We present an accurate free-energy functional for liquid water written in terms of a set of effective potential fields in which fictitious noninteracting water molecules move. The functional contains an \emph{exact} expression of the entropy of noninteracting molecules and thus provides an ideal starting point for the inclusion of complex inter-molecular interactions which depend on the \emph{orientation} of the interacting molecules. We show how an excess free-energy functional can be constructed to reproduce the following properties of water: the dielectric response; the experimental site-site correlation functions; the surface tension; the bulk modulus of the liquid and the variation of this modulus with pressure; the density of the liquid and the vapor phase; and liquid-vapor coexistence. As a demonstration, we present results for the application of this theory to the behavior of liquid water in a parallel plate capacitor. In particular, we make predictions for the dielectric response of water in the nonlinear regime, finding excellent agreement with known data.
In this work we give the analysis of the low frequency dielectric response of selected kaolinitic clays at different water content ranging from dry, over plastic to liquid limit. For all samples investigated, depending on the moisture content, the real part of the dielectric function reflects the abnormal behaviour within the certain frequency region where the negative values of real part of dielectric response have been observed. The anomalies are explained within a generalized conductivity model based on the clay-water electrolyte ions free motion in pore water and their restricted motion ne…
We present a lattice Monte Carlo algorithm based on the one originally proposed by Maggs and Rossetto for simulating electrostatic interactions in inhomogeneous dielectric media. The original algorithm is known to produce attractive interactions between particles of the same dielectric constant in the medium of different dielectric constant. We demonstrate that such interactions are spurious, caused by incorrectly biased statistical weight arising from particle motion during the Monte Carlo moves. We propose a simple parallel tempering algorithm that corrects this unphysical bias. The efficacy…
Investigations of dielectric properties of water in nanoconfinement are highly relevant for various applications. Here, using a simple capacitor model, we show that the low dielectric constant of nanoconfined water found in molecular dynamics simulations can be largely explained by the so-called dielectric dead-layer effect known for ferroelectric nanocapacitors.
One century ago pioneering dielectric results obtained for water and n-alcohols triggered the advent of molecular rotation diffusion theory considered by Debye to describe the primary dielectric absorption in these liquids. Comparing dielectric, viscoelastic, and light scattering results we unambiguously demonstrate that the structural relaxation appears only as a high-frequency shoulder in the dielectric spectra of water. In contrast, the main dielectric peak is related to a supramolecular structure, analogous to the Debye-like peak observed in mono-alcohols.
We define the dielectric constant (susceptibility) that should enter the Maxwell boundary value problem when applied to microscopic dielectric interfaces. The dielectric constant (susceptibility) of the interface is defined by exact linear-response equations involving correlations of statistically fluctuating interface polarization and the Coulomb interaction energy of external charges with the dielectric. The theory is applied to the interface between water and spherical solutes of altering size studied by molecular dynamics (MD) simulations. The effective dielectric constant of interfacial w…
Screening is one of the most important concepts in the study of charged systems. Near a dielectric interface, the ion distribution in a salt solution can be highly nonuniform. Here, we develop a theory that self-consistently treats the inhomogeneous screening effects. At higher concentrations when the bulk Debye screening length is comparable to the Bjerrum length, the double layer structure and interfacial properties are significantly affected by the inhomogeneous screening. In particular, the depletion zone is considerably wider than that predicted by the bulk screening approximation or the …
In this work, the dielectric behavior of water inside charged nanoslit of graphene is studied to analized the water molecules under electrical confinement; through polarizing the nanoslit of graphene, creating an electric field inside the nanopore. How the water molecules are structured under this type of electrical confinement is studied with two force fields of water, the three-site water models here used are the SCP/$ε$ and the FAB/$ε$, the first is a rigid model that improves the SPC model and the second is a flexible model that improves all the force fields of three sites non-polarizables…
We investigate the effect of quenched surface charge disorder on electrostatic interactions between two charged surfaces in the presence of dielectric inhomogeneities and added salt. We show that in the linear weak-coupling regime (i.e., by including mean-field and Gaussian-fluctuations contributions), the image-charge effects lead to a non-zero disorder-induced interaction free energy between two surfaces of equal mean charge that can be repulsive or attractive depending on the dielectric mismatch across the bounding surfaces and the exact location of the disordered charge distribution.
