Universal Reduction in Dielectric Response of Confined Fluids

The Interplay between Dynamics and Structure on the Dielectric Tensor of Nanoconfined Water: Surface Charge and Salinity Effect

Under confinement, the water dielectric constant is a second-order tensor with an abnormally low out-of-plane element. In our work, we investigate the dielectric tensor of an aqueous NaCl solution confined by a quartz slit-pore. The static dielectric constant is determined from local polarization density fluctuations via molecular dynamics simulations. In a pioneering investigation, we evaluate not only the effect of salinity but also surface charge. The parallel dielectric constant decreases with salinity due to dielectric saturation. From a dynamic perspective, the relaxation of water dipoles is slower within the hydration shells of ions. An anisotropic arrangement on the quartz surface results in preferred orientations of interfacial water molecules. By embedding charge, the surface structure changes, and extra dipole fluctuations in one direction may develop anisotropy in the parallel dielectric constant at the interface. Both surface charge and salinity increase the perpendicular dielectric constant. Nevertheless, the surface charge effect is more pronounced and may even recover the bulk dielectric constant value. The electric field established by the charged surface may disturb the planar hydrogen bond network at the interface, increasing out-of-plane dipolar fluctuations. Our work advances the knowledge of confined dielectric behavior, shedding light on the key role that charged surfaces play.

The Hidden Role of the Dielectric Effect in Nanofiltration: A Novel Perspective to Unravel New Ion Separation Mechanisms

Nanofiltration (NF) membranes play a critical role in separation processes, necessitating an in-depth understanding of their selective mechanisms. Existing NF models predominantly include steric and Donnan mechanisms as primary mechanisms. However, these models often fail in elucidating the NF selectivity between ions of similar dimensions and the same valence. To address this gap, an innovative methodology was proposed to unravel new selective mechanisms by quantifying the nominal dielectric effect isolated from steric and Donnan exclusion through fitted pore dielectric constants by regression analysis. We demonstrated that the nominal dielectric effect encompassed unidentified selective mechanisms of significant relevance by establishing the correlation between the fitted pore dielectric constants and these hindrance factors. Our findings revealed that dehydration-induced ion-membrane interaction, rather than ion dehydration, played a pivotal role in ion partitioning within NF membranes. This interaction was closely linked to the nondeformable fraction of hydrated ions. Further delineation of the dielectric effect showed that favorable interactions between ions and membrane functional groups contributed to entropy-driven selectivity, which is a key factor in explaining ion selectivity differences between ions sharing the same size and valence. This study deepens our understanding of NF selectivity and sheds light on the design of highly selective membranes for water and wastewater treatment.

Atomic scale insights into NaCl nucleation in nanoconfined environments

In this work we examine the nucleation from NaCl aqueous solutions within nano-confined environments, employing enhanced sampling molecular dynamics simulations integrated with machine learning-derived reaction coordinates. Through our simulations, we successfully induce phase transitions between solid, liquid, and a hydrated phase, typically observed at lower temperatures in bulk environments. Interestingly, while generally speaking nano-confinement serves to stabilize the solid phase and elevate melting points, there are subtle variations in the thermodynamics of competing phases with the precise extent of confinement. Our simulations explain these findings by underscoring the significant role of water, alongside ion aggregation and subtle, anisotropic dielectric behavior, in driving nucleation within nano-confined environments. This report thus provides a framework for sampling, analyzing and understanding nucleation processes under nano-confinement.

Water and ions in electrified silica nano-pores: a molecular dynamics study

Solid-liquid interfaces (SLIs) are ubiquitous in science and technology from the development of energy storage devices to the chemical reactions occurring in the biological milieu. In systems involving aqueous saline solutions as the liquid, both the water and the ions are routinely exposed to an electric field, whether the field is externally applied, or originating from the natural surface charges of the solid. In the current study a molecular dynamics (MD) framework is developed to study the effect of an applied voltage on the behaviour of ionic solutions located in a ∼7 nm pore between two uncharged hydrophilic silica slabs. We systematically investigate the dielectric properties of the solution and the organisation of the water and ions as a function of salt concentration. In pure water, the interplay between interfacial hydrogen bonds and the applied field can induce a significant reorganisation of the water orientation and densification at the interface. In saline solutions, at low concentrations and voltages the interface dominates the whole system due to the extended Debye length resulting in a dielectric constant lower than that for the bulk solution. An increase in salt concentration or voltage brings about more localized interfacial effects resulting in dielectric properties closer to that of the bulk solution. This suggests the possibility of tailoring the system to achieve the desired dielectric properties. For example, at a specific salt concentration, interfacial effects can locally increase the dielectric constant, something that could be exploited for energy storage.

Nonlinear Effects of Hydrophobic Confinement on the Electronic Structure and Dielectric Response of Water

Fundamental studies of the dielectrics of confined water are critical to understand the ion transport across biological and synthetic nanochannels. The relevance of these fundamental studies, however, surmounts the difficulty of probing water's dielectric constant as a function of a fine variation in confinement. In this work, we explore the computational efficiency of machine learning potentials to derive the confinement effects on the dielectric constant, polarization, and dipole moment of water. Our simulations predict an enhancement of the axial dielectric constant of water under extreme confinement, arising from either the formation of ferroelectric structures of ordered water or larger dipole fluctuations facilitated by the disruption of water's H-bond network. Our study highlights the impact of hydrophobic nanoconfinement on the dielectric constant and on the ionic and electronic structure of water molecules, pointing to the importance of geometric flexibility and electronic polarizability to properly model confinement effects on water.

