{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,10,7]],"date-time":"2026-10-07T21:19:11Z","timestamp":1791407951125,"version":"4.3.3"},"reference-count":54,"publisher":"MDPI AG","issue":"23","license":[{"start":{"date-parts":[[2022,12,5]],"date-time":"2022-12-05T00:00:00Z","timestamp":1670198400000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"name":"DFG","award":["431791331"],"award-info":[{"award-number":["431791331"]}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Molecules"],"abstract":"<jats:p>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.<\/jats:p>","DOI":"10.3390\/molecules27238561","type":"journal-article","created":{"date-parts":[[2022,12,5]],"date-time":"2022-12-05T07:07:31Z","timestamp":1670224051000},"page":"8561","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":13,"title":["Direct Correlation of Surface Tension and Surface Composition of Ionic Liquid Mixtures\u2014A Combined Vacuum Pendant Drop and Angle-Resolved X-ray Photoelectron Spectroscopy Study"],"prefix":"10.3390","volume":"27","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-1442-0239","authenticated-orcid":false,"given":"Ulrike","family":"Paap","sequence":"first","affiliation":[{"name":"Department of Chemistry and Pharmacy, Physical Chemistry II, Friedrich-Alexander-Universit\u00e4t Erlangen-N\u00fcrnberg (FAU), Egerlandstr. 3, 91058 Erlangen, Germany"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Vera","family":"Seidl","sequence":"additional","affiliation":[{"name":"Department of Chemistry and Pharmacy, Inorganic Chemistry, Friedrich-Alexander-Universit\u00e4t Erlangen-N\u00fcrnberg (FAU), Egerlandstr. 1, 91058 Erlangen, Germany"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Karsten","family":"Meyer","sequence":"additional","affiliation":[{"name":"Department of Chemistry and Pharmacy, Inorganic Chemistry, Friedrich-Alexander-Universit\u00e4t Erlangen-N\u00fcrnberg (FAU), Egerlandstr. 1, 91058 Erlangen, Germany"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-9725-8961","authenticated-orcid":false,"given":"Florian","family":"Maier","sequence":"additional","affiliation":[{"name":"Department of Chemistry and Pharmacy, Physical Chemistry II, Friedrich-Alexander-Universit\u00e4t Erlangen-N\u00fcrnberg (FAU), Egerlandstr. 3, 91058 Erlangen, Germany"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-1347-8962","authenticated-orcid":false,"given":"Hans-Peter","family":"Steinr\u00fcck","sequence":"additional","affiliation":[{"name":"Department of Chemistry and Pharmacy, Physical Chemistry II, Friedrich-Alexander-Universit\u00e4t Erlangen-N\u00fcrnberg (FAU), Egerlandstr. 3, 91058 Erlangen, Germany"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"1968","published-online":{"date-parts":[[2022,12,5]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"157","DOI":"10.1016\/S0920-5861(01)00541-7","article-title":"Ionic liquids: Applications in catalysis","volume":"74","author":"Zhao","year":"2002","journal-title":"Catal. Today"},{"key":"ref_2","doi-asserted-by":"crossref","first-page":"2459","DOI":"10.1016\/j.ccr.2004.04.015","article-title":"Ionic liquids in catalysis","volume":"248","author":"Welton","year":"2004","journal-title":"Coord. Chem. Rev."},{"key":"ref_3","doi-asserted-by":"crossref","first-page":"123","DOI":"10.1039\/B006677J","article-title":"Applications of ionic liquids in the chemical industry","volume":"37","author":"Plechkova","year":"2008","journal-title":"Chem. Soc. Rev."