Hydration and sodium-ion binding of trifluoroacetate in aqueous solution

Abstract

Dielectric relaxation (DR) spectra covering the frequency range of 0.2≤v/GHz≤89 have been measured at 25°C for aqueous solutions of sodium trifluoroacetate (NaOAcF3) up to solute concentrations c≲3.6M. In addition to the dominant bulk-water relaxation at ∼20GHz, one or two further modes were detected. The process centered at ∼8GHz was attributed to water molecules hydrating the anions and to the reorientation of the anions themselves. A lower-frequency process at ∼0.6GHz, observed only at c≲1M, 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 ∼20 water molecules were found to be weakly bound by the anion (“slow” water) whereas ∼1–2 water molecules were strongly bound (“frozen”) on the DR time scale. At the saturation limit, c≈ 3.6M, ∼3–4 weakly and ∼2 strongly bound H2O molecules were detected.Additionally, DR spectra have been measured at 25°C for aqueous sodium acetate (NaOAc) solutions up to solute concentrations c≲1M 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.