Effect of the Topology on Wetting and Drying of Hydrophobic Porous Materials

Establishing molecular mechanisms of wetting and drying of hydrophobic porous materials is a general problem for science and technology within the subcategories of the theory of liquids, chromatography, nanofluidics, energy storage, recuperation, and dissipation. In this article, we demonstrate a new way to tackle this problem by exploring the effect of the topology of pure silica nanoparticles, nanotubes, and zeolites. Using molecular dynamics simulations, we show how secondary porosity promotes the intrusion of water into micropores and affects the hydrophobicity of materials. It is demonstrated herein that for nano-objects, the hydrophobicity can be controlled by changing the ratio of open to closed nanometer-sized lateral pores. This effect can be exploited to produce new materials for practical applications when the hydrophobicity needs to be regulated without significantly changing the chemistry or structure of the materials. Based on these simulations and theoretical considerations, for pure silica zeolites, we examined and then classified the experimental database of intrusion pressures, thus leading to the prediction of any zeolite’s intrusion pressure. We show a correlation between the intrusion pressure and the ratio of the accessible pore surface area to total pore volume. The correlation is valid for some zeolites and mesoporous materials. It can facilitate choosing prospective candidates for further investigation and possible exploitation, especially for energy storage, recuperation, and dissipation

Subnanometer Topological Tuning of the Liquid Intrusion/Extrusion Characteristics of Hydrophobic Micropores

Intrusion (wetting)/extrusion (drying) of liquids in/from lyophobic nanoporous systems is key in many fields, including chromatography, nanofluidics, biology, and energy materials. Here we demonstrate that secondary topological features decorating main channels of porous systems dramatically affect the intrusion/extrusion cycle. These secondary features, allowing an unexpected bridging with liquid in the surrounding domains, stabilize the water stream intruding a micropore. This reduces the intrusion/extrusion barrier and the corresponding pressures without altering other properties of the system. Tuning the intrusion/extrusion pressures via subnanometric topological features represents a yet unexplored strategy for designing hydrophobic micropores. Though energy is not the only field of application, here we show that the proposed tuning approach may bring 20–75 MPa of intrusion/extrusion pressure increase, expanding the applicability of hydrophobic microporous materials

Stability and Structure of Hydrated Amorphous Calcium Carbonate

The results of molecular dynamics simulations of hydrated amorphous calcium carbonate (CaCO₃·<i>n</i>H₂O: ACC) are presented. ACC properties were investigated on atomistic, supramolecular, and thermodynamic levels. The clustering of water occluded in the ionic ACC framework was found to be well described by percolation theory, and with a percolation transition for water through ACC at a hydration level, <i>n</i>, of ca. 0.8. Percolation in ACC systems is quantitatively similar to site percolation on a simple cubic lattice where the percolation threshold is observed at <i>p</i>_<i>c</i> = 0.312. Predominantly fourfold tetrahedral molecular coordination of water molecules in the bulk liquid state is changed to sixfold connectivity in ACC. Kinetic stability of ACC is enhanced by dehydration and reaches maximal values when the water content is below the percolation threshold. The computed free energy shows a region of thermodynamic stability of hydrated ACC (1 < <i>n</i> < 6) with respect to calcite and pure water. This region is bounded by two crystallohydrates, monohydrocalcite (<i>n</i> = 1) and ikaite (<i>n</i> = 6), that have lower free energies than ACC. During dehydration at <i>n</i> < 1 the thermodynamic stability of ACC decreases, which favors the processes of nucleation and crystallization. On the other hand, water mobility within ACC also decreases during dehydration, thus making dehydration more difficult. So, the stability of hydrated ACC is controlled by a balance of two opposing factors: kinetics and thermodynamics

Water–Hydrophobic Zeolite Systems

Water intrusion–extrusion in hydrophobic microporous AFI, IFR, MTW and TON pure silica zeolites (zeosils) has been investigated through molecular dynamics (MD) simulations. It was found that intruded water volumes correlate with the free volume of the zeosil unit cells. Calculated adsorption isotherms allowed us to estimate the amounts of water intruded, and deviations from experiments (lower experimental with respect to calculated intrusion pressures) have been explained in terms of connectivity defects in the synthesized materials. Water phase transitions in defectless zeosils occur in a narrow range at high pressure. On the basis of a simple model, we derived a thermodynamic equation that allows one to estimate the intrusion pressure with few parameters, which are easy to obtain, such as fractional free volume of zeosil and the intrusion pressure of a reference system. The structural properties of water clusters inside the zeosil micropores have been interpreted from the analysis of the MD simulations. Compact “bulk-like” clusters form in large channels such as those in AFI and IFR zeosils. The smaller channels of MTW and TON promote the formation of chain-like clusters, which, interestingly, are commensurate with the zeolite channel topology due to a coincidence between the distances of the crystallographic parameter, along the channel, and a maximum in the O–O radial distribution function of bulk water

Water–Hydrophobic Zeolite Systems

Water intrusion–extrusion in hydrophobic microporous AFI, IFR, MTW and TON pure silica zeolites (zeosils) has been investigated through molecular dynamics (MD) simulations. It was found that intruded water volumes correlate with the free volume of the zeosil unit cells. Calculated adsorption isotherms allowed us to estimate the amounts of water intruded, and deviations from experiments (lower experimental with respect to calculated intrusion pressures) have been explained in terms of connectivity defects in the synthesized materials. Water phase transitions in defectless zeosils occur in a narrow range at high pressure. On the basis of a simple model, we derived a thermodynamic equation that allows one to estimate the intrusion pressure with few parameters, which are easy to obtain, such as fractional free volume of zeosil and the intrusion pressure of a reference system. The structural properties of water clusters inside the zeosil micropores have been interpreted from the analysis of the MD simulations. Compact “bulk-like” clusters form in large channels such as those in AFI and IFR zeosils. The smaller channels of MTW and TON promote the formation of chain-like clusters, which, interestingly, are commensurate with the zeolite channel topology due to a coincidence between the distances of the crystallographic parameter, along the channel, and a maximum in the O–O radial distribution function of bulk water