Dielectric permittivity of supercooled volume water has been measured in the range of temperatures from $-20\,^{\circ}$C to $-60\,^{\circ}$C at the frequencies $7.6$~GHz and $9.7$~GHz. The measurements have been made using microwave resonators and wetted silicate sorbents. From the data obtained, the temperature dependences of the relaxation frequencies were updated for the Debye model with two relaxation frequencies. The updated formulae for dielectric permittivity of cold water may be applied to the frequency range $7\ldots200$~GHz.
Recent experiments reporting a very low dielectric permittivity for nanoconfined water have renewed the interest to the structure and dielectric properties of water in narrow gaps. Here, we describe such systems with a minimal Landau-Ginzburg field-theory composed of a nonlocal bulk-determined term and a local water-surface interaction term. We show how the interplay between the boundary conditions and intrinsic bulk correlations encodes dielectric properties of confined water. Our theoretical analysis is supported by molecular dynamics simulations and comparison with the experimental data.
The dielectric relaxation of water in glassy aqueous binary mixtures exhibits an Arrhenius behaviour with a nearly universal activation energy. We here demonstrate that its characteristic relaxation time follows a remarkably general functional dependence on the weight fraction of water for a wide range of molecular systems.
We investigate the temperature dependence of thermodynamic (density, isobaric heat capacity), dynamical (self-diffusion coefficient, shear viscosity), and dielectric properties of several water models, the commonly employed TIP3P water model, well-established 4-point water model TIP4P-2005, and recently developed 4-point water model TIP4P-D. We focus on the temperature range of interest for the field of computational biophysics and soft matter (280-350 K). The 4-point water models lead to a spectacularly improved agreement with experimental data, strongly suggesting that the use of more modern…
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 79 down to 2. If the confining walls are made of a dielectric material, the Hamaker constant reduces almost by 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, i…
We have employed molecular dynamics simulations based on the TIP4P/2005 water model to investigate the local structural, dynamical, and dielectric properties of the two recently reported body-centered-cubic and face-centered-cubic plastic crystal phases of water. Our results reveal significant differences in the local orientational structure and rotational dynamics of water molecules for the two polymorphs. The probability distributions of trigonal and tetrahedral order parameters exhibit a multi-modal structure, implying the existence of significant local orientational heterogeneities, partic…
The static dielectric constant of liquid water is computed using classical force field based molecular dynamics simulation at fixed electric displacement D. The method to constrain the electric displacement is the finite temperature classical variant of the constant-D method developed by Stengel, Spaldin and Vanderbilt (Nat. Phys. 2009, 5: 304). There is also a modification of this scheme imposing fixed values of the macroscopic field E. The method is applied to the popular SPC/E model of liquid water. We compare four different estimates of the dielectric constant, two obtained from fluctuatio…
We present an accurate equation of state for water based on a simple microscopic Hamiltonian, with only four parameters that are well-constrained by bulk experimental data. With one additional parameter for the range of interaction, this model yields a computationally efficient free-energy functional for inhomogeneous water which captures short-ranged correlations, cavitation energies and, with suitable long-range corrections, the non-linear dielectric response of water, making it an excellent candidate for studies of mesoscale water and for use in ab initio solvation methods.
