Dipolar Liquids Research Articles

Erratum: "Long-range orientation correlation in dipolar liquids probed by hyper-Rayleigh scattering" [J. Chem. Phys. 143, 134503 (2015)

First Page

Characterization of conductivity relaxation processes induced by charge dynamics and hydrogen-bond molecular interactions in binary mixtures of propylene carbonate with acetonitrile

Abstract The complex dielectric permittivity, alternating current electrical conductivity, electric modulus and impedance spectra of the binary mixtures of propylene carbonate with acetonitrile have been investigated by employing dielectric relaxation spectroscopy over the frequency range from 20 Hz to 1 MHz at 25 °C. It is found that the real part of dielectric permittivity represents the static dielectric permittivity of these mixtures in the frequency range from 100 kHz to 1 MHz, whereas it starts to increase sharply from static permittivity value and reaches up to five orders of magnitude with the decrease of frequency from 20 kHz to 20 Hz. This behaviour confirms the dominant contribution of absorbed charge contaminants in the dielectric polarization of these dipolar liquids at low frequencies. The electric modulus and impedance spectra of these binary mixtures exhibit the Debye-type dispersion behaviour. Electric double layers and conductivity relaxation processes induced in these mixtures exhibit non-linear behaviour with the mole fraction concentration of the mixtures constituents and the relaxation times of these processes have proportionality with dc electrical conductivity. The static dielectric permittivity, refractive index and excess properties of these dielectric parameters over the entire mixing concentration range of the binary mixtures with temperature variations from 15 °C to 45 °C have also been determined. Results confirm that the heterogeneous intermolecular hydrogen-bond with parallel dipolar ordering is formed between propylene carbonate and acetonitrile in the mixtures. The conductivity activation energy values have been determined from the Arrhenius plots, which have an increase with the increase of propylene carbonate concentration in the binary mixtures.

Free energy functionals for polarization fluctuations: Pekar factor revisited.

The separation of slow nuclear and fast electronic polarization in problems related to electron mobility in polarizable media was considered by Pekar 70 years ago. Within dielectric continuum models, this separation leads to the Pekar factor in the free energy of solvation by the nuclear degrees of freedom. The main qualitative prediction of Pekar's perspective is a significant, by about a factor of two, drop of the nuclear solvation free energy compared to the total (electronic plus nuclear) free energy of solvation. The Pekar factor enters the solvent reorganization energy of electron transfer reactions and is a significant mechanistic parameter accounting for the solvent effect on electron transfer. Here, we study the separation of the fast and slow polarization modes in polar molecular liquids (polarizable dipolar liquids and polarizable water force fields) without relying on the continuum approximation. We derive the nonlocal free energy functional and use atomistic numerical simulations to obtain nonlocal, reciprocal space electronic and nuclear susceptibilities. A consistent transition to the continuum limit is introduced by extrapolating the results of finite-size numerical simulation to zero wavevector. The continuum nuclear susceptibility extracted from the simulations is numerically close to the Pekar factor. However, we derive a new functionality involving the static and high-frequency dielectric constants. The main distinction of our approach from the traditional theories is found in the solvation free energy due to the nuclear polarization: the anticipated significant drop of its magnitude with increasing liquid polarizability does not occur. The reorganization energy of electron transfer is either nearly constant with increasing the solvent polarizability and the corresponding high-frequency dielectric constant (polarizable dipolar liquids) or actually noticeably increases (polarizable force fields of water).

Effects of electric field on thermodynamics and ordering of a dipolar liquid.

We propose that an electric field's role in changing the structural disorder may be investigated by comparing the field-induced entropy decrease, ΔES, against the pressure-induced and cooling-induced entropy decreases, ΔpS and ΔTS, respectively, for the same increase in the dielectric α-relaxation time, Δτα, or in the viscosity. If these three quantities are found to be the same, the change in the number of microstates, Δln Ω = ΔS/R, would be the same whether there is an electric field-induced dipole vector alignment, or not. The available data [S. Samanta and R. Richert, J. Chem. Phys. 142, 044504 (2015)] show that ΔES ≅ ΔpS, and ΔES ≅ ΔTS. We further argue that in the case of conformational disorder without hydrodynamics, as for a flexible molecule's orientationally disordered or plastic crystal, ΔTS would be more negative than ΔES for the same increase in Δτα. For cyclo-octanol plastic crystal, whose octyl-ring would lose some of its dielectrically inactive conformational degrees of freedom on cooling, ΔTS is five-times ΔES. Hence the entropy of such crystals may not be related to their τα, an aspect relevant to certain biopolymer crystals. We also mention other effects of E. The findings are relevant to a number of recent studies on the analysis of the effect of electric field on a liquid's properties. The method can be used to study the role of other entropy-altering variables in liquid crystals and ferromagnetic liquids.

