Molecular Dynamics Simulations of Electrosprayed Water Nanodroplets: Internal Potential Gradients, Location of Excess Charge Centers, and “Hopping” Protons

Abstract

Water nanodroplets charged with excess protons play a central role during electrospray ionization (ESI). In the current study molecular dynamics (MD) simulations were used for gaining insights into the nanodroplet behavior based on classical mechanics. The SPC/E water model was modified to permit the inclusion of protons as highly mobile point charges at minimum computational cost. A spherical trapping potential was assigned to every SPC/E oxygen, thereby allowing the formation of protonated water molecules. Within a tightly packed nanodroplet the individual potential wells merge to form a three-dimensional energy landscape that facilitates rapid proton hopping between water molecules. This approach requires short-range modifications to the standard Coulomb potential for modeling electrostatic proton-water interactions. Simulations on nanodroplets consisting of 1248 water molecules and 10 protons (radius, ca. 21 A) result in a proton diffusion coefficient that is in agreement with the value measured in bulk solution. Radial proton distributions extracted from 1 ns MD runs exhibit a large peak around 14 A, in addition to substantial population density closer to the droplet center. Similar radial distributions were found for nanodroplets charged with Na+ ions. This behavior is dramatically different from that expected on the basis of continuum electrostatic theory, which predicts that excess charge should be confined to a thin layer on the droplet surface. One important contributor to this effect seems to be the ordering of water molecules at the liquid/vacuum interface. This ordering results in an electrical double layer, generating a potential gradient that tends to pull positive charge carriers (such as protons, but also others such as Na+ ions) toward the droplet interior. This deviation from the widely assumed surface charge paradigm could have implications for the mechanism by which protonated analyte ions are formed during ESI.

Matrix Assisted Ionization: New Aromatic and Nonaromatic Matrix Compounds Producing Multiply Charged Lipid, Peptide, and Protein Ions in the Positive and Negative Mode Observed Directly from Surfaces

“…In spite of the importance and world-wide use, the ionization mechanisms of MALDI [4][5][6][7][8] and, to a lesser degree, ESI [9][10][11][12][13] are still in dispute. For MALDI, the two most prominent models are the cluster model [14,15] and the photochemical ionization model [16,17].…”

Molecular dynamics of electrosprayed water nanodroplets containing sodium bis(2‐ethylhexyl)sulfosuccinate

“…Even though each one of these processes is amenable to a qualitative description, an overall picture is hard to come about, especially at the quantitative level. Together with experimental findings for droplets in the micrometer‐size regime, a great deal of detailed and sometimes unexpected information on such processes has been gained by molecular dynamics (MD) simulations especially for nano‐sized droplets …”

“…The most striking results of these studies for aqueous nanodroplets containing simple ionic species are: ions in excess are not located at the droplet surface as expected in terms of continuum electrostatic theory, but are found in a thick shell below the surface leading to complete ion solvation. This peculiar ion distribution involves some ordering of solvent molecules at the droplet surface Shrinkage of nanodroplets, due to water evaporation, occurs through the formation of thorns originated by shape fluctuations …”

“…Together with experimental findings for droplets in the micrometer-size regime, [10] a great deal of detailed and sometimes unexpected information on such processes has been gained by molecular dynamics (MD) simulations especially for nano-sized droplets. [9,11,12] The most striking results of these studies for aqueous nanodroplets containing simple ionic species are:…”

“…This peculiar ion distribution involves some ordering of solvent molecules at the droplet surface. [11] (ii) Shrinkage of nanodroplets, due to water evaporation, occurs through the formation of thorns originated by shape fluctuations. [9,13] (iii) Ejection of each one of the solvated ion through a newly formed water thorn occurs when a critical value of the charge density is reached.…”