Session B25: Behavior of Liquids Confined on the Nanometer Scale I

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Using Grand Canonical Monte Carlo and molecular-dynamics simulation, we examine the phase equilibrium and transport of a gas-expanded liquid under confinement. The system chosen is ethylene-expanded methanol confined in model silica mesopores, but in equilibrium with the bulk mixture -- a system that has received recent interest as a reaction medium, for example, for epoxidation of ethylene. This system was studied at 20^○C and pressures ranging from 5 to 55 bar. In addition, two pore surface chemistries were examined: a hydrophilic pore, in which the silica dangling bonds were terminated by -OH groups, and a model ``hydrophobic'' pore, in which the charges on the pore atoms (including the -OH groups) were turned off. We find that the pressure significantly affects the ethylene mole fraction in the confined mixture. Poresurface chemistry has a significant effect on the composition and transport properties of the confined ethylene-methanol mixture, relative to the bulk. In addition, there are significant qualitative differences between the hydrophilic and hydrophobic pores with regard to the spatial distributions of the confined ethylene and methanol.   

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The natural transition of supercooled liquid water into ice structure is one of the most major phase transitions in the universe. The existence of a critical ice nucleus results in water crystallization known as crystal growth, which is the framework of this study. Using TIP4P/ice atomistic water model, we investigate the crystallization dynamics of water in quasi-bulk and under nanoscale graphene confinements. It is found that the nanoscale texture of the substrate plays a significant role in affecting crystallization kinetics and hence the rate of crystal growth. We understood the presence of nanoscale textures with characteristic feature size smaller than 20 Å enables to impede water from crystallization. To model the crystallization freezing latent heat released at the ice-water interface, we conduct crystallization simulations under NVE ensemble, resemble to what is observed in experiments, during which graphene sheets are thermostated under NVT constraints. In these simulations, comparing to NVT crystallization simuations, freezing released latent heat is removed by the conductive heat transfer with graphene sheets.   

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In extreme confinement, in particular in nanopores, the phase behavior of liquids is dramatically altered, facilitating drying — the formation of a confined vapor phase. Here, the thermodynamics and kinetics of drying are discussed in the light of recent results based on mean field theories and atomistic simulations, clarifying the effect of hydrophobicity, geometry, size, and connectivity of nanopores. The emergence of nanoscale effects not accounted for in macroscopic theories is considered together with the open challenges in the field, which include the presence of dissolved gases and ions in solution. The relevance of such drying phenomena is discussed in different realms; in particular, in biological ion channels, drying of the hydrophobic pore may lead to the block of ionic currents across the membrane, which is known as hydrophobic gating; in solid-state nanoporous materials, drying should be controlled, e.g., in reversed-phase liquid chromatography and in energy applications of heterogeneous lyophobic systems.

A. Giacomello, R. Roth (2020). Adv. Phys.: X, 5, 1817780   

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Ethanol and water are permeable through carbon nanotubes (CNTs). However, in the case of water-ethanol mixtures, distinct layers (or domains) of water and ethanol form inside these nanotubes due to the higher affinity of the ethyl group of ethanol towards carbon [1] and the hydrophobicity of CNTs. Additionally, due to confinement effects on water [2] and ethanol, phase separation may occur depending on the pressure, temperature, and mixing entropy of liquids. In this study, we will analyze the phase separation and formation of domains in liquid mixtures, primarily of ethanol and water, occurring inside CNTs, using ab-initio molecular dynamics (AIMD) to capture both the quantum mechanical effects due to confinement of liquids and the dynamical evolution of these systems. Studying the phase separation of liquid mixtures under confinement may help develop membrane technology for efficient separation of miscible polar liquids and water.
[1] J. Chem. Phys. 145, 124708 (2016)
[2] J. Phys. Chem. C 2016, 120, 26, 14209–14215   

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Recent interest in confinement effects on water touches on diverse areas, ranging from chemical sensing and filtration to modeling of celestial water. Many studies report changes in water behavior when constrained inside carbon nanotubes (CNT) and/or boron nitride nanotubes (BNNT) such as straining of bonds, variance in phases, and transport properties. Our goal is to quantify the dynamics of water at the BNNT interface using in situ characterization at cryogenic temperatures in an aberration-corrected environmental scanning transmission electron microscope (ESTEM) equipped with a K-2 camera and Raman spectrometer. Water is hyper-quenched to liquid nitrogen temperatures below 100 K to allow for the formation of low density amorphous (LDA) ice, a phase of ice similar in structure to liquid water. High resolution images of interactions are then acquired while preserving the overall integrity of the original water structure. Furthermore, Raman spectroscopy was employed to measure the vibrational modes of ice to characterize the phase transitions. We will present a comparison of the structural evolutions of LDA ice with and without the presence of BNNT. Confinement effects of LDA ice inside nanotubes will be discussed.   

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Nanoporous materials are at the heart of numerous important applications: adsorption (gas sensing, drug delivery, chromatography), energy (hydrogen storage, fuel cells and batteries), environment (phase separation, water treatment, nuclear waste storage), Earth science (exchange between the soil and the atmosphere), etc. In this talk, I will present the state-of-the-art on adsorption/condensation and transport in nanoporous materials which possess pore sizes spanning several orders of magnitude (from the sub-nm scale to a few tens of nm).^1,2 I will first discuss the different adsorption regimes encountered when a fluid is set in contact with a porous material: from irreversible adsorption/capillary condensation for the largest pores to reversible/continuous filling for the smallest pores. We will see that simple thermodynamic modeling allows rationalizing these different regimes provided that a description of the shift of the critical point of the confined fluid is taken into account. Then, I will show how transport in such media can be described without having to rely on macroscopic concepts such as hydrodynamics.^3,4,5 Using parameters and coefficients available to experiments, we will see how transport coefficients can be rigorously obtained from simple models in the framework of Statistical Mechanics using models such as free volume theory and intermittent brownian motion.

