Figure 3 . (a) Reduced MeCN number as a function of for
Cl-. (b) Predicted solvation diameters for anions in
comparison with experimental results.58The
red dash line in the right figure is drawn to guide the eye.
To
further examine the accuracy of MDFT for predicting the MeCN structure
in confined space, the local density distribution of MeCN in a nanoslit
is calculated and then compared with simulation results. In the
simulation, the nanoslit is constructed with two parallel graphene
monolayers with separation = 42 Å. The simulation are performed with
standard molecular dynamics simulation package, in which the graphene is
described with full-atomic model, and the three-site
model42 for the MeCN liquid at 298 K is
utilized. The simulation details are provided in the SI .
The same model system is adopted in the MDFT calculation. Technically,
we place the left graphene monolayer at = 0.0 while the right one at
.Figure
4 (a) plots the predicted
normalized
density distributions of MeCN, , along the
direction
from both the MDFT and simulation. The normalized density distribution
from the MDFT is computed by taking average of the angular-dependent
local density over all MeCN orientations.
The
comparison shows that the MDFT prediction well captures the
layer-by-layer structure of MeCN near the wall, in excellent consistency
with the simulation result. For the sake of clear illustration, the
corresponding two-dimensional density map near the wall is depicted inFigure 4 (b) .
Next, we analyze the solvation diameter of ion in confined MeCN. For
reducing the computational cost on the external potential, we
reconstruct the nanoslit with two flat and structureless graphene-like
layers, in which the interaction between the MeCN solvent and neutral
flat wall is described by the 10-4
potential.49 The solute ion is placed at the
center of the nanoslit and immersed in the confined MeCN at ambient
condition.
Taking
Cl- as a case study, we compute the solvation number
and solvation diameter of Cl- in different nanoslits
by using the MDFT, as shown in Figure 4 (c) and (d)respectively.
In the nanoslit with sufficiently
large pore width,
the
solvation number and the solvation diameter recover to their
corresponding bulk values. As the pore width decreases, the solvation
number and the solvation diameter first oscillate around their bulk
values, and then monotonically decrease until reaching the limits. This
oscillative decrement of the solvation diameter unravels the nature of
ion
desolvation
in confined
solvent,
and it’s essentially consistent with the reported variation trend of
coordination number in confined systems. For example, Shao et
al.,45 reported a non-monotonic variation of
the coordination number of Na+ and
K+ in confined water in nanotube as decreasing the
nanotube size.