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.