Direct measurement of ion evaporation kinetics from electrified liquid surfaces

When concentrated solutions of NaI in formamide with electrical conductivit ies K larger than 1.1 S/m are electrosprayed from a Taylor cone-jet in a va cuum, ions are evaporated at substantial rates from the surface of the meni scus and the drops. This constitutes a new source of ions and nanoparticles , where the relative importance of these two contributions is adjustable. T he currents of ions are measured independently from those associated with d rops by a combination of stopping voltage analysis and preferential scatter ing in a gas background. The magnitude E of the electric field at the surfa ce of the drops and at the apex of the cone-jet is controlled through the e lectrical conductivity K of the liquid and its flow rate Q through the jet. E is related through available scaling laws for Taylor cone-jets to the ra tios K/Q or I/Q, where I is the current of drops emitted by the jet. Ion cu rrents are very small or null at typical K/Q values used in the past. A rel atively small initial ion current is attributed to a few particularly sharp features present, perhaps associated with small satellite drops. At still higher K/Q this first ionization source saturates, and ion evaporation from the main drops begins to dominate (E similar to 1 V/nm). E can then be det ermined with little ambiguity, and the associated ion current is also measu red over a broad enough range of electric fields to determine the ionizatio n kinetics. At still higher K/Q the ion current from the drops approaches s aturation, and ion evaporation directly from the meniscus becomes dominant. The total spray current then presents the anomaly of increasing rapidly at decreasing liquid flow rate. The ion current from the meniscus can also be measured in this regime over a broad range of K/Q, with qualitative agreem ent with the ionization measurements from the drops. But the relation estab lished between K/Q and E becomes suspect because ion and drop currents are now comparable. A third approach to infer the ionization rate is based on t he related disappearance of Coulomb explosions of the drops above a critica l K/Q. These results are congruent with the model of Iribarne and Thomson, with an activation barrier for ion evaporation equal to 1.7 eV-(e(3)E/4 pi epsilon(0))(1/2). (C) 2000 American Institute of Physics. [S0021-9606(00)70 126-7].