Hydration of Tetraphenylphosphonium and Tetraphenylborate Ions by Dielectric Relaxation Spectroscopy

Abstract

A systematic study of the dielec. relaxation spectra of aq. solns. of NaBPh4 and Ph4PCl has been made at solute concns. of 0.02 ≤ c/M ≤ 0.82 and 0.20, resp., and over a wide range of frequencies (0.2 ≤ ν/GHz ≤ 89) at 25 °C. The spectra were best described by a superposition of four Debye processes, consisting of a very small ion-pair contribution with an av. relaxation time of about 300 ps, a "slow"-water relaxation at 17 ps, and two bulk-water relaxations at 8 ps and 0.25 ps, resp. The slow-water process has been assigned to the presence of a sheath of water mols. surrounding BPh4- and Ph4P+, whose structure has been enhanced by its proximity to the bulky hydrophobic Ph rings. A structure-making effect on the remaining solvent water can also be obsd. at low concns. More importantly, BPh4- and Ph4P+ show almost identical hydration characteristics, which provides indirect support for the use of the tetraphenylphosphonium tetraphenylborate ref. electrolyte assumption in deriving single-ion thermodn. properties.