Freezing of Water Solvation Dynamics in Nanoconfinement by Reverse Micelles at Room Temperature

Much attention has recently been paid to anomalously low dielectric constants of nanoconfined water between two slabs at room temperature (Fumagalli et al. Science, 2018, 360, 1339). These low values imply that the dipole rotation of the interfacial water on the slab is completely suppressed. Such freezing has so far been observed for water confined between solids. In contrast, it remains unclear whether this holds for water in soft confinement, which is omnipresent naturally and artificially. Here, we address this question using encapsulated reverse micelles with a dye molecule, allowing us to study water sandwiched between the surfactant and dye molecules in solution. Moreover, we examine the solvation related to the dielectric property of water, which is reorientational motion in the hydration layer of the dye molecule, by persistent hole-burning spectroscopy. We first show that the dye molecule is surrounded by water without contact with the surfactant and that the dye molecule has two or three hydration layers on average. We next demonstrate that the solvation dynamics is frozen below the water droplet size of ∼4 nm, whereas they become liquid-like when the RM size is further increased. The average gap distance (∼1.5 nm) for freezing the solvation agrees with the gap distance with no rotational water motions between slabs. Our findings may have biological relevance, providing a new aspect for understanding biological function in cells.

Impact of interfacial curvature on molecular properties of aqueous interfaces

The curvature of soft interfaces plays a crucial role in determining their mechanical and thermodynamic properties, both at macroscopic and microscopic scales. In the case of air/water interfaces, particular attention has recently focused on water microdroplets, due to their distinctive chemical reactivity. However, the specific impact of curvature on the molecular properties of interfacial water and interfacial reactivity has so far remained elusive. Here, we use molecular dynamics simulations to determine the effect of curvature on a broad range of structural, dynamical, and thermodynamical properties of the interface. For a droplet, a flat interface, and a cavity, we successively examine the structure of the hydrogen-bond network and its relation to vibrational spectroscopy, the dynamics of water translation, rotation, and hydrogen-bond exchanges, and the thermodynamics of ion solvation and ion-pair dissociation. Our simulations show that curvature predominantly impacts the hydrogen-bond structure through the fraction of dangling OH groups and the dynamics of interfacial water molecules. In contrast, curvature has a limited effect on solvation and ion-pair dissociation thermodynamics. For water microdroplets, this suggests that the curvature alone cannot fully account for the distinctive reactivity measured in these systems, which are of great importance for catalysis and atmospheric chemistry.

Impedance of nanocapacitors from molecular simulations to understand the dynamics of confined electrolytes

Nanoelectrochemical devices have become a promising candidate technology across various applications, including sensing and energy storage, and provide new platforms for studying fundamental properties of electrode/electrolyte interfaces. In this work, we employ constant-potential molecular dynamics simulations to investigate the impedance of gold-aqueous electrolyte nanocapacitors, exploiting a recently introduced fluctuation-dissipation relation. In particular, we relate the frequency-dependent impedance of these nanocapacitors to the complex conductivity of the bulk electrolyte in different regimes, and use this connection to design simple but accurate equivalent circuit models. We show that the electrode/electrolyte interfacial contribution is essentially capacitive and that the electrolyte response is bulk-like even when the interelectrode distance is only a few nanometers, provided that the latter is sufficiently large compared to the Debye screening length. We extensively compare our simulation results with spectroscopy experiments and predictions from analytical theories. In contrast to experiments, direct access in simulations to the ionic and solvent contributions to the polarization allows us to highlight their significant and persistent anticorrelation and to investigate the microscopic origin of the timescales observed in the impedance spectrum. This work opens avenues for the molecular interpretation of impedance measurements, and offers valuable contributions for future developments of accurate coarse-grained representations of confined electrolytes.

Generalized Helmholtz model describes capacitance profiles of ionic liquids and concentrated aqueous electrolytes

In this work, we propose and validate a generalization of the Helmholtz model that can account for both "bell-shaped" and "camel-shaped" differential capacitance profiles of concentrated electrolytes, the latter being characteristic of ionic liquids. The generalization is based on introducing voltage dependence of both the dielectric constant "ϵr(V)" and thickness "L(V)" of the inner Helmholtz layer, as validated by molecular dynamics (MD) simulations. We utilize MD simulations to study the capacitance profiles of three different electrochemical interfaces: (1) graphite/[BMIm+][BF4-] ionic liquid interface; (2) Au(100)/[BMIm+][BF4-] ionic liquid interface; (3) Au(100)/1M [Na+][Cl-] aqueous interface. We compute the voltage dependence of ϵr(V) and L(V) and demonstrate that the generalized Helmholtz model qualitatively describes both camel-shaped and bell-shaped differential capacitance profiles of ionic liquids and concentrated aqueous electrolytes (in lieu of specific ion adsorption). In particular, the camel-shaped capacitance profile that is characteristic of ionic liquid electrolytes arises simply from combination of the voltage-dependent trends of ϵr(V) and L(V). Furthermore, explicit analysis of the inner layer charge density for both concentrated aqueous and ionic liquid double layers reveal similarities, with these charge distributions typically exhibiting a dipolar region closest to the electrode followed by a monopolar peak at larger distances. It is appealing that a generalized Helmholtz model can provide a unified description of the inner layer structure and capacitance profile for seemingly disparate aqueous and ionic liquid electrolytes.