},{"key":"ref_4","doi-asserted-by":"crossref","first-page":"1","DOI":"10.1016\/j.apcata.2009.10.008","article-title":"Ionic liquids and catalysis: Recent progress from knowledge to applications","volume":"373","author":"Magna","year":"2010","journal-title":"Appl. Catal. A Gen."},{"key":"ref_5","doi-asserted-by":"crossref","first-page":"232","DOI":"10.1039\/C3EE42099J","article-title":"Energy applications of ionic liquids","volume":"7","author":"MacFarlane","year":"2014","journal-title":"Energy Environ. Sci."},{"key":"ref_6","doi-asserted-by":"crossref","first-page":"380","DOI":"10.1007\/s10562-014-1435-x","article-title":"Ionic Liquids in Catalysis","volume":"145","author":"Wasserscheid","year":"2015","journal-title":"Catal. Lett."},{"key":"ref_7","doi-asserted-by":"crossref","first-page":"112038","DOI":"10.1016\/j.molliq.2019.112038","article-title":"Ionic liquids synthesis and applications: An overview","volume":"297","author":"Singh","year":"2020","journal-title":"J. Mol. Liq."},{"key":"ref_8","doi-asserted-by":"crossref","first-page":"32","DOI":"10.1021\/cen-v076n013.p032","article-title":"Designer solvents\u2014Ionic liquids may boost clean technology development","volume":"76","author":"Freemantle","year":"1998","journal-title":"Chem. Eng. News Arch."},{"key":"ref_9","doi-asserted-by":"crossref","first-page":"5247","DOI":"10.1021\/ol702435f","article-title":"Ionic liquids as designer solvents for nucleophilic aromatic substitutions","volume":"9","author":"Newington","year":"2007","journal-title":"Org. Lett."},{"key":"ref_10","doi-asserted-by":"crossref","first-page":"7780","DOI":"10.1039\/c2cs35177c","article-title":"Mixtures of ionic liquids","volume":"41","author":"Niedermeyer","year":"2012","journal-title":"Chem. Soc. Rev."},{"key":"ref_11","doi-asserted-by":"crossref","first-page":"2051","DOI":"10.1039\/c3gc41389f","article-title":"Mixing ionic liquids\u2014\u201csimple mixtures\u201d or \u201cdouble salts\u201d?","volume":"16","author":"Chatel","year":"2014","journal-title":"Green Chem."},{"key":"ref_12","doi-asserted-by":"crossref","first-page":"1101","DOI":"10.1039\/C4SC02931C","article-title":"A physicochemical investigation of ionic liquid mixtures","volume":"6","author":"Clough","year":"2015","journal-title":"Chem. Sci."},{"key":"ref_13","doi-asserted-by":"crossref","first-page":"6357","DOI":"10.1021\/cr500411q","article-title":"Structure and Nanostructure in Ionic Liquids","volume":"115","author":"Hayes","year":"2015","journal-title":"Chem. Rev."},{"key":"ref_14","doi-asserted-by":"crossref","first-page":"5798","DOI":"10.1021\/acs.chemrev.9b00693","article-title":"Microstructural and Dynamical Heterogeneities in Ionic Liquids","volume":"120","author":"Wang","year":"2020","journal-title":"Chem. Rev."},{"key":"ref_15","doi-asserted-by":"crossref","first-page":"1117","DOI":"10.1002\/chem.201904438","article-title":"Temperature-Dependent Surface Enrichment Effects in Binary Mixtures of Fluorinated and Non-Fluorinated Ionic Liquids","volume":"26","author":"Heller","year":"2020","journal-title":"Chem.\u2014A Eur. J."},{"key":"ref_16","doi-asserted-by":"crossref","first-page":"112783","DOI":"10.1016\/j.molliq.2020.112783","article-title":"Pronounced surface enrichment of fluorinated ionic liquids in binary mixtures with methoxy-functionalized ionic liquids","volume":"305","author":"Heller","year":"2020","journal-title":"J. Mol. Liq."},{"key":"ref_17","doi-asserted-by":"crossref","first-page":"497","DOI":"10.1039\/C7FD00175D","article-title":"Determining the composition of the vacuum\u2013liquid interface in ionic-liquid mixtures","volume":"206","author":"Smoll","year":"2018","journal-title":"Faraday Discuss."