Kinetics of diffusion-assisted reactions in microheterogeneous systems

This review is focused on the basic theory of diffusion-assisted reactions in microheterogeneous systems, from porous solids to self-organized colloid s and biomolecules. Rich kinetic behaviors observed experimentally are expl ained in a unified fashion using simple concepts of competing distance and time scales of the reaction and the embedding structure. We mainly consider pseudo-first-order reactions, such as luminescence quenching, described by the Smoluchowski type of equation for the reactant pair distribution funct ion with a sink term defined by the reaction mechanism. Microheterogeneity can affect the microscopic rate constant. It also enters the evolution equa tion through various spatial constraints leading to complicated boundary co nditions and, possibly, to the reduction of dimensionality of the diffusion space. The reaction coordinate and diffusive motion along this coordinate are understood in a general way, depending on the problem at hand. Thus, th e evolution operator can describe translational and rotational diffusion of molecules in a usual sense, it can be a discrete random walk operator when dealing with hopping of adsorbates in solids, or it can correspond to conf ormational fluctuations in proteins. Mathematical formulation is universal but physical consequences can be different. Understanding the principal fea tures of reaction kinetics in microheterogeneous systems enables one to ext ract important structural and dynamical information about the host environm ents by analyzing suitably designed experiments, it helps building effectiv e strategies for computer simulations, and ultimately opens possibilities f or designing systems with controllable reactivity properties. (C) 2001 Else vier Science B.V. All rights reserved.