Hydration of Tetraphenylphosphonium and Tetraphenylborate Ions by Dielectric Relaxation Spectroscopy

Abstract

A systematic study of the dielectric relaxation spectra of aqueous solutions of NaBPh4 and Ph4PCl has been made at solute concentrations of 0.02 < or = c/M < or = 0.82 and 0.20, respectively, and over a wide range of frequencies (0.2 < or = nu/GHz < or = 89) at 25 degrees C. The spectra were best described by a superposition of four Debye processes, consisting of a very small ion-pair contribution with an average 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, respectively. The slow-water process has been assigned to the presence of a sheath of water molecules surrounding BPh4- and Ph4P+, whose structure has been enhanced by its proximity to the bulky hydrophobic phenyl rings. A structure-making effect on the remaining solvent water can also be observed at low concentrations. More importantly, BPh4- and Ph4P+ show almost identical hydration characteristics, which provides indirect support for the use of the tetraphenylphosphonium tetraphenylborate reference electrolyte assumption in deriving single-ion thermodynamic properties.

Hydration and sodium-ion binding of trifluoroacetate in aqueous solution

“…It is satisfying to note that the present DR spectra of NaOAc(aq) for c ≲ 1 M confirm the presence of this low-frequency process and yield amplitudes and relaxation times that are considerably less scattered (Table 2). Interestingly, the present spectra, limited to ν = 89 GHz, did not produce any evidence for the very fast water process centered at 400 GHz [23] which is sometimes [26,27] but not always [12,28] detected in aqueous electrolyte solutions. Nevertheless, the fitted values of ε ∞ ≈ 5-7 (Tables 1 and 2) are considerably larger than the ε ∞ = 3.48 obtained for pure water [23,29], which suggests that the fast water process is still making a minor, if unresolvable, contribution to the present spectra.…”

Charge Asymmetry at Aqueous Hydrophobic Interfaces and Hydration Shells

“…[9,14,15] Although specific ion (i.e. [25] On the other hand, some theoretical calculations have concluded that TPB À ions have a more favorable hydration energy than TPA + ions. Yet, water is often treated as a dielectric continuum.…”

Theoretical study on the hydrophobic and hydrophilic hydration on large solutes: The case of phthalocyanines in water

“…Studies of nuclear magnetic resonance and dielectric relaxation indicate that the average mobility of water molecules in solutions containing hydrophobic solutes decreases. [17][18][19][20] Since these methods measure the global response of water molecules, these techniques cannot distinguish between bulk water and the non-polar solute solvation shells. Recent classical MD simulations carried out by Laage et al 24 on the water reorientational dynamics around hydrophobic solutes with a medium-size and rather spherical-shape, like Me 3 NO or (C 4 H 10 ) 3 COH, find the same type of partial immobilization detected here.…”

“…On the iceberg model, Kauzmann 8 proposed the entropic origin of the hydrophobic attraction among caged hydrophobes in water, widely used to explain biochemical processes mediated by water. [12][13][14] During the last two decades, many studies have attempted to confirm experimentally this model by means of different techniques such as neutron diffraction, dielectric relaxation, a) sanchez@us.es and nuclear magnetic resonance, [15][16][17][18][19][20][21][22][23] without reaching a clear evidence for the primitive model. 12,24 The hydrophobicity has also been studied theoretically.…”