Dielectric relaxation of nitromethane and its mixtures with ethylammonium nitrate: Evidence for strong ion association induced by hydrogen bonding

Binary mixtures of the aprotic protophobic solvent nitromethane (NM) and the protic room-temperature ionic liquid ethylammonium nitrate (EAN) were studied with dielectric relaxation spectroscopy in the two single-phase regions, 0 < xEAN ≤ 0.013 and 0.4 ≤ xEAN < 1, of the binary mixtures at 25 °C. All spectra were well described by a superposition of two relaxation processes whose origins were of composite nature. At low xEAN the solutions behave as a strongly associated electrolyte, displaying moderate ion solvation by nitromethane at the infinite-dilution limit. At high xEAN far from the miscibility gap, the observed dynamics represents the typically observed behaviour of a lubricated ionic liquid smoothly reaching the properties of pure EAN. Additionally, neat NM was investigated in the temperature range of 5 to 65 °C to explore its potential as a calibration standard in dielectric spectroscopy. The obtained 0.05 to 89 GHz spectra were well fitted by a single Debye equation. The dynamics of this medium-permittivity dipolar liquid is governed by rotational diffusion of NM dipoles close to slip boundary conditions with only weak dipole-dipole correlations. Unfortunately, difficulties in obtaining samples of reproducible purity discredit NM as a calibration standard for dielectric measurements.

Dark High Density Dipolar Liquid of Excitons

The possible phases and the nanoscale particle correlations of two-dimensional interacting dipolar particles is a long-sought problem in many-body physics. Here we observe a spontaneous condensation of trapped two-dimensional dipolar excitons with internal spin degrees of freedom from an interacting gas into a high density, closely packed liquid state made mostly of dark dipoles. Another phase transition, into a bright, highly repulsive plasma, is observed at even higher excitation powers. The dark liquid state is formed below a critical temperature Tc ≈ 4.8 K, and it is manifested by a clear spontaneous spatial condensation to a smaller and denser cloud, suggesting an attractive part to the interaction which goes beyond the purely repulsive dipole-dipole forces. Contributions from quantum mechanical fluctuations are expected to be significant in this strongly correlated, long living dark liquid. This is a new example of a two-dimensional atomic-like interacting dipolar liquid, but where the coupling of light to its internal spin degrees of freedom plays a crucial role in the dynamical formation and the nature of resulting condensed dark ground state.

Long-range orientation correlation in dipolar liquids probed by hyper-Rayleigh scattering.

Hyper-Rayleigh scattering (HRS) is sensitive to long-range molecular orientation correlation in isotropic liquids composed of dipolar molecules. The correlation functions that appear in the calculation of HRS mediated by the vector part of the first hyperpolarizability β are the same as the correlation functions for the homogeneous isotropic random vector fields that appear in the description of fluid turbulence. Recent experiments measuring the angle and polarization dependence of HRS from water find a dominant transverse mode contribution with amplitude independent of the scattering wavevector, and this observation of transverse mode HRS strongly constrains the form of the orientation correlation function. Analysis of these HRS results for water determines that the long-range molecular orientation correlation function varies as r(-3±ε) with |ε| < 0.03 on spatial scales up to 2000 nm.

Solvation of narrow pores of graphene-like plates in simple dipolar liquids: Wetting and dewetting behavior and solvent dynamics for varying pore width and solute–solvent interaction

We have investigated the solvation behavior of two parallel graphene-like plates in a Stockmayer liquid by molecular dynamics simulations for varying inter-plate separation and plate–solvent interactions. The solvent structure, orientation and dynamics around the plates and in their confined pores have been calculated. Layering of Stockmayer molecules with dipole vectors parallel to the surfaces is observed in the vicinity of the nonpolar plates. Density profiles reveal wetting and emptying of the narrow pores depending on the nature of solute–solvent interactions. In the interior and outer surface regions, solvent molecules are found to exhibit a significantly different dynamics than the bulk. From an overall perspective, the solvophobic effects observed here can be considered as the generic behavior of dipolar liquids near nonpolar surfaces without the presence of any specific and complex intermolecular interactions such as hydrogen bonds in water and other protic solvents.

