Dipolar Liquids Research Articles

Infrared intensity in dielectric media by an electrostatic model

A change in an infrared intensity in dielectric media is treated by an electrostatic model. The basic model is originally formalized for a dipolar liquid. The model is satisfactorily applied to the infrared intensity of the CH stretching of chloroform, which changes 22 times large in the liquid state at −43 °C as in the gaseous state. A change in the infrared intensity in lithium ammonium tartrate, where a ferroelectric phase transition takes place, is analyzed on the basis of a local polarization above T c or a spontaneous polarization below T c, deducing important quantities on a phase transition. A difference in the infrared intensity of the CBr stretching of 1,10-dibromodecane between the urea clathrate and the crystalline state is analyzed by evaluating electric fields due to bond moments and oscillating dipoles. These analyses confirm the mechanism of the change in the absolute infrared intensity, which originates from an electrostatic interaction with an electric field applied to a molecule or a functional group closely related to a normal mode.

First-principles Calculation of Electrode Surface

Since the electrode interface phenomena have multiscale nature in space and time, the multiscale model from quantum to continuum level should be considered. In this article we describe the structure of the Pt(111)/dipolar liquid interface and the adsorption of sulfur on electrified Au(111) surface. The structure of the Pt(111)/dipolar liquid interface has been investigated by fully self-consistent combination of the first-principles calculation based on quantum mechanics for the metal and the reference hypernetted-chain (RHNC) theory for the liquid. The electronic density profile for the metal, density and orientational structure of liquid molecules, and electrostatic potential across the interface are discussed in detail. A dense layer of liquid molecules, which is ordered in terms of orientation, is formed near the metal surface, but this surface-induced structure extends about only three molecular diameters from the surface. This result is in agreement with the recent experimental observations. For S adsorption on electrified Au(111) surface the screening of the electron at metal surface and the change of the adsorption energy of sulfur are discussed briefly.

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.

The temperature dependent dielectric function of liquid benzene: Interpretation of THz spectroscopy data by molecular dynamics simulation

The dielectric function of liquid benzene at −4 °C, 21 °C, and 65 °C has been calculated from molecular dynamics simulations. The simulated dielectric loss curves reproduce the experimental temperature and density trends. In order to investigate the detailed influence of temperature and density changes as well as the underlying molecular mechanism we have taken advantage of different partitioning and projection schemes for the total dipole moment time correlation function (TCF). The study of the n-body partitioning showed that the temperature dependence of the two- and three-body contributions at t=0 can be explained solely by density change arguments. The molecular projection scheme showed that the dielectric loss is governed by out-of-plane libration at all temperatures. In-plane libration was found to contribute significantly only below 2 THz. Below 1 THz, diffusion, manifested as the negative cross correlation between the out-of-plane and the in-plane TCF’s, plays a role very different from that of the directly observed diffusion in dipolar liquids. It has further been established that it is highly problematic to carry out an analysis of the dielectric loss function in terms of the molecular axis rotational TCF’s which is a common procedure for the absorption spectrum. This problem was, however, solved by employing a molecular projection scheme.

Relative permittivity of dipolar liquids and their mixtures. Comparison of theory and experiment

An empirical extension, based on the Kirkwood–Frohlich equation, of a molecular based second-order perturbation theory is proposed to calculate the relative permittivity of dipolar liquids and mixtures in terms of the molecular dipole moment, the refractive index, density and temperature. Using the resulting equation, and giving an estimation for the dipole moments of the pure components, good agreement is obtained between the theoretical and experimental results for the relative permittivity of mixtures of carbonyl compounds with aprotic liquids. We show that our extension of the Kirkwood–Frohlich equation describes the dielectric properties of mixtures more accurately than the original equation.

Surface charge induced modifications of the structure and dynamics of mixed dipolar liquids at solid–liquid interfaces: A molecular dynamics simulation study

Molecular dynamics simulations are carried out to investigate the structural and dynamical properties of binary Stockmayer liquids near charged solid surfaces at varying surface charge density. The two solvent components differ widely in their polarity. The dipolar mixtures are formed at varying composition and the properties of the interfacial molecules are calculated in terms of several equilibrium and dynamical quantities such as the number density and polarization profiles, electrostriction at surfaces, linear and angular velocity autocorrelation functions, perpendicular (z) and parallel (x,y) components of translational diffusion tensors and rotational diffusion coefficients. The extent of selective adsorption of one species against the other at the surfaces is investigated as a function of surface charge density and composition and its effects on translational and rotational diffusion of interfacial molecules are discussed. The dynamical properties of the interfaces are also compared with those of the bulk.

