Probing non-polarizable liquid/liquid interfaces using scanning ion conductance microscopy

The study of microscopic structure of a liquid/liquid interface is of fundamental importance due to its close relation to the thermodynamics and kinetics of interfacial charge transfer reactions. In this article, the microscopic structure of a non-polarizable water/nitrobenzene (W/NB) interface was evaluated by scanning ion conductance microscope (SICM). Using SICM with a nanometer-sized quartz pipette filled with an electrolyte solution as the probe, the thickness of this type of W/NB interface could be measured at sub-nanometer scale, based on the continuous change of ionic current from one phase to another one. The effects for thicknesses of the non-polarizable W/NB interfaces with different electrolyte concentrations, the Galvani potentials at the interface and the applied potentials on the probe were measured and systematically analyzed. Both experimental setups, that is an organic phase up and an aqueous down, and a reverse version, were employed to acquire the approach curves. These data were compared with those of an ideal polarizable interface under the similar experimental conditions, and several characteristics of non-polarizable interfaces were found. The thickness of a non-polarizable interface increases with the decrease of electrolyte concentration and the increase of applied potential, which is similar to the situation of a polarizable liquid/liquid interface. We also find that the Galvani potential across a non-polarizable interface can also influence the interfacial thickness, this phenomenon is difficult to observe when using polarizable interface. Most importantly, by the comparison of two kinds of liquid/liquid interfaces, we experimentally proved that much more excess ions are gathered in the space charge layer of non-polarizable interfaces than in that of polarizable interfaces. These results are consistent with the predictions of molecular dynamic simulations and X-ray reflectivity measurements.