Journal
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
Volume 115, Issue 6, Pages E1100-E1107Publisher
NATL ACAD SCIENCES
DOI: 10.1073/pnas.1716418115
Keywords
ionic liquids; X-ray reflectivity; surface layering; liquid-like; interdigitated chains
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Funding
- US Department of Energy, Office of Science, Office of Basic Energy Sciences [DE-SC0012704]
- Bundesministerium fur Bildung und Forschung (BMBF) [05k13fk2, 05k16fk1]
- US-Israel Binational Science Foundation, Jerusalem
- ESRF's Partnership for Soft Condensed Matter (PSCM)
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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 rho(e)(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 self-aggregation and bulk-surface structure relations.
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