Perspective: Dynamics of confined liquids

Fluorescence Enhancement of Nonemissive Monodeprotonated Luteolin in a Poly(vinyl alcohol) Film

Solid polymer matrixes can modulate the electronic states of embedded chromophores and have been widely used in flexible optoelectronic and optical materials. Luteolin is one of the most common natural flavonoids, and its neutral and monodeprotonated forms are nonemissive in aqueous solution induced by ultrafast excited-state proton transfer (ESPT) followed by nonradiative relaxation. In this study, we have incorporated luteolin into poly(vinyl alcohol) (PVA) films and studied their fluorescence behaviors. Neutral and one monodeprotonated luteolin coexist in the PVA film. Weak steady-state fluorescence of neutral luteolin peaking at about 440 nm is observed for the first time. In addition, the monodeprotonated luteolin in PVA film exhibits obvious fluorescence peaking at 500 nm, with a fluorescence quantum yield of as high as 0.4 and a fluorescence lifetime of as long as 2.4 ns. Time-dependent density functional theory calculations have determined that the ESPT of neutral luteolin is barrierless but that of monodeprotonated luteolin needs to surmount a barrier, explaining their distinct emission properties. These results indicate the modulation ability of the PVA film in both ground-state deprotonation and ESPT, broadening the application areas of the solid polymer matrix.

Structure and Dynamics of Water in Polysaccharide (Alginate) Solutions and Gels Explained by the Core-Shell Model

In both biological and engineered systems, polysaccharides offer a means of establishing structural stiffness without altering the availability of water. Notable examples include the extracellular matrix of prokaryotes and eukaryotes, artificial skin grafts, drug delivery materials, and gels for water harvesting. Proper design and modeling of these systems require detailed understanding of the behavior of water confined in pores narrower than about 1 nm. We use molecular dynamics simulations to investigate the properties of water in solutions and gels of the polysaccharide alginate as a function of the water content and polymer cross-linking. We find that a detailed understanding of the nanoscale dynamics of water in alginate solutions and gels requires consideration of the discrete nature of water. However, we also find that the trends in tortuosity, permeability, dielectric constant, and shear viscosity can be adequately represented using the "core-shell" conceptual model that considers the confined fluid as a continuum.

Confinement Effects on Reorientation Dynamics of Water Confined within Graphite Nanoslits

Molecular dynamics simulations were used to investigate the reorientation dynamics of water confined within graphite nanoslits of size less than 2 nm, where molecules formed inner and interfacial layers parallel to the confining walls. Significantly related to molecular reorientations, the hydrogen-bond (HB) network of nanoconfined water therein was scrutinized by HB configuration fractions compared to those of bulk water and the influences on interfacial-molecule orientations relative to a nearby C atom plate. The second-rank orientation time correlation functions (OTCFs) of nanoconfined water were calculated and found to follow stretched-exponential, power-law, and power-law decays in a time series. To understand this naïve behavior of reorientation relaxation, the approach of statistical mechanics was adopted in our studies. In terms of the orientation Van Hove function (OVHF), an alternative meaning was given to the second-rank OTCF, which is a measure of the deviation of the OVHF between a molecular system and free molecules in random orientations. Indicated by the OVHFs at related time scales, the stretched-exponential decay of the second-rank OTCF resulted from molecules evacuating out of HB cages formed by their neighbors. After the evacuations, the inner molecules relaxed at relatively fast rates toward random orientations, but the interfacial molecules reoriented at slow rates due to restrictions by hydrophobic interactions with graphite walls. The first power-law decay of the second-rank OTCF was attributed to the distinct relaxation rates of inner and interfacial molecules within a graphite nanoslit. When the inner molecules were completely random in orientation, the second-rank OTCFs changed to another power law decay with a power smaller than the first one.

Trp-cage-1,1,1,3,3,3-Hexafluoro-2-propanol-Water Interactions in a Nanosphere at 298 K

MD simulations of the peptide Trp-cage dissolved in a solvent composed of 28% 1,1,1,3,3,3-hexafluoroisopropanol-water and contained within a nanosphere of 4.2 nm radius are described. To provide a thermal buffer, the nanosphere is submerged in a collection of liquid neopentane molecules, modeled as united atoms. It was found that the HFIP-water mixture demixes when confined under these conditions, with most of the fluoroalcohol becoming strongly associated with the walls of the nanosphere. The remaining HFIP interacts with the peptide and itself in the interior of the nanoshell. Diffusion of HFIP molecules near the surface of the nanoshell is limited, taking place over a small volume, while diffusion of the fluoroalcohol in the center of the nanoshell is more wide-ranging. By contrast, diffusion of the water in the solvent mixture appears to take place equivalently throughout the shell. The conformation of the peptide, distribution of solvent components around it, and the number and duration of the contacts with solvent molecules are calculated and contrasted to results previously reported for the bulk (non-nanocontained) system.

Quantitative Understanding of Liquid Dynamics at Interfaces from a Free-Energy Landscape Perspective

On the basis of molecular dynamics simulations of water and ethanol in nanopores, we devise a methodology to determine the free-energy landscape (FEL) imposed by an interface on an adjoining liquid directly from the particle trajectories. The methodology merely uses the statistical mechanical relation between occupancy and energy and, hence, is particularly suitable in complex situations, e.g., for disordered or rough atomistic interfaces and molecular liquids, as encountered in many biological, geological, and technological situations. Moreover, we show that the thus-obtained FEL enables a quantitative understanding of interface effects on liquid dynamics. Specifically, by determining the local minima and barriers of the FEL and using an Arrhenius-like relation, we reproduce the very strong spatial variation of the structural relaxation time of water and ethanol across nanopores over a broad temperature range. We anticipate that the proposed FEL approach is transferable to various other liquids and interfaces.

