Models of adsorption-induced deformation: ordered materials and beyond

Probing the Limits of Mechanical Stability of the Mesoporous Metal-Organic Framework DUT-76(Cu) by Hydrocarbon Physisorption

The mechanical robustness of MOFs is crucial in most adsorption-related applications. Herein, we investigated the interaction of the mesoporous metal-organic framework DUT-76(Cu) with various C1-C4 hydrocarbons at their boiling points. During adsorption, the pore structure partially collapsed into an amorphous phase while retaining a residual porosity. We employed a combination of multicycle physisorption experiments using different hydrocarbons (methane, ethane, ethylene, propane, propylene, n-butane, and 1,3-butadiene) along with X-ray diffraction, scanning electron microscopy, and total scattering to examine this transition. This methodology allowed us to gain a comprehensive understanding of the effects on the crystal structure, local structure, and macroscopic behavior of the material. Furthermore, we identified specific correlations among the chain length, number of double bonds, and adsorption/desorption cycle stability, which are influenced by adsorption-induced stress. These multicycle adsorption experiments served as semiquantitative tools for assessing the mechanical stability of mesoporous frameworks.

Calcium carbonate hollow microspheres encapsulated cellulose nanofiber/sodium alginate hydrogels as a sequential delivery system

By using folic acid (FA) as the template, calcium carbonate hollow microspheres (CaCO3 HMs) are prepared through the Ostwald ripening process, which can be utilized for the loading of doxorubicin hydrochloride (DOX). The DOX loaded CaCO3 HMs (CaCO3/DOX) are co-encapsulated with ibuprofen (IBU) in the cellulose nanofiber (CNF)/sodium alginate (SA) hydrogels cross-linked by Ca2+. The loading efficiency of DOX in the CaCO3 HMs is 95.7 %, and the loading efficiency of IBU in the hydrogels is 97.0 %. In the weakly alkaline environment (pH ~7.4) that is characteristic of intestinal fluids of human body, the CNF/SA hydrogels are swollen and the encapsulated IBU is first released for pain control, and the release rate of IBU can reach 57.8 %. In the weakly acidic environment (pH ~6.5) that is characteristic of colonic fluids of human body, the CaCO3 HMs are decomposed to release the loaded DOX with a release rate of 50.1 %, which can be used for the treatment of colorectal cancer. The results of release kinetics indicate that the delivery of IBU is governed by first-order model and DOX by zero-order model. The developed sequential delivery system (SDS) can not only enable the release of DOX in colon of human body, but also simultaneously relieve the pain of patients during the chemotherapy of colon cancer.

Kinetics of Adsorption-Induced Deformation of Microporous Carbons

Equilibrium and kinetic behavior of adsorption-induced deformation have attracted a lot of attention in the last few decades. The theoretical and experimental works cover activated carbons, coals, zeolites, glasses, etc. However, most of the theoretical works describe only the equilibrium part of the deformation process or focus on the time evolution of the adsorption process. The present paper aims to cover the existing gap using the thermodynamic framework combined with the diffusion-based description of adsorbate time evolution inside an adsorbent. We obtained self-consistent equations describing equilibrium and out-of-equilibrium adsorption, as well as deformation processes. Further, the obtained equations were verified on the experimental data of carbon dioxide and methane on activated carbons. The model is capable of describing both equilibrium and kinetic adsorption and adsorption-induced deformation data. Additionally, we studied the possible influence of slow relaxation processes in the adsorbent on the adsorption process. The current work helps to interpret experimental data for time-dependent adsorption-induced deformation.

The role of water mobility on water-responsive actuation of silk

Biological water-responsive materials that deform with changes in relative humidity have recently demonstrated record-high actuation energy densities, showing promise as high-performance actuators for various engineering applications. However, there is a lack of theories capable of explaining or predicting the stress generated during water-responsiveness. Here, we show that the nanoscale confinement of water dominates the macroscopic dehydration-induced stress of the regenerated silk fibroin. We modified silk fibroin's secondary structure, which leads to various distributions of bulk-like mobile and tightly bound water populations. Interestingly, despite these structure variations, all silk samples start to exert force when the bound-to-mobile (B/M) ratio of confined water reaches the same level. This critical B/M water ratio suggests a common threshold above which the chemical potential of water instigates the actuation. Our findings serve as guidelines for predicting and engineering silk's WR behavior and suggest the potential of describing the WR behavior of biopolymers through confined water.

