Andrei L. Kolesnikov

Fluids in nanopores are of importance for many engineering applications, including energy storage in supercapacitors, hydrocarbon recovery from unconventional sources, or water desalination. Thermodynamic properties of fluids confined in nanopores differ from the properties of the same fluids in bulk. Density functional theory (DFT) has been widely used for modeling the thermodynamics of confined fluids. However, it is rarely used for calculations of derivative thermodynamic properties. Here, we use a rather simple DFT model for argon based on the Percus–Yevick equation and show that with standard parameterization, it fails to predict derivative properties. However, a slight adjustment in parameters leads to quantitative predictions of isothermal compressibility and thermal expansion coefficient at a selected temperature. Using the adjusted parameterization, we performed the calculations of compressibility of argon confined in carbon slit pores of various sizes and demonstrated that the compressibility of argon in confinement is lower than that in bulk and is pore size dependent. We confirmed the DFT predictions using the Monte Carlo molecular simulations. In addition to isothermal compressibility, we calculated the thermal expansion coefficient of confined argon. Our calculations showed that it behaves similarly to compressibility—it is always lower than the bulk value and gradually increases for smaller pore sizes. For several selected pore sizes, we verified the DFT calculations by Monte Carlo simulations. Overall, our results suggest that the classical DFT can be utilized for calculations of derivative thermodynamic properties of confined fluids, which are computationally challenging to predict using molecular simulations.

N2 adsorption analysis is an established method for the analysis of micro- and mesoporosity of open-porous solids. Thus, all researchers developing open-porous materials such as aerogels apply this commercially available technique. However, due to their extraordinary properties, i.e., huge specific mesopore volumes combined with low mechanical stiffness, aerogels question this method. A prerequisite for N2 adsorption analysis is the determination of a well-equilibrated isotherm. This study investigates experimental parameters and discusses their impact on the resulting isotherms. The findings yield recommendations for the reliable determination of adsorption and desorption isotherms of aerogels. In the case of aerogels, a second factor affecting the shape of the isotherms comes into play, i.e., the superposition of the isotherm and deformation effects resulting from capillary pressure upon mesopore filling. This yields erroneous bimodal pore size distributions (PSD) with their mean values differing from the true ones by up to a factor of 2, thus strongly compromising the strategies for targeted materials synthesis for a given application. This paper uses silica aerogel as a model system and provides, for the first time, an approach for the correction of deformation artifacts upon N2 adsorption analysis from the recorded isotherm only, and thus allows reliable determination of PSDs for compliant mesoporous materials.

The study of physical properties of organophosphorus chemical warfare agents (CWAs) is essential for detection, protection, decontamination and destruction. Due to the extreme toxicity of CWAs, other organophosphorus molecules with similar chemical structures are employed in experimental studies as simulants, such as dimethyl methylphosphonate (DMMP) and diisopropyl methylphosphonate (DIMP). Triethyl phosphate (TEP) can also be considered as an alternative simulant, especially that it is more accessible due to its several industrial applications. Classical molecular simulations complement experimental research on organophosphates. Here, we present the extension of the transferable potentials for phase equilibria (TraPPE) force field to TEP. Using this model, TEP can be parameterized based on interaction parameters from similar organophosphates in conjunction with ab initio calculations. The model was validated by performing molecular dynamics (MD) simulations to predict liquid densities and Gibbs ensemble Monte Carlo simulations to model vapor-liquid equilibrium to predict vapor pressures which were compared with literature data. MD simulations were performed to predict the enthalpy of vaporization, surface tension, viscosity and diffusivity. Additionally, we parameterized and tested trimethyl phosphate (TMP) due to its structural similarity to TEP. Our study helps assess TEP and TMP as alternative CWA simulants, and provides predictions of their physical properties, over a wide range of temperatures, complementing the scarce experimental data. The presented force field can be further utilized for predicting other properties based on molecular simulations, such as adsorption of TEP or TMP.

Adsorption kinetics plays a key role in numerous industrial applications, including gas separation by molecular sieving, water purification, gas storage, and TSA processes. It is accompanied by adsorption-induced deformation, in which molecules adsorbed onto the pore surfaces generate stress, resulting in dimensional changes in the material. Among other things, they control the permeability of coal, influence material degradation during TSA, and complicate the characterization of aerogels. In this work, we present a diffusion-based kinetic model that relies on classical density functional theory to calculate the required equilibrium properties and to account for intraparticle diffusion, external surface barriers, and heat effects. We measured a series of CO₂ uptakes on a carbon molecular sieve (Shirasagi MSC CT-350) at 293 K to verify our model and assess its predictive capabilities. Additionally, data from the literature were used to test the precision of the description of adsorption-induced deformation. We showed that our model can describe and predict both adsorption and deformation kinetic measurements. After the approach was validated, we studied the dependence of strain uptakes on PSD, mass transfer mechanisms, and temperature effects. Our results demonstrate the potential of the proposed approach and can be used to interpret experimental data on adsorption and deformation.

Nanoporous carbons play an important role in different electrochemical applications such as being utilized as electrodes in supercapacitors. Application of electric potential to a porous electrode in electrolyte solution stimulates adsorption or desorption of ions on the electrode surface. Electrosorption causes appearance of solvation pressure in the pores and results in electrode deformation. In this work, using molecular dynamics simulations and the continuum theory based on the modified Poisson-Boltzmann equation, we studied the structure of the electrical double layer in slit graphitic micropores filled with a NaCl aqueous solution, and solvation pressure in these pores. We focused on the behavior of the solvation pressure as a function of pore width and surface charge density. Within molecular dynamics simulations, two different water models were used -- an explicit model based on SPC/E water molecules and an implicit model, i.e., structureless background with fixed dielectric permittivity. The latter allows us to relate molecular dynamics simulation to the continuum theory. Simulations with explicit water show a qualitatively different behavior of the solvation pressure in the 1 and 2 nm pores as a function of the surface charge density. We demonstrated that the value of the solvation pressure is defined by a delicate balance between Van der Waals and electrostatic contributions. We demonstrated that the theory predicts the dependence of the solvation pressure on the pore width, which matches the results of simulations using the implicit water model. Finally, we adapted the continuum theory, developed for adsorption-induced deformation to estimate the deformation of a carbon electrode due to electrosorption. Our results can be used in the further development of nanoporous actuators working based on electrosorption-induced deformation.

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.

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.

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.