Thermodynamic Insights\ninto the Acid Strength of Sulfonate\nLinkers in Confined Space

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