Molecular Insights into Carbon Spacer Length Effects on Sulfobetaine-Phosphoric Acid Interactions: Optimizing Acid...

Phosphoric acid (PA) retention is essential for the efficient and stable operation of PA-doped proton exchange membranes (PEMs) in fuel cells. The strength of the interactions between polymer functional groups and PA is a key determinant of PA retention. In this work, we systematically investigate the interactions between zwitterionic sulfobetaines (SBs) with varying carbon spacer lengths (CSLs) and PA under both anhydrous and hydrated conditions using density functional theory calculations and molecular dynamics (MD) simulations. The results show that both excessively short and long spacers weaken SB-PA interactions, while CSLs of 3-5 yield stronger interactions, with energies exceeding - 50 kcal/mol. Notably, SB with CSL = 4 exhibits the strongest interaction at - 64.2 kcal/mol, attributed to a balance between intramolecular cation-anion interactions within SB and intermolecular interactions with PA. In the presence of water, SBs with CSLs of 3-5 exhibit dominant interactions with PA in SB-PA-water clusters. MD simulations further confirm that SB molecules with CSL of 3-5 maintain strong interactions with PA in aqueous environments. These findings offer molecular insights into the effect of CSL on SB-PA interactions and guide the design of SB-based PEMs with improved PA retention.