The development of autonomous chemical systems that could imitate the properties of living matter, is a challenging problem at the meeting point of materials science and nonequilibrium chemistry. Here we design a multi-channel gel reactor in which out-of-equilibrium conditions are maintained by antagonistic chemical gradients. Our device is a rectangular hydrogel with two or more channels for the flows of separated reactants, which diffuse into the gel to react. The relative position of the channels acts as geometric control parameters, while the concentrations of the chemicals in the channels and the variable composition of the hydrogel, which affects the diffusivity of the chemicals, can be used as chemical control parameters. This flexibility allows finding easily the optimal conditions for the development of nonequilibrium phenomena. We demonstrate this straightforward operation by generating diverse spatiotemporal patterns in different chemical reactions. The use of additional channels can create interacting reaction zones.

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.

Design strategy through linking a driving pH oscillator (master system) to a pH sensitive complexation, precipitation, or protonation equilibrium (slave system) has been widely used to create and control concentration oscillations of chemical entities (e.g., monovalent cations, DNA, nanoparticles) not participating in the pH oscillatory system. No systematic investigation has been carried out on how the components of these equilibria affect the characteristics of the driving pH oscillators, and this feedback effect has been often neglected in previous studies. Here we show that pH sensitive species (hydrogen carbonate, EDTA) through a pH-dependent equilibrium could significantly affect the characteristics (time period and amplitude) of the driving pH oscillators. By varying the concentration of those species we are able to control the strength of the chemical feedback from slave system to master system thus introducing a transition from master-slave coupling to peer-to-peer coupling in linked chemical systems. To illustrate this transition and coupling strategies we investigate two coupled chemical systems, namely, the bromate-sulfite pH oscillator and carbonate-carbon dioxide equilibrium and the hydrogen peroxide-thiosulfate-copper(II) and EDTA complexation equilibrium. As a sign of the peer-to-peer coupling the characteristics of the driving oscillatory systems can be tuned by controlling the feedback strength, and the oscillations can be canceled above a critical value of this parameter.

Spatially localized moving and stationary pH patterns are generated in two-side-fed reaction-diffusion systems.

Reaction–diffusion waves and stationary Turing patterns are observed in closed two-layer gel reactors, where the two compartments are initially filled with complementary sets of reactants of the chlorine dioxide–iodine–malonic acid–poly(vinyl alcohol) reaction. The asymmetrical loading generates concentration gradients and the patterns form at the interface between the two parts. These easy-to-perform experiments allow us to study a wide range of dynamical phenomena without requiring a specific reactor design or the use of sophisticated equipment. To get complementary information on pattern formation in parallel and perpendicular to the direction of the concentration gradients, two geometrically different configurations of compartments are presented. We demonstrate that three variants of the initial distribution of the chemicals can be equally applied, and this flexibility provides a way to introduce additional reagents to perturb the dynamics of the systems. A noticeable increase in the wavelength of Turing patterns and in the period of waves has been induced by adding bromide ions. The interaction of Turing and Hopf modes has been observed as a result of not only the variation of the initial poly(vinyl alcohol) concentration but that of the gradients as well.