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

Abstract

Conductometric investigation of n-tetrabutylammonium hexafluorophosphate (NBu 4PF 6) 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 ( Λ o ) and the association constants ( K A ) were derived from the Lee–Wheaton conductivity equation. The limiting ion conductivities ( λ ± o ) were evaluated using n-tetrabutylammonium tetraphenylborate (NBu 4BPh 4) as a “reference” electrolyte according to the method of Krumgalz. The Stokes’ radii ( r St ) of PF 6 − anion were estimated and used for the calculation of its solvation numbers ( S n ) 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 = ηΛ o ) was found to be almost constant. Slight ionic association was found for NBu 4PF 6 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 S n values indicate weak interactions between PF 6 − and solvent molecules. The only exception was found in the case of MeOH, in which the great S n value can be attributed to a hydrogen bond formation between the fluor atoms of PF 6 − and hydrogen of MeOH molecules.