On the angular velocity slip in nano-flows

The need of reformulation for basic concepts of fluid mechanics in order to account some enhancement phenomena in micro- and nano-flows is presented in the paper. The work is a general formulation of angular velocity slip condition which is developed from primary principles. It is postulated that the different slipping phenomena take place within a thin shell-like layer where a material particle undergoes independently: the slip velocity and angular slip velocity. We suppose also that the molecules rolling at the layer can give a serious contribution to the mass flow enhancement, and therefore, it can be a reason to inclusion of a mathematical modeling for molecules angular momentum. In this paper, we propose the general form of an angular momentum balance for the shell-like layer. This balance is expressed by the surface divergence of an unsymmetric surface stress and an unsymmetric surface couple stress, either by the bulk momentum and bulk moment of momentum or by a surface friction torque. With constitutive relations appropriate for a linear, viscous, isotropic fluid and the constitutive relation for frictional resistance, we obtain generalized Navier-type boundary equations for the slip velocity and the angular slip velocity. As special cases, Poisson’s type boundary condition and Aero–Bulygin–Kuvshlinskii’s angular slip condition are obtained. Proposed here boundary conditions have few applications in modeling of complex nano-flows phenomena. For instant, recently discovered electropumping phenomena by De Luca et al. (J Chem Phys 138:154712-1–154712-10, 2013a) are based on nano-coupling of angular momentum with linear momentum and the electroosmotic draining effect. The theoretical background relies upon different angular velocity slip of dipolar liquids when confined between hydrophobic and hydrophilic solid surfaces—this phenomenon combined with applied rotating electric field leads to pumping of liquid. In our opinion, a concept of angular velocity slip leads to an explanation of the electropumping effect enhancement in nano-channels.

Wetting behavior of nonpolar nanotubes in simple dipolar liquids for varying nanotube diameter and solute-solvent interactions.

Atomistic simulations of model nonpolar nanotubes in a Stockmayer liquid are carried out for varying nanotube diameter and nanotube-solvent interactions to investigate solvophobic interactions in generic dipolar solvents. We have considered model armchair type single-walled nonpolar nanotubes with increasing radii from (5,5) to (12,12). The interactions between solute and solvent molecules are modeled by the well-known Lennard-Jones and repulsive Weeks-Chandler-Andersen potentials. We have investigated the density profiles and microscopic arrangement of Stockmayer molecules, orientational profiles of their dipole vectors, time dependence of their occupation, and also the translational and rotational motion of solvent molecules in confined environments of the cylindrical nanopores and also in their external peripheral regions. The present results of structural and dynamical properties of Stockmayer molecules inside and near atomistically rough nonpolar surfaces including their wetting and dewetting behavior for varying interactions provide a more generic picture of solvophobic effects experienced by simple dipolar liquids without any specific interactions such as hydrogen bonds.

Reference hypernetted chain theory for ferrofluid bilayer: distribution functions compared with Monte Carlo.

Properties of ferrofluid bilayer (modeled as a system of two planar layers separated by a distance h and each layer carrying a soft sphere dipolar liquid) are calculated in the framework of inhomogeneous Ornstein-Zernike equations with reference hypernetted chain closure (RHNC). The bridge functions are taken from a soft sphere (1/r(12)) reference system in the pressure-consistent closure approximation. In order to make the RHNC problem tractable, the angular dependence of the correlation functions is expanded into special orthogonal polynomials according to Lado. The resulting equations are solved using the Newton-GRMES algorithm as implemented in the public-domain solver NITSOL. Orientational densities and pair distribution functions of dipoles are compared with Monte Carlo simulation results. A numerical algorithm for the Fourier-Hankel transform of any positive integer order on a uniform grid is presented.

