Molecular-level DFT analysis of phosphoric acid binding, hydrogen-bond cooperativity, and local proton transfer in poly(vinyl alcohol)/citric acid membrane models
Abstract
A systematic density functional theory study of hydrogen bonding, cooperative binding, and Grotthuss proton transfer in PVA/CA/H₃PO₄ proton exchange membranes is presented. Twenty-three calculations were performed at the B3LYP-D3BJ/6-31G* level using ORCA 6.1.1, encompassing geometry optimisation, potential energy surface scanning, transition state verification, nudged elastic band analysis, natural bond orbital analysis, electron localisation function mapping, and a 65-atom crosslinked wet membrane model. The esterification energy is − 12.4 kJ/mol; single H₃PO₄ binding is − 78.1 kJ/mol; and cooperative binding of two H₃PO₄ molecules reaches − 203.0 kJ/mol. The gas-phase proton transfer barrier of 37.8 kJ/mol is reduced to 19.5 kJ/mol in the fully crosslinked wet environment, with the product state 9.3 kJ/mol more stable than the reactant. NBO bond-order analysis and ELF mapping support a local hydrogen-bond-mediated proton-transfer event, representing an elementary proton-hopping step rather than direct proof of a complete long-range Grotthuss conduction mechanism. A 20-step desorption PES (+ 75.9 kJ/mol) combined with hydration analysis (ΔG = − 278.0 kJ/mol) provides a quantum-mechanical interpretation of partial acid retention. DFT-predicted IR frequencies reproduce all major peaks within 0–110 cm⁻¹.