¹ B. Coasne et al., Chem. Soc. Rev. 42, 4141 (2013).
² B. Coasne, New J. Chem. 40, 4078 (2016).
³ K. Falk et al., Nature Comm. 6, 6949 (2015).
⁴ A. Botan et al., Phys. Rev. E. 91, 032133 (2015).
⁵ T. Lee et al., Nature Comm. 7, 11890 (2016).   

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With the advent of robust techniques to apply electric fields in density-functional calculations, there has been continuous interest to use ab initio molecular dynamics (MD) simulations to study electrically triggered processes, such as electrochemical reactions. Here we introduce a novel approach to sample the canonical ensemble at constant temperature and applied electric potential. Our thermopotentiostat approach can be straightforwardly implemented into any density-functional code. Using thermopotentiostat MD simulations, we compute the dielectric constant of nano-confined water without any assumptions for the dielectric volume. These simulations reveal the existence of a dielectrically dead layer within interfacial water.   

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Drying of nanopores - formation of the vapor phase from the liquid phase in nano-confinement - plays a fundamental role in many different scenarios spanning from biology where ionic currents through ion channels can be blocked by the formation of a bubble inside them, to technology where, e.g., devices for mechanical energy storage can be based on drying properties of nanoporous materials. The presence of hydrophobic solutes in the liquid phase can dramatically modify the kinetics of the drying process. We investigate the role of a dilute hydrophobic gas on the phase behavior of water confined in hydrophobic cylindrical nanopores with diameter 14 Å [1]. With advanced molecular dynamics simulations we characterize the drying free-energy landscape of the systems and we show that a single hydrophobic atom inside the nanopore can decrease and in some cases totally abate the drying free-energy barrier causing the instantaneous drying of the whole nanopore. The identified mechanism provides a possible explanation for the action of volatile anesthetics and strategy for the design of efficient technological devices.
[1] G. Camisasca, A. Tinti, A. Giacomello. Gas-Induced Drying of Nanopores. JPCL 11 (2020), 9171-9177   

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Isolated single-walled carbon nanotubes with diameters between 0.8 and 2.5 nm offer unprecedented opportunities to study phase behavior and dynamics of water and other fluids under nanoscale confinement. Within this size regime of so-called single digit nanopores, fluid phase boundaries are shifted dramatically, slip flow generates huge enhancements over flow predictions, and physical properties like the dielectric constant may vary widely and non-monotonically with confining diameter. Here, we introduce a new experimental platform for lithographic segmentation of carbon nanotubes, generating multiple copies – some empty, some filled, and others partly filled – of the same type of carbon nanotube. We develop Raman spectroscopy techniques that allow unambiguous assignment of carbon nanotubes in real time as either filled or empty at various points along their length. Analysis of carbon nanotube filling under a variety of conditions allows us to address several key questions about the physical chemistry of fluids under nanoscale confinement, including water phase behavior, the formation of nanoscale droplets, comparisons between filling with water and non-aqueous fluids, and the dynamics of capillary condensation inside a single carbon nanotube.   

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Surface thermodynamics has a long rich history with subtleties that still drive research over a century after the foundational work of Gibbs. In recent work, we have examined, using molecular simulation, the role of surface curvature on the surface free energy, $\gamma$, of fluids at convex bounding walls. Here, we extend this work to examine the surface thermodynamics of fluids under confinement by closed curved walls. We present simulation results that demonstrate the dependence of $\gamma$ on the bounding curvature in 2-dimensions, using a model hard-disk fluid contained within circular hard walls of varying size (and curvature). These results are compared to our recent work for the hard-disk fluid at convex circular walls. We also compare to the theoretical predictions of Morphometric Thermodynamics (MT), which gives an expression for the surface free energy that is a linear function of the mean curvature of the surface. While MT is formulated as a theory restricted to convex wall geometries, its applicability to convex surfaces, such as those considered here has never been tested.   

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The structure and properties of deeply super-cooled water have intrigued researchers for many years, and confinement within mesoscopic pores has emerged as a common strategy to study the properties of liquid water in this temperature range. Here, we monitor the FTIR spectrum of HOD as a probe of the structure of super-cooled water confined in mesoporous silica gel. The range of pore sizes investigated spans a reported transition in pore-filling mechanism from axial pore filling, condensation in the full pore followed by extension of the filled section, to radial pore filling where surface layers thicken until full condensation occurs. In the super-cooled range the influence of pore size on the local structure of water was apparent. Evidence of an LDL-like phase with a broad liquid-like OD peak near 2435 cm^-1 was observed in all pore sizes except the smallest pores, 2.2 nm (in contrast to MCM-41 where LDL phase was reported in narrower pores). We explain our findings based on disorder along the pores which limits cooperative interactions of the hydrogen bond network that are required for conversion to an LDL-like phase. This work demonstrates that the occurrence of a liquid-liquid phase transition for water confined in mesoporous materials strongly depends on the pore morphology.