Structure of the Pt(111)/liquid interface: a first-principles/RHNC calculation

Structure of the Pt(111)/dipolar liquid interface has been investigated by fully self-consistent combination of the first-principles calculation based on quantum mechanics for the metal and the reference hypernetted-chain (RHNC) theory for the liquid. The electronic density profile for the metal, density and orientational structure of liquid molecules, and electrostatic potential across the interface are discussed in detail. A dense layer of liquid molecules, which is orientationally ordered, is formed near the metal surface, but this surface-induced structure extends about only three molecular diameters from the surface. This result is in good accord with the recent experimental observations.

Temperature dependence of the dielectric function of C6H6(l) and C6H5CH3(l) measured with THz spectroscopy

We report on an experimental investigation of the temperature dependence of the intermolecular dynamics in liquid benzene and toluene. With the use of THz time domain spectroscopy we measured the complex dielectric function (0.2–3.3 THz) of the liquids, at temperatures between −6 °C and 75 °C. By analyzing the dielectric loss (as opposed to the absorption coefficient) we found three contributions to the dielectric function for toluene and two for benzene. In the dipolar liquid toluene we observed a contribution from rotational diffusion at lower frequencies in addition to the two high-frequency librations also observed in benzene. The temperature and density dependence were different for the two librational bands, probably due to the different effect of three-body interactions for the two processes. Furthermore, we present measurements of the low-frequency depolarized Raman spectra as a function of temperature for benzene and toluene. These have been compared with the dielectric loss at similar temperatures. Moreover, for benzene a similar comparison is made between the dielectric loss and the reduced Raman susceptibility obtained by optical Kerr effect spectroscopy. Libration of the symmetric top axis is found in all spectra.

Structure and dynamics of nonaqueous mixtures of dipolar liquids. I. Infrared and far-infrared spectroscopy

Mixtures of acetone/methanol, acetonitrile/methanol, and acetone/acetonitrile over their whole composition range were studied with infrared and far-infrared (THz) spectroscopy. Experimental spectra of all mixtures were obtained below 120 cm−1, and spectra of methanolic mixtures were also measured from 400 to 1000 cm−1. The mixtures display ideal absorption spectra below 120 cm−1, contrasting with significant nonidealities in the absorption of aqueous mixtures in the same frequency range. Dramatic changes as a function of composition were found in methanolic mixtures at higher frequencies. The high frequency librational band of methanol, centered at 670 cm−1, shifts to substantially lower frequencies upon dilution, indicating marked changes in the librational motion of the hydroxyl hydrogen of methanol. This band is a sensitive probe of the hydrogen bonding environment experienced by methanol molecules.

Structure and dynamics of nonaqueous mixtures of dipolar liquids. II. Molecular dynamics simulations

Molecular dynamics simulations have been used to study mixtures of acetone/methanol, acetonitrile/methanol, and acetone/acetonitrile over their entire composition range. Using the effective pair potentials of the neat liquids, the simulations reproduce much of the experimental spectra presented in the previous paper [D. S. Venables, A. Chiu, and C. A. Schmuttenmaer, J. Chem. Phys. 113, 3243 (2000)]. In addition to the total dipole spectra, autocorrelation functions and their corresponding spectra were calculated for the single dipole moment as well as the translational and rotational velocities of each component in the mixtures. Radial and spatial distribution functions, hydrogen bonding, and methanol chain formation in the mixtures were also analyzed. The red-shift of the high frequency librational band is attributed to methanol chains breaking up into shorter segments, and to hydrogen bonding between methanol and co-solvent molecules. Methanol molecules have a strong tendency to reside in chains, even at low methanol concentrations, and hydrogen bonding is the primary determinant of structure in the mixtures.

Structure of a mixed dipolar liquid near a metal surface: A combined approach of weighted density and perturbative approximations

We study the interfacial structure of a mixed dipolar liquid in contact with a metal surface by using a combined approach of the weighted density and the perturbative approximations. Both the molecular size and the dipole moment of various species can be unequal. The metal surface is treated by using the jellium model. Explicit numerical results are obtained for the interfacial structure of a binary dipolar liquid in contact with a metal surface of varying electron density. The theoretical predictions are compared with the results of Monte Carlo simulations and a good agreement is found for the inhomogeneous density, mole fraction, and polarization profiles of both the species in the interfacial region.