Probing the Behaviour of Fluids Confined in Porous Materials by Neutron Scattering: Applications to CO2 Sequestration and Enhanced Oil and Gas Recovery

The current review presents a discussion on the utility of neutron scattering, with emphasis on neutron total scattering and small-angle neutron scattering (SANS), to explore the structural properties and the phase behaviour of fluids confined in nanopores. The effectiveness of contrast matching SANS on the evaluation of accessibility of porous materials to invading fluids is highlighted too. This review provides also an overview regarding the neutron scattering studies on the structure and the accessibility of greenhouse gases in the complex pore network of geomaterials, with applications to CO2 geological sequestration and enhanced oil and gas recovery.

Effects of Nanoconfinement on Dynamics in Concentrated Aqueous Magnesium Chloride Solutions

Water behavior in various natural and manufactured settings is influenced by confinement in organic or inorganic frameworks and the presence of solutes. Here, the effects on dynamics from both confinement and the addition of solutes are examined. Specifically, water and ion dynamics in concentrated (2.5-4.2 m) aqueous magnesium chloride solutions confined in mesoporous silica (2.8 nm pore diameter) were investigated using polarization selective pump-probe and 2D infrared spectroscopies. Fitting the rotational and spectral diffusion dynamics measured by the vibrational probe, selenocyanate, with a previously developed two-state model revealed distinct behaviors at the interior of the silica pores (core state) and near the wall of the confining framework (shell state). The shell dynamics are noticeably slower than the bulk, or core, dynamics. The concentration-dependent slowing of the dynamics aligns with behavior in the bulk solutions, but the spectrally separated water-associated and Mg2+-associated forms of the selenocyanate probe exhibit different responses to confinement. The disparity in the complete reorientation times is larger upon confinement, but the spectral diffusion dynamics become more similar near the silica surface. The length scales that characterize the transition from surface-influenced to bulk-like behavior for the salt solutions in the pores are discussed and compared to those of pure water and an organic solvent confined in the same pores. These comparisons offer insights into how confinement modulates the properties of different liquids.

The internal structure of gadolinium and perfluorocarbon-loaded polymer nanoparticles affects 19F MRI relaxation times

19F magnetic resonance imaging (19F MRI) is an emerging technique for quantitative imaging in novel therapies, such as cellular therapies and theranostic nanocarriers. Nanocarriers loaded with liquid perfluorocarbon (PFC) typically have a (single) core-shell structure with PFC in the core due to the poor miscibility of PFC with organic and inorganic solvents. Paramagnetic relaxation enhancement acts only at a distance of a few angstroms. Thus, efficient modulation of the 19F signal is possible only with fluorophilic PFC-soluble chelates. However, these chelates cannot interact with the surrounding environment and they might result in image artifacts. Conversely, chelates bound to the nanoparticle shell typically have a minimal effect on the 19F signal and a strong impact on the aqueous environment. We show that the confinement of PFC in biodegradable polymeric nanoparticles (NPs) with a multicore structure enables the modulation of longitudinal (T1) and transverse (T2) 19F relaxation, as well as proton (1H) signals, using non-fluorophilic paramagnetic chelates. We compared multicore NPs versus a conventional single core structure, where the PFC is encapsulated in the core(s) and the chelate in the surrounding polymeric matrix. This modulated relaxation also makes multicore NPs sensitive to various acidic pH environments, while preserving their stability. This effect was not observed with single core nanocapsules (NCs). Importantly, paramagnetic chelates affected both T1 and T219F relaxation in multicore NPs, but not in single core NCs. Both relaxation times of the 19F nucleus were enhanced with an increasing concentration of the paramagnetic chelate. Moreover, as the polymeric matrix remained water permeable, proton enhancement additionally was observed in MRI.

Machine Learning Predictions of Simulated Self-Diffusion Coefficients for Bulk and Confined Pure Liquids

Diffusion properties of bulk fluids have been predicted using empirical expressions and machine learning (ML) models, suggesting that predictions of diffusion also should be possible for fluids in confined environments. The ability to quickly and accurately predict diffusion in porous materials would enable new discoveries and spur development in relevant technologies such as separations, catalysis, batteries, and subsurface applications. In this work, we apply artificial neural network (ANN) models to predict the simulated self-diffusion coefficients of real liquids in both bulk and pore environments. The training data sets were generated from molecular dynamics (MD) simulations of Lennard-Jones particles representing a diverse set of 14 molecules ranging from ammonia to dodecane over a range of liquid pressures and temperatures. Planar, cylindrical, and hexagonal pore models consisted of walls composed of carbon atoms. Our simple model for these liquids was primarily used to generate ANN training data, but the simulated self-diffusion coefficients of bulk liquids show excellent agreement with experimental diffusion coefficients. ANN models based on simple descriptors accurately reproduced the MD diffusion data for both bulk and confined liquids, including the trend of increased mobility in large pores relative to the corresponding bulk liquid.

