Nanoscale Wetting Under Electric Field from Molecular Simulations - Multiscale Molecular Methods in Applied Chemistry - page 164

First, the reported field strengths warrant explanation. In the absence of tin-foil
boundary conditions [ 72 , 73 ], the actual strength of electric field spanning the
aqueous phase, E ( r ), differs significantly from the unperturbed “applied” field E o
due to the opposing effect of water polarization. The actual, dielectrically screened
field E ( r ) is generally nonuniform and is of the order

e r 1 weaker than the
unscreened field E o . In the bulk phase, modeled by Ewald summation with vacuum
boundary conditions [ 72 ], the exact relation (for the absolute values of the field) is
E e r is the relative permittivity [ 74 ]. While constant-voltage

simulation techniques have been presented [ 75 , 76 ], in most cases, E o represents
input information for a simulation experiment and the screened, position dependent,
field E ( r ) can be determine d during the simulation. In the latter case, a viable
estimate of the average field E across the system can be obtained from the observed
polarizatio n of water, measured in terms of the cosine of the alignment angle,
<

¼ 3E o /( e r þ 2), where

[ 77 - 79 ]. When compared with a laboratory measurement, a simula-
tion with fixed applied field E o is akin to an experiment on a system between
electrified surfaces (or capacitor plates) with fixed charge densities. The common
fixed-voltage experiment, on the other hand, corresponds to preselecting the aver-
age field across the system. The implications of the two different constraints have
been discussed by Jia and Hentschke [ 80 ]. In capturing electrostatic screening in
a globally polarized system, simulations employing Ewald periodic conditions are
generally superior to cutoff-based techniques. While the use of a distance cutoff on
the intermolecular interactions mostly gives a satisfactory qualitative description of
the system's responses to the field, dielectric screening is typically underestim a ted,
and the average field E , and apparent voltage V across the system, V

cos y N;i >

DE are
overestimated in this approach [ 68 ]. For clarity, both the exact value of the
unscreened input field E o and the approximate average of the actual field will be
listed in most cases we discuss below.
The second comment concerns the usage of rigid water models such as the
SPC/E model [ 81 ], which by design cannot undergo ionization or react chemically.
We note that actual fields considered here are much too weak to polarize signifi-
cantly, let alone decompose water when any flow of electric current is prevented by
proper insulation. Top end insulators like polymer and silica films with dielectric
strength of up to 5 10 8 Vm 1 can provide more than adequate insulation.

3.2 Resilience of the Hydrogen Bond Network in Polarized Water

For an open system, described by ( 13 ), any field-induced density depletion,
ð@

0, could only be expected in case of dramatic rise in orientational
polarizability of the molecules upon dilution. A mean-field analysis [ 69 , 82 ]ofa
water-mimicking Ising model in electric field explored the assumption
that molecular dipole alignment perturbs hydrogen bonding. Over an interval of
intermediate field strengths, the model-system featured a density drop akin to the

N =@ E o Þ mVT <