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Direct Correlation of Surface Tension and Surface Composition of Ionic Liquid Mixtures-A Combined Vacuum Pendant Drop and Angle-Resolved X-ray Photoelectron Spectroscopy Study. MOLECULES (BASEL, SWITZERLAND) 2022; 27:molecules27238561. [PMID: 36500653 PMCID: PMC9736233 DOI: 10.3390/molecules27238561] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 11/09/2022] [Revised: 11/29/2022] [Accepted: 12/01/2022] [Indexed: 12/12/2022]
Abstract
We investigated the surface tension and surface composition of various mixtures of the two ionic liquids (ILs) 1-methyl-3-octyl-imidazolium hexafluorophosphate [C8C1Im][PF6] and 1,3-bis(polyethylene glycol)imidazolium iodide [(mPEG2)2Im]I in the temperature range from 230 to 370 K under ultraclean vacuum conditions. The surface tension was measured using a newly developed apparatus, and the surface composition was determined by angle-resolved X-ray photoelectron spectroscopy (ARXPS). In the pure ILs, the alkyl chains of [C8C1Im][PF6] and the PEG chains of [(mPEG2)2Im]I are enriched at the IL/vacuum interface. In the mixtures, a strong selective surface enrichment of the alkyl chains occurs, which is most pronounced at low [C8C1Im][PF6] contents. For the surface tension, strong deviations from an ideal mixing behaviour take place. By applying a simple approach based on the surface composition of the mixtures as deduced from ARXPS, we are able to predict and reproduce the experimentally measured temperature-dependent surface tension values with astonishingly high accuracy.
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Purcell SM, Lane PD, D'Andrea L, Elstone NS, Bruce DW, Slattery JM, Smoll EJ, Greaves SJ, Costen ML, Minton TK, McKendrick KG. Surface Structure of Alkyl/Fluoroalkylimidazolium Ionic-Liquid Mixtures. J Phys Chem B 2022; 126:1962-1979. [PMID: 35225614 PMCID: PMC9007465 DOI: 10.1021/acs.jpcb.1c10460] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/19/2023]
Abstract
The gas-liquid interface of ionic liquids (ILs) is critically important in many applications, for example, in supported IL phase (SILP) catalysis. Methods to investigate the interfacial structure in these systems will allow their performance to be improved in a rational way. In this study, reactive-atom scattering (RAS), surface tension measurements, and molecular dynamics (MD) simulations were used to study the vacuum interface of mixtures of partially fluorinated and normal alkyl ILs. The underlying aim was to understand whether fluorinated IL ions could be used as additives to modify the surface structure of one of the most widely used families of alkyl ILs. The series of ILs 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Cnmim][Tf2N]) with n = 4-12 were mixed with a fixed-length, semiperfluorinated analogue (1H,1H,2H,2H-perfluorooctyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C8mimF13][Tf2N]), forming [Cnmim](1-x)[C8mimF13]x[Tf2N] mixtures, where x is the bulk mole fraction of the fluorinated component. The RAS-LIF method combined O-atom projectiles with laser-induced fluorescence (LIF) detection of the product OH as a measure of surface exposure of the alkyl chains. For [C8mim](1-x)[C8mimF13]x[Tf2N] mixtures, RAS-LIF OH yields are below those expected from stoichiometry. There are quantitatively consistent negative deviations from linearity of the surface tension. Both results imply that the lower-surface-tension fluoroalkyl material dominates the surface. A similar deficit is found for alkyl chain lengths n = 4, 6, 8, and 12 and for all (nonzero) x investigated by RAS-LIF. Accessible-surface-area (ASA) analyses of the MD simulations for [Cnmim](1-x)[C8mimF13]x[Tf2N] mixtures qualitatively reproduce the same primary effect of fluoro-chain predominance of the surface over most of the range of n. However, there are significant quantitative discrepancies between MD ASA predictions and experiment relating to the strength of any n-dependence of the relative alkyl coverage at fixed x, and on the x-dependence at fixed n. These discrepancies are discussed in the context of detailed examinations of the surface structures predicted in the MD simulations. Potential explanations, beyond experimental artifacts, include inadequacies in the classical force fields used in the MD simulations or the inability of simple ASA algorithms to capture dynamical factors that influence RAS-LIF yields.