Spin-lattice relaxation time in water/graphene-oxide dispersion

We present the results of the calculations of the spin-lattice relaxation time of water in contact with graphene oxide by means of all-atom molecular dynamics simulations. We fully characterized the water-graphene oxide interaction through the calculation of the relaxation properties of bulk water and of the contact angle as a function of graphene oxide oxidation state and comparing them with the available experimental data. We then extended the calculation to investigate how graphene oxide alters the dynamical and relaxation properties of water in different conditions and concentrations. We show that, despite the diamagnetic nature of the graphene oxide, the confining effects of the bilayers strongly affect the longitudinal relaxation properties of interfacial water, which presents a reduced dynamics due to hydrogen bonds with oxygen groups on graphene oxide. This property makes graphene oxide an interesting platform to investigate water dynamics in confined geometries and an alternative contrast-agent for magnetic resonance imaging applications, especially in view of the possibility to functionalize graphene oxide from theranostic perspectives.

Unusual microwave heating of water in reverse micellar solution

Microwaves (MWs) are widely used for heating food, accelerating chemical reactions, drying materials, therapies, and so on. Water molecules absorb MWs and produce heat because of their substantial electric dipole moments. Recently, increasing attention has been paid to accelerating various catalytic reactions in water-containing porous materials using MW irradiation. Here, a critical question is whether water in nanoscale pores generates heat in the same way as liquid water. Is it valid that MW-heating behaviors of nanoconfined water are estimated solely by a dielectric constant of liquid water? There are almost no studies regarding this question. Here, we address it using reverse micellar (RM) solutions. Reverse micelles are water-containing nanoscale cages formed by self-assembled surfactant molecules in oil. We measured real-time temperature changes of liquid samples within a waveguide under MW irradiation at 2.45 GHz and at MW intensities of ~ 3 to ~ 12 W/cm². We found that the heat production and its rate per unit volume of water in the RM solution are about one order of magnitude larger than those of liquid water at all the MW intensities examined. This indicates that water spots that are much hotter than liquid water under MW irradiation at the same intensity, are formed in the RM solution. Our findings will give fundamental information to develop effective and energy-saving chemical reactions in nanoscale reactors with water under MW irradiation, and to study MW effects on various aqueous mediums with nanoconfined water. Furthermore, the RM solution will serve as a platform to study the impact of nanoconfined water on MW-assisted reactions.

Nanoconfinement facilitates reactions of carbon dioxide in supercritical water

The reactions of CO2 in water under extreme pressure-temperature conditions are of great importance to the carbon storage and transport below Earth's surface, which substantially affect the carbon budget in the atmosphere. Previous studies focus on the CO2(aq) solutions in the bulk phase, but underground aqueous solutions are often confined to the nanoscale, and nanoconfinement and solid-liquid interfaces may substantially affect chemical speciation and reaction mechanisms, which are poorly known on the molecular scale. Here, we apply extensive ab initio molecular dynamics simulations to study aqueous carbon solutions nanoconfined by graphene and stishovite (SiO2) at 10 GPa and 1000 ~ 1400 K. We find that CO2(aq) reacts more in nanoconfinement than in bulk. The stishovite-water interface makes the solutions more acidic, which shifts the chemical equilibria, and the interface chemistry also significantly affects the reaction mechanisms. Our findings suggest that CO2(aq) in deep Earth is more active than previously thought, and confining CO2 and water in nanopores may enhance the efficiency of mineral carbonation.

Confinement Effects on Distribution and Transport of Neutral Solutes in a Small Hydrophobic Nanopore

Molecular dynamics simulations are used to study confinement effects in small cylindrical silica pores with extended hydrophobic surface functionalization as realized, for example, in reversed-phase liquid chromatography (RPLC) columns. In particular, we use a 6 nm cylindrical and a 10 nm slit pore bearing the same C₁₈ stationary phase to compare the conditions inside the smaller-than-average pores within an RPLC column to column-averaged properties. Two small, neutral, apolar to moderately polar solutes are used to assess the consequences of spatial confinement for typical RPLC analytes with water (W)-acetonitrile (ACN) mobile phases at W/ACN ratios between 70/30 and 10/90 (v/v). The simulated data show that true bulk liquid behavior, as observed over an extended center region in the 10 nm slit pore, is not recovered within the 6 nm cylindrical pore. Instead, the ACN-enriched solvent layer around the C₁₈ chain ends (the ACN ditch), a general feature of hydrophobic interfaces equilibrated with aqueous-organic liquids, extends over the entire pore lumen of the small cylindrical pore. This renders the entire pore a highly hydrophobic environment, where, contrary to column-averaged behavior, neither the local nor the pore-averaged sorption and diffusion of analytes scales directly with the W/ACN ratio of the mobile phase. Additionally, the solute polarity-related discrimination between analytes is enhanced. The consequences of local ACN ditch overlap in RPLC columns are reminiscent of ion transport in porous media with charged surfaces, where electrical double-layer overlap occurring locally in smaller pores leads to discrimination between co- and counterionic species.

