Dielectric Properties Of Liquid Water Research Articles

Machine learning the electric field response of condensed phase systems using perturbed neural network potentials

The interaction of condensed phase systems with external electric fields is of major importance in a myriad of processes in nature and technology, ranging from the field-directed motion of cells (galvanotaxis), to geochemistry and the formation of ice phases on planets, to field-directed chemical catalysis and energy storage and conversion systems including supercapacitors, batteries and solar cells. Molecular simulation in the presence of electric fields would give important atomistic insight into these processes but applications of the most accurate methods such as ab-initio molecular dynamics (AIMD) are limited in scope by their computational expense. Here we introduce Perturbed Neural Network Potential Molecular Dynamics (PNNP MD) to push back the accessible time and length scales of such simulations. We demonstrate that important dielectric properties of liquid water including the field-induced relaxation dynamics, the dielectric constant and the field-dependent IR spectrum can be machine learned up to surprisingly high field strengths of about 0.2 V Å−1 without loss in accuracy when compared to ab-initio molecular dynamics. This is remarkable because, in contrast to most previous approaches, the two neural networks on which PNNP MD is based are exclusively trained on molecular configurations sampled from zero-field MD simulations, demonstrating that the networks not only interpolate but also reliably extrapolate the field response. PNNP MD is based on rigorous theory yet it is simple, general, modular, and systematically improvable allowing us to obtain atomistic insight into the interaction of a wide range of condensed phase systems with external electric fields.

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.

Temperature dependent optical and dielectric properties of liquid water studied by terahertz time-domain spectroscopy

Terahertz waves have found wide applications in the biological and medical areas. Since water is one of the most important ingredients in the biological systems, it is necessary to investigate its optical and dielectric properties in the terahertz range. These fundamental physical parameters play an important role in many applications, such as biological imaging and hyperthermia applications, and are temperature dependent. In order to shed more light on the property of temperature dependence, we performed a systematic investigation by changing the temperature of the water sample and using a terahertz time-domain spectrometer. The experimental results show that when the temperature increases from 275 K to 340 K, the refractive index and the complex dielectric constant of water increase noticeable, with the absorption coefficient increased observably. A good agreement is found between experimental data and the double Debye model.

Polar-solvation classical density-functional theory for electrolyte aqueous solutions near a wall.

A precise description of the structural and dielectric properties of liquid water is critical to understanding the physicochemical properties of solutes in electrolyte solutions. In this article, a mixture of ionic and dipolar hard spheres is considered to account for water crowding and polarization effects on ionic electrical double layers near a uniformly charged hard wall. As a unique feature, solvent hard spheres carrying a dipole at their centers were used to model water molecules at experimentally known concentration, molecule size, and dipolar moment. The equilibrium ionic and dipole density profiles of this electrolyte aqueous model were calculated using a polar-solvation classical density-functional theory (PSCDFT). These profiles were used to calculate the charge density distribution, water polarization, dielectric permittivity function, and mean electric potential profiles as well as differential capacitance, excess adsorptions, and wall-fluid surface tension. These results were compared with those corresponding to the pure dipolar model and unpolar primitive solvent model of electrolyte aqueous solutions to understand the role that water crowding and polarization effects play on the structural and thermodynamic properties of these systems. Overall, PSCDFT predictions are in agreement with available experimental data.

Vibrational polarizabilities of hydrogen-bonded water

The vibrational polarizabilities and the related molecular properties of hydrogen-bonded water are analyzed theoretically, taking the case of (water)30 clusters as an example case. It is shown that some off-diagonal dipole derivatives are large for the translations of incompletely hydrogen-bonded molecules, and this is reasonably explained by the scheme of intermolecular charge fluxes induced along hydrogen bonds. In total, because of these intermolecular charge fluxes, molecular translations give rise to the vibrational polarizability of 2.8–3.3 a03 per molecule, which is as large as about 40% of the electronic polarizability, mainly in the frequency region below 300cm−1. Adding the contributions of the molecular rotations (librations) and the translation–rotation cross term, the total polarizability (electronic+vibrational) at ∼100cm−1 is slightly larger than the double of that at >4000cm−1. The relation of these results to some observed time- and frequency-dependent dielectric properties of liquid water is briefly discussed.

