Disintegration Mechanisms of Charged Aqueous Nanodroplets Studied by Simulations and Analytical Models

Abstract

The mechanism of fragmentation processes in aqueous nanodroplets charged with ions is studied by molecular dynamics (MD) simulations. By using constant-temperature MD, the evaporation of the water is naturally taken into account and sequences of ion fragmentation events are observed. The size of the critical radius of the charged droplet just before the fragmentation and the distribution of the sizes of the fragments are estimated. Comparison of the Rayleigh critical radius for fragmentation and simulation data is within 0.23 nm. This seemingly small difference arises from a large difference in the number of water molecules that makes fragmentation an activated process as in the ion evaporation mechanism (IEM). This finding is in agreement with the predictions of Labowsky et al. [Anal. Chim. Acta 2000, 406, 105-118] for charged aqueous drops. The size of the daughter droplets is larger than the prediction of Born's theory by 0.1 to 0.15 nm. The nature and the dynamics of the intermediate states of the fragmentation process characterized by a bridge formed between the mother droplet and the evaporating ion or thorned structures where the ion sits on the tip are important for the outcome of the size-distribution of the fragments, while they are is missing in Born's theory.

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

“…Distinct from these experimental and theoretical efforts, a number of researchers, beginning with Znamenskiy et al 20 and including the works of Consta and colleagues [21][22][23] as well as Konermann and colleagues [24][25][26] , have used Molecular Dynamic (MD) simulations to directly observed, computationally, ions evaporating from nanodrops. Notable amongst these prior works, Ichiki & Consta 23 computed the approximate radii of evaporating, solvated chloride ions from water drops in simulations, finding good agreement with model predictions of Labowsky et al 17 .…”

A simple model for the disintegration of highly charged solvent droplets during electrospray ionization

“…For deriving eq 1, Rayleigh [32] assumed that fragmentation is triggered by shape oscillations. Droplet deformations were described in terms of spherical harmonics, and he concluded that the fragmentation channel having the lowest ⌬E ts is associated with quadrupole (prolate-oblate [9]) oscillations, whereas higher order deformations are less effective at mediating disintegration [42]. For a neutral droplet (f ( ( ϭ 0) disintegration is highly endothermic due to the increase in total surface area.…”

“…E lectrospray ionization (ESI) mass spectrometry (MS) has evolved into one of the most versatile and widely used analytical techniques. The ESI process itself has been subject of numerous studies (see, e.g., [1][2][3][4][5][6][7][8][9][10][11][12] and references therein). Analyte solution is passed through a metal capillary to which high voltage has been applied, and solvent droplets emanate from the tip of a Taylor cone at the capillary outlet [11].…”

“…(1) How exactly can the interplay of Coulomb forces and surface tension be interpreted, considering that the two factors do not even share the same physical units? (2) What is the reason for the generally observed asymmetry in product size, considering that Rayleigh's mechanism [32] should favor the formation of two equally sized fragments [9,18,34,35]? (3) What are the consequences of offspring droplets being ejected from the tip of extended protrusions, instead of being emitted from the surface of a more or less spherical parent droplet?…”

“…More recently, the rapid solidification of charged nanodroplets using sol-gel techniques has provided a means to capture transient droplet shapes and study them by microscopy [17]. Also, molecular dynamics simulations represent an interesting approach in this area [9,10]. All those studies reveal that disintegration proceeds through intermediate structures where a parent droplet carries one or two thin protrusions that can be considerably longer than the parent diameter.…”