Nanoscale Wetting Under Electric Field from Molecular Simulations - Multiscale Molecular Methods in Applied Chemistry - page 172

of O(10 2 )V ˚ 1 . The comparison between wetting free energies at different pore
widths shows that the effect of the field grows with the thickness of the polarized
water slab; however, the free energy change with the angle is essentially

independent of the pore width. Given the smaller width amounts to about four
layers of water molecules, it is clear that the angle dependence is dominated by pure
surface-layer effects. The variation of the free energy with the angle produces an
aligning torque of (absolute) magnitude

f

tð E ; fÞ¼ð@ F =@fÞ A wl @sð E ; fÞ=@f (17)

where F is the free energy and A wl is the area of solid wall/liquid interface. The
torque on nanopore walls arises solely due to anisotropic water/wall interactions
and independently of any direct interaction between the wall material and the field.
Importantly, because a major part of angular forces on water molecules reflects
orientational preferences of hydrogen-bonding, a nonzero torque can exist even
when there is no dielectric contrast between the pore material and water.

4.3.2 Dispersed Nanoparticles

The orienting effect, discussed above, is present in other geometries including
nonspherical nanoparticles in a dispersion. As orientational forces between
the solvent molecules and the particle surfaces couple with those imposed by the
applied field, they augment the classical effect ( 16 ), and enhance the trend to align
the nanoparticle surface with the field. This expectation, based on surface thermo-
dynamics calculations for aqueous confinements [ 53 ], is confirmed in molecular
dynamics simulations of freely rotating nano-platelets suspended in water under
external field [ 108 ]. Orientational forces are found to exceed continuum theory
predictions [ 129 ] by a factor close to two [ 108 ], providing a direct measure of the
molecular mechanism neglected in macroscopic theories. Enhanced torques can
considerably facilitate the use of electric field in tuning suspension structure and
thus, for a supersaturated regime, also the structure of any emerging crystalline
phase. For materials science as well as for the design of electro-mechanical sensors,
it is essential to estimate also the dynamics of nanoparticle orientation. Reorienta-
tion time of a 2-3 nm wide graphene-like platelet under the (actual) field of
~0.03 V ˚ 1 is of O(10 2 ) ps. A very interesting result is an approximate balancing
between increased hydrodynamic friction and the electric torque upon particle size
scaling. The field-induced reorientation dynamics therefore depends only weakly
on the particle size and remains fast O(10 2 -10 3 ps) even for comparatively big
O(10) nm particles; these results can be extrapolated to even bigger sizes not
accessible by molecular dynamics simulations with explicit solvent. Apart from
the torque enhancement due to hydration-shell molecules, the observed dynamic
behavior conforms well to predictions [ 129 ] from classical hydrodynamics.