},{"key":"ref_18","doi-asserted-by":"crossref","first-page":"1962","DOI":"10.1021\/acs.jpcb.1c10460","article-title":"Surface Structure of Alkyl\/Fluoroalkylimidazolium Ionic\u2013Liquid Mixtures","volume":"126","author":"Purcell","year":"2022","journal-title":"J. Phys. Chem. B"},{"key":"ref_19","doi-asserted-by":"crossref","first-page":"2571","DOI":"10.1002\/adma.201100211","article-title":"Surface Science and Model Catalysis with Ionic Liquid-Modified Materials","volume":"23","author":"Libuda","year":"2011","journal-title":"Adv. Mater."},{"key":"ref_20","doi-asserted-by":"crossref","first-page":"464","DOI":"10.1016\/j.susc.2009.12.023","article-title":"Surface characterization of imidazolium ionic liquids by high-resolution Rutherford backscattering spectroscopy and X-ray photoelectron spectroscopy","volume":"604","author":"Hashimoto","year":"2010","journal-title":"Surf. Sci."},{"key":"ref_21","doi-asserted-by":"crossref","first-page":"9386","DOI":"10.1021\/la061248q","article-title":"Ionic Liquids in Vacuo:\u2009 Analysis of Liquid Surfaces Using Ultra-High-Vacuum Techniques","volume":"22","author":"Smith","year":"2006","journal-title":"Langmuir"},{"key":"ref_22","doi-asserted-by":"crossref","first-page":"2136","DOI":"10.1039\/b921580h","article-title":"Gas\u2013liquid interface of room-temperature ionic liquids","volume":"39","author":"Santos","year":"2010","journal-title":"Chem. Soc. Rev."},{"key":"ref_23","doi-asserted-by":"crossref","first-page":"19237","DOI":"10.1021\/jp9056797","article-title":"Surface Structural Study on Ionic Liquids Using Metastable Atom Electron Spectroscopy","volume":"113","author":"Iwahashi","year":"2009","journal-title":"J. Phys. Chem. C"},{"key":"ref_24","doi-asserted-by":"crossref","first-page":"5158","DOI":"10.1021\/cr100114t","article-title":"Photoelectron Spectroscopy of Ionic Liquid-Based Interfaces","volume":"110","author":"Lovelock","year":"2010","journal-title":"Chem. Rev."},{"key":"ref_25","doi-asserted-by":"crossref","first-page":"11502","DOI":"10.1021\/jp106582t","article-title":"Surfactant-Mediated Ion Exchange and Charge Reversal at Ionic Liquid Interfaces","volume":"114","author":"Chen","year":"2010","journal-title":"J. Phys. Chem. B"},{"key":"ref_26","doi-asserted-by":"crossref","first-page":"5010","DOI":"10.1039\/c2cp24087d","article-title":"Recent developments in the study of ionic liquid interfaces using X-ray photoelectron spectroscopy and potential future directions","volume":"14","year":"2012","journal-title":"Phys. Chem. Chem. Phys."},{"key":"ref_27","doi-asserted-by":"crossref","first-page":"4404","DOI":"10.1039\/C4SC00640B","article-title":"The ionic liquid\u2013vacuum outer atomic surface: A low-energy ion scattering study","volume":"5","author":"Fearn","year":"2014","journal-title":"Chem. Sci."},{"key":"ref_28","first-page":"1761266","article-title":"Ultrathin ionic liquid films on metal surfaces: Adsorption, growth, stability and exchange phenomena","volume":"5","author":"Lexow","year":"2020","journal-title":"Adv. Phys. X"},{"key":"ref_29","unstructured":"Wandelt, K. (2020). Surfaces of Ionic Liquids. Surface and Interface Science, Wiley-VCH Verlag GmbH & Co.KGaA."