Mixing coarse-grained and fine-grained water in molecular dynamics simulations of a single system

The use of a supra-molecular coarse-grained (CG) model for liquid water as solvent in molecular dynamics simulations of biomolecules represented at the fine-grained (FG) atomic level of modelling may reduce the computational effort by one or two orders of magnitude. However, even if the pure FG model and the pure CG model represent the properties of the particular substance of interest rather well, their application in a hybrid FG/CG system containing varying ratios of FG versus CG particles is highly non-trivial, because it requires an appropriate balance between FG-FG, FG-CG, and CG-CG energies, and FG and CG entropies. Here, the properties of liquid water are used to calibrate the FG-CG interactions for the simple-point-charge water model at the FG level and a recently proposed supra-molecular water model at the CG level that represents five water molecules by one CG bead containing two interaction sites. Only two parameters are needed to reproduce different thermodynamic and dielectric properties of liquid water at physiological temperature and pressure for various mole fractions of CG water in FG water. The parametrisation strategy for the FG-CG interactions is simple and can be easily transferred to interactions between atomistic biomolecules and CG water.

Evaluation of Water Permittivity Models From Ground-Based Observations of Cold Clouds at Frequencies Between 23 and 170 GHz

Accurate retrievals of liquid water path (LWP) from passive microwave radiometers rely on the use of radiative transfer models to describe the absorption of radiation by various atmospheric components. When clouds are present, atmospheric absorption is affected by the dielectric properties of liquid water. In this paper, we use measurements from four microwave radiometers to assess four models of the complex permittivity of water. The observations are collected at five frequencies between 23.8 and 170 GHz. The purpose of the study is to compare measurements of microwave absorption with model computations in supercooled liquid clouds that have temperatures between 0 °C and -30 °C. Models of liquid water permittivity in this temperature range suffer from a lack of laboratory measurements and are generally derived from the extrapolation of available data. An additional rationale for this work is to examine to what degree the use of different dielectric models affects the retrieval of LWP in supercooled liquid clouds. Inaccuracies in modeling the water permittivity at low temperatures are likely one of the largest sources of retrieval uncertainty in supercooled clouds, uncertainty that could offset the advantages offered by the enhanced sensitivity of channels at frequencies at and above 90 GHz relative to lower frequencies.

Terahertz reflection spectroscopy of Debye relaxation in polar liquids [Invited

Terahertz (THz) radiation interacts strongly with the intermolecular hydrogen-bond network in aqueous liquids. The dielectric properties of liquid water and aqueous solutions in the THz spectral region are closely linked to the microscopic dynamics of the liquid solution, and hence THz spectroscopy offers an important insight into fundamental intermolecular interactions in polar liquids. At the same time, the strong and characteristic interaction between THz radiation and liquids offers a methodology for the classification of liquids inside containers, and hence the THz region is suitable for remote detection of some of the properties of bottled liquids. Here we present a review of THz spectroscopy and modeling of water-ethanol mixtures, and establish a link between the dielectric function of water-ethanol mixtures and some of their thermodynamic properties. We then review how the knowledge of the dielectric function of aqueous mixtures can be used for inspection of liquids inside bottles. Finally we draw up some of the limits to the applicability of THz reflection spectroscopy in the identification of dangerous liquids.

Dynamic polarizability, Cauchy moments, and the optical absorption spectrum of liquid water: A sequential molecular dynamics/quantum mechanical approach

The dynamic polarizability and optical absorption spectrum of liquid water in the 6-15 eV energy range are investigated by a sequential molecular dynamics (MD)/quantum mechanical approach. The MD simulations are based on a polarizable model for liquid water. Calculation of electronic properties relies on time-dependent density functional and equation-of-motion coupled-cluster theories. Results for the dynamic polarizability, Cauchy moments, S(-2), S(-4), S(-6), and dielectric properties of liquid water are reported. The theoretical predictions for the optical absorption spectrum of liquid water are in good agreement with experimental information.

Retro-modelling of a dual resonant applicator and accurate dielectric properties of liquid water from −20 °C to +100 °C

A critical analysis is made of literature reports on the dielectric properties of pure water from liquid water supercooled at −20 °C, to +100 °C, in principle in the frequency range 0 to 3 GHz. Measurements using complete multi-step numerical modelling of a dual resonant cavity at about 920 MHz and 2230 MHz are presented. The measured data at about +20 °C are used as reference for the calculation of data at other temperatures. Due to the high resolution and considerations of various error sources, the resulting accuracy becomes high and allows the construction of improved empirical formulae for the Debye relaxation behaviour.

