title: Effect of the environmental humidity on the bulk, interfacial and nanoconfined properties of an ionic liquid.
abstract: With reference to our previous surface-force study on 1-hexyl-3-methylimidazolium ethylsulfate ([HMIM] EtSO4) using an extended surface forces apparatus, which showed an ordered structure within the nanoconfined dry ionic liquid (IL) between mica surfaces that extended up to ∼60 nm from the surface, this work focuses on the influence of the environmental humidity on the bulk, interfacial and nanoconfined structure of [HMIM] EtSO4. Infrared spectroscopy and rheometry reflect the changes in chemical and physical properties of the bulk IL due to the uptake of water when exposed to ambient humidity, while wide-angle X-ray scattering shows a mild swelling of the bulk nanostructure, and the AFM sharp tip reveals an additional surface layer at the mica-IL interface. When the water-containing [HMIM] EtSO4 is nanoconfined between two mica surfaces, no long-range order is detected, in contrast to the results obtained for the dry IL, which demonstrates that the presence of water can prevent the liquid-to-solid transformation of this IL. A combination of techniques and the calculated Bjerrum length indicate that water molecules weaken interionic electrostatic and hydrogen-bonding interactions, which lessens ion-ion correlations. Our work shows that the solid-like behavior of the nanoconfined IL strongly depends on the presence of absorbed water and hence, it has implications with regard to the correct interpretation of laboratory studies and their extension to real applications in lubrication.
authors.affiliation: Dept. of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign, IL-61801 Urbana, USA. rosae@illinois.edu.
authors.forename: L Andres
authors.initials: LA
authors.lastname: Jurado
authors.affiliation: Dept. of Materials Science and Engineering, University of Illinois at Urbana-Champaign, IL-61801 Urbana, USA.
authors.forename: Hojun
authors.initials: H
authors.lastname: Kim
authors.affiliation: Lab. for Surface Science and Technology, Dept. of Materials, ETH Zurich, CH-8093 Zurich, Switzerland and Dipartimento di Scienze Chimiche e Geologiche, Università di Cagliari, 09042 Cagliari, Italy.
authors.forename: Antonella
authors.initials: A
authors.lastname: Rossi
authors.affiliation: Lab. for Surface Science and Technology, Dept. of Materials, ETH Zurich, CH-8093 Zurich, Switzerland.
authors.forename: Andrea
authors.initials: A
authors.lastname: Arcifa
authors.affiliation: Dept. of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, IL-61801 Urbana, USA.
authors.forename: Jonathon K
authors.initials: JK
authors.lastname: Schuh
authors.affiliation: Lab. for Surface Science and Technology, Dept. of Materials, ETH Zurich, CH-8093 Zurich, Switzerland.
authors.forename: Nicholas D
authors.initials: ND
authors.lastname: Spencer
authors.affiliation: Dept. of Materials Science and Engineering, University of Illinois at Urbana-Champaign, IL-61801 Urbana, USA.
authors.forename: Cecilia
authors.initials: C
authors.lastname: Leal
authors.affiliation: Dept. of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, IL-61801 Urbana, USA.
authors.forename: Randy H
authors.initials: RH
authors.lastname: Ewoldt
authors.affiliation: Dept. of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign, IL-61801 Urbana, USA. rosae@illinois.edu.
authors.forename: Rosa M
authors.initials: RM
authors.lastname: Espinosa-Marzal