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.