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Title: Surface structure evolution in a homologous series of ionic liquids

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 in this paper 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 ρ 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 surfacemore » layer. For n = 22, a different surface phase is observed above melting. Finally, our results also impact general liquid-phase issues like supramolecular self-aggregation and bulk–surface structure relations.« less
Authors:
 [1] ; ORCiD logo [2] ;  [3] ;  [4] ;  [4] ;  [3] ;  [5] ;  [6] ;  [7] ; ORCiD logo [1]
  1. Bar-Ilan Univ., Ramat Gan (Israel). Physics Dept. Inst. of Nanotechnology and Advanced Materials
  2. European Synchrotron Radiation Facility (ESRF), Grenoble (France). The European Synchrotron and Partnership for Soft Condensed Matter (PSCM)
  3. Kiel Univ. (Germany). Inst. for Experimental and Applied Physics. Ruprecht Haensel Lab.
  4. Kiel Univ. (Germany). Inst. for Experimental and Applied Physics
  5. SLAC National Accelerator Lab., Menlo Park, CA (United States). Stanford Synchrotron Radiation Lab. (SSRL). Materials Science Division
  6. European Synchrotron Radiation Facility (ESRF), Grenoble (France)
  7. Brookhaven National Lab. (BNL), Upton, NY (United States). National Synchrotron Light Source II (NSLS-II)
Publication Date:
Report Number(s):
BNL-203505-2018-JAAM
Journal ID: ISSN 0027-8424
Grant/Contract Number:
SC0012704; 05k13fk2; 05k16fk1
Type:
Accepted Manuscript
Journal Name:
Proceedings of the National Academy of Sciences of the United States of America
Additional Journal Information:
Journal Volume: 115; Journal Issue: 6; Journal ID: ISSN 0027-8424
Publisher:
National Academy of Sciences, Washington, DC (United States)
Research Org:
Brookhaven National Lab. (BNL), Upton, NY (United States); Kiel Univ. (Germany); Bar-Ilan Univ., Ramat Gan (Israel)
Sponsoring Org:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22); German Federal Ministry of Education and Research (BMBF); US-Israel Binational Science Foundation (BSF)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; ionic liquids; X-ray reflectivity; surface layering; liquid-like; interdigitated chains
OSTI Identifier:
1433970

Haddad, Julia, Pontoni, Diego, Murphy, Bridget M., Festersen, Sven, Runge, Benjamin, Magnussen, Olaf M., Steinruck, Hans-Georg, Reichert, Harald, Ocko, Benjamin M., and Deutsch, Moshe. Surface structure evolution in a homologous series of ionic liquids. United States: N. p., Web. doi:10.1073/pnas.1716418115.
Haddad, Julia, Pontoni, Diego, Murphy, Bridget M., Festersen, Sven, Runge, Benjamin, Magnussen, Olaf M., Steinruck, Hans-Georg, Reichert, Harald, Ocko, Benjamin M., & Deutsch, Moshe. Surface structure evolution in a homologous series of ionic liquids. United States. doi:10.1073/pnas.1716418115.
Haddad, Julia, Pontoni, Diego, Murphy, Bridget M., Festersen, Sven, Runge, Benjamin, Magnussen, Olaf M., Steinruck, Hans-Georg, Reichert, Harald, Ocko, Benjamin M., and Deutsch, Moshe. 2018. "Surface structure evolution in a homologous series of ionic liquids". United States. doi:10.1073/pnas.1716418115.
@article{osti_1433970,
title = {Surface structure evolution in a homologous series of ionic liquids},
author = {Haddad, Julia and Pontoni, Diego and Murphy, Bridget M. and Festersen, Sven and Runge, Benjamin and Magnussen, Olaf M. and Steinruck, Hans-Georg and Reichert, Harald and Ocko, Benjamin M. and Deutsch, Moshe},
abstractNote = {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 in this paper 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. Finally, our results also impact general liquid-phase issues like supramolecular self-aggregation and bulk–surface structure relations.},
doi = {10.1073/pnas.1716418115},
journal = {Proceedings of the National Academy of Sciences of the United States of America},
number = 6,
volume = 115,
place = {United States},
year = {2018},
month = {1}
}