Dynamics of polarization relaxation in a dipolar mixture at a solid–liquid interface

Dynamics of polarization relaxation in a dipolar mixture at solid–liquid interface is investigated theoretically by using time dependent density functional theory. The time dependence of the building up of polarization of different dipolar species next to a newly charged solid surface is calculated by solving both Markovian and non-Markovian equations of motion for density and momentum variables. The inhomogeneity of the interface is taken into account in the dynamical theory. The theory also includes the various intra- and interspecies molecular correlations and the surface–molecule interactions. Both the molecular size and the dipole moment of various species in the dipolar mixture can be unequal. The initial inhomogeneity of the interface is calculated by using a weighted density functional theory. Explicit numerical results are obtained for the polarization relaxation in a binary dipolar liquid at varying distances from the solid surface and the dynamics of interfacial relaxation of the two species are compared with that in the bulk phase. A slowing down of the rate of relaxation is found for interfacial molecules of both the species. However, the extent of slowing down is found to be different for different species. Also, the rate of relaxation is found to change nonmonotonically with distance from the surface due to the layering of solvent molecules at the interface. © 2000 American Institute of Physics.

Interfacial structure of a mixed dipolar liquid in contact with a charged solid surface

We develop a nonlinear theory for the calculation of interfacial structural properties of a dipolar mixture in contact with a charged solid surface. Both the molecular sizes and the dipole moments of various species can be unequal. Explicit numerical results are obtained for the interfacial structure of a binary dipolar liquid in contact with a charged surface of varying surface charge density. The dipole moments of the two species are also varied. The density profiles of both the species are found to be highly inhomogeneous and oscillatory near the solid surface. The more polar species is found to exhibit a positive electrostriction at the surface with an increasing surface electrostatic field. An opposite behavior is observed for the less polar species. The polarization profiles reveal pronounced orientational order of the solvent molecules of both the species near the charged surface. The contact polarizations of the more polar and the less polar species show, respectively, a more than linear and a less than linear increase with increasing surface charge density. The predictions of the present theory for a given set of parameter values are compared with the results of Monte Carlo simulation of the same system and a good agreement is found for the inhomogneous density and polarization profiles of both the species.

Heterogeneous relaxation patterns in supercooled liquids studied by solvation dynamics

We have measured the solvation dynamics of a dipolar supercooled liquid near its glass transition in a temperature range in which the average structural relaxation time varies more than four orders of magnitude. The analysis of the time dependent average emission energy and the inhomogeneous linewidth of the S0<--T1(0-0) transition reveals that the orientation correlation decay pattern intrinsic in each relaxing unit is associated with a stretching exponent beta(intr)=1.00+/-0.08 in the entire range T(g)</=T</=T(g)+6 K. Our analysis also allows one to detect fluctuations in terms of the resulting apparent homogeneity within the long time tail of the decay. Even at times significantly exceeding the average structural relaxation time, no sign of a transition towards purely exponential or otherwise homogeneous behavior could be observed. This implies that even at t approximately 50<tau>(KWW) the individual time constants remain correlated to their initial values at t=0.

Dielectric properties and structure of water at room temperature. New experimental data in 5 Hz–13 Mhz frequency range

The dielectric permittivity of destilled water was measured in the frequency range of 5 Hz to 13 MHz with different thicknesses of the sample and for various oscillator levels. The two electrode method was used with plate condenser. A simple model, composed of a dipole liquid with domain structure is described. Basing on the model, we show possible ways of polarization. The present conclusions relate dielectric properties of water which result from the model. We propose a new conductivity model for dipole liquid with domain structure. Measurements of dielectric permittivity are consistent with the model. It is shown that experimental dielectric permittivity curves are a superposition two Debye's curves with different relaxation times and two different values of static dielectric constant. Debye's resonance for lower frequency is associated with oxygen's domain structure and for higher frequency with hydrogen's domain structure. We observe an increase of dielectric permittivity with time associated with ordering of the structure after filling. For thin samples we can observe damping of the Debye's resonance and interaction of the domains' structures.

