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 the 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 th...

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Bibliographic Details
Published in:Faraday discussions 2018-01, Vol.206, p.497-522
Main Authors: Smoll, E J, Tesa-Serrate, M A, Purcell, S M, D'Andrea, L, Bruce, D W, Slattery, J M, Costen, M L, Minton, T K, McKendrick, K G
Format: Article
Language:English
Subjects:
Anions
Cations
Chains
Computer simulation
Ionic liquids
Laser induced fluorescence
Legal issues
Molecular dynamics
Order parameters
Surface tension
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Summary:The vacuum-liquid interfaces of a number of ionic-liquid mixtures have been investigated using the 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 ([C mim] ) 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 [C mim] and [C mim] with a common bis(trifluoromethylsulfonyl)imide ([NTf ] ) 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.
ISSN:1359-6640
1364-5498
DOI:10.1039/c7fd00175d