Simultaneous ion and neutral evaporation in aqueous nanodrops: experiment, theory, and molecular dynamics simulations

Abstract

We use a combination of tandem ion mobility spectrometry (IMS-IMS, with differential mobility analyzers), molecular dynamics (MD) simulations, and analytical models to examine both neutral solvent (H2O) and ion (solvated Na + ) evaporation from aqueous sodium chloride nanodrops. For experiments, nanodrops were produced via electrospray ionization (ESI) of an aqueous sodium chloride solution. Two nanodrops were examined in MD simulations: a 2,500 water molecule nanodrop with 68 Na + and 60 Cl -ions (an initial net charge of z = +8), and (2) a 1,000 water molecule nanodrop with 65 Na + and 60 Cl -ions (an initial net charge of z = +5). Specifically, we used MD simulations to examine the validity of a model for the neutral evaporation rate incorporating both the Kelvin (surface curvature) and Thomson (electrostatic) influences, while both MD simulations and experimental measurements were compared to predictions of the ion evaporation rate equation of Labowsky et al [Anal Chim Acta, 2000, 406, 105-118]. To within a single fit parameter, we find excellent agreement between simulated and modeled neutral evaporation rates for nanodrops with solute volume fractions below 0.30. Similarly, MD simulation inferred ion evaporation rates are in excellent agreement with predictions based on the Labowsky et al equation. Measurements of the sizes and charge states of ESI generated NaCl clusters suggest that the charge states of these clusters are governed by ion evaporation, however, ion evaporation appears to have occurred with lower activation energies in experiments than was anticipated based on analytical calculations as well as MD simulations. Several possible reasons for this discrepancy are discussed.

The Relation Between Ejection Mechanism and Ion Abundance in the Electric Double Layer of Drops

“…The scaling of the data implies that the observation of the single-ion ejection from a minute nanodrop does not warrant an IEM mechanism. Our view is different from that that has been presented in the literature 80,81 where the ejection of simple ions from droplets composed of ≈ 1000 H 2 O is considered evidence of the ion evaporation mechanism (in the sense of Iribarne-Thompson and Labowsky et al). We elaborate more on this point in the next section.…”

“…Higashi et al have found excellent agreement 80 of MD evaporation simulations at T = 460 K with the IEM Labowsky et al model. 25 The simulated droplets comprise 2500 and 1000 water molecules with a mixture of Na + and Clions at a super-saturation.…”

“…Minute droplet size or enrichment of the outer solvent layers in ions under rapid evaporation signals caution when simulations are used to validate models of the ion-evaporation mechanism. 80 When evaporation is fast ions reside in non-equilibrium positions in the droplet's outer layer. Thus, their solvation does not obey the Born solvation model that underlies the ionevaporation models.…”

“…83 Higashi et al have performed atomistic simulations at T = 460 K of charged nanodrops comprised 2500 and 1000 H 2 O molecules, Na + ions and Clions in order to provide direct evidence of the IEM. 80 Temperature determines to a great extent the events in a droplet's lifetime. Even though temperature of T = 460 K in simulations appears to be high it is still not clear whether it is unrealistic because of conflicting experimental data on droplet temperature.…”

Activation of sub 2 nm Water Soluble and Insoluble Standard Ions with Saturated Vapors of Butanol in a Boosted TSI ultrafine CPC

“…The monomer and dimer of the THABr are separated by another small peak (at 418 Volts) more 'visible' in the Figure 3B in which the CPC is used as detector thanks to its sensitivity. This peak appears when the DMA is working in closed loop without dryer to remove volatile solvent (methanol, ethanol or acetonitrile) vapors introduced in the sheath gas by the droplets produced in the electrospray source [24,49,50]. That small peak is produced by the solvated monomer of the THABr with these vapors and eventually water vapor in case the carrier gas used through the electrospray source is not dry N 2 or air from a cylinder.…”