The influence of an electric field on the wetting of an SiO2 film by ionic liquids

The electrowetting of a dielectric SiO₂ film 100 nm thick by drops (D = 2–3 mm) of [C₄mIm][PF₆], [C₆mIm][PF₆], and [C₆mIm][BTI] ionic liquids was studied at |U| ≤ 60 V in a ～10^?8 mbar vacuum. Electrocapillary curves of the dependence of the wetting angle on electric field potential were constructed with an accuracy of ±1 deg. In conformity with the Young-Lippman equation, the wetting angle θ° decreased by the parabolic law from 51° to 43° for [C₄mIm][PF₆], from 48° to 38° for [C₆mIm][PF₆], and from 35° to 27° for [C₆mIm][BTI] as |U| increased at 298 K. The electrocapillary curve branches were situated symmetrically in the (θ°, U) coordinates with respect to the line passing through the point U = 0; that is, zero-charge potential is zero for the electrowetting of the dielectric film by the ionic liquids. The capacitance of the double electrical layer at the ionic liquid-dielectric interface was determined. This value was found to be 4.65, 2.93, and 1.73 μF/m² for the electrowetting of the SiO₂ film at 298 K by the ionic liquids specified, respectively.