Stably electrospraying highly conductive sodium chloride aqueous solution coated with outer ionic liquid using coaxial capillary

Abstract

Concentrated aqueous solutions with high conductivity have been proven difficult to be stably electrosprayed mainly due to their large surface tension, which will easily trigger air discharge. The ionic liquid with lower surface tension is involatile, viscous, and highly conductive and can easily achieve stable electrospray. This paper presents a more convenient method to ionize the concentrated aqueous solution than the previous attempts and proves the feasibility of coaxially electrospraying two highly conductive liquids (∼1 S/m). Our method utilizes a coaxial capillary to coat the highly conducting concentrated sodium chloride (NaCl) aqueous solution (0.67 S/m) with a thin layer of immiscible ionic liquid to form a compound cone and achieve the stable coaxial electrospray. The compound cone remains static at a suitable flow rate (0.1–1 nl/s) of the ionic liquid. At a higher or lower flow rate, the compound cone elongates or shrinks and eventually vibrates. Due to the high conductivity of both liquids, the spray current does not follow the current scaling law. The spray current is mostly dominated by the NaCl solution and counterintuitively decreases as the flow rate of the ionic liquid increases due to the frequent vibration of the compound cone. The highly conducting thin layer of the ionic liquid is the key to achieving stable coaxial electrospray. It lowers the surface tension of the compound cone and shields the external electric field acting at the inner aqueous solution; thus, the air discharge of the aqueous solution is suppressed, and the stable compound cone can be formed.