Unambiguous characterization of in-plane dielectric response in nanoconfined liquids: water as a case study
Donostia International Physics Center (DIPC)（多诺西亚国际物理中心）；Department of Applied Physics, Engineering School of Gipuzkoa, Basque Country University, UPV/EHU（巴斯克地区大学工程学校吉普斯夸应用物理系）
The surface of a polar liquid presents a special environment for the solvation and organization of charged solutes, which differ from bulk behaviors in important ways. These differences have motivated many attempts to understand electrostatic response at aqueous interfaces in terms of a spatially varying dielectric permittivity, typically concluding that the dielectric constant of interfacial water is significantly lower than in the bulk liquid. Such analyses, however, are complicated by the potentially nonlocal nature of dielectric response over the short length scales of interfacial heteroge…
Dipolar susceptibility of interfacial water and the corresponding interface dielectric constant were calculated from numerical molecular dynamics simulations for neutral and charged states of buckminsterfullerene C$_{60}$. Dielectric constants in the range 10--22, depending on temperature and solute charge, were found. The hydration water undergoes a structural crossover as a function of the solute charge. Its main signatures include the release of dangling O-H bonds pointing toward the solute and the change in the preferential orientations of hydration water from those characterizing hydropho…
Despite water is the most studied substance in the Earth, it is not completely understood why its structural and dynamical properties give rise to some anomalous behaviors. Interesting properties emerge when experiments at low temperatures and/or high pressures, are performed. Here we report dielectric measurements of cold water under constrained conditions, i.e. water that below the melting point can not freeze. The inner pressure shifts the α relaxation peak to similar frequencies as seen in ice Ih. Also, when we reach the triple point at 251 K, ice III seems to form. As far as we know, this…
The goal of this work is to propose a simple continuous model that captures the dielectric properties of water at the nanometric scale. We write an electrostatic energy as a functional of the polarisation field containing a term in $P^4$ and non-local Gaussian terms. Such an hamiltonian can reproduce two key properties of water: the saturation of the polarisation response of water in the presence of a strong electrostatic field and the nanometric dipolar correlations of the solvent molecules modifying the long range van der waals interaction. This model explores thus two fundamental aspects th…
Isothermal-isobaric molecular dynamics simulations have been performed to examine an ample set of properties of the model water-N,N-dimethylformamide (DMF) mixture as a function of composition. The SPC-E and TIP4P-Ew water models together with two united atom models for DMF [Chalaris M., Samios J., J. Chem. Phys., 2000, 112, 8581; Cordeiro J., Int. J. Quantum Chem., 1997, 65, 709] were used. Our principal analyses concern the behaviour of structural properties in terms of radial distribution functions, and the number of hydrogen bonds between molecules of different species as well as thermodyn…
In the supercooled regime at elevated pressure two forms of liquid water, high-density (HDL) and low-density (LDL), have been proposed to be separated by a coexistence line ending at a critical point, but a connection to ambient conditions has been lacking. Here we perform large-scale molecular dynamics simulations and demonstrate that the underlying potential energy surface gives a strictly bimodal characterization of the molecules at all temperatures as spatially inhomogeneous either LDL- or HDL-like with a 3:1 predominance for HDL at ambient conditions. The Widom line, indicating maximum fl…
Gigahertz-to-terahertz spectroscopy of macromolecules in aqueous environments provides an important approach for identifying their global and transient molecular structures, as well as directly assessing hydrogen-bonding. We report dielectric properties of zwitterionic dodecylphosphocholine (DPC) micelles in aqueous solutions over a wide frequency range, from 50 MHz to 1.12 THz. The dielectric relaxation spectra reveal different polarization mechanisms at the molecular level, reflecting the complexity of DPC micelle-water interactions. We have made a deconvolution of the spectra into different…
This paper discusses the nanopores emerging and developing in a liquid dielectric under the action of the ponderomotive electrostrictive forces in a nonuniform electric field. It is shown that the gradient of the electric field in the vicinity of the rupture (cavitation nanopore) substantially increases and determines whether the rupture grows or collapses. The cavitation rupture in the liquid (nanopore) tends to stretch along the lines of the original field. The mechanism of the breakdown associated with the generation of secondary ruptures in the vicinity of the poles of the nanopore is prop…
