Indexed on: 22 Apr '17Published on: 12 Apr '17Published in: Journal of Physical Chemistry C
The electrical double layers (EDLs) in the mixtures of room-temperature ionic liquids and water play an important role in many applications but have only begun to receive widespread attention recently. Here, we report the molecular dynamics simulations of EDLs near rigid polyanion nanorods immersed in electrolytes containing ionic liquids [C2mim][TfO], water, and Na+ ions. When the water content in bulk electrolyte is high, the EDLs near the rods are similar to those in aqueous electrolytes except that TfO– ions accumulate notably near the cation layer adsorbed on the rods. When the water content in the bulk electrolyte becomes very low, even though water is greatly enriched in the interfacial region, the microenvironment in the region offers weak dielectric screening and the EDL exhibits features common to those in neat ionic liquids, e.g., charge overscreening. Na+ ions are readily adsorbed on the sulfonate groups of the polyanion rod in the range of water contents explored here but can experience an energy barrier when moving from the bulk electrolyte to the rod surface if the water content is very low. Introducing Na+ ions into the system displaces C2mim+ ions from the rod’s surface. However, the number of displaced C2mim+ ions is far less than the Na+ newly adsorbed on the rod, and thus, charge overscreening is enhanced. We highlight the important role of ion–ion correlations in determining the EDL characteristics and their response to the variation of mixture electrolytes’ water/Na+ ion contents revealed here and discuss their technical implications.