Mild-Temperature Supercritical Water Confined in Hydrophobic Metal-Organic Frameworks.

Fluids under extreme confinement show characteristics significantly different from those of their bulk counterpart. This work focuses on water confined within the complex cavities of highly hydrophobic metal-organic frameworks (MOFs) at high pressures. A combination of high-pressure intrusion-extrusion experiments with molecular dynamic simulations and synchrotron data reveals that supercritical transition for MOF-confined water takes place at a much lower temperature than in bulk water, ∼250 K below the reference values. This large shifting of the critical temperature (Tc) is attributed to the very large density of confined water vapor in the peculiar geometry and chemistry of the cavities of Cu2tebpz (tebpz = 3,3',5,5'-tetraethyl-4,4'-bipyrazolate) hydrophobic MOF. This is the first time the shift of Tc is investigated for water confined within highly hydrophobic nanoporous materials, which explains why such a large reduction of the critical temperature was never reported before, neither experimentally nor computationally.

Mechanism of Water Intrusion into Flexible ZIF-8: Liquid Is Not Vapor.

Zeolitic Imidazolate Frameworks (ZIF) find application in storage and dissipation of mechanical energy. Their distinctive properties linked to their (sub)nanometer size and hydrophobicity allow for water intrusion only under high hydrostatic pressure. Here we focus on the popular ZIF-8 material investigating the intrusion mechanism in its nanoscale cages, which is the key to its rational exploitation in target applications. In this work, we used a joint experimental/theoretical approach combining in operando synchrotron experiments during high-pressure intrusion experiments, molecular dynamics simulations, and stochastic models to reveal that water intrusion into ZIF-8 occurs by a cascade filling of connected cages rather than a condensation process as previously assumed. The reported results allowed us to establish structure/function relations in this prototypical microporous material, representing an important step to devise design rules to synthesize porous media.

Effect of Crystallite Size on the Flexibility and Negative Compressibility of Hydrophobic Metal-Organic Frameworks.

Flexible nanoporous materials are of great interest for applications in many fields such as sensors, catalysis, material separation, and energy storage. Of these, metal-organic frameworks (MOFs) are the most explored thus far. However, tuning their flexibility for a particular application remains challenging. In this work, we explore the effect of the exogenous property of crystallite size on the flexibility of the ZIF-8 MOF. By subjecting hydrophobic ZIF-8 to hydrostatic compression with water, the flexibility of its empty framework and the giant negative compressibility it experiences during water intrusion were recorded via in operando synchrotron irradiation. It was observed that as the crystallite size is reduced to the nanoscale, both flexibility and the negative compressibility of the framework are reduced by ∼25% and ∼15%, respectively. These results pave the way for exogenous tuning of flexibility in MOFs without altering their chemistries.

Effect of coarse graining in water models for the study of kinetics and mechanisms of clathrate hydrates nucleation and growth.

Clathrate hydrates are crystalline inclusion compounds wherein a water framework encages small guest atoms/molecules within its cavities. Among the others, methane clathrates are the largest fossil fuel resource still available. They can also be used to safely transport gases and can also form spontaneously under suitable conditions plugging pipelines. Understanding the crystallization mechanism is very important, and given the impossibility of experimentally identifying the atomistic path, simulations played an important role in this field. Given the large computational cost of these simulations, in addition to all-atom force fields, scientists considered coarse-grained water models. Here, we have investigated the effect of coarse-graining, as implemented in the water model mW, on the crystallization characteristics of methane clathrate in comparison with the all-atom TIP4P force field. Our analyses revealed that although the characteristics directly depending on the energetics of the water models are well reproduced, dynamical properties are off by the orders of magnitude. Being crystallization a non-equilibrium process, the altered kinetics of the process results in different characteristics of crystalline nuclei. Both TIP4P and mW water models produce methane clathrate nuclei with some amount of the less stable (in the given thermodynamic conditions) structure II phase and an excess of pentagonal dodecahedral cages over the tetrakaidecahedral ones regarding the ideal ratio in structure I. However, the dependence of this excess on the methane concentration in solution is higher with the former water model, whereas with the latter, the methane concentration in solution dependence is reduced and within the statistical error.

Classical nucleation of vapor between hydrophobic plates.

In this work, an extended classical nucleation theory (CNT), including line tension, is used to disentangle classical and non-classical effects in the nucleation of vapor from a liquid confined between two hydrophobic plates at a nanometer distance. The proposed approach allowed us to gauge, from the available simulation work, the importance of elusive nanoscale effects, such as line tension and non-classical modifications of the nucleation mechanism. Surprisingly, the purely macroscopic theory is found to be in quantitative accord with the microscopic data, even for plate distances as small as 2 nm, whereas in extreme confinement (<1.5 nm), the CNT approximations proved to be unsatisfactory. These results suggest how classical nucleation theory still offers a computationally inexpensive and predictive tool useful in all domains where nanoconfined evaporation occurs-including nanotechnology, surface science, and biology.

