Complete removal of water from room-temperature ionic liquids is nearly impossible. For the electrochemical applications of ionic liquids, how water is distributed in the electrical double layers when the bulk liquids are not perfectly dry can potentially determine whether key advantages of ionic liquids, such as a wide electrochemical window, can be harnessed in practical systems. In this paper, we study the adsorption of water on electrode surfaces in contact with humid, imidazolium-based ionic liquids using molecular dynamics simulations. The results revealed that water molecules tend to accumulate within sub-nanometer distance from charged electrodes. At low amount of water in the bulk, the distributions of ions and of electrostatic potential in the double layer are affected weakly by the presence of water, but the spatial distribution of water molecules is strongly dependent on both. The preferential positions of water molecules in double layers are determined by the balance of several factors: the tendency to follow the positions of the maximal absolute value of the electrical field, the association with their ionic surroundings, and the propensity to settle at positions where more free space is available. The balance between these factors changes with charging the electrode, but the adsorption of water generally increases with voltage. The ion specificity of water electrosorption is manifested in the stronger presence of water near positive electrodes (where anions are the counterions) than near negative electrodes (where cations are counterions). These predictions await experimental verification.

Molecular simulation has emerged as an essential tool for modern-day research, but obtaining proper results and making reliable conclusions from simulations requires adequate sampling of the system under consideration. To this end, a variety of methods exist in the literature that can enhance sampling considerably, and increasingly sophisticated, effective algorithms continue to be developed at a rapid pace. Implementation of these techniques, however, can be challenging for experts and non-experts alike. There is a clear need for software that provides rapid, reliable, and easy access to a wide range of advanced sampling methods and that facilitates implementation of new techniques as they emerge. Here we present SSAGES, a publicly available Software Suite for Advanced General Ensemble Simulations designed to interface with multiple widely used molecular dynamics simulations packages. SSAGES allows facile application of a variety of enhanced sampling techniques-including adaptive biasing force, string methods, and forward flux sampling-that extract meaningful free energy and transition path data from all-atom and coarse-grained simulations. A noteworthy feature of SSAGES is a user-friendly framework that facilitates further development and implementation of new methods and collective variables. In this work, the use of SSAGES is illustrated in the context of simple representative applications involving distinct methods and different collective variables that are available in the current release of the suite. The code may be found at: https://github.com/MICCoM/SSAGES-public.

We report detailed simulation results on the formation dynamics of an electrical double layer (EDL) inside an electrochemical cell featuring room-temperature ionic liquids (RTILs) enclosed between two planar electrodes. Under relatively small charging currents, the evolution of cell potential from molecular dynamics (MD) simulations during charging can be suitably predicted by the Landau-Ginzburg-type continuum model proposed recently (Bazant et al 2011 Phys. Rev. Lett. 106 046102). Under very large charging currents, the cell potential from MD simulations shows pronounced oscillation during the initial stage of charging, a feature not captured by the continuum model. Such oscillation originates from the sequential growth of the ionic space charge layers near the electrode surface. This allows the evolution of EDLs in RTILs with time, an atomistic process difficult to visualize experimentally, to be studied by analyzing the cell potential under constant-current charging conditions. While the continuum model cannot predict the potential oscillation under such far-from-equilibrium charging conditions, it can nevertheless qualitatively capture the growth of cell potential during the later stage of charging. Improving the continuum model by introducing frequency-dependent dielectric constant and density-dependent ion diffusion coefficients may help to further extend the applicability of the model. The evolution of ion density profiles is also compared between the MD and the continuum model, showing good agreement.

Electrically driven ionic transport of room-temperature ionic liquids (RTILs) through nanopores is studied using atomistic simulations. The results show that in nanopores wetted by RTILs a gradual dewetting transition occurs upon increasing the applied voltage, which is accompanied by a sharp increase in ionic current. These phenomena originate from the solvent-free nature of RTILs and are in stark contrast with the transport of conventional electrolytes through nanopores. Amplification is possible by controlling the properties of the nanopore and RTILs, and we show that it is especially pronounced in charged nanopores. The results highlight the unique physics of nonequilibrium transport of RTILs in confined geometries and point to potential experimental approaches for manipulating ionic transport in nanopores, which can benefit diverse techniques including nanofluidic circuitry and nanopore analytics.