Suppressing Cooperative Water Condensation through Nucleation-Controlled HIPF Architectures

Abstract Hydrogen-bonded ionic-pair frameworks (HIPFs) provide a versatile platform for controlling interfacial hydration through molecular organization. Here, two crystalline melamine–trimesic acid (ME–TMA) HIPFs structures with fiber-like and plate-like morphologies, denoted F-HIPF and P-HIPF, were obtained by varying the nucleation conditions. Both materials exhibit low N2-accessible surface areas of 7.5 and 2.5 m2 g–1, respectively, providing a low-porosity platform in which hydration behavior can be separated from extensive permanent-pore filling. F-HIPF shows smooth, nearly reversible water uptake, whereas P-HIPF develops pronounced high-humidity hysteresis associated with more cooperative water retention. Direct, water-vapor adsorption and desorption, calorimetry reveals distinct loading-dependent enthalpic and entropic signatures. Operando difference infrared spectroscopy further demonstrates reversible hydration and different temperature dependences of the normalized initial response. Rate-derived kinetic descriptors indicate an enthalpically favorable but entropically penalized response for F-HIPF and an almost athermal, kinetically entropy-favored response for P-HIPF. When deposited on oxidized carbon fibers, F-HIPF increases irradiation-induced evaporation relative to bare CF, showing that a developed permanent pore network is not required for the observed enhancement.