Physical Chemistry

Conical shape fluctuations determine the rate of ion-evaporation and the emitted cluster-size distribution from multiple-charged droplets



The ion-evaporation mechanism (IEM) is perceived to be a major pathway for disintegration of multi-ion charged droplets found in atmospheric and sprayed aerosols. However, the precise mechanism of IEM and the effect of the nature of the ions in the emitted cluster-size distribution have not yet been established despite its broad use in mass spectrometry and atmospheric chemistry over past half century. Here we present a systematic study of the emitted ion-cluster distribution in relation to their spatial distribution in the parent droplet using atomistic modeling. It is found that in the parent droplet, multiple kosmotropic and weakly polarizable chaotropic ions (Cs+) are buried deeper within the droplet than polarizable chaotropic ions (Cl-, I-). This differentiation in the ion location is only captured by a polarizable model. It is demonstrated that the emitted cluster-size distribution is determined by dynamic conical deformations and not by the equilibrium ion-depth within the parent droplet as the IEM models assume. Critical factors that determine the cluster-size distribution such as the charge sign asymmetry that have not been considered in models and in experiments are presented. We argue that the existing IEM analytical models do not establish a clear difference between IEM and Rayleigh fission. We propose a shift in the existing view for IEM from the equilibrium properties of the parent droplet to the chemistry in the conical shape fluctuations that serve as the centers for ion-emission. Consequently, chemistry in the conical fluctuations may also be a key element to explain charge states of macromolecules in mass spectrometry and may have potential applications in catalysis due to the electric field in the conical region.

Version notes

Data were added for the fluoride and cesium ions. Additional analysis of the data was included.


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Supplementary material

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Supporting Information
(S1) Details of the computational methods and models. (S2) Radial distribution functions (RDFs) for droplets with multiple ions. (S3) RDFs for droplets with a sole ion. (S4) Charge distribution in droplets with multiple ions. (S5) Charge distribution in droplets with a sole ion. (S6) Typical snapshots of conical fluctuations from the simulations.