Dielectric Relaxation Study of the Ion Solvation and Association of NaCF3SO3/ Mg(CF3SO3)2/ and Ba(ClO4)2 in N,N-Dimethylformamide

摘要

Solutions of sodium trifluoromethanesulfonate, magnesium trifluoromethanesulfbnate, and barium perchlorate in N,N-dimethylformamide (DMF) have been investigated using broadband dielectric relaxation spectroscopy at 25 °C. All spectra were dominated by a solvent relaxation process centered at ～15 GHz but also exhibited one (for NaCF3SO3) or two (for the 2:1 salts) low-amplitude processes, centered at frequencies below 2 GHz, that could be attributed to the presence of ion pairs. Effective solvation numbers calculated from the solvent relaxation amplitudes indicated strong solvation of all three cations, with evidence for the formation of a second solvation sheath for Mg~(2+) and possibly Ba~(2+). Detailed analysis of the solute-related processes showed that solvent-shared ion pairs (SIPs) were formed in NaCF3SO3 solutions in DMF. The data for Mg(CF3SO3)2 and Ba(ClO4)2 solutions were not definitive but, consistent with the solvation evidence, favored the presence of double solvent-separated ion pairs and SIPs. Overall association constants, K_A, were small for all three salts in DMF and increased in the order: NaCF3SO3 < Ba(ClO4)2 < Mg(CF3SO3)2.