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Affiliation(s)
- Simon M Purcell
- Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, U.K
| | - Paul D Lane
- Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, U.K
| | - Lucía D'Andrea
- Department of Chemistry, University of York, Heslington, York YO10 5DD, U.K
| | - Naomi S Elstone
- Department of Chemistry, University of York, Heslington, York YO10 5DD, U.K
| | - Duncan W Bruce
- Department of Chemistry, University of York, Heslington, York YO10 5DD, U.K
| | - John M Slattery
- Department of Chemistry, University of York, Heslington, York YO10 5DD, U.K
| | - Eric J Smoll
- Department of Chemistry and Biochemistry, Montana State University, Bozeman, Montana 59717, United States
| | - Stuart J Greaves
- Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, U.K
| | - Matthew L Costen
- Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, U.K
| | - Timothy K Minton
- Ann and H.J. Smead Department of Aerospace Engineering Sciences, University of Colorado Boulder, Boulder, Colorado 80303, United States
| | - Kenneth G McKendrick
- Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, U.K
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Fu X, Wang S, Huang Y, Yang X, Liu Q, Zheng Q. Densities and apparent molar volumes of diluent solutions of [EmimNTf2], [BmimNTf2], and [BmmimNTf2] in DEGDME and TEGDME. J Mol Liq 2021. [DOI: 10.1016/j.molliq.2021.117328] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/15/2022]
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Oz E, Sahin O, Okur HI, Suzer S. Surface Propensity of Anions in a Binary Ionic-Liquid Mixture Assessed by Full-Range Angle-Resolved X-ray Photoelectron Spectroscopy and Surface-Tension Measurements. Chemphyschem 2020; 21:2397-2401. [PMID: 32955763 DOI: 10.1002/cphc.202000750] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/01/2020] [Revised: 09/18/2020] [Indexed: 11/09/2022]
Abstract
Angle-resolved X-ray photoelectron spectroscopy and contact-angle measurements guided by a signal attenuation model are utilized to extract molar composition and anion enrichment in the vacuum interface of a binary ionic liquid mixture, having a common quaternary ammonium cation and two different anions. By using the intensity ratio of the F1s peaks belonging to the two different anions recorded at the full electron take-off angle range, from 0° to 80°, we have determined that only a fractionally covered and anion enriched surface layer can predict the AR-XPS data, which is also consistent with surface tension measurements. Moreover, the more bulky and non-spherical anion enrichment is evident even at the conventional and the so assumed bulk sensitive take-off angle of 0°. This methodology provides a surface enrichment factor of the molecular ions and clearly serves as an experimental evidence for recently debated surface layering and/or island structure in ionic liquid systems.
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Affiliation(s)
- Erdinc Oz
- Department of Chemistry, Bilkent University, 06800, Ankara, Turkey
| | - Ozgur Sahin
- Department of Chemistry, Bilkent University, 06800, Ankara, Turkey
| | - Halil I Okur
- Department of Chemistry, Bilkent University, 06800, Ankara, Turkey
| | - Sefik Suzer
- Department of Chemistry, Bilkent University, 06800, Ankara, Turkey
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Yang D, Huang Y, Wang X, He R, Zhou G, Chen X, Yang Z. Different Hydrogen Bond Changes Driven by Surface Segregation Behavior of Imidazolium-Based Ionic Liquid Mixture at the Liquid-Vacuum Interface. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2020; 36:11798-11808. [PMID: 32962350 DOI: 10.1021/acs.langmuir.0c01501] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/11/2023]
Abstract
In this work, molecular dynamics (MD) simulations were carried out to study the behaviors of a binary ionic liquid (IL) mixture consisting of equimolar [C2C1Im][BF4] and [C4C1Im][BF4], as well as two corresponding pure ILs, at the liquid-vacuum interface. Our simulation results show that the competition of nonpolar interactions between different alkyl chains of two cations results in an obvious surface segregation behavior of the IL mixture at the interface, indicating an enhanced aggregation of the [C4C1Im]+ cations but a weakened aggregation of the [C2C1Im]+ cations at the outermost surface. More interestingly, different hydrogen bond (HB) changes between two imidazolium cations at the interface can be driven by such surface segregation behavior, where the [C2C1Im]+ cations rather than the [C4C1Im]+ ones have more and stronger HBs with the [BF4]- anions by comparison with the corresponding pure ILs at the interface. Meanwhile, it is interesting to find that such a stronger HB would lower the rotations of the imidazolium rings of interfacial [C2C1Im]+ cations. By contrast, the [C4C1Im]+ cations at the outermost surface rotate faster owing to their weaker HB. In addition, the orientation analysis uncovers that there is a major decrease for the orderliness of interfacial [C2C1Im]+ cations, but a minor decrease for that of interfacial [C4C1Im]+ cations, from the pure IL to the IL mixture. Such distinct results are closely related to the surface segregation between the [C2C1Im]+ and [C4C1Im]+ cations in the IL mixture and their interfacial HB properties. Thus, our simulation results afford a deep insight into the surface segregation effect on the HB behavior of the imidazolium-based IL mixture at liquid-vacuum interface.
