How Activated Carboxylic Acids Can Drive Dissipative Systems

Dissipative (non-equilibrium) chemical systems whose properties are transitorily changed by light or chemical stimuli are increasingly investigated. Among chemical stimuli, activated carboxylic acids (ACAs) are used to drive acid–base-based dissipative systems. Here, we give a comprehensive description of the operation mechanisms of such systems. Three types of systems are identified: systems under dissipative conditions (Type 1), energy ratchets (Type 2), and non-equilibrium steady state (NESS) systems (Type 3). Type 1 systems are driven from an equilibrium state to another via protonation by the ACA. However, this new equilibrium is transient because decarboxylation of the ACA conjugate base and back proton transfer rapidly restore the initial state. In Type 2 systems, after ACA consumption, the system is brought into an out-of-equilibrium state. Consequently, part of the free energy change due to the ACA decarboxylation is transferred to the system. Differently from Types 1 and 2, in Type 3 systems, ACA decarboxylation is part of the cyclic network; when fuel and waste species are chemostatted, a NESS can be reached displaying kinetic asymmetry.