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Polymers of intrinsic microporosity (PIMs) are a unique class of soft materials, which, unlike hard materials such as zeolites or carbons, are capable of both vapor adsorption (nanopore filling) and absorption (polymer plasticization/swelling). While adsorption is governed by pore structure and surface functionalization, adsorption depends on the chemical composition of the matrix material. Since both adsorption and absorption increase sorbent mass (vapor uptake), gravimetric and volumetric characterization methods exhibit severe limitations in isotherm interpretation. Thus, distinguishing between vapor adsorption and absorption remains a key challenge for understanding sub‐nanometer‐scale processes, which play a crucial role in many emerging applications of PIMs, including gas separation, water purification, organic solvent nanofiltration, and electrochemical energy storage/conversion. Herein, an alternative approach is presented based on in situ ellipsometric read out and concurrent optical modeling of adsorption and absorption. Ellipsometry is highly sensitive to changes in thickness and optical density of the thin film sorbents, enabling the acquisition of both adsorption and absorption isotherms. This study showcases four PIM sorbents with varied material chemistry, responding to various vapors. Their ad‐/absorption contributions are studied and disentangled experimentally, with nanopore confinement and swelling models based on classical physisorption and polymer Flory‐like theory.
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Expanding Cluster, Enhancing Adsorption: Investigating the Role of Electrostatic Configurations on Water Vapor Adsorption
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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- PAR ID:
- 10654704
- Publisher / Repository:
- American Chemical Society
- Date Published:
- Journal Name:
- Langmuir
- Volume:
- 41
- Issue:
- 27
- ISSN:
- 0743-7463
- Page Range / eLocation ID:
- 17668 to 17678
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1021/acs.langmuir.5c01303
