Taylor cones of ionic liquids from capillary tubes as sources of pure ions: The role of surface tension and electrical conductivity

Abstract

The emissions of Taylor cones from a wide range of ionic liquids (ILs) have been tested in vacuo in an attempt to identify what physical properties favor the purely ionic regime (PIR). This regime is well known in the case of Taylor cones of liquid metals. For nonmetallic liquids, it has been previously observed in conventional (capillary tube) electrospray sources at room temperature only for the room temperature molten salt (ionic liquid) EMI–BF4 (EMI=1-ethyl-3-methylimidazolium). A large number of other ILs and their mixtures have been studied here, most of which (but not all) are unable to reach the PIR at room temperature. Based on these results and additional theoretical considerations, strong support is assembled for the notion that the PIR is favored by ILs not only of high electrical conductivity but also of high surface tension. This hypothesis is confirmed by tests with three recently synthesized ILs, EMI–GaCl4, EMI–C(CN)3, and EMI–N(CN)2, all of which combine exceptional surface tension and electrical conductivity, and all of which reach the PIR at room temperature far more readily than EMI–BF4.

Fragmentation in Time-of-Flight Spectrometry-Based Calculations of Ionic Electrospray Thruster Performance

“…Because such lenses focus a relatively narrow energy range [10], ions that fragment before the lens would not, and typically do not, appear in the TOF traces reported. However, in [18,19], no Einzel lens was applied and, similar to our findings, ∼30% or more of the solvated species may have fragmented based on nonzero TOF trace slopes between the t 0 and t 1 flight times for several ILs, including EMI-BF 4 .…”

A method to determine the onset voltage of single and arrays of electrospray emitters

“…This snap-over behavior could also explain why for liquids with low conductivities, such as EMI-Tf 2 N ͑0.88 S/m͒, purely ionic modes have been observed when sprayed using a needle ͑high fluid impedance, limited flow rate͒ and droplet mode when sprayed using a capillary. [18][19][20] In the first case the surface remains at equipotential during snap-over, which occurs more slowely than charge relaxation, while in the second case the surface cannot be assumed to be an equipotential due to the liquid's high flow rate.…”

Tailoring the hydraulic impedance of out-of-plane micromachined electrospray sources with integrated electrodes

“…Recent studies with the ionic liquids 1-ethyl-3-methylimidazolium tetrafluoroborate ͑EMI-BF 4 ͒ and 1-ethyl-3-methylimidazolium bis͑trifluorom-ethylsulfonyl͒imide ͑EMI-Tf 2 N, also referred to as EMI-Im͒, show that when sprayed from a capillary under vacuum conditions, a purely ionic regime ͑PIR͒ is only reached at low flow rates ͑Ӷ1 nL/ s͒ while at higher flow rates mixed droplet-ion emission is observed. [6][7][8][9] We have developed a method to tailor the hydraulic impedance in microfabricated silicon capillary emitters by creating a "porous" structure inside the capillary, thus combining the advantages of internally wetted capillaries with flow rate matching of externally wetted emitters. Similar to the method described by Valaskovic and Ehrenfeld 10 for loading capillaries with diameters below 300 m with particulate materials, in our case without applying any pressure, the modification of the hydraulic impedance is achieved by introducing silica microspheres into the capillaries and fixing them by means of a silanization step using silicon tetrachloride ͑SiCl 4 ͒ gas.…”