Computational dielectric spectroscopy on solid-solution interface by time-dependent voltage applied molecular dynamics simulation

A frequency-dependent dielectric constant characterizes the dielectric response of a medium and also represents the time scale of system's collective dynamics. Although it is valuable not only academically but also practically for developing advanced devices, getting the value of a solution at the interface with a solid or electrode surface is challenging both experimentally and computationally. Here, we propose a computational method that imitates the dielectric spectroscopy and AC impedance measurement. It combines a time-dependent voltage applied molecular dynamics simulation with an equivalent circuit representation of a system composed of a solution confined between two identical electrodes. It gives the frequency-dependent dielectric constants of the bulk solution and the interface simultaneously. Unlike the conventional method, it does not require computation of a dipole autocorrelation function and its Fourier transformation. Application of the method on a system of water confined between polarizable Pt electrodes gives the static dielectric constant and the relaxation time of the bulk water in good agreement with previous simulation results and experimental values. In addition, it gives a much smaller static dielectric constant at the interface, consistent with previous observations. The outline of the dielectric dispersion curve of the interface seems similar to that of the bulk, but the relaxation time is several times faster.

Origin of dielectric polarization suppression in confined water from first principles

It has long been known that the dielectric constant of confined water should be different from that in bulk. Recent experiments have shown that it is vanishingly small, however the origin of the phenomenon remains unclear. Here we used ab initio molecular dynamics simulations (AIMD) and AIMD-trained machine-learning potentials to understand water's structure and electronic properties underpinning this effect. For the graphene and hexagonal boron-nitride substrates considered, we find that it originates in the spontaneous anti-parallel alignment of the water dipoles in the first two water layers near the solid interface. The interfacial layers exhibit net ferroelectric ordering, resulting in an overall anti-ferroelectric arrangement of confined water. Together with constrained hydrogen-bonding orientations, this leads to much reduced out-of-plane polarization. Furthermore, we directly contrast AIMD and simple classical force-field simulations, revealing important differences. This work offers insight into a property of water that is critical in modulating surface forces, the electric-double-layer formation and molecular solvation, and shows a way to compute it.

From ab initio to continuum: Linking multiple scales using deep-learned forces

We develop a deep learning-based algorithm, called DeepForce, to link ab initio physics with the continuum theory to predict concentration profiles of confined water. We show that the deep-learned forces can be used to predict the structural properties of water confined in a nanochannel with quantum scale accuracy by solving the continuum theory given by Nernst-Planck equation. The DeepForce model has an excellent predictive performance with a relative error less than 7.6% not only for confined water in small channel systems (L < 6 nm) but also for confined water in large channel systems (L = 20 nm) which are computationally inaccessible through the high accuracy ab initio molecular dynamics simulations. Finally, we note that classical Molecular dynamics simulations can be inaccurate in capturing the interfacial physics of water in confinement (L < 4.0 nm) when quantum scale physics are neglected.

Thermodynamic Analysis of Capillary and Electric Field Effects on Liquid-Vapor Equilibrium: A Study on the Water-Ethanol Mixture

This study investigates phase equilibrium manipulation in nonideal mixtures through a combined capillary and external electric field approach. Utilizing thermodynamic principles, an expression is established for estimating the equilibrium liquid mole fraction in a confined system subjected to a localized electric field within a capillary that is filled with a liquid phase in equilibrium with its vapor counterpart. Applied to a water-ethanol system, the model suggests large shifts in the equilibrium liquid mole fraction of water due to the electric field and capillary effects. These findings reveal that while the capillary's influence remains negligible for radii exceeding 10 nm, capillaries of smaller dimensions, when exposed to electric fields of around 300 MV/m, can amplify the equilibrium liquid water mole fraction by up to 55%. This suggests the potential for phase equilibrium control through larger capillaries and lower electric fields, while intriguing complexities arise at very small radii.

Anisotropic pressure effects on nanoconfined water within narrow graphene slit pores

There is an increasing interest toward disclosing and explaining confinement effects on liquids, such as water or aqueous solutions, in slit pore setups. Particularly puzzling are the changes of physical and chemical properties in the nanoconfinement regime where no bulk-like water phase exists between the two interfacial water layers such that the density profile across the slit pore becomes highly stratified, ultimately leading to bilayer and monolayer water. These changes must be quantified with respect to some meaningful reference state of water, the most natural one being bulk water at the same pressure and temperature conditions. However, bulk water is a homogeneous liquid with isotropic properties, whereas water confined in slit pores is inhomogeneous, implying anisotropic properties as described by the perpendicular and parallel components of the respective tensors. In the case of pressure, the inhomogeneous nature of the setup results in a well-defined difference between the perpendicular and parallel pressure tensor components that is uniquely determined by the interfacial tension being a thermodynamic property. For bilayer water constrained in graphene slit pores that are only about 1 nm wide, we demonstrate that there exists a thermodynamic point where the pressure tensor of the inhomogeneous fluid, nanoconfined water, is effectively isotopic and the pressure is thus scalar as in the homogeneous fluid, bulk water. This specific point of vanishing effective interfacial tension is proposed to serve as a well-defined reference state to compare the properties of nanoconfined liquids to those of the corresponding bulk liquid at the same (isotropic) pressure and temperature conditions. In future work, this idea could be applied to assess confinement effects on chemical reactivity in aqueous solutions as well as to other nanoconfined liquids in other pores such as layered minerals.

Dielectric response of confined water films from a classical density functional theory perspective

We re-examine the problem of the dielectric response of highly polar liquids such as water in confinement between two walls using simple two-variable density functional theory involving number and polarisation densities. In the longitudinal polarisation case where a perturbing field is applied perpendicularly to the walls, we show that the notion of the local dielectric constant, although ill-defined at a microscopic level, makes sense when coarse-graining over the typical size of a particle is introduced. The approach makes it possible to study the effective dielectric response of thin liquid films of various thicknesses in connection with the recent experiments of Fumagalli et al., [Science, 2018, 360, 1339-1342], and to discuss the notion of the interfacial dielectric constant. We argue that the observed properties as a function of slab dimensions, in particular the very low dielectric constants of the order of 2-3 measured for thin slabs of ∼1 nm thickness do not highlight any special properties of water but can be recovered for a generic polar solvent having similar particle size and the same high dielectric constant. Regarding the transverse polarisation case where the perturbing field is parallel to the walls, the associated effective dielectric constant as a function of slab dimensions reaches bulk-like values at much shorter widths than in the longitudinal case. In both cases, we find an oscillatory behaviour for slab thicknesses in the one nanometer range due to packing effects.

