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FDIONIC18 Determining the Composition of the Vacuum-Liquid Interface in Ionic-Liquid Mixtures


The vacuum-liquid interfaces of a number of ionic-liquid mixtures have been investigated using a combination of reactive-atom scattering with laser-induced fluorescence detection (RAS-LIF), selected surface tension measurements, and molecular dynamics (MD) simulations. The mixtures are based on the widespread 1-alkyl-3-methylimidazolium ([Cnmim]+) cation, including mixed cations which differ in chain length or chemical functionality with a common anion; and different anions for a common cation. RAS-LIF results imply that the surface compositions exhibit a general form of non-stoichiometric behaviour that mimics the well-known Henry’s and Raoult’s laws at low and high mole fraction, respectively. The Extended Langmuir model provides a moderately good single-parameter fit, but higher-order terms are required for an accurate description. The quantitative relationship between RAS-LIF and surface tension, which probes the surface composition only indirectly, is explored for mixtures of [C2mim]+ and [C12mim]+ with a common bis(trifluoromethylsulfonyl)imide ([NTf2]-) anion. Extended Langmuir model fits to surface tension data are broadly consistent with those to RAS-LIF; however, several other common approaches to extracting surface compositions from measured surface tensions result in much larger discrepancies. MD simulations suggest that RAS-LIF faithfully reports on the alkyl-chain exposure at the surface, which is only subtly modified by composition-dependent structural reorganisation.

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Publication details

The article was received on 22 May 2017, accepted on 09 Jun 2017 and first published on 14 Jun 2017

Article type: Paper
DOI: 10.1039/C7FD00175D
Citation: Faraday Discuss., 2017, Accepted Manuscript
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    FDIONIC18 Determining the Composition of the Vacuum-Liquid Interface in Ionic-Liquid Mixtures

    E. Smoll, M. T. Serrate, S. Purcell, L. D'Andrea, D. W. Bruce, J. M. Slattery, M. Costen, T. Minton and K. G. McKendrick, Faraday Discuss., 2017, Accepted Manuscript , DOI: 10.1039/C7FD00175D

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