Abstract
Dielectric relaxation (DR) spectra covering the frequency range of 0.2 <= v/GHz <= 89 have been measured at 25 degrees C for aqueous solutions of sodium trifluoroacetate (NaOAcF3) up to solute concentrations c less than or similar to 3.6 M. In addition to the dominant bulk-water relaxation at similar to 20 GHz, one or two further modes were detected. The process centered at similar to 8 GHz was ...
Abstract
Dielectric relaxation (DR) spectra covering the frequency range of 0.2 <= v/GHz <= 89 have been measured at 25 degrees C for aqueous solutions of sodium trifluoroacetate (NaOAcF3) up to solute concentrations c less than or similar to 3.6 M. In addition to the dominant bulk-water relaxation at similar to 20 GHz, one or two further modes were detected. The process centered at similar to 8 GHz was attributed to water molecules hydrating the anions and to the reorientation of the anions themselves. A lower-frequency process at similar to 0.6 GHz, observed only at c less than or similar to 1 M. was assigned to the presence of a small amount of doubly solvent-separated ion pairs superimposed by ion-cloud relaxation. Hydration numbers were calculated from the water relaxation amplitudes. At infinite dilution similar to 20 water molecules were found to be weakly bound by the anion ("slow" water) whereas similar to 1-2 water molecules were strongly bound ("frozen") on the DR time scale. At the saturation limit, c approximate to 3.6 M, similar to 3-4 weakly and similar to 2 strongly bound H2O molecules were detected. Additionally, DR spectra have been measured at 25 degrees C for aqueous sodium acetate (NaOAc) solutions up to solute concentrations c less than or similar to 1 M to supplement previously published data [H. M. A. Rahman et al., J. Phys. Chem. B 116 (2012) 314]. These new data now allow quantitative evaluation of the low-frequency solute-related relaxation also detected for this salt. For both electrolytes, NaOAcF3(aq) and NaOAc(aq), upper limits for their association constants were determined. (C) 2012 Elsevier B.V. All rights reserved.
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