Measuring Protein Conformation at Aqueous Interfaces with 2D Infrared Spectroscopy of Emulsions

Determining the secondary and tertiary structures of proteins at aqueous interfaces is crucial for understanding their function, but measuring these structures selectively at the interface is challenging. Here we demonstrate that two-dimensional infrared (2D-IR) spectroscopy of protein stabilized emulsions offers a new route to measuring interfacial protein structure with high levels of detail. We prepared hexadecane/water oil-in-water emulsions stabilized by model LK peptides that are known to fold into either α-helix or β-sheet conformations at hydrophobic interfaces and measured 2D-IR spectra in a transmission geometry. We saw clear spectral signatures of the peptides folding at the interface, with no detectable residue from remaining bulk peptides. Using 2D spectroscopy gives us access to correlation and dynamics data, which enables structural assignment in cases where linear spectroscopy fails. Using the emulsions allows one to study interfacial spectra with standard transmission geometry spectrometers, bringing the richness of 2D-IR to the interface with no additional optical complexity.

Pore size effects on surface charges and interfacial electrostatics of mesoporous silicas

Water in confinement becomes more structured than bulk water, and its properties, such as the dielectric constant, change. It remains unclear, however, how the interfacial reactions in confinement, such as the adsorption of ions on the surfaces of small pores, differ from those in larger spaces. We focused on the deprotonation reaction of hydroxyl groups, a fundamental surface reaction, and investigated the dependence of the surface charge density on pore size by determining the surface charge densities of six types of mesoporous silicas with micropores and mesopores at different ionic strengths and pH levels from batch titration tests. The surface complexation model assuming a potential distribution based on the Poisson-Boltzmann equation in cylindrical coordinates was fitted to the obtained surface charge densities to relate the electrostatics near the surface to the surface reaction. The results showed that the absolute values of the surface charge densities decreased with decreasing pore diameter due to the overlap of the electrical double layers. Furthermore, the capacitance of the Stern layer optimized by fitting decreased with decreasing pore diameter, especially in pores smaller than 4 nm in diameter, which suggested that the dielectric constants of water decreased near the surfaces of small pores.

Extended Wertheim theory predicts the anomalous chain length distributions of divalent patchy particles under extreme confinement

Colloidal patchy particles with divalent attractive interaction can self-assemble into linear polymer chains. Their equilibrium properties in 2D and 3D are well described by Wertheim's thermodynamic perturbation theory which predicts a well-defined exponentially decaying equilibrium chain length distribution. In experi- mental realizations, due to gravity, particles sediment to the bottom of the suspension forming a monolayer of particles with a gravitational height smaller than the particle diameter. In accordance with experiments, an anomalously high monomer concentration is observed in simulations which is not well understood. To account for this observation, we interpret the polymerization as taking place in a highly confined quasi-2D plane and extend the Wertheim thermodynamic perturbation theory by defining addition reactions constants as functions of the chain length. We derive the theory, test it on simple square well potentials, and apply it to the experimental case of synthetic colloidal patchy particles immersed in a binary liquid mixture that are described by an accurate effective critical Casimir patchy particle potential. The important interaction parameters entering the theory are explicitly computed using the integral method in combination with Monte Carlo sampling. Without any adjustable parameter, the predictions of the chain length distribution are in excellent agreement with explicit simulations of self-assembling particles. We discuss generality of the approach, and its application range.

Confinement of molecular materials using a solid-state loading method: a route for exploring new physical states and their subsequent transformation highlighted by caffeine confined to SBA-15 pores

Using the innovative solid-state loading (milling-assisted loading, MAL) method to confine caffeine to cylindrical pores (SBA-15, ∅ = 6 nm) gives the opportunity to explore the original physical states of caffeine and their subsequent transformation using low-frequency Raman spectroscopy, powder X-ray diffraction and microcalorimetry investigations. It was shown that MAL makes possible the loading of the selected form in the polymorphism of caffeine. While form II has similar structural and dynamics properties in confined and bulk forms, the confined rotator phase (form I) exhibits clear differences with the bulk form inherent to its orientational disorder. Interestingly, the two confined forms of caffeine undergo an exothermic disordering transformation upon heating into a physical state at the border between a nanocrystallized orientationally disordered phase and an amorphous state, not existing in the bulk form. The melting of this new physical state was observed at 150 °C, i.e. 85 degrees below the melting temperature of the bulk form I, thus demonstrating the confinement of caffeine. It was also found that the liquid confined to pores of 6 nm mean diameter recrystallizes upon cooling, which can be explained by the very disordered nature of the recrystallized state.

Altered relaxation dynamics of excited state reactions by confinement in reverse micelles probed by ultrafast fluorescence up-conversion

Chemical reactions in confined environments are important in areas as diverse as heterogenous catalysis, environmental chemistry and biochemistry, yet they are much less well understood than the equivalent reactions in either the gas phase or in free solution. The understanding of chemical reactions in solution was greatly enhanced by real time studies of model reactions, through ultrafast spectroscopy (especially when supported by molecular dynamics simulation). Here we review some of the efforts that have been made to adapt this approach to the investigation of reactions in confined media. Specifically, we review the application of ultrafast fluorescence spectroscopy to measure reaction dynamics in the nanoconfined water phase of reverse micelles, as a function of the droplet radius and the charge on the interface. Methods of measurement and modelling of the reactions are outlined. In all of the cases studied (which are focused on ultrafast intramolecular reactions) the effect of confinement was to suppress the reaction. Even in the largest micelles the result in the bulk aqueous phase was not usually recovered, suggesting an important role for specific interactions between reactant and environment, for example at the interface. There was no simple one-to-one correspondence with direct measures of the dynamics of the confined phase. Thus, understanding the effect of confinement on reaction rate appears to require not only knowledge of the dynamics of the reaction in solutions and the effect of confinement on the medium, but also of the interaction between reactant and confining medium.

