Thermodynamic Insights into the Acid Strength of Sulfonate Linkers in Confined Space

Sulfonic acid groups exhibit a strong ability to deprotonate in aqueous environments and facilitate proton conduction through the membrane. However, the membrane’s hydration level is a critical factor influencing both the acid strength and the efficiency of proton conduction mechanisms. In this study, we employ ab initio molecular dynamics (AIMD) simulations augmented with metadynamics to explore the thermodynamic drivers governing the acid strength, highlighting the complex interactions between solvent molecules and functional groups within confinement at different interlinker spacings. Our computational studies reveal that a minimum of four water molecules are required to deprotonate these ethyl sulfonic acid (ESA) units, while internal energy drives the transition. For these ESA units, the tendency to accept and donate acidic protons is correlated to the dynamic behavior of hydronium ions. The distribution of a hydronium ion in the vicinity of ESA units is also found to be more sensitive to interlinker spacing than the water content itself. Therefore, the interplay of interlinker spacing and hydration level is crucial to further optimize the application of these systems, thereby facilitating the development of next-generation technologies in fields such as heterogeneous catalysis, proton exchange membranes, and energy conversion systems.