Finding Infinities in Nanoconfined Geothermal Electrolyte Static Dielectric Properties and Implications on Ion Adsorption/Pairing

Infinities should naturally occur in the dielectric responses of ionic solutions relevant to many geochemical, energy storage, and electrochemical applications at a strictly zero frequency. Using molecular dynamics simulations cross-referenced with coarse-grained Monte Carlo models, using nanoslit pore models at hydrothermal conditions, and treating confined mobile charges as polarization, we demonstrate the far reaching consequences. The dielectric permittivity profile perpendicular to the slit (ϵ⊥(z)) increases, not decreases, with ionic concentration, unlike in the more widely studied megahertz-to-gigahertz frequency range. In confined electrolytes, the divergences in ϵ⊥(z) correctly describe crossovers between bulk- and surface-dominated dielectric behavior. Nanoconfinement at low ionic concentrations changes monovalent ion energetics by 1-2 kJ/mol, but no dielectric property studied so far is universally correlated to ion adsorption or ion-ion interactions. We caution that infinities signal violation of the "electrical insulator" dielectric assumption.

Liquid-activated quantum emission from pristine hexagonal boron nitride for nanofluidic sensing

Liquids confined down to the atomic scale can show radically new properties. However, only indirect and ensemble measurements operate in such extreme confinement, calling for novel optical approaches that enable direct imaging at the molecular level. Here we harness fluorescence originating from single-photon emitters at the surface of hexagonal boron nitride for molecular imaging and sensing in nanometrically confined liquids. The emission originates from the chemisorption of organic solvent molecules onto native surface defects, revealing single-molecule dynamics at the interface through the spatially correlated activation of neighbouring defects. Emitter spectra further offer a direct readout of the local dielectric properties, unveiling increasing dielectric order under nanometre-scale confinement. Liquid-activated native hexagonal boron nitride defects bridge the gap between solid-state nanophotonics and nanofluidics, opening new avenues for nanoscale sensing and optofluidics.

Ion and water adsorption to graphene and graphene oxide surfaces

Graphene and graphene oxide (GO) are two particularly promising nanomaterials for a range of applications including energy storage, catalysis, and separations. Understanding the nanoscale interactions between ions and water near graphene and GO surfaces is critical for advancing our fundamental knowledge of these systems and downstream application success. This minireview highlights the necessity of using surface-specific experimental probes and computational techniques to fully characterize these interfaces, including the nanomaterial, surrounding water, and any adsorbed ions, if present. Key experimental and simulation studies considering water and ion structures near both graphene and GO are discussed. The major findings are: water forms 1-3 hydration layers near graphene; ions adsorb electrostatically to graphene under an applied potential; the chemical and physical properties of GO vary considerably depending on the synthesis route; and these variations influence water and ion adsorption to GO. Lastly, we offer outlooks and perspectives for these research areas.

Dielectric exclusion, an éminence grise

Dielectric exclusion has long been well-established as the key mechanism in membrane desalination, critical for delivering the required levels of salt rejection, also playing important role in electro-membrane processes, nanofluidics, and biomimetics. Unfortunately, its elusive nature and many features, such as dependence on the pore size, membrane hydration, and ion size and charge, make it deceivingly similar to the other ion exclusions mechanisms, steric and Donnan, which has led to much controversy and misconceptions. Starting from the Born model and the concept of self-energy, the present paper reviews and highlights the physical basis of dielectric exclusion, its main features and the ways it may be looked at. It discusses what makes the dielectric exclusion both similar and distinctly different from the other mechanism and its synergy and intimate connection with other phenomena, such as Donnan exclusion, permeability-selectivity upper-bound, and selectivity of charged membranes towards uncharged solutes. The paper also addresses subjects that still cause much controversy at present, such as appropriate measures of ionic radii and the subtle distinction between the dielectric exclusion and primary ion hydration. It also points to gaps that need to be bridged towards more complete theory. The points addressed here are important for understanding, modeling and development of various next-generation separation technologies including water purification, resource recovery and reuse, and green energy generation and storage.

Ion solvation as a predictor of lanthanide adsorption structures and energetics in alumina nanopores

Adsorption reactions at solid-water interfaces define elemental fate and transport and enable contaminant clean-up, water purification, and chemical separations. For nanoparticles and nanopores, nanoconfinement may lead to unexpected and hard-to-predict products and energetics of adsorption, compared to analogous unconfined surfaces. Here we use X-ray absorption fine structure spectroscopy and operando flow microcalorimetry to determine nanoconfinement effects on the energetics and local coordination environment of trivalent lanthanides adsorbed on Al2O3 surfaces. We show that the nanoconfinement effects on adsorption become more pronounced as the hydration free energy, ΔGhydr, of a lanthanide decreases. Neodymium (Nd3+) has the least exothermic ΔGhydr (-3336 kJ·mol-1) and forms mostly outer-sphere complexes on unconfined Al2O3 surfaces but shifts to inner-sphere complexes within the 4 nm Al2O3 pores. Lutetium (Lu3+) has the most exothermic ΔGhydr (-3589 kJ·mol-1) and forms inner-sphere adsorption complexes regardless of whether Al2O3 surfaces are nanoconfined. Importantly, the energetics of adsorption is exothermic in nanopores only, and becomes endothermic with increasing surface coverage. Changes to the energetics and products of adsorption in nanopores are ion-specific, even within chemically similar trivalent lanthanide series, and can be predicted by considering the hydration energies of adsorbing ions.

