Determining the composition of the vacuum–liquid interface in ionic-liquid mixtures. (25th September 2017)

• Record Type: Journal Article
• 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.
• Is Part Of: Faraday discussions. Volume 206(2018)
• Page Start: 497
• Page End: 522
• Publication Date: 2017-09-25
• Subjects: Chemistry -- Periodicals; Metallurgy -- Periodicals; Electrochemistry -- Periodicals; 540
• DOI: 10.1039/c7fd00175d
• Languages: English
• ISSNs: 1359-6640

Abstract

Abstract : The vacuum–liquid interfaces of a number of ionic-liquid mixtures have been investigated using a combination of RAS-LIF, selected surface tension measurements, and molecular dynamics simulations. Abstract : 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 n 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 [C2 mim] + and [C12 mim] + 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.