(113c) The Structural Ordering of Ionic Liquids Nanoconfined Between Charged Walls

In recent years, room temperature ionic liquids (RTILs) have been widely studied due to their unique properties such as negligible volatility and high electrochemical and thermal stability. The behavior of RTILs at surfaces and in confined geometries is also of great interest for use in various electrochemical applications, such as super-capacitors[1].  However, the structure and behavior of the RTILs in small, confined geometries is still not well understood, especially when the RTIL is interfaced with a charged surface[2]. The ordering of RTILs confined between charged walls is dictated by several competing factors, e.g., interactions between the charged species in the fluid, interactions between charged fluid species and the charged walls, aggregation behavior of alkyl side-chains, and geometric effects of the pore. In this work, we perform molecular dynamics (MD) simulations that examine the behavior and trends of model confined RTILs.  We perform atomistically detailed simulations of several commonly used RTILs (e.g., [CnC1im][NTf2], where n is the length of the side-chain) confined between mica walls.  We combine these results with MD simulations of a coarse-grained, minimal model, to efficiently establish trends over a wide range of statepoints.  Specifically, we investigate the impact of wall charge, pore separation, and side-chain interactions on the self-assembly within the pore. Our atomistic and coarse-grained simulations provide similar results, where we observe several clear structural ordering motifs. Under certain conditions, we find close agreement with ordering schematics previously proposed in the literature[2].  We conclude by comparing our simulated results to surface force apparatus measurements of the intersurface forces across the RTIL film for matching [CnC1im][NTf2] systems. 

[1] G. Feng and P.T. Cummings, J. Phys. Chem. Lett., 2011, 2, 2859-2864

[2] S. Perkin, et al. Chem. Commun., 2011, 47, 6572–6574