Consequences of Cylindrical Pore Geometry for Interfacial Phenomena in Reversed-Phase Liquid Chromatography

The interfacial phenomena behind analyte separation in a reversed-phase liquid chromatography column take place nearly exclusively inside the silica mesopores. Their cylindrical geometry can be expected to shape the properties of the chromatographic interface with consequences for the analyte density distribution and diffusivity. To investigate this topic through molecular dynamics simulations, we introduce a cylindrical pore inside a slit pore configuration, where the inner curved and outer planar silica surface bear the same bonded phase. The present model replicates an average-sized (9 nm) mesopore in an endcapped C₁₈ column equilibrated with a mobile phase of 70/30 (v/v) water/acetonitrile. Simulations performed for ethylbenzene and acetophenone show that the surface curvature shifts the bonded phase and analyte density toward the pore center, decreases the solvent density in the bonded-phase region, increases the acetonitrile excess in the interfacial region, and considerably enhances the surface diffusivity of both analytes. Overall, the cylindrical pore provides a more hydrophobic environment than the slit pore. Ethylbenzene density is decidedly increased in the cylindrical pore, whereas acetophenone density is nearly equally distributed between the cylindrical and slit pore. The cylindrical pore geometry thus sharpens the discrimination between the apolar and moderately polar analytes while enhancing the mass transport of both.

Transferable Gaussian Attractive Potentials for Organic/Oxide Interfaces

Organic/oxide interfaces play an important role in many areas of chemistry and in particular for lubrication and corrosion. Molecular dynamics simulations are the method of choice for providing complementary insight to experiments. However, the force fields used to simulate the interaction between molecules and oxide surfaces tend to capture only weak physisorption interactions, discarding the stabilizing Lewis acid/base interactions. We here propose a simple complement to the straightforward molecular mechanics description based on "out-of-the-box" Lennard-Jones potentials and electrostatic interactions: the addition of an attractive Gaussian potential between reactive sites of the surface and heteroatoms of adsorbed organic molecules, leading to the Gaussian Lennard-Jones (GLJ) potential. The interactions of four oxygenated and four amine molecules with the typical and widespread hematite and γ-alumina surfaces are investigated. The root mean square deviation (RMSD) for all probed molecules is only 5.7 kcal/mol, which corresponds to an error of 23% over hematite. On γ-alumina, the RMSD is 11.2 kcal/mol using a single parameter for all five chemically inequivalent surface aluminum atoms. Applying GLJ to the simulation of organic films on oxide surfaces demonstrates that the mobility of the surfactants is overestimated by the simplistic LJ potential, while GLJ and other qualitatively correct potentials show a strong structuration and slow dynamics of the surface films, as could be expected from the first-principles adsorption energies for model head groups.

Disjoining Pressure of Water in Nanochannels

The disjoining pressure of water was estimated from wicking experiments in 1D silicon dioxide nanochannels of heights of 59, 87, 124, and 1015 nm. The disjoining pressure was found to be as high as ∼1.5 MPa while exponentially decreasing with increasing channel height. Such a relation resulting from the curve fitting of experimentally derived data was implemented and validated in computational fluid dynamics. The implementation was then used to simulate bubble nucleation in a water-filled 59 nm nanochannel to determine the nucleation temperature. Simultaneously, experiments were conducted by nucleating a bubble in a similar 58 nm nanochannel by laser heating. The measured nucleation temperature was found to be in excellent agreement with the simulation, thus independently validating the disjoining pressure relation developed in this work. The methodology implemented here integrates experimental nanoscale physics into continuum simulations thus enabling numerical study of various phenomena where disjoining pressure plays an important role.

Ultrafast Dynamics and Liquid Structure in Mesoporous Silica: Propagation of Surface Effects in a Polar Aprotic Solvent

Enhancement of processes ranging from gas sorption to ion conduction in a liquid can be substantial upon nanoconfinement. Here, the dynamics of a polar aprotic solvent, 1-methylimidazole (MeIm), in mesoporous silica (2.8, 5.4, and 8.3 nm pore diameters) were examined using femtosecond infrared vibrational spectroscopy and molecular dynamics simulations of a dilute probe, the selenocyanate (SeCN^-) anion. The long vibrational lifetime and sensitivity of the CN stretch enabled a comprehensive investigation of the relatively slow time scales and subnanometer distance dependences of the confined dynamics. Because MeIm does not readily donate hydrogen bonds, its interactions in the hydrophilic silanol pores differ more from the bulk than those of water confined in the same mesopores, resulting in greater structural order and more dramatic slowing of dynamics. The extent of surface effects was quantified by modified two-state models used to fit three spatially averaged experimental observables: vibrational lifetime, orientational relaxation, and spectral diffusion. The length scales and the models (smoothed step, exponential decay, and simple step) describing the transitions between the distinctive shell behavior at the surface and the bulk-like behavior at the pore interior were compared to those of water. The highly nonuniform distributions of the SeCN^- probe and antiparallel layering of MeIm revealed by the simulations guided the interpretation of the results and development of the analytical models. The results illustrate the importance of electrostatic effects and H-bonding interactions in the behavior of confined liquids.

