Ion-pair or ion-cloud relaxation? On the origin of small-amplitude low-frequency relaxations of weakly associating aqueous electrolytes

A detailed dielectric relaxation study of aqueous solutions of sodium chloride at 25 degrees C, covering the wide frequency range of 0.2 <=nu/GHz <= 89 is presented. Special attention was given to the low concentration region (c <= 1 mol L-1), as a low-frequency, small-amplitude relaxation appears at those concentrations. It is shown that this mode is not an artifact. A detailed discussion of its features suggests that for the studied NaCl solutions this relaxation partly arises from ion-pair reorientation. However, there is convincing evidence that ion-cloud relaxation also significantly contributes. Since the latter effect, thought to be of negligible amplitude so far, is ubiquitous to all electrolyte solutions it affects the determination of association constants of weakly associating electrolytes with dielectric spectroscopy. For the studied NaCl solutions the bulk-water relaxation was also analyzed. As expected, the cooperative water dynamics speeds up with increasing salt concentration, albeit to a lesser extent than assumed so far. More accurate data for the amount of H2O strongly bound by Na+ were obtained, suggesting that at infinite dilution approximately 5.2 water molecules are frozen by the cation. (C) 2012 Elsevier B.V. All rights reserved.