Provisional Theory of Nanoscale Water Dielectrics, J. Biol. Phys. & Chem. 13, 9-11 (2013) (original) (raw)
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On the Dynamic and Electronic Polarization Corrections to Dielectric Constant of Water
The Journal of Physical Chemistry A
The standard approach to calculating the dielectric constant from molecular dynamics (MD) simulations employs a variant of the Kirkwood−Froḧlich methodology. Many popular nonpolarizable models of water, such as TIPnP, give a reasonable agreement with the experimental value of 78. However, it has been argued in the literature that the dipole moments of these models are effective, being smaller than the real dipole of a liquid water molecule by about a factor of el ε , or roughly 2. If the total or corrected dipole moment is used in calculations, the dielectric constant comes out nearly twice as large, i.e., in the range of 160, which is twice as high as the experimental value. Here we discuss possible reasons for such a discrepancy. One approach takes into account dynamic corrections due to the dependence of the dielectric response of the medium producing the reaction field on the time scale of dipole fluctuations computed in the Kirkwood−Froḧlich method. When dynamic corrections are incorporated into the computational scheme, a much better agreement with the experimental value of the dielectric constant is found when the corrected (real) dipole moment of liquid water is used. However, a formal analysis indicates that the static properties, such as dielectric constant, should not depend on dynamics. We discuss the resulting conundrum and related issues of simulations of electrostatic interactions using periodic boundary conditions in the context of our findings.
Tunable dielectric constant of water at the nanoscale
Physical Review E, 2015
Using molecular dynamics simulations, the influence of the surface charge density of a nanotube on the static dielectric permittivity of confined water was reported. Whereas the dielectric anisotropy between the radial and axial directions of water confined in hydrophilic and hydrophobic membranes and the increase in axial dielectric permittivity with respect to the bulk value have previously been described, we found that an increase in the surface charge density leads to a drastic decrease in into the axial direction. The decrease in is accompanied by a strong slowdown in the rotational dynamics of water molecules. We show that this effect is due to the strong orientation of water molecules induced by the surface charge. Thus, by controlling the surface charge in nanotubes and nanocavities, it is possible to tune the dielectric permittivity of confined fluids at the nanoscale.
The Journal of Chemical Physics, 2019
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 that have to be included in implicit models of electrolytes for a relevant description of electrostatic interactions at nanometric scales.
Dielectric response of the water hydration shell of an ion: a field theory approach
2019
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 that have to be included in implicit models of electrolytes for a relevant description of electrostatic interactions at nanometric scales.
Physical Review Materials, 2021
There is a long-standing question about the molecular configuration of interfacial water molecules in the proximity of solid surfaces, particularly carbon atoms which plays a crucial role in electrochemistry and biology. In this study, the dielectric, structural and dynamical properties of confined water placed between two parallel graphene walls at different inter distances from the Angstrom scale to few tens of nanometer have been investigated using molecular dynamics. For dielectric properties of water, we show that the perpendicular component of water dielectric constant drastically decreases under sub 2 nm spatial confinement. The achieved dielectric constant data through linear response and fluctuation-dissipation theory, are consistent with recent reported experimental results. 1 By determining the charge density as well as fluctuations in the number of atoms, we provide a molecular rationale for the behavior of perpendicular dielectric response function. We also interpret the behavior of the dielectric response in terms of the presence of dangling O-H bonds of waters. By examining the residence time and lateral diffusion constant of water under confinement, we reveal that the water molecules tend to keep their hydrogen bond networks at the interface of water-graphene. We also found consistency between lateral diffusion and z-component of variance in the center of mass of the system as a function of confinement.
Colloid and Polymer Science, 2014
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We critically review the literature on the Debye absorption peak of liquid water and the excess response found on the high frequency side of the Debye peak. We find a lack of agreement on the microscopic phenomena underlying both of these features. To better understand the molecular origin of Debye peak we ran large scale molecular dynamics simulations and performed several different distance-dependent decompositions of the low frequency dielectric spectra, finding that it involves processes that take place on scales of 1-2 nm. We also calculated the k-dependence of the Debye relaxation, finding it to be highly dispersive. These findings are inconsistent with models that relate Debye relaxation to local processes such as the rotation/translation of molecules after H-bond breaking. We introduce the " spectrumfitter " Python package for fitting dielectric spectra and analyze different ways of fitting the high frequency excess, such as including one or two additional Debye peaks. We propose using the generalized Lydanne-Sachs-Teller (gLST) equation as a way of testing the physicality of model dielectric functions. Our gLST analysis indicates that fitting the excess dielectric response of water with secondary and tertiary Debye relaxations is problematic. We suggest that a distribution of Debye and oscillatory modes or truncated power-law is the correct way to fit the excess response. Our work is consistent with the recent theory of Popov et al. (2016) that Debye relaxation is due to the propagation of Bjerrum-like defects in the hydrogen bond network, similar to the mechanism in ice.
Anomalously low dielectric constant of confined water
The dielectric constant e of interfacial water has been predicted to be smaller than that of bulk water (e ≈ 80) because the rotational freedom of water dipoles is expected to decrease near surfaces, yet experimental evidence is lacking. We report local capacitance measurements for water confined between two atomically flat walls separated by various distances down to 1 nanometer. Our experiments reveal the presence of an interfacial layer with vanishingly small polarization such that its out-of-plane e is only~2. The electrically dead layer is found to be two to three molecules thick. These results provide much-needed feedback for theories describing water-mediated surface interactions and the behavior of interfacial water, and show a way to investigate the dielectric properties of other fluids and solids under extreme confinement.
The Journal of Chemical Physics, 1998
The longitudinal frequency and wave-vector dependent complex dielectric response function (k,)ϭ1Ϫ1/⑀(k,) is calculated in a broad range of k values by means of molecular dynamics computer simulation for a central force model of water. Its imaginary part, i.e., Im͕⑀(k,)͖/͉⑀(k,)͉ 2 , shows two main contributions in the region of small k values: Debye-like orientational relaxation in the lower frequency part of the spectrum and a damped librational resonance at the high frequency wing. The Debye relaxation time does not follow a de Gennes-like pattern: (k) goes through a maximum at kϷk*Ϸ1.7 Å Ϫ1 , while the static polar structure factor S(k) peaks at kϷ3 Å Ϫ1 . The resonance frequency (k) and the decay decrement ␥(k) show a dispersion law, indicative of a decaying optical-like mode, the libron. With an approximate normal mode approach, we analyze the origin of this mode on a molecular level which shows that it is due to a damped propagation of molecular orientational vibrations through the network of hydrogen bonds. At high k the decay, due to dissipation of collective into single particle motions, dominates. The static dielectric function is calculated on the basis of the response function spectra via the Kramers-Kronig relation. In the small k region ⑀(k) decreases from the macroscopic value ⑀ Ϸ80 to a value Ϸ15, i.e. it exhibits a Lorentzian-type behavior. This behavior is shown to be determined by higher order multipole correlation functions. In the intermediate and high k range, our results on ⑀(k) and (k) are in excellent agreement with data extracted from experimental partial pair correlation functions: ⑀(k) exhibits two divergence points on the k axis with a range of negative values in between where a maximum in (k) is found with max (k)ӷ1, indicative of overscreening. Consequences of quantum corrections to (k) with respect to a purely classical calculation are discussed and consequences are shown for the interaction energy between hydrated ions.