Advanced Materials for Energy-Water Systems: The Central Role of Water/Solid Interfaces in Adsorption, Reactivity, and Transport

The structure, chemistry, and charge of interfaces between materials and aqueous fluids play a central role in determining properties and performance of numerous water systems. Sensors, membranes, sorbents, and heterogeneous catalysts almost uniformly rely on specific interactions between their surfaces and components dissolved or suspended in the water-and often the water molecules themselves-to detect and mitigate contaminants. Deleterious processes in these systems such as fouling, scaling (inorganic deposits), and corrosion are also governed by interfacial phenomena. Despite the importance of these interfaces, much remains to be learned about their multiscale interactions. Developing a deeper understanding of the molecular- and mesoscale phenomena at water/solid interfaces will be essential to driving innovation to address grand challenges in supplying sufficient fit-for-purpose water in the future. In this Review, we examine the current state of knowledge surrounding adsorption, reactivity, and transport in several key classes of water/solid interfaces, drawing on a synergistic combination of theory, simulation, and experiments, and provide an outlook for prioritizing strategic research directions.

Confinement-Controlled Aqueous Chemistry within Nanometric Slit Pores

In this Focus Review, we put the spotlight on very recent insights into the fascinating world of wet chemistry in the realm offered by nanoconfinement of water in mechanically rather rigid and chemically inert planar slit pores wherein only monolayer and bilayer water lamellae can be hosted. We review the effect of confinement on different aspects such as hydrogen bonding, ion diffusion, and charge defect migration of H⁺(aq) and OH^-(aq) in nanoconfined water depending on slit pore width. A particular focus is put on the strongly modulated local dielectric properties as quantified in terms of anisotropic polarization fluctuations across such extremely confined water films and their putative effects on chemical reactions therein. The stunning findings disclosed only recently extend wet chemistry in particular and solvation science in general toward extreme molecular confinement conditions.

Diffusion of confined fluids in microporous zeolites and clay materials

Fluids exhibit remarkable variation in their structural and dynamic properties when they are confined at the nanoscopic scale. Various factors, including geometric restriction, the size and shape of the guest molecules, the topology of the host, and guest-host interactions, are responsible for the alterations in these properties. Due to their porous structures, aluminosilicates provide a suitable host system for studying the diffusion of sorbates in confinement. Zeolites and clays are two classes of the aluminosilicate family, comprising very ordered porous or layered structures. Zeolitic materials are important due to their high catalytic activity and molecular sieving properties. Guest molecules adsorbed by zeolites display many interesting features including unidimensional diffusion, non-isotropic rotation, preferred orientation and levitation effects, depending on the guest and host characteristics. These are useful for the separation of hydrocarbons which commonly exist as mixtures in nature. Similarly, clay materials have found application in catalysis, desalination, enhanced oil recovery, and isolation barriers used in radioactive waste disposal. It has been shown that the bonding interactions, level of hydration, interlayer spacing, and number of charge-balancing cations are the important factors that determine the nature of diffusion of water molecules in clays. Here, we present a review of the current status of the diffusion mechanisms of various adsorbed species in different microporous zeolites and clays, as investigated using quasielastic neutron scattering and classical molecular dynamics simulation techniques. It is impossible to write an exhaustive review of the subject matter, as it has been explored over several decades and involves many research topics. However, an effort is made to cover the relevant issues specific to the dynamics of different molecules in microporous zeolites and clay materials and to highlight a variety of interesting features that are important for both practical applications and fundamental aspects.

Formation of unexpected S-S covalent bonds in H2S dimers under confinement

The present work investigates the effect of confinement on the hydrogen bonding interactions in H2S dimers. The interiors of different sized fullerenes (C60, C70, C84, and C120) have been used to model the effect of confinement. It was found that as the degree of confinement increases, the hydrogen bonding between H2S molecules disappears and sulphur-sulphur interactions appear. We obtained clear computational evidence that, inside C60, the H2S dimer is bound by a covalent bond between two sulphur atoms. It was also found that the strength of the S-S bond inside fullerenes is linked to the amount of charge transfer from the H2S dimer to the fullerene.

Simulations of the IR and Raman spectra of water confined in amorphous silica slit pores

Water in nano-scale confining environments is a key element in many biological, material, and geological systems. The structure and dynamics of the liquid can be dramatically modified under these conditions. Probing these changes can be challenging, but vibrational spectroscopy has emerged as a powerful tool for investigating their behavior. A critical, evolving component of this approach is a detailed understanding of the connection between spectroscopic features and molecular-level details. In this paper, this issue is addressed by using molecular dynamics simulations to simulate the linear infrared (IR) and Raman spectra for isotopically dilute HOD in D₂O confined in hydroxylated amorphous silica slit pores. The effect of slit-pore width and hydroxyl density on the silica surface on the vibrational spectra is also investigated. The primary effect of confinement is a blueshift in the frequency of OH groups donating a hydrogen bond to the silica surface. This appears as a slight shift in the total (measurable) spectra but is clearly seen in the distance-based IR and Raman spectra. Analysis indicates that these changes upon confinement are associated with the weaker hydrogen-bond accepting properties of silica oxygens compared to water molecules.

