Publication: Molecular Dynamics Simulations of Electrosprayed Water Nanodroplets: Internal Potential Gradients, Location of Excess Charge Centers, and 'Hopping' Protons
All || By Area || By Year| Title | Molecular Dynamics Simulations of Electrosprayed Water Nanodroplets: Internal Potential Gradients, Location of Excess Charge Centers, and 'Hopping' Protons | Authors/Editors* | E. Ahadi and L. Konermann |
|---|---|
| Where published* | J. Phys. Chem. B |
| How published* | Journal |
| Year* | 2009 |
| Volume | 113 |
| Number | |
| Pages | 7071-7080 |
| Publisher | |
| Keywords | MD Electrospray protons |
| Link | http://pubs.acs.org/doi/abs/10.1021/jp810599f |
| 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 protonâwater interactions. Simulations on nanodroplets consisting of 1248 water molecules and 10 protons (radius, ca. 21 Ã ) 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 Ã , 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. |
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