Direct Evaluation of the Electrokinetic Properties of Electrolytes in a Nanochannel using Electrical Impedance Spectroscopy

An electric double layer is the local distribution of ions created at any liquid and solid interface. The thickness of the electric double layer is typically in nanometers, so that it is dominant in the nanoscale. Because the size of the nanospace is the same order of magnitude as the thickness of the electric double layer, the nanospace should be partially/totally filled with an electric double layer. Therefore, the investigation of the electrokinetics inside the electric double layer in nanospaces, such as nanopores and nanochannels, is very important for electrical/electrochemical analysis and for the manipulation of molecules and nanoparticles in nanofluidic systems. Here, electrical impedance spectroscopy was used to comprehensively investigate how the ionic concentration and the volume of the nanospace influenced the electrical properties of various ionic solutions in a nanochannel. The electrokinetic properties and the thickness of the electric double layer in the nanospace were determined to be dependent on the hydration diameter, mobility, and ionic strength of the various ion species. The electric double layer overlaps under particular conditions, such as with sufficiently narrow channels, and the qualitative properties of this were consistent with the Debye-Huckel-Onsager equation. These results should be a significant contribution to the improvement of the analysis and manipulation of molecules and particles in nanochannels, which could lead to further development in the application of nanofluidics.