Dielectric capacity, liquid water content, and pore structure of thawing–freezing materials

A capacitive sensor-based experimental approach is worked out to study the ice/water phase change in cohesive porous media subject to freezing and thawing. This technique relies upon the dielectric properties of liquid water, ice, air, and mineral substrate in the radio-frequency range. A semi-empirical method based upon the Lichtenecker model and combining drying and freezing tests provides an accurate estimation of the liquid water content versus the temperature in freezing cement pastes. This estimation is further analysed with the help of thermoporometry concepts in order to characterize the pore size distribution and the specific surface area. The results range in the same order of magnitude as those assessed from gravimetric sorption/desorption isotherms.

Enhancement of the Wolf Damped Coulomb Potential: Static, Dynamic, and Dielectric Properties of Liquid Water from Molecular Simulation

The recently introduced damped Coulomb potential [Wolf, D.; Keblinski, P.; Phillpot, S. R.; Eggebrecht, J. J. Chem. Phys. 1999, 110, 8254] is analyzed and slightly modified to obtain a significant improvement of dielectric properties when applied to simulations of dipolar liquids. It is used in molecular dynamics simulations of TIP3P and SPC water models. The results are compared with simulations using Ewald summation and the CHARMM shifted-force potential. A transferable scheme for parametrization of damping constants and cutoff radii is given. With a cutoff distance of only 9 A, static properties such as average potential energy, particle density, and radial distribution functions, dynamic properties such as velocity autocorrelation function and dipole moment relaxation, and the dielectric constant are found to be in good agreement with the reference Ewald simulations.

Heat and Mass Transfer during Microwave Steam Treatment of Contaminated Soils

A time-dependent mechanistic model has been developed to describe the coupled heat and mass transfer during microwave steam treatment of contaminated soils. The model consists of multiphase mass and energy conservation equations, along with various temperature-dependent parameterizations of important physical state properties. The coupled nonlinear equations were solved numerically by applying a macroscopic control volume method with the appropriate boundary and initial conditions and using a finite-difference scheme. Microwave heating experiments of soil samples were performed. The effects of microwave power, soil sample depth, and soil type on water evaporation rate and temperature were measured with time. The modeled results are in good agreement with the experimental data. The results revealed that microwave power density and soil sample depth significantly affect the microwave evaporation process of water from soils. Water evaporation rate and temperature variation are complex and associated with the dielectric properties of liquid water distributed within the soil. The deeper the soil sample, the more nonuniform the evaporation process, although nonuniformities diminish as the evaporation process approaches completion. Finally it was verified that soil types and their thermal properties play a minor role, provided that the soil minerals are microwave transparent.

Hydrogen Network Fluctuations and the Microwave Dielectric Properties of Liquid Water

A short tutorial is given on interactions of water with electromagnetic fields, particularly in the microwave frequency range. Properties of the water molecule are sketched and attributes of the percolating tetrahedral hydrogen bond network of liquid water are presented. Characteristics in the polarization noise of liquid water are discussed and are related to the dielectric spectrum of water. The principal dielectric relaxation in the microwave region is represented by a Debye-type spectrum, the parameter values of which are given as a function of temperature in tabular form. At 19°C, as an example, the complex dielectric water spectrum is shown in an extended frequency range, also covering the near millimetre and submillimetre wave region up to 2 THz. A wait-and-switch model of dielectric relaxation, as suggested by computer simulation studies, is briefly outlined, in order to illustrate the mechanisms behind the hydrogen network fluctuations.

Spectroscopic and dielectric properties of liquid water: A molecular dynamics simulation study

The spectroscopic and dielectric properties of liquid water under an ambient condition are studied via a molecular dynamics (MD) computer simulation method. By employing the recent TAB/10D potential model [B. D. Bursulaya, J. Jeon, D. A. Zichi, and H. J. Kim, J. Chem. Phys. 108, 3286 (1997)], the evolving solvent electronic structure is incorporated into the simulation. Thus both the induced dipole and polarizability variations of individual water molecules with the fluctuating nuclear configuration are accounted for. The MD results on far-IR absorption, depolarized Raman scattering (DRS) and optical Kerr effect (OKE) spectroscopy are in reasonable agreement with experiments. It is found that the nonlinear electronic response of water to its fluctuating environment plays an important role in the DRS and OKE; it significantly enhances the contribution of the water librational motions to the spectra, compared with that of hindered translations. This indicates that not only molecular dynamics but also accompanying electronic structure modulations are essential to quantitative understanding of various electronic spectroscopy. The effects of H/D isotope substitution are briefly discussed.

