Pressure and solvent isotope effects on the mobility of monovalent cations in water

Limiting molar conductivities of alkali metal chlorides (LiCl to CsCl) and tetraalkylammonium bromides (Me4NBr to Bu4NBr) in H2O and D2O were determined at 25 °C as a function of pressure up to 196.1 MPa. The limiting molar conductivities of the ions were obtained with the aid of transference numbers of KCl under high pressure, and transformed into the residual friction coefficients Δζobs to see what kinds of factors are important in the mechanism of ion migration. The pressure and solvent isotope effects on Δζobs of the Li+ ion agree qualitatively with the predictions of the Hubbard–Onsager (HO) dielectric friction theory, which indicates that dielectric friction plays an important role for smaller ions. However, the Cs+ ion shows opposite pressure and solvent isotope effects. For the R4N+ ions, Δζobs increases with an increase in the ionic radius and the pressure, opposite to the predictions of the HO theory. The increase in Δζobs for large R4N+ ions with increasing pressure suggests that the structure of the hydrophobic hydration shell is not weakened by pressure as much as that of bulk water. To make sure of this, the rotational correlation times of water molecules in pure D2O (τc°) and coordinated to the Bu4N+ ions (τc+) were estimated from 2H NMR spin-lattice relaxation times (T1) of D2O molecules at high pressure. The pressure dependence of τ+c/τ°c was in qualitative agreement with that of Δζobs(Bu4N+).