Electrostatic correlations and variable permittivity of electrolytes are essential for exploring many chemical and physical properties of interfaces in aqueous solutions. We propose a continuum electrostatic model for the treatment of these effects in the framework of the self-consistent field theory. The model incorporates a space-or field-dependent dielectric permittivity and an excluded ion-size effect for the correlation energy. This results in a self-energy modified Poisson-Nernst-Planck or Poisson-Boltzmann equation together with state equations for the self energy and the dielectric fun…
We have investigated thermodynamic and dynamic properties as well as the dielectric constant of water-metha\-nol model mixtures in the entire range of composition by using constant pressure molecular dynamics simulations at ambient conditions. The SPC/E and TIP4P/Ew water models are used in combination with the OPLS united atom modelling for methanol. Changes of the average number of hydrogen bonds between particles of different species and of the fractions of differently bonded molecules are put in correspondence with the behavior of excess mixing volume and enthalpy, of self-diffusion coeffi…
We present a critical comparison of the dielectric properties of three models of water - TIP4P/2005, TIP4P/2005f and TTM3F. Dipole spatial correlation is measured using the distance dependent Kirkwood function along with one dimensional and two dimensional dipole correlation functions. We find that the introduction of flexibility alone does not significantly affect dipole correlation and only affects $\varepsilon(ω)$ at high frequencies. By contrast the introduction of polarizability increases dipole correlation and yields a more accurate $\varepsilon(ω)$. Additionally the introduction of pola…
We generalize the predictions for attractions between over-all neutral surfaces induced by charge fluctuations/correlations to non-uniform systems that include dielectric discontinuities, as is the case for mixed charged lipid membranes in an aqueous solution. We show that the induced interactions depend in a non-trivial way on the dielectric constants of membrane and water and show different scaling with distance depending on these properties. The generality of the calculations also allows us to predict under which dielectric conditions the interaction will change sign and become repulsive.
In this Letter, we present a simple model of aqueous interfacial molecular structure and we use this model to isolate the effects of hydrogen bonding on the dielectric properties of the liquid water-vapor interface. By comparing this model to the results of atomistic simulation we show that the anisotropic distribution of molecular orientations at the interface can be understood by considering the behavior of a single water molecule interacting with the average interfacial density field via an empirical hydrogen bonding potential. We illustrate that the depth dependence of this orientational a…
Using dielectric spectroscopy and differential scanning calorimetry, we have performed a detailed investigation of the influence of water uptake on the translational and reorientational glassy dynamics in the typical ionic liquid 1-Butyl-3-methyl-imidazolium chloride. From a careful analysis of the measured dielectric permittivity and conductivity spectra, we find a significant acceleration of cation reorientation and a marked increase of the ionic conductivity for higher water contents. The latter effect mainly arises due to a strong impact of water content on the glass temperature, which for…
Dielectric spectra (10^4-10^11 Hz) of water and ice at 0 °C are considered in terms of proton conductivity and compared to each other. In this picture, the Debye relaxations, centered at 1/τ_W ~ 20 GHz (in water) and 1/τ_I ~ 5 kHz (in ice), are seen as manifestations of diffusion of separated charges in the form of H3O+ and OH- ions. The charge separation results from the self-dissociation of H2O molecules, and is accompanied by recombination in order to maintain the equilibrium concentration, N. The charge recombination is a diffusion-controlled process with characteristic lifetimes of τ_W an…
Isotopic Fingerprints of Proton-mediated Dielectric Relaxation in Solid and Liquid Water
Austrian Institute of Technology（奥地利技术研究所）；Israel Institute of Technology（以色列理工学院）；University of Washington（华盛顿大学）；École Polytechnique Fédérale de Lausanne（洛桑联邦理工学院）；Hamburg University of Technology（汉堡工业大学）
Friction at water-carbon interfaces remains a major puzzle with theories and simulations unable to explain experimental trends in nanoscale waterflow. A recent theoretical framework -- quantum friction (QF)-- proposes to resolve these experimental observations by considering nonadiabatic coupling between dielectric fluctuations in water and graphitic surfaces. Here, using a classical model that enables fine-tuning of the solid's dielectric spectrum, we provide evidence from simulations in general support of QF. In particular, as features in the solid's dielectric spectrum begin to overlap with…