Alternative schemes for the inclusion of a reaction-field correction into molecular dynamics simulations: Influence on the simulated energetic, structural, and dielectric properties of liquid water

Different schemes for treating the electrostatic interactions in molecular dynamics simulations are investigated: charge-group truncation with or without reaction-field correction, atomic truncation with or without reaction-field correction, and Ewald summation. When a reaction-field correction is applied, the influence of the size of the radius selected for the spherical boundary to the continuum is also considered. The different schemes are applied to simple point charge water simulations, and simulated energetic, transport, structural, and dielectric properties are compared. It is concluded that (i) the inclusion of a reaction-field correction in a charge-group truncation scheme induces significant changes in different types of properties, and that a number of properties are not identical to those observed using the Ewald scheme, (ii) when the reaction-field correction is included in an atomic truncation scheme instead, the agreement with the Ewald results is in general improved, and (iii) the increase (decrease) of the radius of the boundary to the continuum by 0.1 (nm) with respect to the cutoff radius induces in both cases a degradation of the simulated properties. Special attention is paid to the calculation of the dielectric permittivity from the simulations. Due to the finite size of the statistical ensembles considered, this property is not assumed to be isotropic, and the degree of anisotropy is used instead as a test for convergence. Since the incorporation of the reaction-field correction into an atomic truncation scheme leads, when systems of high dielectric permittivity are considered, to electrostatic interactions which implicitly contain a (physically reasonable) shifting function and properties which are comparable to those obtained using the Ewald method, this scheme is a clear improvement over a charge-group-based truncation when a reaction-field correction is used in molecular dynamics simulations of noncharged systems.

Integral equations and simulation studies of waterlike models

Central force potentials designed to model water are investigated by means of integral equation theories and computer simulations. We consider an associating neutral model which yields molecules with the geometry of the water molecule and that incorporates an effective pair potential aimed at describing the hydrogen bond interaction. In addition we study a charged version of the Hamiltonian, which provides a realistic description of the properties of liquid water. These models are analyzed in the full association limit, which is obtained by imposing a number of bonds per particle compatible with the water geometry, i.e., 2 and 1 for OH and HH correlations. The structure of the neutral model presents remarkable resemblances with that obtained using realistic models of water. In addition, the atomic Ornstein–Zernike theory along with the hypernetted-chain closure provides an accurate description of the structure of this anisotropic molecular system. The consideration of the full association limit introduces important improvements in the theoretical description of the charged central force model. The performance of this approach in the prediction of thermodynamic, structural, and dielectric properties of liquid water is investigated and the results compared with simulation and experimental data.

Reaction Field Effects on the Simulated Properties of Liquid Water

Abstract The effects of including a reaction field contribution on the structure and dynamics of liquid water have been investigated using molecular dynamics simulations. Reaction field effects are determined for two models of water, the simple point charge (SPC) model and the extended simple point charge (SPC/E) model, and at two temperatures (277 K and 300 K). Inclusion of the reaction field leads to a reduced system density, an increase in translational diffusion, which is model dependent, an increase in internal energy, and an increase in rotational diffusion rates, in addition to the large (known) changes in the dielectric properties of liquid water. It is concluded that continued use of the reaction field technique should involve a reparameterization of the water model and not merely a merging with the original model parameters.

DIELECTRIC PROPERTIES OF MATERIALS AND MEASUREMENT TECHNIQUES

ABSTRACT The dielectric properties of materials are defined, and the nature of their dependence on moisture content, frequency of the applied electric field, temperature of the material, and density of particulate materials is discussed. Dielectric properties of liquid water are summarized. The influence of moisture content, frequency, temperature, and density on the values of the dielectric constant and loss factor, real and imaginary parts of the relative complex permittivity, respectively, are illustrated for several different kinds of materials. Examples include dielectric properties for the cereal grains corn and wheat, pecans, and pulverized coal. Techniques for the measurement of dielectric properties of materials at high frequencies and microwave frequencies useful for dielectric heating and drying applications are discussed briefly, and numerous publications describing these methods in greater detail are cited for reference.

Computer Simulation of the Dielectric Properties of Liquid Water

Abstract Monte Carlo simulations of water in the NVT ensemble using three models (SPC, TIP4P and TIPS2) are reported. The internal energy, dielectric constant, and the site-site radial distribution functions of liquid water (temperature 300 K and mass density 1 gm cc−1) were calculated and compared with experiment. It was found that of the three intermolecular potential models, SPC gives the best dielectric constant. Since SPC also yields acceptable results for the energy and structure, it is judged to be the best among the three models studied.