Hydrophobicity of molecular-scale textured surfaces: The case of zeolitic imidazolate frameworks, an atomistic perspective.

Hydrophobicity has proven fundamental in an inexhaustible amount of everyday applications. Material hydrophobicity is determined by chemical composition and geometrical characteristics of its macroscopic surface. Surface roughness or texturing enhances intrinsic hydrophilic or hydrophobic characteristics of a material. Here we consider crystalline surfaces presenting molecular-scale texturing typical of crystalline porous materials, e.g., metal-organic frameworks. In particular, we investigate one such material with remarkable hydrophobic qualities, ZIF-8. We show that ZIF-8 hydrophobicity is driven not only by its chemical composition but also its sub-nanoscale surface corrugations, a physical enhancement rare amongst hydrophobes. Studying ZIF-8's hydrophobic properties is challenging as experimentally it is difficult to distinguish between the materials' and the macroscopic corrugations' contributions to the hydrophobicity. The computational contact angle determination is also difficult as the standard "geometric" technique of liquid nanodroplet deposition is prone to many artifacts. Here, we characterise ZIF-8 hydrophobicity via: (i) the "geometric" approach and (ii) the "energetic" method, utilising the Young-Dupré formula and computationally determining the liquid-solid adhesion energy. Both approaches reveal nanoscale Wenzel-like bathing of the corrugated surface. Moreover, we illustrate the importance of surface linker termination in ZIF-8 hydrophobicity, which reduces when varied from sp3 N to sp2 N termination. We also consider halogenated analogues of the methyl-imidazole linker, which promote the transition from nanoWenzel-like to nanoCassie-Baxter-like states, further enhancing surface hydrophobicity. Present results reveal the complex interface physics and chemistry between water and complex porous, molecular crystalline surfaces, providing a hint to tune their hydrophobicity.

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.

Giant Negative Compressibility by Liquid Intrusion into Superhydrophobic Flexible Nanoporous Frameworks.

Materials or systems demonstrating negative linear compressibility (NLC), whose size increases (decreases) in at least one of their dimensions upon compression (decompression) are very rare. Materials demonstrating this effect in all their dimensions, negative volumetric compressibility (NVC), are exceptional. Here, by liquid porosimetry and in situ neutron diffraction, we show that one can achieve exceptional NLC and NVC values by nonwetting liquid intrusion in flexible porous media, namely in the ZIF-8 metal-organic framework (MOF). Atomistic simulations show that the volumetric expansion is due to the presence of liquid in the windows connecting the cavities of ZIF-8. This discovery paves the way for designing novel materials with exceptional NLC and NVC at reasonable pressures suitable for a wide range of applications.

Crystal-Size-Induced Band Gap Tuning in Perovskite Films.

A comprehensive picture explaining the effect of the crystal size in metal halide perovskite films on their opto-electronic characteristics is currently lacking. We report that perovskite nanocrystallites exhibit a wider band gap due to concurrent quantum confinement and size dependent structural effects, with the latter being remarkably distinct and attributed to the perturbation from the surface of the nanocrystallites affecting the structure of their core. This phenomenon might assist in the photo-induced charge separation within the perovskite in devices employing mesoporous layers as they restrict the size of nanocrystallites present in them. We demonstrate that the crystal size effect is widely applicable as it is ubiquitous in different compositions and deposition methods employed in the fabrication of state-of-the-art perovskite solar cells. This effect is a convenient and effective way to tune the band gap of perovskites.

Molecular Basis of CLC Antiporter Inhibition by Fluoride.

CLC channels and transporters conduct or transport various kinds of anions, with the exception of fluoride, which acts as an effective inhibitor. Here, we performed sub-nanosecond DFT-based QM/MM simulations of the E. coli anion/proton exchanger ClC-ec1 and observed that fluoride binds incoming protons within the selectivity filter, with excess protons shared with the gating glutamate E148. Depending on E148 conformation, the competition for the proton can involve either a direct F-/E148 interaction or the modulation of water molecules bridging the two anions. The direct interaction locks E148 in a conformation that does not allow for proton transport, and thus inhibits protein function.

Focus Article: Theoretical aspects of vapor/gas nucleation at structured surfaces.

