Statistical Analysis of Droplet Charge Acquired during Contact with Electrodes in Strong Electric Fields.

Aqueous droplets acquire charge when they contact electrodes in high-voltage electric fields. Although many researchers have investigated droplet charging under various conditions, the droplet charges are typically reported simply in terms of a mean and standard deviation. Here, we show that droplets often acquire significantly less charge for a single contact compared to the previous and subsequent contacts. These "low-charge events," which are not observed with charging of metal balls, yield up to a 60% decrease in charge acquired by the droplet and occur regardless of the applied field strength, droplet conductivity, or droplet volume. In all cases examined here, the occurrence of low-charge events to good approximation follows a negative binomial distribution (i.e., a Pascal distribution) with a mean probability of 13%. We further demonstrate that approximately 16% of charging events are characterized by "irregular" Taylor cone dynamics, suggesting that instabilities in the electrically driven deformation of the approaching liquid interface may be responsible for the low-charge events. The results indicate that workers using systems involving droplet charging should take into account the high likelihood of droplets randomly acquiring less charge than expected.

Droplet Conductivity Strongly Influences Bump and Crater Formation on Electrodes during Charge Transfer.

Aqueous droplets acquire charge when they contact electrodes in high voltage electric fields, but the exact mechanism of charge transfer is not understood. Recent work by Elton et al. revealed that electrodes are physically pitted during charge transfer with aqueous droplets. The pits are believed to result when a dielectric breakdown arc occurs as a droplet approaches the electrode and the associated high current density transiently locally melts the electrode, leaving distinct crater-like deformations on the electrode surface. Here we show that the droplet conductivity strongly modulates the pitting morphology but has little effect on the amount of charge transferred. Electron and atomic force microscopy shows that deionized water droplets yield no observable deformations, but as the salt concentration in the droplet increases above 10-3 M, the deformations become increasingly large. The observed intensity of the flash of light released during the dielectric breakdown arc also increases with droplet conductivity. Surprisingly, despite the large difference in pitting morphology and corresponding arc intensity, droplets of any conductivity acquire similar amounts of charge. These results suggest that the energy transferred during dielectric breakdown is primarily responsible for electrode pitting rather than the total amount of energy released during charge transfer.

Measurement of Charge Transfer to Aqueous Droplets in High Voltage Electric Fields.

The electric charge acquired by aqueous droplets when they contact an electrode is a crucial parameter in experimental and industrial applications where electric fields are used to manipulate droplet motion and coalescence. For unclear reasons, many investigators have found that aqueous droplets acquire significantly more positive than negative charge. Extant techniques for determining the droplet charge typically rely on a hydrodynamic force balance that depends on accurate characterization of the drag forces acting on the droplet. Here we present an alternative methodology for measuring the droplet charge via direct measurement of the electric current. As the droplet approaches the electrode the current is observed to gradually increase, followed by a large pulse when the droplet makes apparent contact. We interpret the transient current signals as the superposition of the natural response of an RLC circuit and an induced current described by the Shockley-Ramo theory. Nonlinear regression of the observed current to the theoretical model allows for the droplet charge to be extracted, independent of any assumptions about the force balance on the droplet. We demonstrate that regression of the current signal yields charge values that are on average within 4% of charges measured via a force balance. We use the chronocoulometric methodology to investigate how the charge varies with the applied potential, and we demonstrate that deionized water droplets contacting planar electrodes acquire on average 69% more positive charge than negative charge.

Pb(II)-promoted amide cleavage: mechanistic comparison to a Zn(II) analogue.

Two new Pb(II) complexes of the amide-appended nitrogen/sulfur epppa (N-((2-ethylthio)ethyl)-N-((6-pivaloylamido-2-pyridyl)methyl)-N-((2-pyridyl)methyl)amine) chelate ligand, [(epppa)Pb(NO3)2] (4-NO3) and [(epppa)Pb(ClO4)2] (4-ClO4), were prepared and characterized. In the solid state, 4-NO3 exhibits κ(5)-epppa chelate ligand coordination as well as the coordination of two bidentate nitrate ions. In acetonitrile, 4-NO3 is a 1:1 electrolyte with a coordinated NO3(-), whereas 4-ClO4 is a 1:2 electrolyte. Treatment of 4-ClO4 with 1 equiv Me4NOH·5H2O in CH3CN:CH3OH (3:5) results in amide methanolysis in a reaction that is akin to that previously reported for the Zn(II) analogue [(epppa)Zn](ClO4)2 (3-ClO4). (1)H NMR kinetic studies of the amide methanolysis reactions of 4-ClO4 and 3-ClO4 as a function of temperature revealed free energies of activation of 21.3 and 24.5 kcal/mol, respectively. The amide methanolysis reactions of 4-ClO4 and 3-ClO4 differ in terms of the effect of the concentration of methanol (saturation kinetics for 4-ClO4; second-order behavior for 3-ClO4), the observation of a small solvent kinetic isotope effect (SKIE) only for the reaction of the Zn(II)-containing 3-ClO4, and the properties of an initial intermediate isolated from each reaction upon treatment with Me4NOH·5H2O. These experimental results, combined with computational studies of the amide methanolysis reaction pathways of 4-ClO4 and 3-ClO4, indicate that the Zn(II)-containing 3-ClO4 initially undergoes amide deprotonation upon treatment with Me4NOH·5H2O. Subsequent amide protonation from coordinated methanol yields a structure containing a coordinated neutral amide and methoxide anion from which amide cleavage can then proceed. The rate-determining step in this pathway is either amide protonation or protonation of the leaving group. The Pb(II)-containing 4-ClO4 instead directly forms a neutral amide-containing, epppa-ligated Pb(II)-OH/Pb(II)-OCH3 equilibrium mixture upon treatment with Me4NOH·5H2O in methanol. The rate-determining step in the amide methanolysis pathway of 4-ClO4 is nucleophilic attack of the Pb(II)-OCH3 moiety on the coordinated amide. Overall, it is the larger size of the Pb(II) center and the availability of coordination positions that enable direct formation of a Pb(II)-OH/Pb(II)-OCH3 mixture versus the initial amide deprotonation identified in the reaction of the Zn(II)-containing 3-ClO4.