The Electric Double Layer and the Electrophoretic Motion of an Ionic Liquid in a Dielectric Liquid

Abstract

An ionic liquid (IL) is a salt in the liquid state in room temperature. ILs consist of large organic cation and relatively small organic or inorganic anion. ILs have unique physical, chemical, and electrochemical properties, such as no vapor pressure or wide electrochemical window. Therefore ILs can be applied many scientific or industrial field. ILs are contact with other dielectric liquid and electrically manipulated in many application including microfluidic application. In this thesis, we focused the electromechanical properties of ILs such as the electric double layer. The modified Poison Boltzmann equation is solved for the interface of an IL and a dielectric liquid. The charge density and the ionic composition change in the electric double layer are also derived. The double layer of the Taylor cone is solved using the analytical result and is complemented by a numerical computation. A conducting droplet in a dielectric liquid is charged with contact of the electrode under electric field. And the charged droplet is translated electrophoretically between the two electrodes. The interfacial tension of an IL-silicone oil system is measured using the deformation of this translating droplet under the uniform electric field. The electrophoretic motion of a droplet of an IL is observed, and the electrophoretic mobility is estimated experimentally. Large mobility could be explained using the over-charging phenomenon due to the deformation of the droplet on the electrode.When a charged droplet is translated in oil by electrophoresis, the droplet follows a straight line between two electrodes. However, if an oil-air interface is close to a droplet, the droplet follows an upwardly concave parabolic pathway. By using the leaky dielectric model, we have verified that this phenomenon is caused by the distorted electric field due to the difference in permittivities. Furthermore, it is enhanced by the accumulated free charges on the oil-air interface due to the difference in the electric conductivities of oil and air. Finally, we suggest a complementary arrangement of electrodes for realizing a straight pathway of the droplet near the interface.