Chemistry Reference
In-Depth Information
3 Molecular Thermodynamics ................................................................ 161
3.1 Water in Hydrophobic Confinement and Applied Field . . ........................... 161
3.2 Resilience of the Hydrogen Bond Network in Polarized Water .................... 164
4 New Effects at the Nanoscale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166
4.1 Effects of Field Direction and Polarity on the Wetting Properties .................. 167
4.2 Wetting Free Energy . . ................................................................ 168
4.3 Water-Mediated Ordering of a Nanomaterial . . . ..................................... 170
5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174
1
Introduction
Modulation of solid/liquid interfacial tension by the applied electric field is cur-
rently enjoying explosive growth in a wide range of applications: from electrospray
ionization and ink-jet printing to electrical control of optical devices [
1
,
2
]. Under-
standing the influence of an applied electric field on interfacial properties of water
is of great interest to workers in the field of microfluidics [
3
,
4
], in particular the
electrowetting on dielectric (EWOD) [
5
]. There is also great interest from the
biology perspective since strong fields
E
(water dipole energy
E
d
comparable
to thermal energy
k
B
T
) arise in ion channels of cell membranes [
6
-
10
], in mem-
brane electroporation [
11
,
12
], and at the active site on an enzyme [
13
]. Recent
experiments [
14
-
17
] investigated the effect of electric field on contact angle, which
also potentially impacts the stability of liquid-liquid interfaces [
18
] and may be
pertinent to carbon nanotube sieves of O (1 nm) thickness [
19
]. There are excellent
review articles on electrowetting from macroscopic perspective that the reader is
referred to [
16
,
20
-
23
].
The advent of micro- and nanoporous materials sparked renewed interest in
wetting techniques including electrowetting in nanomaterials whose high surface-
to-volume ratio makes these media especially difficult to permeate with water.
Rapid developments in nanofluidics warrant a transition from continuum to mole-
cular level descriptions [
24
]. Computer simulation offers unique possibilities for
investigating molecular-level phenomena difficult to probe experimentally [
25
].
In this review chapter we focus on nanoscale effects that can currently be probed
best via molecular simulations. These tools give us the predictive power to discover
novel effects operating at short length scales.
The chapter is organized as follows. We start with macroscopic thermodynamic
predictions and discuss the phase behavior of confined liquids in general in the
absence of applied electric field. The primary focus is on capillary evaporation,
a phenomenon that can be reversed in the presence of the electric field. The reader
is directed to extensive excellent reviews [
26
] of capillary condensation. Next we
focus on the combined effect of confinement and electric field on liquids structure
and thermodynamics, water in particular, its stability against evaporation, and
resilience of the hydrogen bond network in polarized water. We devote increased
attention to issues of external conditions, as they determine how the system
responds to applied electric field. We concentrate on systems maintaining
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