Accessing the usefulness of atomic adsorption configurations in predicting the adsorption properties of molecules with machine learning

Malone, Walter; von_der_Heyde, Johnathan; Kara, Abdelkader

doi:10.1039/D3CP06312G

Electrostatic configurations─the spatial arrangement of charged sites within an adsorbent─can profoundly influence the adsorbent’s interaction with water and the resulting cluster formation and their orientation. This design feature can serve as a tuning parameter for water vapor adsorption to achieve the desired isotherm behavior. Hence, understanding the role of electrostatic configurations in water vapor adsorption can inform many established and emerging areas concerning the water-energy nexus and water security. In this work, we apply continuous fractional component grand canonical Monte Carlo (CFC-GCMC) to perform water adsorption simulations in idealized cylindrical nanopores across five different charge configurations with varying pore sizes (1, 1.1, and 1.2 nm) and charge magnitudes (∼±0.39–1.17). The alternating along (AA) configuration (positive charges in the inner ring and negative charges in the outer ring while alternating in the z-direction) demonstrates higher water uptake at saturation, and water adsorption starts at a much lower pressure than other configurations. Analysis of the water clustering pattern in AA reveals both radial and axial expansions of water clusters, which facilitates accommodation of extra water molecules. Increasing the charge magnitude shifts the type-V isotherm inflection point to lower pressure, thereby increasing the hydrophilic nature of the cylinder. Probing different energetic interactions and electrostatic potentials of the configuration suggests the unique relaxation of the water clusters in the AA patterned cylinders. Investigating the effect of charge magnitude and pore size provides more insight into their hydrophilic nature. Finally, analyzing the hydrogen bonding and adsorbed phase characteristics at saturation hints at strong ordering induced by pore confinements and electrostatic configurations compared with bulk liquid water. The simulations show that tailored charge arrangements can enhance adsorption by facilitating uptake at a lower pressure and achieving a higher water capacity at saturation. This study presents original insights into the interplay of electrostatic configuration, pore size, and charge strength in controlling water vapor adsorption within nanopores and the resulting confined water vapor structure.

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