Surface Phases and Surface Freezing in an Ionic Liquid

Diego Pontoni; Julia Haddad; Bridget M. Murphy; Sven Festersen; Oleg Konovalov; Benjamin M. Ocko; Moshe Deutsch

doi:10.1021/acs.jpcc.9b00286

Surface Phases and Surface Freezing in an Ionic Liquid

Diego Pontoni, Julia Haddad, Bridget M. Murphy, Sven Festersen, Oleg Konovalov, Benjamin M. Ocko, Moshe Deutsch

Department of Physics

Research output: Contribution to journal › Article › peer-review

15 Scopus citations

Abstract

Room temperature ionic liquids (RTILs), a novel class of liquid salts, are intensively studied for their basic science and numerous emerging applications. When undercooled, RTILs comprising long alkyl chains often exhibit liquid crystal (LC) bulk phases. However, only one molecular-resolution experimental structure study was published for their LC surface phases. We measured the temperature evolution of another LC surface phase, using surface specific Å-resolution X-ray methods. This phase's existence range, 90 °C, much exceeds the corresponding bulk phase's 3 °C. Its thickness, L, confirms the theory-predicted logarithmic temperature dependence, with an amplitude equaling the bulk correlation length. Surprisingly, at L's divergence temperature, a ?20 Å thick, hexagonally packed, crystalline monolayer forms at, and fully covers, the sample's surface. It is identified as a surface-frozen Langmuir-Gibbs film and fundamentally differs from the only reported RTIL surface crystal, a Coulomb-dominated, four-layer, island phase, covering only 5%-15% of the surface.

Original language	English
Pages (from-to)	3058-3066
Number of pages	9
Journal	Journal of Physical Chemistry C
Volume	123
Issue number	5
DOIs	https://doi.org/10.1021/acs.jpcc.9b00286
State	Published - 7 Feb 2019

Bibliographical note

Publisher Copyright:
Copyright © 2019 American Chemical Society.

Funding

We thank ESRF for beamtime at ID15 and ID10, ESRF’s Partnership for Soft Condensed Matter (PSCM) for support, Drs. V. Honkimakï and T. Buslaps (ESRF) for assistance and advice, H. Gottlieb and M. Afri (Bar-Ilan) for NMR consultation, and G. Shmul (Weizmann Institute) for HR-ESI-MS measurements and insightful discussions. We also thank PETRA III at DESY, a member of Helmohltz Association (HGF), for beamtime at P08, using the LISA instrument, funded by BMBF grants 05K13FK2 and 05K16FK1.

Funders	Funder number
Helmohltz Association
Hugh Green Foundation
Bundesministerium für Bildung und Forschung	05K16FK1, 05K13FK2

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title = "Surface Phases and Surface Freezing in an Ionic Liquid",

abstract = "Room temperature ionic liquids (RTILs), a novel class of liquid salts, are intensively studied for their basic science and numerous emerging applications. When undercooled, RTILs comprising long alkyl chains often exhibit liquid crystal (LC) bulk phases. However, only one molecular-resolution experimental structure study was published for their LC surface phases. We measured the temperature evolution of another LC surface phase, using surface specific {\AA}-resolution X-ray methods. This phase's existence range, 90 °C, much exceeds the corresponding bulk phase's 3 °C. Its thickness, L, confirms the theory-predicted logarithmic temperature dependence, with an amplitude equaling the bulk correlation length. Surprisingly, at L's divergence temperature, a ?20 {\AA} thick, hexagonally packed, crystalline monolayer forms at, and fully covers, the sample's surface. It is identified as a surface-frozen Langmuir-Gibbs film and fundamentally differs from the only reported RTIL surface crystal, a Coulomb-dominated, four-layer, island phase, covering only 5%-15% of the surface.",

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AU - Murphy, Bridget M.

AU - Festersen, Sven

AU - Konovalov, Oleg

AU - Ocko, Benjamin M.

AU - Deutsch, Moshe

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AB - Room temperature ionic liquids (RTILs), a novel class of liquid salts, are intensively studied for their basic science and numerous emerging applications. When undercooled, RTILs comprising long alkyl chains often exhibit liquid crystal (LC) bulk phases. However, only one molecular-resolution experimental structure study was published for their LC surface phases. We measured the temperature evolution of another LC surface phase, using surface specific Å-resolution X-ray methods. This phase's existence range, 90 °C, much exceeds the corresponding bulk phase's 3 °C. Its thickness, L, confirms the theory-predicted logarithmic temperature dependence, with an amplitude equaling the bulk correlation length. Surprisingly, at L's divergence temperature, a ?20 Å thick, hexagonally packed, crystalline monolayer forms at, and fully covers, the sample's surface. It is identified as a surface-frozen Langmuir-Gibbs film and fundamentally differs from the only reported RTIL surface crystal, a Coulomb-dominated, four-layer, island phase, covering only 5%-15% of the surface.

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Surface Phases and Surface Freezing in an Ionic Liquid

Abstract

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