Anomalous Freezing of Low-Dimensional Water Confined in Graphene Nanowrinkles

Various properties of water are affected by confinement as the space-filling of the water molecules is very different from bulk water. In our study, we challenged the creation of a stable system in which water molecules are permanently locked in nanodimensional graphene traps. For that purpose, we developed a technique, nitrocellulose-assisted transfer of graphene grown by chemical vapor deposition, which enables capturing of the water molecules below an atomically thin graphene membrane structured into a net of regular wrinkles with a lateral dimension of about 4 nm. After successfully confining water molecules below a graphene monolayer, we employed cryogenic Raman spectroscopy to monitor the phase changes of the confined water as a function of the temperature. In our experiment system, the graphene monolayer structured into a net of fine wrinkles plays a dual role: (i) it enables water confinement and (ii) serves as an extremely sensitive probe for phase transitions involving water via graphene-based spectroscopic monitoring of the underlying water structure. Experimental findings were supported with classical and path integral molecular dynamics simulations carried out on our experimental system. Results of simulations show that surface premelting of the ice confined within the wrinkles starts at ∼200 K and the melting process is complete at ∼240 K, which is far below the melting temperature of bulk water ice. The processes correspond to changes in the doping and strain in the graphene tracked by Raman spectroscopy. We conclude that water can be confined between graphene structured into nanowrinkles and silica substrate and its phase transitions can be tracked via Raman spectral feature of the encapsulating graphene. Our study also demonstrated that peculiar behavior of liquids under spatial confinement can be inspected via the optical response of atomically thin graphene sensors.

Dielectric Spectroscopy and Application of Mixing Models Describing Dielectric Dispersion in Clay Minerals and Clayey Soils

The number of sensors, ground-based and remote, exploiting the relationship between soil dielectric response and soil water content continues to grow. Empirical expressions for this relationship generally work well in coarse-textured soils but can break down for high-surface area and intricate materials such as clayey soils. Dielectric mixing models are helpful for exploring mechanisms and developing new understanding of the dielectric response in porous media that do not conform to a simple empirical approach, such as clayey soils. Here, we explore the dielectric response of clay minerals and clayey soils using the mixing model approach in the frequency domain. Our modeling focuses on the use of mixing models to explore geometrical effects. New spectroscopic data are presented for clay minerals (talc, kaolinite, illite and montmorillonite) and soils dominated by these clay minerals in the 1 MHz–6 GHz bandwidth. We also present a new typology for the way water is held in soils that we hope will act as a framework for furthering discussion on sensor design. We found that the frequency-domain response can be mostly accounted for by adjusting model structural parameters, which needs to be conducted to describe the Maxwell–Wagner (MW) relaxation effects. The work supports the importance of accounting for soil structural properties to understand and predict soil dielectric response and ultimately to find models that can describe the dielectric–water content relationship in fine-textured soils measured with sensors.

Manipulating the physical states of confined ibuprofen in SBA-15 based drug delivery systems obtained by solid-state loading: Impact of the loading degree

Using the Milling-Assisted Loading (MAL) solid-state method for loading a poorly water-soluble drug (ibuprofen, IBP) within the SBA-15 matrix has given the opportunity to manipulate the physical state of drugs for optimizing bioavailability. The MAL method makes it easy to control and analyze the influence of the degree of loading on the physical state of IBP inside the SBA-15 matrix with an average pore diameter of 9.4 nm. It was found that the density of IBP molecules in an average pore size has a direct influence on both the glass transition and the mechanism of crystallization. Detailed analyzes of the crystallite distribution and melting by Raman mapping, x-ray diffraction, and differential scanning calorimetry have shown that the crystals are localized in the core of the channel and surrounded by a liquid monolayer. The results of these complementary investigations have been used for determining the relevant parameters (related to the SBA-15 matrix and to the IBP molecule) and the nature of the physical state of the confined matter.

Anomaly of the dielectric function of water under confinement and its role in van der Waals interactions

We present a theoretical calculation of the changes in the Hamaker constant due to the anomalous reduction of the static dielectric function of water. Under confinement, the dielectric function of water decreases from a bulk value of 80 down to 2. If the confining walls are made of a dielectric material, the Hamaker constant reduces by almost 90%. However, if the confinement is realized with metallic plates, there is little change in the Hamaker constant. Additionally, we show that confinement can be used to decreases the Debye screening length without changing the salt concentration. This in turn is used to change the Hamaker constant in the presence of electrolytes.

Interactions of water confined in a metal-organic framework as studied by a combined approach of Raman, FTIR, and IR electroabsorption spectroscopies and multivariate curve resolution analysis

Water in nanoconfinement shows distinct properties that are markedly different from those of bulk water. These unique properties stem not only from the water-water interaction but also from the interactions between water and the surrounding confining environment. Here we used a combined approach of vibrational spectroscopies (Raman, FTIR, and IR electroabsorption) and a multivariate curve resolution technique to study the interactions of water in a heterogeneous confining environment within a prototype of pillared layer-type metal-organic frameworks (MOFs), CPL-1 ([Cu2(pzdc)2(pyz)]n, where pzdc = 2,3-pyrazinedicarboxylate, pyz = pyrazine). The OH stretching Raman spectrum of hydrated CPL-1 microcrystals revealed that the adsorbed water molecules resemble the subpopulation of bulk water whose hydrogen bond is weak. Multivariate curve resolution analysis of FTIR spectra monitoring water desorption from CPL-1 allowed for accurate assignments of the framework's carboxylate vibrational modes associated with water-filled and empty nanopores of the MOF, and for quantitative determination of the number fraction of these pores. Furthermore, building on the assignments so made, IR electroabsorption measurements showed that the hydrogen-bonding interaction with water adsorbed in CPL-1 has little impact on the response to electric fields of the framework vibrational modes. The present findings altogether provide a solid basis of elucidating water confined in CPL-1 and demonstrate the potential of the combined vibrational spectroscopic method for interrogating the interactions within MOFs.