Orientational Ordering in Nano-confined Polar Liquids

Water and other polar liquids exhibit nanoscale structuring near charged interfaces. When a polar liquid is confined between two charged surfaces, the interfacial solvent layers begin to overlap, resulting in solvation forces. Here, we perform molecular dynamics simulations of polar liquids with different dielectric constants and molecular shapes and sizes confined between charged surfaces, demonstrating strong orientational ordering in the nanoconfined liquids. To rationalize the observed structures, we apply a coarse-grained continuum theory that captures the orientational ordering and solvation forces of those liquids. Our findings reveal the subtle behavior of different nanoconfined polar liquids and establish a simple law for the decay length of the interfacial orientations of the solvents, which depends on their molecular size and polarity. These insights shed light on the nature of solvation forces, which are important in colloid and membrane science, scanning probe microscopy, and nano-electrochemistry.

Fluids and Electrolytes under Confinement in Single-Digit Nanopores

Confined fluids and electrolyte solutions in nanopores exhibit rich and surprising physics and chemistry that impact the mass transport and energy efficiency in many important natural systems and industrial applications. Existing theories often fail to predict the exotic effects observed in the narrowest of such pores, called single-digit nanopores (SDNs), which have diameters or conduit widths of less than 10 nm, and have only recently become accessible for experimental measurements. What SDNs reveal has been surprising, including a rapidly increasing number of examples such as extraordinarily fast water transport, distorted fluid-phase boundaries, strong ion-correlation and quantum effects, and dielectric anomalies that are not observed in larger pores. Exploiting these effects presents myriad opportunities in both basic and applied research that stand to impact a host of new technologies at the water-energy nexus, from new membranes for precise separations and water purification to new gas permeable materials for water electrolyzers and energy-storage devices. SDNs also present unique opportunities to achieve ultrasensitive and selective chemical sensing at the single-ion and single-molecule limit. In this review article, we summarize the progress on nanofluidics of SDNs, with a focus on the confinement effects that arise in these extremely narrow nanopores. The recent development of precision model systems, transformative experimental tools, and multiscale theories that have played enabling roles in advancing this frontier are reviewed. We also identify new knowledge gaps in our understanding of nanofluidic transport and provide an outlook for the future challenges and opportunities at this rapidly advancing frontier.

Dielectric Properties of Nanoconfined Water from Ab Initio Thermopotentiostat Molecular Dynamics

We discuss how to include our recently proposed thermopotentiostat technique [Deissenbeck et al. Phys. Rev. Lett. 2021, 126, 136803] into any existing ab initio molecular dynamics (AIMD) package. Using thermopotentiostat AIMD simulations in the canonical NVTΦ ensemble at a constant electrode potential, we compute the polarization bound charge and dielectric response of interfacial water from first principles.

Exact polarization energy for clusters of contacting dielectrics

The induced surface charges appear to diverge when dielectric particles form close contacts. Resolving this singularity numerically is prohibitively expensive because high spatial resolution is needed. We show that the strength of this singularity is logarithmic in both inter-particle separation and dielectric permittivity. A regularization scheme is proposed to isolate this singularity, and to calculate the exact cohesive energy for clusters of contacting dielectric particles. The results indicate that polarization energy stabilizes clusters of open configurations when permittivity is high, in agreement with the behavior of conducting particles, but stabilizes the compact configurations when permittivity is low.

Variable Interfacial Water Nanosized Arrangements Measured by Atomic Force Microscopy

While there seems to be broad agreement that cluster formation does exist near solid surfaces, its presence at the liquid/vapor interface is controversial. We report experimental studies we have carried out on interfacial water attached on hydrophobic and hydrophilic surfaces. Nanosized steps in the measured force vs distance to the surface curves characterize water cluster profiles. An expansion of the interfacial structure with time is observed; the initial profile extent is typically ∼1 nm, and for longer times expanded structures of ∼70 nm are observed. Our previous results showed that the interfacial water structure has a relative permittivity of ε ≈ 3 at the air/water interface homogeneously increasing to ε ≈ 80 at 300 nm inside the bulk, but here we have shown that the interfacial dielectric permittivity may have an oscillating profile describing the spatial steps in the force vs distance curves. This low dielectric permittivity arrangements of clusters extend the region with ε ≈ 3 inside bulk water and exhibit a behavior similar to that of water networks that expand in time.

Ion Specificity of Confined Ion-Water Structuring and Nanoscale Surface Forces in Clays

Ion specificity and related Hofmeister effects, which are ubiquitous in aqueous systems, can have spectacular consequences in hydrated clays, where ion-specific nanoscale surface forces can determine large-scale cohesive swelling and shrinkage behaviors of soil and sediments. We have used a semiatomistic computational approach and examined sodium, calcium, and aluminum counterions confined with water between charged surfaces representative of clay materials to show that ion-water structuring in nanoscale confinement is at the origin of surface forces between clay particles which are intrinsically ion-specific. When charged surfaces strongly confine ions and water, the amplitude and oscillations of the net pressure naturally emerge from the interplay of electrostatics and steric effects, which cannot be captured by existing theories. Increasing confinement and surface charge densities promote ion-water structures that increasingly deviate from the ions' bulk hydration shells, being strongly anisotropic, persistent, and self-organizing into optimized, nearly solid-like assemblies where hardly any free water is left. Under these conditions, strongly attractive interactions can prevail between charged surfaces because of the dramatically reduced dielectric screening of water and the highly organized water-ion structures. By unravelling the ion-specific nature of these nanoscale interactions, we provide evidence that ion-specific solvation structures determined by confinement are at the origin of ion specificity in clays and potentially a broader range of confined aqueous systems.