},{"key":"ref_30","doi-asserted-by":"crossref","first-page":"3726","DOI":"10.1002\/cphc.201300719","article-title":"Temperature-Dependent Surface-Enrichment Effects of Imidazolium-Based Ionic Liquids","volume":"14","author":"Kolbeck","year":"2013","journal-title":"ChemPhysChem"},{"key":"ref_31","doi-asserted-by":"crossref","first-page":"1694","DOI":"10.1016\/j.susc.2010.06.016","article-title":"Surface segregation in binary mixtures of imidazolium-based ionic liquids","volume":"604","author":"Souda","year":"2010","journal-title":"Surf. Sci."},{"key":"ref_32","doi-asserted-by":"crossref","first-page":"224701","DOI":"10.1063\/1.4838376","article-title":"Surface structures of binary mixtures of imidazolium-based ionic liquids using high-resolution Rutherford backscattering spectroscopy and time of flight secondary ion mass spectroscopy","volume":"139","author":"Nakajima","year":"2013","journal-title":"J. Chem. Phys."},{"key":"ref_33","doi-asserted-by":"crossref","first-page":"542","DOI":"10.1016\/j.molliq.2017.01.073","article-title":"Surface structures of binary mixture of ionic liquids","volume":"230","author":"Nakajima","year":"2017","journal-title":"J. Mol. Liq."},{"key":"ref_34","doi-asserted-by":"crossref","first-page":"2397","DOI":"10.1002\/cphc.202000750","article-title":"Surface Propensity of Anions in a Binary Ionic-Liquid Mixture Assessed by Full-Range Angle-Resolved X-ray Photoelectron Spectroscopy and Surface-Tension Measurements","volume":"21","author":"Oz","year":"2020","journal-title":"ChemPhysChem"},{"key":"ref_35","doi-asserted-by":"crossref","first-page":"621","DOI":"10.1016\/j.jcis.2007.06.003","article-title":"Surface tensions of imidazolium based ionic liquids: Anion, cation, temperature and water effect","volume":"314","author":"Freire","year":"2007","journal-title":"J. Colloid Interface Sci."},{"key":"ref_36","doi-asserted-by":"crossref","first-page":"57","DOI":"10.1016\/j.fluid.2008.01.002","article-title":"Applying a QSPR correlation to the prediction of surface tensions of ionic liquids","volume":"265","author":"Gardas","year":"2008","journal-title":"Fluid Phase Equilibria"},{"key":"ref_37","doi-asserted-by":"crossref","first-page":"829","DOI":"10.1039\/C1CS15146K","article-title":"Surface tension of ionic liquids and ionic liquid solutions","volume":"41","author":"Tariq","year":"2012","journal-title":"Chem. Soc. Rev."},{"key":"ref_38","doi-asserted-by":"crossref","first-page":"6130","DOI":"10.1021\/acs.langmuir.6b00209","article-title":"Influence of Nanosegregation on the Surface Tension of Fluorinated Ionic Liquids","volume":"32","author":"Shimizu","year":"2016","journal-title":"Langmuir"},{"key":"ref_39","first-page":"610","article-title":"The tension of composite fluid surfaces and the mechanical stability of films of fluid","volume":"86","author":"Hardy","year":"1912","journal-title":"Proc. R. Soc. Lond. Ser. A Contain. Pap. Math. Phys. Character"},{"key":"ref_40","doi-asserted-by":"crossref","first-page":"2221","DOI":"10.1021\/ja02268a002","article-title":"The constitution and fundamental properties of solids and liquids. Part I. Solids","volume":"38","author":"Langmuir","year":"1916","journal-title":"J. Am. Chem. Soc."},{"key":"ref_41","doi-asserted-by":"crossref","first-page":"1848","DOI":"10.1021\/ja02254a006","article-title":"The constitution and fundamental properties of solids and liquids. II. Liquids","volume":"39","author":"Langmuir","year":"1917","journal-title":"J. Am. Chem. Soc."},{"key":"ref_42","doi-asserted-by":"crossref","first-page":"451","DOI":"10.1021\/cr60024a002","article-title":"Forces Near the Surfaces of Molecules","volume":"6","author":"Langmuir","year":"1930","journal-title":"Chem. Rev."},{"key":"ref_43","unstructured":"Langmuir, I. (2022, October 20). Nobel Lecture. NobelPrize.org. Available online: https:\/\/www.nobelprize.org\/prizes\/chemistry\/1932\/langmuir\/lecture\/."