Functional Rhythmic Chemical Systems Governed by pH-Driven Kinetic Feedback

Hydrogen ion autocatalytic reactions, especially in combination with an appropriate negative feedback process, show a wide range of dynamical phenomena, like clock behavior, bistability, oscillations, waves, and stationary patterns. The temporal or spatial variation of pH caused by these reactions is often significant enough to control the actual state (geometry, conformation, reactivity) or drive the mechanical motion of coupled pH-sensitive physico-chemical systems. These autonomous operating systems provide nonlinear chemistry's most reliable applications, where the hydrogen ion autocatalytic reactions act as engines. This review briefly summarizes the nonlinear dynamics of these reactions and the different approaches developed to properly couple the pH-sensitive units (e. g., pH-sensitive equilibria, gels, molecular machines, colloids). We also emphasize the feedback of the coupled processes on the dynamics of the hydrogen ion autocatalytic reactions since the way of coupling is a critical operational issue.