Modified Lucas-Washburn theory for fluid filling in nanotubes

Ultrafast water transport in carbon nanotubes (CNTs) has drawn a great deal of attention in a number of applications, such as water desalination, power generation, and biomolecule detection. With the recent experimental advances in water filling of isolated CNTs, the Lucas-Washburn theory for capillary rise in tubes needs to be revisited for a better understanding of the physics and dynamics of water filling in CNTs. Here, the Lucas-Washburn theory is corrected for the hydrodynamic entrance effects as well as the variation of capillary pressure and hydrodynamic properties with the radius and length of CNTs. Due to the large slippage in CNTs, inclusion of the entrance effects is important particularly for the initial stages of filling where a L∝t scaling, as opposed to L^{2}∝t, is observed in our molecular dynamics (MD) simulations. The corrected Lucas-Washburn theory is shown to predict the water filling dynamics in CNTs as observed in MD simulations. With the corrected theory, we achieve a better understanding of capillary rise and water filling dynamics in CNTs.

Dielectric Properties of Water in Charged Nanopores

In this study, we examine the spectral dielectric properties of liquid water in charged nanopores over a wide range of frequencies (0.3 GHz to 30 THz) and pore widths (0.3 to 5 nm). This has been achieved using classical molecular dynamics simulations of hydrated Na-smectite, the prototypical swelling clay mineral. We observe a drastic (20-fold) and anisotropic decrease in the static relative permittivity of the system as the pore width decreases. This large decrement in static permittivity reflects a strong attenuation of the main Debye relaxation mode of liquid water. Remarkably, this strong attenuation entails very little change in the time scale of the collective relaxation. Our results indicate that water confined in charged nanopores is a distinct solvent with a much weaker collective nature than bulk liquid water, in agreement with recent observations of water in uncharged nanopores. Finally, we observe remarkable agreement between the dielectric properties of the simulated clay system against a compiled set of soil samples at various volumetric water contents. This implies that saturation may not be the sole property dictating the dielectric properties of soil samples, rather that the pore-size distribution of fully saturated nanopores may also play a critically important role.

Molecular Simulations to Elucidate Transport Phenomena in Polymeric Membranes

Despite decades of dominance in separation technology, progress in the design and development of high-performance polymer-based membranes has been incremental. Recent advances in materials science and chemical synthesis provide opportunities for molecular-level design of next-generation membrane materials. Such designs necessitate a fundamental understanding of transport and separation mechanisms at the molecular scale. Molecular simulations are important tools that could lead to the development of fundamental structure-property-performance relationships for advancing membrane design. In this Perspective, we assess the application and capability of molecular simulations to understand the mechanisms of ion and water transport across polymeric membranes. Additionally, we discuss the reliability of molecular models in mimicking the structure and chemistry of nanochannels and transport pathways in polymeric membranes. We conclude by providing research directions for resolving key knowledge gaps related to transport phenomena in polymeric membranes and for the construction of structure-property-performance relationships for the design of next-generation membranes.

Tuning the Dielectric Response of Water in Nanoconfinement through Surface Wettability

The tunable polarity of water can be exploited in emerging technologies including catalysis, gas storage, and green chemistry. Recent experimental and theoretical studies have shown that water can be rendered into an effectively apolar solvent under nanoconfinement. We furthermore demonstrate, through molecular simulations, that the static dielectric constant of water can be modified by changing the wettability of the confining material. We find the out-of-plane dielectric response to be highly sensitive to the level of confinement and can be reduced up to 40× , in accordance with experimental data. By altering the surface wettability from superhydrophilic to superhydrophobic, we observe a 36% increase for the out-of-plane and a 31% decrease for the in-plane dielectric constants. Our findings demonstrate the feasibility of tunable water polarity, a phenomenon with great potential for scientific and technological impact.

A molecular perspective on induced charges on a metallic surface

Understanding the response of the surface of metallic solids to external electric field sources is crucial to characterize electrode-electrolyte interfaces. Continuum electrostatics offer a simple description of the induced charge density at the electrode surface. However, such a simple description does not take into account features related to the atomic structure of the solid and to the molecular nature of the solvent and of the dissolved ions. In order to illustrate such effects and assess the ability of continuum electrostatics to describe the induced charge distribution, we investigate the behavior of a gold electrode interacting with sodium or chloride ions fixed at various positions, in a vacuum or in water, using all-atom constant-potential classical molecular dynamics simulations. Our analysis highlights important similarities between the two approaches, especially under vacuum conditions and when the ion is sufficiently far from the surface, as well as some limitations of the continuum description, namely, neglecting the charges induced by the adsorbed solvent molecules and the screening effect of the solvent when the ion is close to the surface. While the detailed features of the charge distribution are system-specific, we expect some of our generic conclusions on the induced charge density to hold for other ions, solvents, and electrode surfaces. Beyond this particular case, the present study also illustrates the relevance of such molecular simulations to serve as a reference for the design of improved implicit solvent models of electrode-electrolyte interfaces.

