Self-dissociation and protonic charge transport in water and

Abstract

A comprehensive survey on experimental techniques, results and theoretical interpretations concerning the self-dissociation and protonic charge transport in water and ice is given. Recent investigations of fast protolytic reactions in pure water and aqueous solutions by means of relaxation techniques complete our knowledge about state and kinetic properties of the proton in this medium. In comparison here with our experience regarding the same properties in ice crystals are far less complete, as usual techniques of aqueous solutions are not applicable. Direct measurements of individual properties of ‘excess’ and ‘defect’ protons in ice (mobilities, concentrations, reaction rates) are presented. The proton transport in hydrogen-bonded media is completely different from normal ionic migration and corresponds more to electronic transport processes in semi-conductors. Generally the proton transport through hydrogen bonds includes two processes: (1) The formation (or rearrangement) of (H-bond) structure with orientation, favourable for a proton transition, and (2) the charge transfer within the H bond. The first step is rate determining in water, whereas the second one is decisive for the charge transport in ice. The requirements for a theoretical treatment therefore are (1) for water: a theory of ‘structural diffusion’ of the H-bonded hydration complex of H3O+, and (2) for ice: a (quantum-mechanical) theory of the protonic motion within the potential well of the H bond. The mechanism of structural diffusion provides an explanation of the anomalous H3O+ and OH- mobility and their recombination rate in water. The difference between protonic and normal ionic charge transport occurs most obviously in the absolute values of mobilities in ice. The proton mobility in ice differs by many orders of magnitude from that of normal ions, but only by a factor of about 50 from electronic mobilities in some metals and semi-conductors. Further arguments, demonstrating the analogy between protonic and electronic charge transport are given. The reaction kinetics of protolytic systems and the fast proton transport in H-bonded systems are of certain importance with respect to biological problems.