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Affiliation(s)
- Deshuai Yang
- Institute of Advanced Materials (IAM), State-Province Joint Engineering Laboratory of Zeolite Membrane Materials, College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang 330022, People's Republic of China
| | - Yiping Huang
- Institute of Advanced Materials (IAM), State-Province Joint Engineering Laboratory of Zeolite Membrane Materials, College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang 330022, People's Republic of China
- School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, People's Republic of China
| | - Xueping Wang
- Institute of Advanced Materials (IAM), State-Province Joint Engineering Laboratory of Zeolite Membrane Materials, College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang 330022, People's Republic of China
| | - Ruiyao He
- Institute of Advanced Materials (IAM), State-Province Joint Engineering Laboratory of Zeolite Membrane Materials, College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang 330022, People's Republic of China
| | - Guobing Zhou
- Institute of Advanced Materials (IAM), State-Province Joint Engineering Laboratory of Zeolite Membrane Materials, College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang 330022, People's Republic of China
| | - Xiangshu Chen
- Institute of Advanced Materials (IAM), State-Province Joint Engineering Laboratory of Zeolite Membrane Materials, College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang 330022, People's Republic of China
| | - Zhen Yang
- Institute of Advanced Materials (IAM), State-Province Joint Engineering Laboratory of Zeolite Membrane Materials, College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang 330022, People's Republic of China
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Costa R, Pereira CM, Silva AF, Brevet PF, Benichou E. Ordering and Nonideality of Air-Ionic Liquid Interfaces in Surface Second Harmonic Generation. J Phys Chem B 2020; 124:3954-3961. [PMID: 32324418 DOI: 10.1021/acs.jpcb.9b12025] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
The air-ionic liquid interface for a series of ionic liquids involving imidazolium cations [Cnmim] with different alkyl chain lengths (n = 2, 4, 6, 8, 10, and 12) and the same [NTf2] imide anion has been studied by polarization-resolved second harmonic generation (SHG). An increase as a function of the alkyl chain length of the orientational parameter reveals the increasing ordering of the air-pure ionic liquid interfaces although it is not possible to disentangle the change in mean tilt angle from a change in the tilt angle probability distribution width. Besides, the study of the air-mixed ([C12mim])x([C2mim])1-x[NTf2] ionic liquid interface clearly demonstrates the interfacial nonideality of the mixed ionic liquids. The long alkyl chain cation perturbs the interface as seen from the orientational parameter and displaces the short alkyl chain one for bulk mixture contents as low as 10%. At higher long alkyl chain cation bulk mixture contents, the interface behaves close to a pure long alkyl chain ionic liquid.
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Affiliation(s)
- Renata Costa
- Departamento de Quı̀mica e Bioquı̀mica, CIQUP-Physical Analytical Chemistry and Electrochemistry Group, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
| | - Carlos M Pereira
- Departamento de Quı̀mica e Bioquı̀mica, CIQUP-Physical Analytical Chemistry and Electrochemistry Group, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
| | - A Fernando Silva
- Departamento de Quı̀mica e Bioquı̀mica, CIQUP-Physical Analytical Chemistry and Electrochemistry Group, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
| | - Pierre-François Brevet
- Université de Lyon, Université Claude Bernard Lyon, CNRS, Institut Lumière Matière, F-69622 Villeurbanne, France
| | - Emmanuel Benichou
- Université de Lyon, Université Claude Bernard Lyon, CNRS, Institut Lumière Matière, F-69622 Villeurbanne, France
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Heller BS, Paap U, Maier F, Steinrück HP. Pronounced surface enrichment of fluorinated ionic liquids in binary mixtures with methoxy-functionalized ionic liquids. J Mol Liq 2020. [DOI: 10.1016/j.molliq.2020.112783] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/01/2022]
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Heller BSJ, Lexow M, Greco F, Shin S, Partl G, Maier F, Steinrück HP. Temperature-Dependent Surface Enrichment Effects in Binary Mixtures of Fluorinated and Non-Fluorinated Ionic Liquids. Chemistry 2020; 26:1117-1126. [PMID: 31633251 PMCID: PMC7027538 DOI: 10.1002/chem.201904438] [Citation(s) in RCA: 9] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/26/2019] [Indexed: 11/05/2022]
Abstract
Using angle-resolved X-ray photoelectron spectroscopy (ARXPS), we investigate the topmost nanometers of various binary ionic liquid (IL) mixtures at different temperatures in the liquid state. The mixtures consist of ILs with the same [PF6 ]- anion but two different cations, namely 3-methyl-1-(3,3,4,4,4-pentafluorobutyl)imidazolium hexafluorophosphate, [PFBMIm][PF6 ], and 1-butyl-3-methylimidazolium hexafluorophosphate, [C4 C1 Im][PF6 ], with 10, 25, 50 and 75 mol % content of [PFBMIm][PF6 ]. We observe a preferential enrichment of the fluorinated chain in the topmost layer, relative to the bulk composition, which is most pronounced for the lowest content of [PFBMIm][PF6 ]. Upon cooling the mixtures stepwise from 95 °C until surface charging effects in XPS indicate solidification, we observe a pronounced increase in surface enrichment of the fluorinated chain with decreasing temperature in the liquid state. In contrast to the mixtures with lower [PFBMIm][PF6 ] contents, cooling the 75 mol % mixture additionally shows an abrupt decrease of the fluorinated chain signal before complete solidification occurs, which is assigned to partial precipitation effects.