We present a simple, unified theory for the self-energy of an ion near a dielectric interface. Our theory accounts for both the short-range (solvation) and long-range (image force) electrostatic forces, charge polarization induced by these forces, and the cavity energy. In contrast to previous models, our self energy is continuous across the interface and thus applicable to both the water and air (oil) sides of the interface. With no fitting parameters, we predict the specific ion effect on the interfacial affinity of halogen anions at the water/air interface, and the strong adsorption of hydr…
The density and orientational relaxation of bulk water can be separately studied by depolarized light scattering (DLS) and dielectric spectroscopy (DS), respectively. Here we ask the question of what are the leading collective modes responsible for polarization anisotropy relaxation (DLS) and dipole moment relaxation (DS) of solutions involving mostly hydrophobic solute-water interfaces. We study, by atomistic molecular dynamics simulations, the dynamics and structure of hydration water interfacing N-Acetyl-leucine-methylamide (NALMA) dipeptide. The DLS response of the solution is consistent w…
The Infrared Spectrum is used as an experimental data target, to improved the TIP4P/$ε$, adding harmonic potential U(r) in all bonds and harmonic potential U(θ) in the angle formed by the hydrogens and oxygen atoms of the water molecule. The flexibility in the molecule gives the ability of the water molecules to change, around different temperatures and pressures, their structure and in the bulk liquid the distribution of the dipole moment. This distribution helps to reproduce better the experimental data that the rigid models can not describe. The rigid water models of 3 and 4 sites have limi…
We consider the development of pre-breakdown cavitation nanopores appearing in the dielectric fluid under the influence of the electrostrictive stresses in the inhomogeneous pulsed electric field. It is shown that three characteristic regions can be distinguished near the needle electrode. In the first region, where the electric field gradient is greatest, the cavitation nanopores, occurring during the voltage nanosecond pulse, may grow to the size at which an electron accelerated by the field inside the pores can acquire enough energy for excitation and ionization of the liquid on the opposit…
The effect of the applied trajectory length on the convergence of the static dielectric constant and the self-diffusion coefficient were examined for the SPC/E water model in the NVT ensemble with different system size at 293 K. Very long simulation times of 6-8 ns were employed in order to track the convergence of these properties. Temperature dependence and isotope effects, via using D$_2$O instead of H$_2$O, were also investigated. A simulation for the polarizable SWM4-DP model was also carried out to compare the effect of different potential models. Radial distribution functions and the ne…
In spite of a large number of papers published in the last twenty years in the literature, the basic model of liquid water is still controversial. We have measured the dielectric permittivity of pure water and aqueous chlorides solutions in the range 0.2 to 1.5 THz and we have found that the overall dielectric behavior can be satisfactorily described with the weighted average of two single time constant Debye functions. The weights of the two components have been obtained by infrared spectra analysis and are related to two populations of water molecules having a different long range organizati…
We demonstrate that Casimir-Polder energies between noble gas atoms (dissolved in water) and oil-water interfaces are highly surface specific. Both repulsion (e.g. hexane) and attraction (e.g. glycerine and cyclodecane) is found with different oils. For several intermediate oils (e.g. hexadecane, decane, and cyclohexane) both attraction and repulsion can be found in the same system. Near these oil-water interfaces the interaction is repulsive in the non-retarded limit and turns attractive at larger distances as retardation becomes important. These highly surface specific interactions may have …
We studied dielectric properties of nano-sized liquid water samples confined in polymerized silicates MCM-41 characterized by the porous sizes \sim 3-10nm. We report the direct measurements of the dielectric constant by the dielectric spectroscopy method at frequencies 25Hz-1MHz and demonstrate clear signatures of the second-order phase transition of ferroelectric nature at temperatures next to the λ- point in the bulk supercooled water. The presented results support the previously developed polar liquid phenomenology and hence establish its applicability to model actual phenomena in liquid wa…
