Altered polar character of nanoconfined liquid water

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Altered polar character of nanoconfined liquid water

Sayantan Mondal, Subhajit Acharya, and Biman Bagchi

Phys. Rev. Research 1, 033145 – Published 3 December 2019

Abstract

Atomistic molecular dynamics simulation study of dipolar fluids confined to spherical nanocavities of radii ranging from $R_{c} = 1$ to 4 nm reveals a surprisingly small Kirkwood correlation factor $(g_{K})$ in water, but not so in dipolar Stockmayer fluid. This results in an ultrafast relaxation of the total dipole moment time correlation function (DMTCF) of water. The static dielectric constant of water under nanoconfinement exhibits a strong dependence on size with a remarkably low value even at $R_{c} = 3 nm$ and a slow convergence to the bulk value because of surface-induced long-range orientational correlations. Interestingly, DMTCF exhibits a bimodal $1 / f$ noise power spectrum.

Received 26 February 2019
Revised 3 June 2019

DOI:https://doi.org/10.1103/PhysRevResearch.1.033145

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Confinement Dielectric properties

Water

Molecular dynamics

Condensed Matter, Materials & Applied PhysicsPolymers & Soft Matter

Authors & Affiliations

Sayantan Mondal , Subhajit Acharya, and Biman Bagchi ^*

Solid State and Structural Chemistry Unit, Indian Institute of Science, Bengaluru, India

^*Corresponding author: bbagchi@iisc.ac.in

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Issue

Vol. 1, Iss. 3 — December - December 2019

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Images

Figure 1
Static dielectric constant against the inverse of the number of molecules $(1 / N)$ for aqueous nanocavities with (a) LJ-12,6 atomistic walls, (b) LJ-10,4,3 walls, (c) LJ-9,3 walls, and (d) SF with LJ-9,3 walls. The convergence for water is extremely slow. Extrapolations using a cubic polynomial provide asymptotic values of 67.9, 57.5, and 70.4, respectively (thermodynamic limit). However, the convergence for SF is remarkably fast. Insets: Schematic two-dimensional cross sections of the nanocavities. Penetration of water molecules into the soft spheres (yellow regions) and inaccessible regions (orange) inside the cavity invoke errors in the volume calculation. Data represented as $[ɛ (N_{wat})]$ (SM S4 and S5 [35]).
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Figure 2
$ɛ$ of water against the inverse of effective radius aqueous nanocavities. Surfaces are described by (a) LJ-12,6, (b) LJ-10,4,3, and (c) LJ-9,3 potentials. The sensitivity of $ɛ$ to $R_{eff}$ is strong. After the divergentlike behavior $ɛ$ becomes negative. (d) Wave-vector $(k)$ -dependent dielectric function calculated for dipolar hard-sphere liquid using mean spherical approximation shows similar divergencelike behavior (replotted from Ref. [42]).
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Figure 3
(a) Total DMTCF of water inside spherical nanocavities. The relaxation under confinement is approximately 20 times faster than the bulk (dashed line). (b) First rank orientational TCF for nanoconfined water which shows a slightly faster decay than the bulk. The distinction is prominent, especially for $R_{c} = 1 nm$ . However, $C_{1} (t)$ converges to the bulk response with increasing cavity size. (c) Total DMTCFs of SF inside spherical nanocavities show approximately four times faster relaxation. (d) First rank orientational TCF for nanoconfined SF. Unlike water, the confined particles show a slower decay, however, with comparable timescales.
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Figure 4
Time evolution of angles created by the total dipole moment vectors of two hemispheres with (a) $X$ axis and (b) $Y$ axis for an $R_{c} = 4 nm$ aqueous system with atomistic LJ-12,6 walls. (c) and (d) show similar plots for an $R_{c} = 4 nm$ SF system with LJ-9,3 walls. In SF, the fluctuations are short lived. These plots show strong anticorrelated dipole flips that result in enormous cancellations. Blue and red represent northern and southern hemispheres, respectively.
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Figure 5
(a) Self- and cross-DMTCF among eight grids inside an aqueous nanocavity of $R_{c} = 4 nm$ . The amplitudes of self-terms are approximately four to ten times higher than that of the cross terms. However, there are 8 self- and 56 cross terms that construct $〈 M (0) M (t) 〉$ . As a result, the negative contributions from anticorrelated cross terms predominate at longer times. This makes the net relaxation (red dashes) drastically fast. (b) Bimodal $1 / f$ character of the power spectrum of dipole moment fluctuation under confinement.
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