Figure 5. Pore size-sensitive oriented structures of EMI cations and TFSI anions in carbon nanopores of 0.7 nm.
b, Definition of molecular vectors L, S and T. For EMI cation, L passes through both N atoms in the imidazolium ring and S is normal to L in the imidazolium ring. For TFSI anion, L passes through both S atoms in S-N-S bonding and S is normal to the S-N-S plane. d, Examples of molecular orientation for the L vector of cations to z-axis as <P2(cosθ)> = -0.5 (perpendicular), 0 (random) and 1 (parallel). (a, e) Density profiles of EMI (a) and TFSI (e) ions across the slit width. a, 83% of EMI cations located in the grey region (i) with double peaks due to the cation's T vector being well-aligned to the pore walls in two configurations, as shown in the inset. Cations adjacent to the pore walls contribute only 9% of the total density in the orange region (ii). Their long and small molecular axes (vector L and S) are oriented parallel to pore walls. The residual cations between the grey and orange regions have a vector L parallel to pore walls. e, 78% of TFSI anions locate in the grey region (i). In that region, anions have a vector T parallel to pore walls, whereas vectors L and S have no preferred orientation, as shown in the inset. Anions adjacent to the pore walls (4% of the total density) in the orange region (iii) have vectors L and S oriented parallel to the pore walls. The residual anions in the light green region (ii) have vectors L and T parallel to pore walls. (c, f) Order parameter <P2(cosθ)> changes of L, S and T vectors against z-axis. The colored regions are the same as in (a) or (e).