},{"key":"ref_44","doi-asserted-by":"crossref","first-page":"4408","DOI":"10.1021\/acs.langmuir.7b04237","article-title":"Probing the Surface Tension of Ionic Liquids Using the Langmuir Principle","volume":"34","author":"Shimizu","year":"2018","journal-title":"Langmuir"},{"key":"ref_45","doi-asserted-by":"crossref","first-page":"144","DOI":"10.1007\/s10765-020-02720-w","article-title":"Surface Tension and Viscosity of Binary Mixtures of the Fluorinated and Non-fluorinated Ionic Liquids [PFBMIm][PF6] and [C4C1Im][PF6] by the Pendant Drop Method and Surface Light Scattering","volume":"41","author":"Koller","year":"2020","journal-title":"Int. J. Thermophys."},{"key":"ref_46","doi-asserted-by":"crossref","unstructured":"Paap, U., Kre\u00df, B., Steinr\u00fcck, H.P., and Maier, F. (2022). Probing Surface and Interfacial Tension of Ionic Liquids in Vacuum with the Pendant Drop and Sessile Drop Method. Int. J. Mol. Sci., 23.","DOI":"10.3390\/ijms232113158"},{"key":"ref_47","doi-asserted-by":"crossref","first-page":"1330","DOI":"10.1039\/b713584j","article-title":"Angle-resolved X-ray photoelectron spectroscopy of the surface of imidazolium ionic liquids","volume":"10","author":"Lockett","year":"2008","journal-title":"Phys. Chem. Chem. Phys."},{"key":"ref_48","doi-asserted-by":"crossref","first-page":"1905","DOI":"10.1039\/b920804f","article-title":"Insights into the surface composition and enrichment effects of ionic liquids and ionic liquid mixtures","volume":"12","author":"Maier","year":"2010","journal-title":"Phys. Chem. Chem. Phys."},{"key":"ref_49","doi-asserted-by":"crossref","first-page":"045105","DOI":"10.1063\/1.4942943","article-title":"Dual analyzer system for surface analysis dedicated for angle-resolved photoelectron spectroscopy at liquid surfaces and interfaces","volume":"87","author":"Niedermaier","year":"2016","journal-title":"Rev. Sci. Instrum."},{"key":"ref_50","doi-asserted-by":"crossref","first-page":"100041","DOI":"10.1016\/j.jil.2022.100041","article-title":"Bis-polyethylene glycol-functionalized imidazolium ionic liquids: A multi-method approach towards bulk and surface properties","volume":"2","author":"Seidl","year":"2022","journal-title":"J. Ion. Liq."},{"key":"ref_51","doi-asserted-by":"crossref","first-page":"1733","DOI":"10.1002\/cphc.201800216","article-title":"Surface Enrichment in Equimolar Mixtures of Non-Functionalized and Functionalized Imidazolium-Based Ionic Liquids","volume":"19","author":"Heller","year":"2018","journal-title":"ChemPhysChem"},{"key":"ref_52","doi-asserted-by":"crossref","first-page":"8682","DOI":"10.1021\/jp902978r","article-title":"Influence of Different Anions on the Surface Composition of Ionic Liquids Studied Using ARXPS","volume":"113","author":"Kolbeck","year":"2009","journal-title":"J. Phys. Chem. B"},{"key":"ref_53","doi-asserted-by":"crossref","first-page":"211","DOI":"10.1002\/sia.740030506","article-title":"Empirical Atomic Sensitivity Factors for Quantitative Analysis by Electron Spectroscopy for Chemical Analysis","volume":"3","author":"Wagner","year":"1981","journal-title":"Surf. Interface Anal."},{"key":"ref_54","doi-asserted-by":"crossref","first-page":"80","DOI":"10.1021\/je060247x","article-title":"High-Pressure Densities and Derived Thermodynamic Properties of Imidazolium-Based Ionic Liquids","volume":"52","author":"Gardas","year":"2006","journal-title":"J. Chem. Eng. 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