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Issue 7, 2017
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Structural and electrostatic effects at the surfaces of size- and charge-selected aqueous nanodrops

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Abstract

The effects of ion charge, polarity and size on the surface morphology of size-selected aqueous nanodrops containing a single ion and up to 550 water molecules are investigated with infrared photodissociation (IRPD) spectroscopy and theory. IRPD spectra of M(H2O)n where M = La3+, Ca2+, Na+, Li+, I, SO42− and supporting molecular dynamics simulations indicate that strong interactions between multiply charged ions and water molecules can disrupt optimal hydrogen bonding (H-bonding) at the nanodrop surface. The IRPD spectra also reveal that “free” OH stretching frequencies of surface-bound water molecules are highly sensitive to the ion's identity and the OH bond's local H-bond environment. The measured frequency shifts are qualitatively reproduced by a computationally inexpensive point-charge model that shows the frequency shifts are consistent with a Stark shift from the ion's electric field. For multiply charged cations, pronounced Stark shifting is observed for clusters containing ∼100 or fewer water molecules. This is attributed to ion-induced solvent patterning that extends to the nanodrop surface, and serves as a spectroscopic signature for a cation's ability to influence the H-bond network of water located remotely from the ion. The Stark shifts measured for the larger nanodrops are extrapolated to infinite dilution to obtain the free OH stretching frequency of a surface-bound water molecule at the bulk air–water interface (3696.5–3701.0 cm−1), well within the relatively wide range of values obtained from SFG measurements. These cluster measurements also indicate that surface curvature effects can influence the free OH stretching frequency, and that even nanodrops without an ion have a surface potential that depends on cluster size.

Graphical abstract: Structural and electrostatic effects at the surfaces of size- and charge-selected aqueous nanodrops

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Publication details

The article was received on 01 Feb 2017, accepted on 17 May 2017 and first published on 19 May 2017


Article type: Edge Article
DOI: 10.1039/C7SC00481H
Citation: Chem. Sci., 2017,8, 5201-5213
  • Open access: Creative Commons BY license
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    Structural and electrostatic effects at the surfaces of size- and charge-selected aqueous nanodrops

    R. J. Cooper, J. T. O'Brien, T. M. Chang and E. R. Williams, Chem. Sci., 2017, 8, 5201
    DOI: 10.1039/C7SC00481H

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