Weakly Anisotropic Dielectric Properties of Water Droplets at the Nanoscale

The slab-confined water at the nanoscale exhibits anomalous dielectric properties compared to bulk water, for example, significantly low dielectric constant. In this work, we study the dielectric properties of nanoscale water droplets at room temperature using molecular dynamics simulations. We find that the nanoscale water droplets feature weakly anisotropic dielectric constant: the radial component of dielectric constants is distinctly smaller than the tangential component although they both decrease with reducing droplet size in a similar way. Such dielectric behavior is closely related to the orientational preference of water molecules near the convex surface. The molecular dipole prefers to slightly orientate toward the interior of droplets in contrast to the out-of-plane preference for free-standing water films and slab-confined water, which suppresses the fluctuation of dipole moments in the radial direction. Meanwhile, it facilitates the formation of the open hydrogen-bond network in the surface layer and ultimately leads to the relatively weak suppression of tangential fluctuations. The differential suppression is responsible for the anisotropic dielectric constant of water droplets. This anisotropic characteristic is also found in dielectric relaxation: both the radial and the tangential relaxation are consistently slowed down upon approaching surface but the latter is universally slower.

A Model Electron Transfer Reaction in Confined Aqueous Solution

Liquid water confined within nanometer-sized channels exhibits a strongly reduced local dielectric constant perpendicular to the wall, especially at the interface, and this has been suggested to induce faster electron transfer kinetics at the interface than in the bulk. We study a model electron transfer reaction in aqueous solution confined between graphene sheets with classical molecular dynamics. We show that the solvent reorganization energy is reduced at the interface compared to the bulk, which explains the larger rate constant. However, this facilitated solvent reorganization is due to the partial desolvation by the graphene sheet of the ions involved in the electron transfer and not to a local dielectric constant reduction effect.

Bulk and interfacial nanostructure and properties in deep eutectic solvents: Current perspectives and future directions

Deep eutectic solvents (DESs) are a tailorable class of solvents that are rapidly gaining scientific and industrial interest. This is because they are distinct from conventional molecular solvents, inherently tuneable via careful selection of constituents, and possess many attractive properties for applications, including catalysis, chemical extraction, reaction media, novel lubricants, materials chemistry, and electrochemistry. DESs are a class of solvents composed solely of hydrogen bond donors and acceptors with a melting point lower than the individual components and are often fluidic at room temperature. A unique feature of DESs is that they possess distinct bulk liquid and interfacial nanostructure, which results from intra- and inter-molecular interactions, including coulomb forces, hydrogen bonding, van der Waals interactions, electrostatics, dispersion forces, and apolar-polar segregation. This nanostructure manifests as preferential spatial arrangements of the different species, and exists over several length scales, from molecular- to nano- and meso-scales. The physicochemical properties of DESs are dictated by structure-property relationships; however, there is a significant gap in our understanding of the underlying factors which govern their solvent properties. This is a major limitation of DES-based technologies, as nanostructure can significantly influence physical properties and thus potential applications. This perspective provides an overview of the current state of knowledge of DES nanostructure, both in the bulk liquid and at solid interfaces. We provide definitions which clearly distinguish DESs as a unique solvent class, rather than a subset of ILs. An appraisal of recent work provides hints towards trends in structure-property relationships, while also highlighting inconsistencies within the literature suggesting new research directions for the field. It is hoped that this review will provide insight into DES nanostructure, their potential applications, and development of a robust framework for systematic investigation moving forward.

Entropic Stabilization of Water at Graphitic Interfaces

The thermodynamic stability of water next to graphitic surfaces is of fundamental interest, as it underlies several natural phenomena and important industrial processes. It is commonly assumed that water wets graphite more than graphene due to increased, favorable van der Waals interactions between the interfacial water molecules with multiple carbon sheets. Here, we employed extensive computer simulations and analysis of the molecular correlation functions to show that the interfacial water thermodynamics is in fact dominated by surface entropy. We show that on graphite, destabilization of the interfacial hydrogen bond network leads to an overcompensating increase in population of low frequency translational and librational modes, which is ultimately responsible for the increased interfacial stability compared to graphene. The spectroscopic signature of this effect is an enhancement of the modes near 100 and 300 cm^-1. This subtle interplay between entropy and surface binding may have important consideration for interpretations of various phenomena, including the hydrophobic effect.

Dielectric Susceptibility of Water in the Interface

It has long been anticipated that dielectric constants of polar liquids are reduced in the interfacial layer. Recent experiments and computer simulations support these expectations. A strong reduction of the dielectric constant is found in the direction perpendicular to a planar substrate, while the parallel response is bulk-like. This Perspective highlights recent theoretical calculations and simulations with an eye on relating them to properties observable in the laboratory. The average interface dielectric constant computed from simulations connects to thin films experiments, but this cannot be extended to screening of charges. In contrast to dielectric theories where a single dielectric constant gauges both the polarization energy and screening, these two signatures of dielectric polarization diverge on the molecular scale. The reduction of the dielectric constant of water in thin films is currently viewed as a combined effect of geometric confinement imposed by the substrate and the reconstruction of water hydrogen bonds in the surface layer.

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.

Confined Water's Dielectric Constant Reduction Is Due to the Surrounding Low Dielectric Media and Not to Interfacial Molecular Ordering

Liquid water confined within nanometer-sized channels exhibits a surprisingly low dielectric constant along the direction orthogonal to the channel walls. This is typically assumed to result from a pronounced heterogeneity across the sample: the dielectric constant would be bulk-like everywhere except at the interface, where it would be dramatically reduced by strong restrictions on interfacial molecules. Here we study the dielectric properties of water confined within graphene slit channels via classical molecular dynamics simulations. We show that the permittivity reduction is not due to any important alignment of interfacial water molecules, but instead to the long-ranged anisotropic dipole correlations combined with an excluded-volume effect of the low-dielectric confining material. The bulk permittivity is gradually recovered only over several nanometers due to the impact of long-range electrostatics, rather than structural features. This has important consequences for the control of, e.g., ion transport and chemical reactivity in nanoscopic channels and droplets.

