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crystalline or amorphous structures [Henderson, 2002]. On most close-packed sur-
faces, H 2 O adsorption forms a bilayer structure of hexagonal rings that aligns to the
registry of the metal surface [Henderson, 2002; Michaelides et al., 2003a].
Surprisingly, molecular dynamics and Monte Carlo simulations have overwhel-
mingly shown that the structure of the metal - water interface at ambient temperatures
and full solvation bears a striking resemblance to the metal - water interfaces described
above under UHV conditions (for a review, see Taylor and Neurock [2005]).
Hydrogen bonding creates time-fluctuating, metastable water networks at the discontinu-
ity between the phases, and simulations have shown that these have a permanence of
several picoseconds [Izvekov et al., 2001; Raghavan et al., 1991]. A number of these
simulations use generic potentials in which the identity of the metal is omitted from
the actual simulation (see, e.g., Christou et al. [1981]). More recent ab initio molecular
dynamics simulations, however, paint a similar picture [Izvekov et al., 2001; Taylor,
2009b; Vassilev et al., 2005]. Figure 4.5 shows the results of ab initio density functional
molecular dynamics simulations of the Cu(111) - H 2 O interface, providing a clear indi-
cation of local structure, including the presence of bilayers extending up to about 1 nm
from the interface, with a permanence of at least several picoseconds [Taylor, 2009b].
Accordingly, Neurock and co-workers have developed models for the electro-
chemical interface that retain this concept of hexagonal structure over close-packed
metal surfaces [Filhol and Neurock, 2006; Taylor et al., 2006c]. With the use of a
screening charge as described in Section 4.3, the sensitivity of the structural parameters
of water with respect to the electrochemical environment were explored [Taylor et al.,
2006a]. The predominant effect stems from the polar nature of the water molecule, in
which the water molecules are observed to rotate as a function of the applied potential.
Figure 4.5 The trajectory of water molecules during an ab initio simulation of the electrode/
electrolyte interface. The disentanglement and then reorganization of water molecules into
bilayers can be clearly seen as time progresses [Taylor, 2009b].
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