Solvent Relaxation
Processes in Aqueous Solutions of Sodium Malonate and Sodium
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.
Andrew Tromans, Peter M. May and Glenn Hefter,
* Takaaki Sato and Richard
J. Chem. Phys. B 108 (2004) 13789 - 13795