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Title: Extraordinary Slowing of Structural Dynamics in Thin Films of a Room Temperature Ionic Liquid

Abstract

The role that interfaces play in the dynamics of liquids is a fundamental scientific problem with vast importance in technological applications. From material science to biology, e.g., batteries to cell membranes, liquid properties at interfaces are frequently determinant in the nature of chemical processes. For most liquids, like water, the influence of an interface falls off on a ~1 nm distance scale. Room temperature ionic liquids (RTILs) are a vast class of unusual liquids composed of complex cations and anions that are liquid salts at room temperature. They are unusual liquids with properties that can be finely tuned by selecting the structure of the cation and anion. RTILs are being used or developed in applications such as batteries, CO 2 capture, and liquids for biological processes. Here, it is demonstrated quantitatively that the influence of an interface on RTIL properties is profoundly different from that observed in other classes of liquids. The dynamics of planar thin films of the room temperature ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BmimNTf 2), were investigated using two-dimensional infrared spectroscopy (2D IR) with the CN stretch of SeCN as the vibrational probe. The structural dynamics (spectral diffusion) of the thin films with controlled nanometer thicknesses weremore » measured and compared to the dynamics of the bulk liquid. The samples were prepared by spin coating the RTIL, together with the vibrational probe, onto a surface functionalized with an ionic monolayer that mimics the structure of the BmimNTf 2. Near-Brewster’s angle reflection pump–probe geometry 2D IR facilitated the detection of the exceedingly small signals from the films, some of which were only 14 nm thick. Even in quarter micron (250 nm) thick films, the observed dynamics were much slower than those of the bulk liquid. Using a new theoretical description, the correlation length (exponential falloff of the influence of the interfaces) was found to be 28 ± 5 nm. This very long correlation length, ~30 times greater than that of water, has major implications for the use of RTILs in devices and other applications.« less

Authors:
ORCiD logo [1];  [1]; ORCiD logo [1]; ORCiD logo [1]
  1. Department of Chemistry, Stanford University, Stanford, California 94305, United States
Publication Date:
Research Org.:
Stanford Univ., CA (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES); US Air Force Office of Scientific Research (AFOSR); National Science Foundation (NSF)
OSTI Identifier:
1462131
Alternate Identifier(s):
OSTI ID: 1498667
Grant/Contract Number:  
FG03-84ER13251; FA9550-16-1-0104; ECCS-1542152
Resource Type:
Journal Article: Published Article
Journal Name:
ACS Central Science
Additional Journal Information:
Journal Name: ACS Central Science Journal Volume: 4 Journal Issue: 8; Journal ID: ISSN 2374-7943
Publisher:
American Chemical Society (ACS)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; 36 MATERIALS SCIENCE

Citation Formats

Nishida, Jun, Breen, John P., Wu, Boning, and Fayer, Michael D. Extraordinary Slowing of Structural Dynamics in Thin Films of a Room Temperature Ionic Liquid. United States: N. p., 2018. Web. doi:10.1021/acscentsci.8b00353.
Nishida, Jun, Breen, John P., Wu, Boning, & Fayer, Michael D. Extraordinary Slowing of Structural Dynamics in Thin Films of a Room Temperature Ionic Liquid. United States. doi:10.1021/acscentsci.8b00353.
Nishida, Jun, Breen, John P., Wu, Boning, and Fayer, Michael D. Mon . "Extraordinary Slowing of Structural Dynamics in Thin Films of a Room Temperature Ionic Liquid". United States. doi:10.1021/acscentsci.8b00353.
@article{osti_1462131,
title = {Extraordinary Slowing of Structural Dynamics in Thin Films of a Room Temperature Ionic Liquid},
author = {Nishida, Jun and Breen, John P. and Wu, Boning and Fayer, Michael D.},
abstractNote = {The role that interfaces play in the dynamics of liquids is a fundamental scientific problem with vast importance in technological applications. From material science to biology, e.g., batteries to cell membranes, liquid properties at interfaces are frequently determinant in the nature of chemical processes. For most liquids, like water, the influence of an interface falls off on a ~1 nm distance scale. Room temperature ionic liquids (RTILs) are a vast class of unusual liquids composed of complex cations and anions that are liquid salts at room temperature. They are unusual liquids with properties that can be finely tuned by selecting the structure of the cation and anion. RTILs are being used or developed in applications such as batteries, CO2 capture, and liquids for biological processes. Here, it is demonstrated quantitatively that the influence of an interface on RTIL properties is profoundly different from that observed in other classes of liquids. The dynamics of planar thin films of the room temperature ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BmimNTf2), were investigated using two-dimensional infrared spectroscopy (2D IR) with the CN stretch of SeCN– as the vibrational probe. The structural dynamics (spectral diffusion) of the thin films with controlled nanometer thicknesses were measured and compared to the dynamics of the bulk liquid. The samples were prepared by spin coating the RTIL, together with the vibrational probe, onto a surface functionalized with an ionic monolayer that mimics the structure of the BmimNTf2. Near-Brewster’s angle reflection pump–probe geometry 2D IR facilitated the detection of the exceedingly small signals from the films, some of which were only 14 nm thick. Even in quarter micron (250 nm) thick films, the observed dynamics were much slower than those of the bulk liquid. Using a new theoretical description, the correlation length (exponential falloff of the influence of the interfaces) was found to be 28 ± 5 nm. This very long correlation length, ~30 times greater than that of water, has major implications for the use of RTILs in devices and other applications.},
doi = {10.1021/acscentsci.8b00353},
journal = {ACS Central Science},
issn = {2374-7943},
number = 8,
volume = 4,
place = {United States},
year = {2018},
month = {7}
}

Journal Article:
Free Publicly Available Full Text
Publisher's Version of Record at 10.1021/acscentsci.8b00353

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Cited by: 2 works
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Figures / Tables:

Figure 1 Figure 1: (A) Preparation procedure for BmimNTf2 RTIL thin films. An SiO2 surface is first functionalized with an ionic monolayer that mimics the structure of BmimNTf2 RTIL. BmimNTf2 dissolved in methanol is then spin coated onto the functionalized surface. For 2D IR measurements, BmimSeCN was mixed with BmimNTf2 with 1:10more » ratio as a vibrational probe. (B) Optical microscope image of ∼301 nm thick BmimNTf2 film. (C) Micro-Raman spectrum acquired at the center of the image with 100× objective. The features above >700 cm−1 arise from the BmimNTf2 RTIL. The particularly strong band at ∼740 cm−1 is assigned to the NS stretch mode of the NTf2 anion. (D) Variation of the NS stretch Raman band intensity across the green horizontal line in part B, indicating that the RTIL film thickness is reasonably constant across the 200 μm range.« less

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Figures/Tables have been extracted from DOE-funded journal article accepted manuscripts.