3.1. Solvation Diameter in Confined MeCN
Both the radial distribution
function and radial orientation distribution of MeCN surrounding
Na+ or Cl-in
free space are calculated by using the MDFT in comparison with the
relevant MD simulations.57 The same set of
system parameters is employed in the MDFT calculations: temperatureT = 298 K and bulk MeCN number density =
1.139×10-2 molecule/Å3(equivalently, the mass density,
0.777
g/cm3),
and
the LJ parameters are presented in the SI .
Figure
2 (a) and (b) show the radial distribution functions of MeCN, ,
surrounding Na+ and Cl-,
respectively.
Here
is the radial distance to the center of solute ion. The radial
orientation distribution function of MeCN is calculated as:
, (12)
where is the orientation-dependent radial distribution function of MeCN
solvent, and
following
our previous work36 it can be obtained
straightforwardly from the local structure of MeCN solvent described
with the orientation-dependent local density. The first peaks of radial
distribution functions for Na+ and
Cl- are remarkably larger than the other peaks,
indicating that the solvation radii of ions are closely dependent on the
MeCN solvent within the first solvation
shell.
Figure 2 (c) and (d) plot the radial orientation distribution
functions of MeCN surrounding Na+ and
Cl-, respectively.
The
radial orientation distribution functions display an oscillatory
feature,
showing
that the radial density distribution is sensitive to the orientation
angle. Namely, at different orientation angles, the radial density
distributions can be very different.
Indeed,
owing to the strong electrostatic interaction between the solute ion and
MeCN solvent, the nitrogen site of MeCN orients preferentially towards
Na+ within the first solvation shell. Opposite
situation can be found with the solute of Cl-.
Although there is a small deviation
between the MDFT predictions and MD simulation results,
the density distributions within the
first solvation shell are in excellent agreements.
This shows that the MDFT provides
an accurate and feasible method to calculate the local density and
orientation angle of MeCN
surrounding solute anion or cation.