Effect of tip curvature on ionic emissions from Taylor cones of ionic liquids from externally wetted tungsten tips

Several ionic liquids (ILs) of relatively high electrical conductivity (K) and surface tension (γ) are electrosprayed under vacuum from electrochemically sharpened and roughened tungsten wires with tip radii R varying from 2 to 80 μm. All our tips exhibit a purely ionic emission, confirming the versatility of these ionic liquid ion sources (ILIS) previously demonstrated by Lozano and Martínez-Sánchez using R∼20 μm. A drastic increase in ion current from 50 to 1000 nA results when increasing R from 2.5 up to 29 μm and above, offsetting the considerable disadvantage in current emission level previously observed for ILIS (∼200 nA) versus conventional internally fed capillary tube emitters (∼1000 nA). Experiments with shielding electrodes show that this increase in current is not due to a reduction in space charge effects associated to the higher onset voltages required at smaller curvatures. It results from a reduction in flow impedance at increasing R. Long-term stable Taylor cone operation was not achieved with any of the ILs tested, even when alternating the tip voltage to minimize electrochemical reactions.