The Role of Fine-Tuned Surface Functionality on Adsorption Thermodynamics of Nanoconfined Water within Periodic Mesoporous Organosilicas

High Resolution Image Download MS PowerPoint Slide In this study, the adsorption enthalpy and entropy of water in the nanoconfinement of periodic mesoporous organosilicas with fine-tuned surface chemistry were investigated by integrating thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) with a continuous-flow humidity generator, providing a direct method to better understand the water adsorption process at very low surface coverage. For comparison, the commonly used indirect isosteric method was employed, yielding generally comparable results to the simultaneous DSC-TGA measurements despite differing experimental conditions. Only the calorimetric method revealed that not only more hydrophilic materials exhibit high absolute values of adsorption enthalpy of water at the first surface contact but also hydrophobic materials, as they contain surface silanol groups that can strongly interact with water until these sites are saturated. Beyond this initial humidity region of water-surface contact, both methods yield largely consistent results at higher relative humidity, clearly distinguishing between hydrophilic and hydrophobic surface behavior. More hydrophobic materials have lower absolute values of adsorption enthalpy, reflecting weaker interaction strength and less structuring before reaching the condensation enthalpy of water (−44 kJ mol –1 ), while hydrophilic materials exhibit higher absolute values of adsorption enthalpy before water–water interactions dominate. Additionally, results from the isosteric method indicate that, at higher surface coverage, hydrophilic materials exert a long-range structural influence extending into distant water layers. Furthermore, the influence of surface functionality and temperature on water vapor sorption isotherms was analyzed, revealing the temperature independence of both the water aggregate size prior to, and the relative pressure at, the inflection point of capillary condensation. In contrast, the desorption branch primarily governs the temperature dependence of the adsorption–desorption hysteresis. This study presents the first detailed enthalpic analysis of water adsorption in PMOs using a continuous-flow calorimetric method, providing a foundation for future research on water-surface functionality interactions either at different loadings under isothermal conditions or across varying temperatures.