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Affiliation(s)
- Bettina S J Heller
- Lehrstuhl für Physikalische Chemie II, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstraße 3, 91058, Erlangen, Germany
| | - Matthias Lexow
- Lehrstuhl für Physikalische Chemie II, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstraße 3, 91058, Erlangen, Germany
| | - Francesco Greco
- Lehrstuhl für Physikalische Chemie II, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstraße 3, 91058, Erlangen, Germany
| | - Sunghwan Shin
- Lehrstuhl für Physikalische Chemie II, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstraße 3, 91058, Erlangen, Germany
| | - Gabriel Partl
- Institut für Allgemeine, Anorganische und Theoretische Chemie, Leopold-Franzens-Universität Innsbruck, Innrain 80-82, 6020, Innsbruck, Austria
| | - Florian Maier
- Lehrstuhl für Physikalische Chemie II, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstraße 3, 91058, Erlangen, Germany
| | - Hans-Peter Steinrück
- Lehrstuhl für Physikalische Chemie II, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstraße 3, 91058, Erlangen, Germany
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Moon D. Recent advances in MEIS. SURF INTERFACE ANAL 2019. [DOI: 10.1002/sia.6708] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Affiliation(s)
- DaeWon Moon
- Department of New BiologyDaegu Gyeongbuk Institute of Science and Technology Daegu Republic of Korea
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Heller BSJ, Kolbeck C, Niedermaier I, Dommer S, Schatz J, Hunt P, Maier F, Steinrück HP. Surface Enrichment in Equimolar Mixtures of Non-Functionalized and Functionalized Imidazolium-Based Ionic Liquids. Chemphyschem 2018; 19:1733-1745. [PMID: 29645340 PMCID: PMC6175172 DOI: 10.1002/cphc.201800216] [Citation(s) in RCA: 13] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/12/2018] [Indexed: 11/30/2022]
Abstract
For equimolar mixtures of ionic liquids with imidazolium‐based cations of very different electronic structure, we observe very pronounced surface enrichment effects by angle‐resolved X‐ray photoelectron spectroscopy (XPS). For a mixture with the same anion, that is, 1‐methyl‐3‐octylimidazolium hexafluorophosphate+1,3‐di(methoxy)imidazolium hexafluorophosphate ([C8C1Im][PF6]+[(MeO)2Im][PF6]), we find a strong enrichment of the octyl chain‐containing [C8C1Im]+ cation and a corresponding depletion of the [(MeO)2Im]+ cation in the topmost layer. For a mixture with different cations and anions, that is, [C8C1Im][Tf2N]+[(MeO)2Im][PF6], we find both surface enrichment of the [C8C1Im]+ cation and the [Tf2N]− (bis[(trifluoromethyl)sulfonyl]imide) anion, while [(MeO)2Im]+ and [PF6]− are depleted from the surface. We propose that the observed behavior in these mixtures is due to a lowering of the surface tension by the enriched components. Interestingly, we observe pronounced differences in the chemical shifts of the imidazolium ring signals of the [(MeO)2Im]+ cations as compared to the non‐functionalized cations. Calculations of the electronic structure and the intramolecular partial charge distribution of the cations contribute to interpreting these shifts for the two different cations.
