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

► Precise dielectric spectra for aqueous NaCl solutions covering the frequency range 0.2 ≤ ν /GHz ≤ 89 reveal the presence of a weak solute relaxation. ► This study provides first reliable experimental evidence for ion-cloud relaxation in aqueous 1:1 electrolytes. ► In addition to ion-cloud relaxation, the reorientation of solvent-shared ion pairs contributes to the solute mode. ► Consequences for the determination of association constants for weakly associating electrolytes are discussed. ► Improved effective hydration numbers, Zib(Na+), are presented for the cation.