KINETICS OF NONSTATIONARY, DIFFUSION-INFLUENCED REVERSIBLE-REACTIONS IN SOLUTION

The statistical nonequilibrium thermodynamic theory of diffusion-influenced reactions is extended to nonstationary situations. Coupled dynamic equations for the average concentrations and the radial distribution function are derived, and, in the low density limit, applied to study the approach of the reversible reaction A + B reversible C to equilibrium. Two types of rate coefficients for the bimolecular reaction are discussed: (i) molecular rate coefficient describing the rates of elementary reactive events, and (ii) phenomenological rate constants defined via the macroscopic rate equations. In contrast to the phenomenological rate constant, the molecular forward rate coefficient ceases to depend on diffusion when the reaction reaches equilibrium. If the relaxation time for the reaction is much greater than that for diffusion, the classical expressions of Eigen for the linearized relaxation rate near equilibrium are recovered. A close relationship between the classical approach, the pseudo-steady-state approximation, and Onsager's regression hypothesis is demonstrated. The relation between the present results and those recently put forward in the literature is discussed.