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Affiliation(s)
- Bettina S J Heller
- Lehrstuhl für Physikalische Chemie II, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstraße 3, 91058, Erlangen, Germany
| | - Claudia Kolbeck
- Lehrstuhl für Physikalische Chemie II, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstraße 3, 91058, Erlangen, Germany
| | - Inga Niedermaier
- Lehrstuhl für Physikalische Chemie II, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstraße 3, 91058, Erlangen, Germany
| | - Sabine Dommer
- Lehrstuhl für Organische Chemie I, Friedrich-Alexander-Universität Erlangen-Nürnberg, Nikolaus-Fiebiger-Straße 10, 91058, Erlangen, Germany
| | - Jürgen Schatz
- Lehrstuhl für Organische Chemie I, Friedrich-Alexander-Universität Erlangen-Nürnberg, Nikolaus-Fiebiger-Straße 10, 91058, Erlangen, Germany
| | - Patricia Hunt
- Chemistry Department, Imperial College London, SW72AZ, London, United Kingdom
| | - Florian Maier
- Lehrstuhl für Physikalische Chemie II, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstraße 3, 91058, Erlangen, Germany
| | - Hans-Peter Steinrück
- Lehrstuhl für Physikalische Chemie II, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstraße 3, 91058, Erlangen, Germany
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Innovative aspects of protein stability in ionic liquid mixtures. Biophys Rev 2018; 10:841-846. [PMID: 29549586 DOI: 10.1007/s12551-018-0411-x] [Citation(s) in RCA: 27] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/01/2018] [Accepted: 03/05/2018] [Indexed: 10/17/2022] Open
Abstract
Mixtures of ionic liquids (ILs) have attracted our attention because of their extraordinary performances in extraction technologies and in absorbing large amount of CO2 gas. It has been observed that when two or more ILs are mixed in different proportions, a new solvent is obtained which is much better than that of each component of ILs from which the mixture is obtained. Within a mixture of ILs, several unidentified interactions occur among several ions which give rise to unique solvent properties to the mixture. Herein, in this review, we have highlighted the utilization of the advantageous properties of the IL mixtures in protein stability studies. This approach is exceptional and opens new directions to the use of ILs in biotechnology.
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Smoll EJ, Tesa-Serrate MA, Purcell SM, D'Andrea L, Bruce DW, Slattery JM, Costen ML, Minton TK, McKendrick KG. Determining the composition of the vacuum-liquid interface in ionic-liquid mixtures. Faraday Discuss 2018; 206:497-522. [PMID: 28944811 DOI: 10.1039/c7fd00175d] [Citation(s) in RCA: 18] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
The vacuum-liquid interfaces of a number of ionic-liquid mixtures have been investigated using the combination of reactive-atom scattering with laser-induced fluorescence detection (RAS-LIF), selected surface tension measurements, and molecular dynamics (MD) simulations. The mixtures are based on the widespread 1-alkyl-3-methylimidazolium ([Cnmim]+) cation, including mixed cations which differ in chain length or chemical functionality with a common anion; and different anions for a common cation. RAS-LIF results imply that the surface compositions exhibit a general form of non-stoichiometric behaviour that mimics the well-known Henry's and Raoult's laws at low and high mole fraction, respectively. The extended Langmuir model provides a moderately good single-parameter fit, but higher-order terms are required for an accurate description. The quantitative relationship between RAS-LIF and surface tension, which probes the surface composition only indirectly, is explored for mixtures of [C2mim]+ and [C12mim]+ with a common bis(trifluoromethylsulfonyl)imide ([NTf2]-) anion. Extended Langmuir model fits to surface tension data are broadly consistent with those to RAS-LIF; however, several other common approaches to extracting surface compositions from measured surface tensions result in much larger discrepancies. MD simulations suggest that RAS-LIF faithfully reports on the alkyl-chain exposure at the surface, which is only subtly modified by composition-dependent structural reorganisation.
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Affiliation(s)
- E J Smoll
- Department of Chemistry and Biochemistry, Montana State University, Bozeman, Montana 59717, USA.
| | - M A Tesa-Serrate
- Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh, EH14 4AS, UK.
| | - S M Purcell
- Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh, EH14 4AS, UK.
| | - L D'Andrea
- Department of Chemistry, University of York, Heslington, York, YO10 5DD, UK
| | - D W Bruce
- Department of Chemistry, University of York, Heslington, York, YO10 5DD, UK
| | - J M Slattery
- Department of Chemistry, University of York, Heslington, York, YO10 5DD, UK
| | - M L Costen
- Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh, EH14 4AS, UK.
| | - T K Minton
- Department of Chemistry and Biochemistry, Montana State University, Bozeman, Montana 59717, USA.
| | - K G McKendrick
- Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh, EH14 4AS, UK.
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