ABSTRACT

Proton-jump mechanism has been discussed by Conway et al., and it has been revealed that the isotope effect on the excess proton conductance λ°_E(H⁺)/λ°_E(D⁺) is important in determining the rate-determining step for the proton-jump mechanism. In order to examine the Conway model [λ°_E(H⁺)/λ°_E(D⁺) = √2] in detail and to disclose the important factors on the dynamics of the proton-jump mechanism, the limiting molar conductances ٨° of DCl, KOD, and KCl in heavy water (D₂O) and those of HCl, KOH, and KCl in light water (H₂O) were determined as a function of pressure (0.1 to 196.1 MPa) and temperature (5 to 65°C). Furthermore, ٨° of HCl and KCl   in aqueous mixtures [ethanol-, tert- butyl alcohol (TBA)-, and 1,4-dioxane-water mixtures] were obtained at 25°C under high pressure to examine the effect of an organic substance on the dynamics of the proton jump in relation to the water structure in these mixtures.

In pure water, the pressure coefficients of λ°_E(D⁺) and λ°_E(H⁺) were larger than those of λ°_E(OD-) and λ°_E(OH^-) at each temperature, while the temperature coefficients of λ°_E(D⁺) and λ°_E(H⁺) were smaller than those of λ°_E(OD^-) and λ°_E(OH^-) at each pressure. Concerning the isotope effect on λ°_E, both λ°_E(H⁺)/λ°_E(D⁺) and λ°_E(OH^-)/λ°_E(OD^-) decrease with increasing pressure and temperature, being contrary to the Conway model. We have pointed out that it is important to take into account the difference in the pre-rotation of water molecules adjacent to the D₃O⁺ (H₃O⁺) and the OD^- (OH^-) ion to explain these pressure and temperature effects on λ°_E. Furthermore, the isotope effect on λ°_E is determined not only by the reciprocal square root of the mass between a hydrogen and a deuterium atom ( √2), but also by the difference in strength of hydrogen bonds between D₂O and H₂O.

The pressure effect on λ°_E (H⁺) is considered to be related to the stability of water structure in aqueous mixtures, i. e., the more developed is the water structure at 1 atm, the more prominent is the increase in λ°_E (H⁺) with pressure. From the pressure effect on λ°_E(H⁺) in aqueous mixtures, it has been found that ethanol and TBA molecules stabilize the water structure in the water-rich region whereas an addition of a small amount of dioxane to water results in the breakage of hydrogen-bonded networks of water. The ability of an organic substance to stabilize the water structure is important in determining the dynamics of the proton jump in aqueous mixtures.