Phase transitions of octamethylcyclotetrasiloxane confined inside aluminosilicate and silicate nanoporous matrices

We report the melting behaviour of a dipolar cyclic siloxane liquid: octamethylcyclotetrasiloxane (OMCTS) confined in three mesoporous silica matrices: Al-SBA-15, SBA-15 and CPG glasses, using differential scanning calorimetry and dielectric spectroscopy. We investigate the influence of acid sites on the adsorptive properties of mesoporous silica materials, which were synthesized by applying Pluronic-type polymers as pore-creating agents. Aluminosilicate matrices have been synthesized by direct synthesis procedure using aluminium chloride. These materials characterized by N2 sorption measurements, and the small-angle X-ray scattering data exhibit the same hexagonal P6 mm structure with a mean mesopores size of 4.6 nm (Al-SBA-15) and 4.9 nm (SBA-15). The controlled pore glasses used in this experiment have pores of mean diameter of 7.5 nm. For all systems studied, the OMCTS melting point in pores has been found to decrease with decreasing pore diameter. This result is in qualitative agreement with that obtained in molecular simulation where the adsorbate-wall interactions are weak compared to the adsorbate–adsorbate interactions.

Distinctive Features of Water–Dimethyl Sulfoxide Mixtures: Dielectric Spectra Revisited

Broadband dielectric spectra (1 MHz to 80 GHz) are reported for dimethyl sulfoxide–water mixtures in the complete composition range. The spectra are evaluated in order to yield information about the underlying relaxation time distribution, the principal relaxation time, and the static permittivity of the liquids. With a view of gaining insights into the structure and microdynamics of water in its different states of interaction, these parameters are compared to such for mixtures of water with other protic and aprotic dipolar liquids as well as with non-polar substances. Also used for information about structure fluctuations are ultrasonic spectra of the dimethyl sulfoxide–water system (0.2 MHz to 2.75 GHz). A notable result is the composition dependence of the principal dielectric relaxation time. In conformity with the wait-and-switch model, it increases at low DMSO content and thus reflects the decrement in the concentration of hydrogen-bonding sites. The decreasing relaxation time at high DMSO content is clearly a result of the reduced association due to the reduced concentration of hydrogen-donating sites.

Changes in Permittivity and Density of Molecular Liquids under High Pressure

We collected and analyzed the density and permittivity of 57 nonpolar and dipolar molecular liquids at different temperatures (143 sets) and pressures (555 sets). No equation was found that could accurately predict the change to polar liquid permittivity by the change of its density in the range of the pressures and temperatures tested. Consequently, the influence of high hydrostatic pressure and temperature on liquid permittivity may be a more complicated process compared to density changes. The pressure and temperature coefficients of permittivity can be drastically larger than the pressure and temperature coefficients of density, indicating that pressure and particularly temperature significantly affect the structure of molecular liquids. These changes have less influence on the density change but can strongly affect the permittivity change. The clear relationship between the tangent and secant moduli of the permittivity curvatures under pressure for various molecular liquids at different temperatures was obtained, from which one can calculate the Tait equation coefficients from the experimental values of the pressure influence on the permittivity at ambient pressure.

Dielectric relaxation in ionic liquids: Role of ion-ion and ion-dipole interactions, and effects of heterogeneity

A semi-molecular theory for studying the dielectric relaxation (DR) dynamics in ionic liquids (ILs) has been developed here. The theory predicts triphasic relaxation of the generalized orientational correlation function in the collective limit. Relaxation process involves contributions from dipole-dipole, ion-dipole, and ion-ion interactions. While the dipole-dipole and ion-ion interactions dictate the predicted three relaxation time constants, the relaxation amplitudes are determined by dipole-dipole, ion-dipole, and ion-ion interactions. The ion-ion interaction produces a time constant in the range of 5-1000μs which parallels with the conductivity dominated dielectric loss peak observed in broadband dielectric measurements of ILs. Analytical expressions for two time constants originating from dipolar interactions in ILs match exactly with those derived earlier for dipolar solvents. The theory explores relations among single particle rotational time, collective rotational time, and DR time for ILs. Use of molecular volume for the rotating dipolar ion of a given IL leads to a predicted DR time constant much larger than the slowest DR time constant measured in experiments. In contrast, similar consideration for dipolar liquids produces semi-quantitative agreement between theory and experiments. This difference between ILs and common dipolar solvents has been understood in terms of extremely low effective rotational volume of dipolar ion, argued to arise from medium heterogeneity. Effective rotational volumes predicted by the present theory for ILs are in general agreement with estimates from experimental DR data and simulation results. Calculations at higher temperatures predict faster relaxation time constants reducing the difference between theory and experiments.