Heterogeneous nucleation is the preferential means of formation of a new phase. Gas and vapor nucleation in fluids under confinement or at textured surfaces is central for many phenomena of technological relevance, such as bubble release, cavitation, and biological growth. Understanding and developing quantitative models for nucleation is the key to control how bubbles are formed and to exploit them in technological applications. An example is the in silico design of textured surfaces or particles with tailored nucleation properties. However, despite the fact that gas/vapor nucleation has been investigated for more than one century, many aspects still remain unclear and a quantitative theory is still lacking; this is especially true for heterogeneous systems with nanoscale corrugations, for which experiments are difficult. The objective of this focus article is analyzing the main results of the last 10-20 years in the field, selecting few representative works out of this impressive body of the literature, and highlighting the open theoretical questions. We start by introducing classical theories of nucleation in homogeneous and in simple heterogeneous systems and then discuss their extension to complex heterogeneous cases. Then we describe results from recent theories and computer simulations aimed at overcoming the limitations of the simpler theories by considering explicitly the diffuse nature of the interfaces, atomistic, kinetic, and inertial effects.

Mechanism of the Cassie-Wenzel transition via the atomistic and continuum string methods.

The string method is a general and flexible strategy to compute the most probable transition path for an activated process (rare event). We apply here the atomistic string method in the density field to the Cassie-Wenzel transition, a central problem in the field of superhydrophobicity. We discuss in detail the mechanism of wetting of a submerged hydrophobic cavity of nanometer size and its dependence on the geometry of the cavity. Furthermore, we analyze the algorithmic analogies between the continuum "interface" string method and CREaM [Giacomello et al., Phys. Rev. Lett. 109, 226102 (2012)], a method inspired by the string that allows for a faster and simpler computation of the mechanism and of the free-energy profiles of the wetting process.

Clathrate structure-type recognition: Application to hydrate nucleation and crystallisation.

For clathrate-hydrate polymorphic structure-type (sI versus sII), geometric recognition criteria have been developed and validated. These are applied to the study of the rich interplay and development of both sI and sII motifs in a variety of hydrate-nucleation events for methane and H2S hydrate studied by direct and enhanced-sampling molecular dynamics (MD) simulations. In the case of nucleation of methane hydrate from enhanced-sampling simulation, we notice that already at the transition state, ∼80% of the enclathrated CH4 molecules are contained in a well-structured (sII) clathrate-like crystallite. For direct MD simulation of nucleation of H2S hydrate, some sI/sII polymorphic diversity was encountered, and it was found that a realistic dissipation of the nucleation energy (in view of non-equilibrium relaxation to either microcanonical (NVE) or isothermal-isobaric (NPT) distributions) is important to determine the relative propensity to form sI versus sII motifs.

Massively parallel molecular dynamics simulation of formation of clathrate-hydrate precursors at planar water-methane interfaces: insights into heterogeneous nucleation.

The formation of methane-hydrate precursors at large planar water-methane interfaces has been studied using massively parallel molecular dynamics in systems of varying size from around 10 000 to almost 7 × 10(6) molecules. This process took two distinct steps. First, the concentration of solvated methane clusters increases just inside the aqueous domain via slow diffusion from the methane-water interface, forming "clusters" of solvated methane molecules. Second, the re-ordering process of solvated methane and water molecules takes place in a manner very roughly consistent with the "blob" hypothesis, although with important differences, to form hydrate precursors, necessary for subsequent hydrate nucleation and crystallisation. It was found that larger system sizes serve to promote the formation rate of precursors.

Relaxation of a steep density gradient in a simple fluid: comparison between atomistic and continuum modeling.

We compare dynamical nonequilibrium molecular dynamics and continuum simulations of the dynamics of relaxation of a fluid system characterized by a non-uniform density profile. Results match quite well as long as the lengthscale of density nonuniformities are greater than the molecular scale (~10 times the molecular size). In presence of molecular scale features some of the continuum fields (e.g., density and momentum) are in good agreement with atomistic counterparts, but are smoother. On the contrary, other fields, such as the temperature field, present very large difference with respect to reference (atomistic) ones. This is due to the limited accuracy of some of the empirical relations used in continuum models, the equation of state of the fluid in the present example.

Multiscale biomolecular simulations in the exascale era.

The complexity of biological systems and processes, spanning molecular to macroscopic scales, necessitates the use of multiscale simulations to get a comprehensive understanding. Quantum mechanics/molecular mechanics (QM/MM) molecular dynamics (MD) simulations are crucial for capturing processes beyond the reach of classical MD simulations. The advent of exascale computing offers unprecedented opportunities for scientific exploration, not least within life sciences, where simulations are essential to unravel intricate molecular mechanisms underlying biological processes. However, leveraging the immense computational power of exascale computing requires innovative algorithms and software designs. In this context, we discuss the current status and future prospects of multiscale biomolecular simulations on exascale supercomputers with a focus on QM/MM MD. We highlight our own efforts in developing a versatile and high-performance multiscale simulation framework with the aim of efficient utilization of state-of-the-art supercomputers. We showcase its application in uncovering complex biological mechanisms and its potential for leveraging exascale computing.

