Modulation of an Induced Charge Density Gradient in the Room-Temperature Ionic Liquid BMIM⁺BF₄^–

In an earlier article ( Langmuir 2016, 32, 9507–9512), we reported the existence of an induced charge density gradient, ρ f, in a room-temperature ionic liquid (RTIL, BMIM + BF 4 – ) normal to a charged planar silica surface. In this work, we demonstrate experimental control over the sign and magnitude of the gradient. The spatial extent of ρ f can exceed 100 μm from the charged surface. We characterized ρ f through the rotational diffusion time constant gradient of a cationic chromophore in the room-temperature ionic liquid (RTIL). The sign and magnitude of ρ f in BMIM + BF 4 – is linked directly to the surface charge density of the electrode, which can be controlled. We used transparent conductive electrodes (FTO and ITO coated on glass) as supports and demonstrated that control over the electrode surface charge carrier density can influence the magnitude and sign of ρ f . There are limitations to this approach based on the FTO and ITO properties, and we demonstrate these limits experimentally.