We experimentally study the transition from steady flow to unsteady flow in a quasi-2D granular heap when small amounts of water are added to monodisperse glass spheres. Particles flow uniformly down both sides of the heap for low water content, but unsteady flow occurs as the water content increases. The unsteady flow mode consists of a non-depositing downslope avalanche and an upslope propagating granular jump. The transition from steady to unsteady flow occurs when the slope exceeds a critical angle as a result of water-induced cohesion. Under unsteady flow conditions, the deposited heap co…
The energy functional, the governing partial differential equation(s) (PDE), and the boundary conditions need to be consistent with each other in a modeling system. In electrolyte solution study, people usually use a free energy form of an infinite domain system (with vanishing potential boundary condition) and the derived PDE(s) for analysis and computing. However, in many real systems and/or numerical computing, the objective domain is finite, and people still use the similar energy form, PDE(s) but with different boundary conditions, which may cause inconsistency. In this work, (1) we prese…
The low-frequency collective vibrational modes in proteins as well as the protein-water interface have been suggested as dominant factors controlling the efficiency of biochemical reactions and biological energy transport. It is thus crucial to uncover the mystery of hydration structure and dynamics as well as their coupling to collective motions of proteins in aqueous solutions. Here we report dielectric properties of aqueous bovine serum albumin protein solutions as a model system using an extremely sensitive dielectric spectrometer with frequencies spanning from megahertz to terahertz. The …
Diffusion-Oscillatory Dynamics in Liquid Water on Data of Dielectric Spectroscopy
A.M. Prokhorov General Physics Institute, Russian Academy of Sciences（俄罗斯科学院A.M.普罗霍罗夫普通物理研究所）；Physical Department, M.V. Lomonosov Moscow State University（莫斯科国立大学物理系）
Using a combination of dielectric spectroscopy and solid-state deuteron NMR, the hydration water dynamics of connective tissue proteins is studied at sub-ambient temperatures. In this range, the water dynamics follows an Arrhenius law. A scaling analysis of dielectric losses, 'two-phase' NMR spectra, and spin-lattice relaxation times consistently yield evidence for a Gaussian distribution of energy barriers. With the dielectric data as input, random-walk simulations of a large-angle, quasi-isotropic water reorientation provide an approximate description of stimulated-echo data on hydrated elas…
We present a technique to locally and rapidly heat water drops in microfluidic devices with microwave dielectric heating. Water absorbs microwave power more efficiently than polymers, glass, and oils due to its permanent molecular dipole moment that has a large dielectric loss at GHz frequencies. The relevant heat capacity of the system is a single thermally isolated picoliter drop of water and this enables very fast thermal cycling. We demonstrate microwave dielectric heating in a microfluidic device that integrates a flow-focusing drop maker, drop splitters, and metal electrodes to locally d…
Broadband Dielectric Spectroscopy on dry Poly(vinyl alcohol)/Poly(vinylidene fluoride) blends reinforced with Nano-Graphene Platelets at combined pressure and temperature
Spherical confinement can alter the properties of a dipolar fluid in several different ways. In an atomistic molecular dynamics simulation study of two different dipolar liquids (SPC/E water and a model Stockmayer fluid) confined to nanocavities of different radii ranging from Rc=1nm to 4nm, we find that the Kirkwood correlation factor remains surprisingly small in water, but not so in model Stockmayer liquid. This gives rise to an anomalous ultrafast relaxation of the total dipole moment time correlation function (DMTCF). The static dielectric constant of water under nanoconfinement (computed…
We present the results of numerical simulations of the electrostatics and dynamics of water hydration shells surrounding Kihara cavities given by a Lennard-Jones (LJ) layer at the surface of a hard-sphere cavity. The local dielectric response of the hydration layer substantially exceeds that of bulk water, with the magnitude of the dielectric constant peak in the shell increasing with the growing cavity size. The polar shell propagates into bulk water to approximately the cavity radius. The statistics of the electrostatic field produced by water inside the cavity follow linear response and app…
Molecular dynamics of two water models, SPC/E and TIP3P, at a number of temperatures is used to test the Kivelson-Madden equation connecting single-particle and collective dielectric relaxation times through the Kirkwood factor. The relation is confirmed by simulations and used to estimate the nonlinear effect of the electric field on the dielectric relaxation time. We show that the main effect of the field comes through slowing down of the single-particle rotational dynamics and the relative contribution of the field-induced alteration of the Kirkwood factor is insignificant for water. Theori…