Static dielectric permittivity of ionic liquids ultraconfined in carbon nanotubes

In this work the parallel component of the static dielectric permittivity, ϵ II of ionic liquids ultraconfined into flexible carbon nanotubes of radius of 1.2 nm and 2.4 nm is evaluated from molecular dynamics simulations. We show an enhancement of ϵ II with respect to bulk value and a counter-intuitive temperature dependence. Indeed an increase of ϵ II as a function of the temperature opposed to a bulk behavior is evidenced. Increase in ϵ II is the result of the strong orientation of ionic liquid close to the pore wall. The temperature dependence is the consequence of the thermal fluctuations increasing the dipolar fluctuations such that the strong orientation is conserved. Eventually, we show a molecular stacking between [C4mim+][Tf2N−] and CNT decreasing dipolar fluctuations close to the CNT surface reducing ϵ II .

Nanostructure of a deep eutectic solvent at solid interfaces

HYPOTHESIS

Deep eutectic solvents (DESs) are an attractive class of tunable solvents. However, their uptake for relevant applications has been limited due to a lack of detailed information on their structure-property relationships, both in the bulk and at interfaces. The lateral nanostructure of the DES-solid interfaces is likely to be more complex than previously reported and requires detailed, high-resolution investigation.

EXPERIMENTS

We employ a combination of high-resolution amplitude-modulated atomic force microscopy and molecular dynamics simulations to elucidate the lateral nanostructure of a DES at the solid-liquid interface. Specifically, the lateral and near-surface nanostructure of the DES choline chloride:glycerol is probed at the mica and highly-ordered pyrolytic graphite interfaces.

FINDINGS

The lateral nanostructure of the DES-solid interface is heterogeneous and well-ordered in both systems. At the mica interface, the DES is strongly ordered via polar interactions. The adsorbed layer has a distinct rhomboidal symmetry with a repeat spacing of ~0.9 nm comprising all DES species. At the highly ordered pyrolytic graphite interface, the adsorbed layer appears distinctly different, forming an apolor-driven row-like structure with a repeat spacing of ~0.6 nm, which largely excludes the chloride ion. The interfacial nanostructure results from a delicate balance of substrate templating, liquid-liquid interactions, species surface affinity, and packing constraints of cations, anions, and molecular components within the DES. For both systems, distinct near-surface nanostructural layering is observed, which becomes more pronounced close to the substrate. The surface nanostructures elucidated here significantly expand our understanding of DES interfacial behavior and will enhance the optimization of DES systems for surface-based applications.

Anomalous dielectric response of nanoconfined water

In order to develop a microscopic level understanding of the anomalous dielectric properties of nanoconfined water (NCW), we study and compare three different systems, namely, (i) NCW between parallel graphene sheets (NCW-GSs), (ii) NCW inside graphene covered nanosphere (NCW-Sph), and (iii) a collection of one- and two-dimensional constrained Ising spins with fixed orientations at the termini. We evaluate the dielectric constant and study the scaling of ε with size by using linear response theory and computer simulations. We find that the perpendicular component remains anomalously low at smaller inter-plate separations (d) over a relatively wide range of d. For NCW-Sph, we could evaluate the dielectric constant exactly and again find a low value and a slow convergence to the bulk. To obtain a measure of surface influence into the bulk, we introduce and calculate correlation lengths to find values of ∼9 nm for NCW-GS and ∼5 nm for NCW-Sph, which are surprisingly large, especially for water. We discover that the dipole moment autocorrelations exhibit an unexpected ultrafast decay. We observe the presence of a ubiquitous frequency of ∼1000 cm^-1, associated only with the perpendicular component for NCW-GS. This (caging) frequency seems to play a pivotal role in controlling both static and dynamic dielectric responses in the perpendicular direction. It disappears with an increase in d in a manner that corroborates with the estimated correlation length. A similar observation is obtained for NCW-Sph. Interestingly, one- and two-dimensional Ising model systems that follow Glauber spin-flip dynamics reproduce the general characteristics.

Confinement-Induced Enhancement of Parallel Dielectric Permittivity: Super Permittivity Under Extreme Confinement

Enhancement of parallel (x-y plane) dielectric permittivity of confined fluids has been shown previously. However, a theoretical model that explains this enhancement is lacking thus far. In this study, using statistical-mechanical theories and molecular dynamics simulations, we show an explicit relation between the parallel dielectric permittivity, density variations, and dipolar correlations for protic and aprotic fluids confined in slit-like channels. We analyze the importance of dipolar correlations on enhancement of parallel dielectric permittivity inside large channels and extreme confinements. In large channels, beyond the interfacial region, dipolar correlations exhibit bulk-like behavior. Under extreme confinement, the correlations become stronger to the extent that they give rise to a giant increase in the parallel dielectric permittivity. This sudden increase in dielectric permittivity can be a signature of a liquid transition into higher-ordered structures and has important consequences for understanding ion transport, molecular dissociation, and chemical reactions inside nanoconfined environments.

Water Layer at Hydrophobic Surface: Electrically Dead but Dynamically Alive?

The origin of the anomalous low value of the static dielectric constant (SDC) of confined water has been addressed and unearthed. While the low value is partly due to the different dielectric boundaries, a significant role is played by the "electrically dead layer" (EDL). As the observed dielectric constant is the harmonic mean of the grid-wise SDCs, the first layer, having the smallest SDC, makes a disproportionately large contribution. This enhanced contribution, in turn, arises from the orientationally ordered surface water molecules. They exhibit reduced fluctuations in collective dipole moment, as the molecules remain partly caged due to water-surface interactions. This phenomenon is found to be universal. We study the structure and dynamics of the water molecules which characterize the EDL. We demonstrate that while the EDL remains alive at a molecular level, with a finite residence time, it displays time scales not substantially different compared to the distant water layers.