Surface structure evolution in a homologous series of ionic liquids

Julia Haddad, Diego Pontoni, Bridget M. Murphy, Sven Festersen, Benjamin Runge, Olaf M. Magnussen, Hans Georg Steinrück, Harald Reichert, Benjamin M. Ocko, Moshe Deutsch

Research output: Contribution to journalArticlepeer-review

40 Scopus citations

Abstract

Interfaces of room temperature ionic liquids (RTILs) are important for both applications and basic science and are therefore intensely studied. However, the evolution of their interface structure with the cation's alkyl chain length n from Coulomb to van der Waals interaction domination has not yet been studied for even a single broad homologous RTIL series. We present here such a study of the liquid-air interface for n = 2 to 22, using angstrom-resolution X-ray methods. For n<6, a typical "simple liquid" monotonic surface-normal electron density profile ρe(z) is obtained, like those of water and organic solvents. For n>6, increasingly more pronounced nanoscale self-segregation of the molecules' charged moieties and apolar chains yields surface layering with alternating regions of headgroups and chains. The layering decays into the bulk over a few, to a few tens, of nanometers. The layering periods and decay lengths, their linear n dependence, and slopes are discussed within two models, one with partial-chain interdigitation and the other with liquid-like chains. No surface-parallel long-range order is found within the surface layer. For n = 22, a different surface phase is observed above melting. Our results also impact general liquid-phase issues like supramolecular selfaggregation and bulk-surface structure relations.

Original languageEnglish
Pages (from-to)E1100-E1107
JournalProceedings of the National Academy of Sciences of the United States of America
Volume115
Issue number6
DOIs
StatePublished - 6 Feb 2018

Bibliographical note

Funding Information:
We thank A. Martinelli and A. Triolo for sharing numerical data; V. Honkimaki, T. Buslaps, M. Di Michiel, A. Checco, andO. Konovalov for discussions and assistance; ESRF, APS, and PETRA III for beamtime and ESRF's Partnership for Soft Condensed Matter (PSCM); US Department of Energy, Office of Science, Office of Basic Energy Sciences Contract DE-SC0012704 (to B.M.O.); Bundesministerium für Bildung und Forschung (BMBF) Grants 05k13fk2 and 05k16fk1 (to O.M.M, B.M.M, S.F, and B.R.); and the US-Israel Binational Science Foundation, Jerusalem (M.D.) for support.

Funding Information:
DE-SC0012704 (to B.M.O.); Bundesministerium für Bildung und Forschung (BMBF) Grants 05k13fk2 and 05k16fk1 (to O.M.M, B.M.M, S.F, and B.R.); and the US–Israel Binational Science Foundation, Jerusalem (M.D.) for support.

Keywords

  • Interdigitated chains
  • Ionic liquids
  • Liquid-like
  • Surface layering
  • X-ray reflectivity

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