Tuning Wetting-Dewetting Thermomechanical Energy for Hydrophobic Nanopores via Preferential Intrusion.

Heat and the work of compression/decompression are among the basic properties of thermodynamic systems. Being relevant to many industrial and natural processes, this thermomechanical energy is challenging to tune due to fundamental boundaries for simple fluids. Here via direct experimental and atomistic observations, we demonstrate, for fluids consisting of nanoporous material and a liquid, one can overcome these limitations and noticeably affect both thermal and mechanical energies of compression/decompression exploiting preferential intrusion of water from aqueous solutions into subnanometer pores. We hypothesize that this effect is due to the enthalpy of dilution manifesting itself as the aqueous solution concentrates upon the preferential intrusion of pure water into pores. We suggest this genuinely subnanoscale phenomenon can be potentially a strategy for controlling the thermomechanical energy of microporous liquids and tuning the wetting/dewetting heat of nanopores relevant to a variety of natural and technological processes spanning from biomedical applications to oil-extraction and renewable energy.

Exploring the Heat of Water Intrusion into a Metal-Organic Framework by Experiment and Simulation.

Wetting of a solid by a liquid is relevant for a broad range of natural and technological processes. This process is complex and involves the generation of heat, which is still poorly understood especially in nanoconfined systems. In this article, scanning transitiometry was used to measure and evaluate the pressure-driven heat of intrusion of water into solid ZIF-8 powder within the temperature range of 278.15-343.15 K. The conditions examined included the presence and absence of atmospheric gases, basic pH conditions, solid sample origins, and temperature. Simultaneously with these experiments, molecular dynamics simulations were conducted to elucidate the changing behavior of water as it enters into ZIF-8. The results are rationalized within a temperature-dependent thermodynamic cycle. This cycle describes the temperature-dependent process of ZIF-8 filling, heating, emptying, and cooling with respect to the change of internal energy of the cycle from the calculated change in the specific heat capacity of the system. At 298 K the experimental heat of intrusion per gram of ZIF-8 was found to be -10.8 ± 0.8 J·g-1. It increased by 19.2 J·g-1 with rising temperature to 343 K which is in a reasonable match with molecular dynamic simulations that predicted 16.1 J·g-1 rise. From these combined experiments, the role of confined water in heat of intrusion of ZIF-8 is further clarified.

Bubbles enable volumetric negative compressibility in metastable elastocapillary systems.

Although coveted in applications, few materials expand when subject to compression or contract under decompression, i.e., exhibit negative compressibility. A key step to achieve such counterintuitive behaviour is the destabilisations of (meta)stable equilibria of the constituents. Here, we propose a simple strategy to obtain negative compressibility exploiting capillary forces both to precompress the elastic material and to release such precompression by a threshold phenomenon - the reversible formation of a bubble in a hydrophobic flexible cavity. We demonstrate that the solid part of such metastable elastocapillary systems displays negative compressibility across different scales: hydrophobic microporous materials, proteins, and millimetre-sized laminae. This concept is applicable to fields such as porous materials, biomolecules, sensors and may be easily extended to create unexpected material susceptibilities.

Geometry as a catalyst: how vapor cavities nucleate from defects.

The onset of cavitation is strongly enhanced by the presence of rough surfaces or impurities in the liquid. Despite decades of research, the way the geometry of these defects promote the nucleation of bubbles and its effect on the kinetics of the process remains largely unclear. We present here a comprehensive explanation of the catalytic action that roughness elements exert on the nucleation process for both pure vapor cavities and gas ones. This approach highlights that nucleation may follow nontrivial paths connected with a sharp decrease of the free energy barriers as compared to flat surfaces. Furthermore, we demonstrate the existence of intermediate metastable states that break the nucleation process in multiple steps; these states correspond to what is commonly known as cavitation nuclei. A single dimensionless parameter, the nucleation number, is found to control this rich phenomenology. The devised theory allows one to quantify the effect of the geometry and hydrophobicity of surface asperities on nucleation. Within the same framework, it is possible to treat both vapor cavitation, which is relevant, e.g., for organic liquids, and gas-promoted cavitation, which is commonly encountered in water. The theory is shown to be valid from the nano- to the macroscale.