Enhanced Surface Interaction of Water Confined in Hierarchical Porous Polymers Induced by Hydrogen Bonding

Abstract

Hierarchical porous polymer systems are increasingly applied to catalysis, bioengineering, or separation technology because of the versatility provided by the connection of mesopores with percolating macroporous structures. Nuclear magnetic resonance (NMR) is a suitable technique for the study of such systems as it can detect signals stemming from the confined liquid and translate this information into pore size, molecular mobility, and liquid-surface interactions. We focus on the properties of water confined in macroporous polymers of ethylene glycol dimethacrylate and 2-hydroxyethyl methacrylate [poly(EGDMA-co-HEMA)] with different amounts of cross-linkers, in which a substantial variation of hydroxyl groups is achieved. As soft polymer scaffolds may swell upon saturation with determined liquids, the use of NMR is particularly important as it measures the system in its operational state. This study combines different NMR techniques to obtain information on surface interactions of water with hydrophilic polymer chains. A transition from a surface-induced relaxation in which relaxivity depends on the pore size to a regime where the organic pore surface strongly restricts water diffusion is observed. Surface affinities are defined through the molecular residence times near the network surface.

Modulation of Hydrophilic/Hydrophobic Character of Porous Environments in Metal–Organic Frameworks via Direct Polymer Capping Probed by NMR Diffusion Measurements

“…28 Moreover, similar confinement effect has been observed for porous polymer matrices with surface affinity toward water adsorption. 50 Figure 4 shows schematically the proposed structures for ZIF-8/PABA composites allowing for a small fraction of water molecules to be adsorbed on top of the polymer-coated nanocrystals, which would continuously exchange with the vapor phase in contact with the intercrystallite space, leading to the observation of a fast decay in the STE experiment.…”

Environmental Applications of Engineered Materials with Nanoconfinement

“…For example, when considering the removal of pollutants in water treatment, a confined reaction system involves a series of interplaying factors, such as the hydration properties of a pollutant, the coordination environment of surface functional groups or metallic sites, the pollutant–surface interactions, the interference of coexisting ions, and the redox potential. The properties of the background water molecules can also be modulated by nanoconfinement, giving rise to the formation of strong hydrogen-bonded water networks, reduced mobility of water molecules, phase transition to ice-like crystalline structures, , reduced dielectric constant, and enhanced bonding to surfaces . This example illustrates the complexity and lingering uncertainties surrounding the interpretation of confinement-based environmental systems for even the most pervasive molecules.…”

“…The properties of the background water molecules can also be modulated by nanoconfinement, giving rise to the formation of strong hydrogen-bonded water networks, 25 reduced mobility of water molecules, 26 phase transition to ice-like crystalline structures, 27,28 reduced dielectric constant, 29 and enhanced bonding to surfaces. 30 This example illustrates the complexity and lingering uncertainties surrounding the interpretation of confinementbased environmental systems for even the most pervasive molecules.…”

Hydration of MgO-Based Cement: Water Dynamics by 1H Fast Field-Cycling NMR Relaxometry

“…For NMR relaxation of water protons in confining systems, mechanisms in which water molecules are adsorbed on and desorbed from the solid surface moving on the surface by translational diffusion and at the same time changing their orientation, have been proposed. ,,,,, In particular, the “reorientations mediated by translational displacements” model (RMTD), in which adsorbate molecules adopt preferential orientations relative to the local surface, has been suggested by Kimmich et al ,,,, and demonstrated to be valid for several confining systems, such as porous media, particle aggregates, and hydrogels. ,− …”