Molecular dynamics simulations of simple dipolar liquids in spherical cavity: Effects of confinement on structural, dielectric, and dynamical properties

The equilibrium and dynamical properties of Stockmayer liquids confined in a spherical cavity are investigated by means of molecular dynamics simulations. The simulations are carried out at varying density and cavity size. Various equilibrium and time dependent quantities such as the spatial and orientational density profiles, dielectric constants, average energies, pressures, components of translational diffusion tensors parallel and perpendicular to the cavity surface, rotational diffusion coefficients and several time correlation functions are calculated and the effects of confinement on the above properties are discussed. The density profiles are found to be highly inhomogeneous near the cavity wall, and the dielectric constant of the liquids in cavity is found to be significantly smaller than that of the bulk phases. The diffusion along the surface normal and also the dipolar orientational relaxation of solvent molecules in cavity are found to slow down because of confinement. The dynamics of solvation of a newly created charge distribution in the cavity is also studied and the results are compared with the dynamics of solvation in bulk solvent. The solvation in the cavity is found to occur at a much slower rate.

Structure of dipolar liquids near charged solid surfaces: A nonlinear theory based on a density functional approach and Monte Carlo simulations

A nonlinear theory for the calculation of density and polarization of dipolar solvents near charged surfaces is developed by using a density functional approach. The theory is based on a weighted density approximation for the isotropic part of the density and the anisotropic (or dipolar) part is calculated by using a perturbative approach. The theory, however, retains the full nonlinear dependence on the surface charge density or the external field. Explicit numerical results are obtained for different values of the external field. It is found that the number density of the solvent near the surface increases with increasing surface charge density indicating the presence of electrostriction. The polarization is found to increase nonlinearly with the external field and exhibits the presence of dielectric saturation at high field strength. The predictions of the present nonlinear theory are compared with the results of Monte Carlo simulations and a good agreement is found for both the density and polarization near charged surfaces. The present simulations clearly reveal a nonlinear behavior of dipolar molecules in presence of the charged surfaces. Also, the nonlinear effects are found to be especially important for interfacial molecules, in agreement with the predictions of the analytical theory presented here.

Structure and dynamics of mixed dipolar liquids near solid surfaces: a molecular dynamics simulation study

The structural and dynamical properties of binary Stockmayer liquids near solid surfaces are investigated by means of molecular dynamics simulations. The two solvent components differ widely in their polarity and they are mixed at varying mole fraction. The dipolar mixtures are confined between two planar solid surfaces and the properties of the interfacial molecules are calculated in terms of several equilibrium and time dependent quantities such as the number density, orientational structure, perpendicular ( z) and parallel ( x, y) components of translational diffusion tensors, rotational diffusion coefficients and various time correlation functions of both the species. The extent of selective adsorption of one species against the other at the surface is investigated and its effects on various dynamical properties of the interfaces are discussed. The dynamics of solvation of a newly created charge distribution near a solid surface is also studied and the results are compared with the dynamics of solvation in mixed solvent in the bulk phase. The solvation at the interface is found to occur at a much slower rate primarily because of the selective adsorption of the less polar solvent at the solid surface and redistribution of solvent molecules near the solute.

Solvation Dynamics in Nonassociated Polar Solvents

Ultrafast solvation dynamics in three nonassociated polar solvents, namely, acetonitrile, dimethyl sulfoxide, and acetone, have been studied by using the molecular hydrodynamic theory. For solvation in acetonitrile, the solvent memory function required for this study has been obtained from recent dielectric relaxation measurements of Venabales and Schuttenmaer; earlier theoretical studies used only the Kerr relaxation data. As the latter provides only an indirect information regarding the polar dynamical response of the dipolar liquid, it fails to provide a fully quantitative description of the solvation time correlation function, S(t). The present study with full dielectric data, on the other hand, gives excellent agreement with the experimental results. The theory shows that the ultrafast part of the solvation dynamics originates almost entirely from the high-frequency component of dielectric relaxation (with time constant 0.177 ps), although the latter represents only a small part of the latter. For DMSO and acetone, however, the present theory predicts a decay slower than the experimental observation. It is proposed that for these two solvents specific chromophore−solvent interactions might be responsible for the large discrepancy. On the basis of the theory, two experimental studies have also been proposed.

Solvation Dynamics of a Quadrupolar Solute in Dipolar Liquids

A microscopic calculation of solvation dynamics of dipolar and quadrupolar solutes in liquid water and acetonitrile is presented. The solvation is found to he biphasic. The calculated solvation time correlation function of ionic quadrupolar solute (K+) in water is in good agreement with re cent computer simulation results. Present study reveals some interesting aspects of quadrupolar solvation dynamics which differ significantly from that of ionic and dipolar solvation.