Bridging Microscopic Dynamics and Hydraulic Permeability in Mechanically-Deformed Nanoporous Materials

In the field of nanoconfined fluids, there are striking examples of deformation/transport coupling in which mechanical solicitation of the confining solid and dynamics of the confined fluid impact each other. While this intriguing behavior can be harnessed for applications (e.g., energy storage, phase separation, catalysis), the underlying mechanisms remain to be understood. Here, using molecular simulations, we investigate fluid flow in deformable nanoporous materials subjected to external mechanical stresses. We show that the pore mechanical properties significantly affect fluid flow as they lead to significant pore deformations and different fluid organization at the solid surface. Despite such mechanical effects, we show that the fluid thermodynamic properties (i.e., adsorption) can be linked consistently to Darcy's law for the permeability by invoking a pore size definition based on the concept of Gibbs' dividing surface. In particular, regardless of the solid stiffness and applied external stress, all data can be rationalized by accounting for the fluid viscosity and slippage at the solid surface (independently of a specific pore size definition). Using such a formalism, we establish that the intimate relation─derived using the linear response theory─between collective diffusivity and hydraulic permeability remains valid. This allows linking consistently microscopic dynamics experiments and macroscopic permeability experiments on fluid flow in deformable nanoporous materials.

What Drives Deformation of Smart Nanoporous Materials During Adsorption and Electrosorption?

Nanoporous solids have high surface area, so processes at the surface affect the sample as a whole. When guest species adsorb in nanopores, be they molecules adsorbing from the gas phase, or ions adsorbing from solution, they cause material deformation. While often undesired, adsorption- or electrosorption-induced deformation provides a potential for nanoporous materials to be used as actuators. Progress in this direction requires understanding the mechanisms of adsorption- or electrosorption-induced deformation. These two processes are rarely discussed together, and this Perspective aims to fill this gap to some extent, focusing on driving forces for both processes. Typically the main driving force for both is the solvation (disjoining) pressure, acting normally to the pore walls. However, in some cases, solvation pressure is not sufficient to describe the effects even qualitatively. We highlight examples in which the surface stress acting along the solid surface is an additional driving force for deformation.

Temperature Evolution of Sorbonorit-4 Methane-Induced Deformation through the Eyes of Classical Density Functional Theory

Activated carbons are widely used industrial adsorbents due to their attractive sorption properties. Although extensive research on activated carbon has been carried out for several centuries, some aspects of the adsorption-induced deformation of activated carbon remain unclear. The puzzling temperature dependence of the methane-induced deformation of activated carbon is investigated in the present work. Several experimental studies have shown that an increase in temperature leads to a reversal of the sign of adsorption strain at low pressures, i.e., the contraction turns into an expansion. Here we suggest a possible explanation for this effect by applying classical density functional theory to the adsorption isotherms of nitrogen, carbon dioxide, and methane as well as to methane-induced deformation isotherms. Our calculations show that the adsorption stress generated in the smallest pores predominates at higher temperatures and leads to material swelling. Lowering the temperature, on the other hand, leads to a predominance of larger pores and compression of the activated carbon material. We also investigated the possibility of determining the pore size distribution from methane-induced deformation and adsorption data and the predictive capabilities of our theoretical approach.

Noether's second theorem and covariant field theory of mechanical stresses in inhomogeneous ionic liquids

In this paper, we present a covariant approach that utilizes Noether's second theorem to derive a symmetric stress tensor from the grand thermodynamic potential functional. We focus on the practical case where the density of the grand thermodynamic potential is dependent on the first and second coordinate derivatives of the scalar order parameters. Our approach is applied to several models of inhomogeneous ionic liquids that consider electrostatic correlations of ions or short-range correlations related to packing effects. Specifically, we derive analytical expressions for the symmetric stress tensors of the Cahn-Hilliard-like model, Bazant-Storey-Kornyshev model, and Maggs-Podgornik-Blossey model. All of these expressions are found to be consistent with respective self-consistent field equations.

Macroscopic forces in inhomogeneous polyelectrolyte solutions

In this paper, we present a self-consistent field theory of macroscopic forces in spatially inhomogeneous flexible chain polyelectrolyte solutions. We derive an analytical expression for a stress tensor which consists of three terms: isotropic hydrostatic stress, electrostatic (Maxwell) stress, and stress rising from conformational entropy of polymer chains-conformational stress. We apply our theory to the description of polyelectrolyte solutions confined in a conductive slit nanopore and observe anomalous behavior of disjoining pressure and electric differential capacitance.