Studies of ion solvation and ion association of n-tetrabutylammonium hexafluorophosphate and n-tetrabutylammonium tetraphenylborate in various solvents

Conductometric investigation of n-tetrabutylammonium hexafluorophosphate (NBu4PF6) was performed in methanol (MeOH), ethanol (EtOH), 2-propanol (2-PrOH), acetone (ACE), 3-pentanone (PEN), tetrahydrofurane (THF), dimethylsulfoxide (DMSO), N,N-dimethylformamide (DMF), dichloromethane (DCM), acetonitrile (ACN) and N-methylformamide (NMF) at 298.15 K. The limiting molar conductivities (Λo) and the association constants (KA) were derived from the Lee–Wheaton conductivity equation. The limiting ion conductivities ( λ ± o ) were evaluated using n-tetrabutylammonium tetraphenylborate (NBu4BPh4) as a “reference” electrolyte according to the method of Krumgalz. The Stokes’ radii (rSt) of PF6− anion were estimated and used for the calculation of its solvation numbers (Sn) in investigated solvent media. The mobility of ions was found to be completely controlled by the bulk viscosity (η) and consequently the Walden product (W = ηΛo) was found to be almost constant. Slight ionic association was found for NBu4PF6 in solvent media with dielectric constant ɛ > 16 while the association becomes more significant in THF which has the lowest dielectric constant (ɛ = 7.5). The Sn values indicate weak interactions between PF6− and solvent molecules. The only exception was found in the case of MeOH, in which the great Sn value can be attributed to a hydrogen bond formation between the fluor atoms of PF6− and hydrogen of MeOH molecules.