Ground-state and dynamical properties of two-dimensional dipolar Fermi liquids

We study the ground-state properties of a two-dimensional spin-polarized fluid of dipolar fermions within the Euler–Lagrange Fermi-hypernetted-chain approximation. Our method is based on the solution of a scattering Schrödinger equation for the “pair amplitude” g(r), where g(r) is the pair distribution function. A key ingredient in our theory is the effective pair potential, which includes a bosonic term from Jastrow–Feenberg correlations and a fermionic contribution from kinetic energy and exchange, which is tailored to reproduce the Hartree–Fock limit at weak coupling. Very good agreement with recent results based on quantum Monte Carlo simulations is achieved over a wide range of coupling constants up to the liquid-to-crystal quantum phase transition. Using the fluctuation–dissipation theorem and a static approximation for the effective inter-particle interactions, we calculate the dynamical density–density response function, and furthermore demonstrate that an undamped zero-sound mode exists for any value of the interaction strength, down to infinitesimally weak couplings.

Interpretation of the Electric Impedance Spectra Recorded for Liquids in the Presence of Ionic and Displacement Currents

The paper shows that the impedance spectrum of ionically conducting dipolar liquid is composed of two bands which correspond to the two dynamic electric phenomena occurring in the liquid: the ionic current and the displacement current. The relative intensity of these two bands strongly depends on the relation between the corresponding relaxation times, τion and τdispl. The data presented in the paper shows that in spite of differences in the intensities, the band due to the ionic conductivity is always placed in the lower frequency part of the impedance spectrum. So, for ω → 0, the real part of the impedance corresponds to the direct current conductivity (σDC) of the liquid studied, as expected. The problem is illustrated with the experimental impedance spectra recorded for liquid supramolecular polymer N,N′-di(2-ethylhexyl)urea (EHU).

Influence of short range potential on field induced chain aggregation in low density dipolar particles

We investigate, by Monte Carlo simulation, the effect of the steepness of the short range repulsive potential on mesostructure formation in dipolar particles submitted to a strong external field. Columnar clusters made of several dipolar chains are only observed when the short-range potential is sufficiently steep. The confinement of the dipolar liquid in a slit geometry instead of bulk conditions suppresses the formation of columns.

Effects of electric field on the entropy, viscosity, relaxation time, and glass-formation

By using the known formalism for the effect of an externally applied electric field, E, on thermodynamics of a dielectric material, we calculated the field-induced configurational entropy factor, ΔSconf (E)/E(2), of 50 dipolar liquids, including those whose static permittivity, εs, decreases on cooling. The field induced change, ΔSconf (E), is found to be experimentally detectable only when E is on the order of 10(5) V∕cm, a value less than the dielectric breakdown field strength of some liquids but in the range of nonlinear dielectric response. We argue that the dielectric response is formally nonlinear already for E > 0, and then show that the difference between the Langevin-function and the extrapolated linear response is < 0.15% for E in the 10(5) V∕cm range. Therefore, such high E values may be used to estimate ΔSconf (E). We conclude that (i) for E in the 10(5) V∕cm range, ΔSconf (E) is high enough to produce a measurable change in the viscosity and relaxation time of some ultraviscous liquids with prominent dipolar interactions, thereby changing their glass formation temperature, and (ii) application of E would reversibly transform, isothermally, some liquids to glass, and transform some glasses to liquid. Finally, we suggest that the effect of E can be used to determine the merits of the models for non-Arrhenius kinetics.

The vapor-liquid interface potential of (multi)polar fluids and its influence on ion solvation

The interface between the vapor and liquid phase of quadrupolar-dipolar fluids is the seat of an electric interfacial potential whose influence on ion solvation and distribution is not yet fully understood. To obtain further microscopic insight into water specificity we first present extensive classical molecular dynamics simulations of a series of model liquids with variable molecular quadrupole moments that interpolates between SPC/E water and a purely dipolar liquid. We then pinpoint the essential role played by the competing multipolar contributions to the vapor-liquid and the solute-liquid interface potentials in determining an important ion-specific direct electrostatic contribution to the ionic solvation free energy for SPC/E water-dominated by the quadrupolar and dipolar parts-beyond the dominant polarization one. Our results show that the influence of the vapor-liquid interfacial potential on ion solvation is strongly reduced due to the strong partial cancellation brought about by the competing solute-liquid interface potential.