The joint effect of surface polarity and concentration on the structure and dynamics of acetonitrile solution: a molecular dynamics simulation study

The interfacial properties of the acetonitrile (ACN)-water-silica interface have great implications in both liquid chromatography and heterogeneous catalysis. We have performed molecular dynamics (MD) simulations of ACN and water binary solutions to give a comprehensive study of the collective effect of silica surface polarity and ACN concentration on interfacial structures and dynamics by tuning both surface charges and ACN concentration. MD simulation results indicate that many properties in the liquid-solid interface region undergo a monotonic change as the silica surface is tuned from polar to apolar due to the weakening of hydrogen bonding, while their dependence on ACN concentration is presumably governed by the preferential adsorption of water at the silica surface over ACN. However, at apolar surfaces, the interfacial structures of both water and ACN behave like the liquid-vapor interface, and this resemblance leads to an enrichment of ACN at the interface as well as accelerated dynamics, which is very different from that in the bulk solution. The organization of ACN molecules at both polar and apolar surfaces can be attributed to the amphiphilic nature of ACN, by which the micro-heterogeneity domain formed can persist both in the bulk and at the liquid-solid interface. Moreover, extending diffusion analysis to the second layer of the interface shows that the interfacial transport pathways at polar surfaces are likely very different from that of apolar surfaces. These simulation results give a full spectrum description of the ACN/water liquid-solid interface at the microscopic level and will be helpful for explaining related spectroscopic experiments and understanding the microscopic mechanisms of separation protocols in current chromatography applications.

Lubricated friction around nanodefects

The lubrication properties of nanoconfined liquids underpin countless natural and industrial processes. However, our current understanding of lubricated friction is still limited, especially for nonideal interfaces exhibiting nanoscale chemical and topographical defects. Here, we use atomic force microscopy to explore the equilibrium and dynamical behavior of a model lubricant, squalane, confined between a diamond tip and graphite in the vicinity of an atomic step. We combine high-resolution imaging of the interface with highly localized shear measurements at different velocities and temperatures to derive a quantitative picture of the lubricated friction around surface defects. We show that defects tend to promote local molecular order and increase friction forces by reducing the number of stable molecular configurations in their immediate vicinity. The effect is general, can propagate over hundreds of nanometers, and can be quantitatively described by a semiempirical model that bridges the molecular details and mesoscale observations.

Dynamics of Confined Short-Chain alkanol in MCM-41 by Dielectric Spectroscopy: Effects of matrix and system Treatments and Filling Factor

The dynamics of n-propanol confined in regular MCM-41 matrix with the pore size Dpore = 40 Å, under various matrix conditioning and sample confining conditions, using broadband dielectric spectroscopy (BDS), is reported. First, various drying procedures with the capacitor filling under air or N2 influence the BDS spectra of the empty MCM-41 and the confined n-PrOH/MCM-41 systems, but have a little effect on the maximum relaxation time of the main process. Finally, various filling factors of n-PrOH medium in the optimally treated MCM-41 system lead to unimodal or bimodal spectra interpreted in terms of the two distinct dynamic phases in the confined states.

Closest-Packing Water Monolayer Stably Intercalated in Phyllosilicate Minerals under High Pressure

The directional hydrogen-bond (HB) network and nondirectional van der Waals (vdW) interactions make up the specificity of water. Directional HBs could construct an ice-like monolayer in hydrophobic confinement even in the ambient regime. Here, we report a water monolayer dominated by vdW interactions confined in a phyllosilicate interlayer under high pressure. Surprisingly, it was in a thermodynamically stable state coupled with bulk water at the same pressure (P) and temperature (T), as revealed by the thermodynamic integration approach on the basis of molecular dynamics (MD) simulations. Both classical and ab initio MD simulations showed water O atoms were stably trapped and exhibited an ordered hexagonal closest-packing arrangement, but OH bonds of water reoriented frequently and exhibited a specific two-stage reorientation relaxation. Strikingly, hydration in the interlayer under high pressure had no relevance with surface hydrophilicity rationalized by the HB forming ability, which, however, determines wetting in the ambient regime. Intercalated water molecules were trapped by vdW interactions, which shaped the closest-packing arrangement and made hydration energetically available. The high pressure-volume term largely drives hydration, as it compensates the entropy penalty which is restricted by a relatively lower temperature. This vdW water monolayer should be ubiquitous in the high pressure but low-temperature regime.

Water as a tuneable solvent: a perspective

Water is the most sustainable solvent, but its polarity limits the solubility of non-polar solutes. Confining water in hydrophobic nanopores could be a way to modulate water solvent properties and enable using water as tuneable solvent (WaTuSo).

The effect of surface polarity on the structure and collective dynamics of liquid ethanol

Typical configurations of ethanol during polarity modulation.