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Pontoni D, DiMichiel M, Deutsch M. Binary mixtures of homologous room-temperature ionic liquids: Nanoscale structure evolution with alkyl lengths’ difference. J Mol Liq 2022. [DOI: 10.1016/j.molliq.2022.118874] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/05/2023]
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Pontoni D, DiMichiel M, Deutsch M. Binary mixtures of homologous room-temperature ionic liquids: Temperature and composition evolution of the nanoscale structure. J Mol Liq 2021. [DOI: 10.1016/j.molliq.2021.116587] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022]
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Haddad J, Pontoni D, Murphy BM, Festersen S, Runge B, Magnussen OM, Steinrück HG, Reichert H, Ocko BM, Deutsch M. Surface structure evolution in a homologous series of ionic liquids. Proc Natl Acad Sci U S A 2018; 115:E1100-E1107. [PMID: 29358372 PMCID: PMC5819424 DOI: 10.1073/pnas.1716418115] [Citation(s) in RCA: 35] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Interfaces of room temperature ionic liquids (RTILs) are important for both applications and basic science and are therefore intensely studied. However, the evolution of their interface structure with the cation's alkyl chain length [Formula: see text] from Coulomb to van der Waals interaction domination has not yet been studied for even a single broad homologous RTIL series. We present here such a study of the liquid-air interface for [Formula: see text], using angstrom-resolution X-ray methods. For [Formula: see text], a typical "simple liquid" monotonic surface-normal electron density profile [Formula: see text] is obtained, like those of water and organic solvents. For [Formula: see text], increasingly more pronounced nanoscale self-segregation of the molecules' charged moieties and apolar chains yields surface layering with alternating regions of headgroups and chains. The layering decays into the bulk over a few, to a few tens, of nanometers. The layering periods and decay lengths, their linear [Formula: see text] dependence, and slopes are discussed within two models, one with partial-chain interdigitation and the other with liquid-like chains. No surface-parallel long-range order is found within the surface layer. For [Formula: see text], a different surface phase is observed above melting. Our results also impact general liquid-phase issues like supramolecular self-aggregation and bulk-surface structure relations.
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Affiliation(s)
- Julia Haddad
- Physics Department, Bar-Ilan University, Ramat Gan 5290002, Israel
- Institute of Nanotechnology and Advanced Materials, Bar-Ilan University, Ramat Gan 5290002, Israel
| | - Diego Pontoni
- European Synchrotron Radiation Facility, The European Synchrotron and Partnership for Soft Condensed Matter (PSCM), 38000 Grenoble, France
| | - Bridget M Murphy
- Institute for Experimental and Applied Physics, Kiel University, 24118 Kiel, Germany
- Ruprecht Haensel Laboratory, Kiel University, 24098 Kiel, Germany
| | - Sven Festersen
- Institute for Experimental and Applied Physics, Kiel University, 24118 Kiel, Germany
| | - Benjamin Runge
- Institute for Experimental and Applied Physics, Kiel University, 24118 Kiel, Germany
| | - Olaf M Magnussen
- Institute for Experimental and Applied Physics, Kiel University, 24118 Kiel, Germany
- Ruprecht Haensel Laboratory, Kiel University, 24098 Kiel, Germany
| | - Hans-Georg Steinrück
- Stanford Synchrotron Radiation Laboratory (SSRL) Materials Science Division, Stanford Linear Accelerator Center (SLAC) National Accelerator Laboratory, Menlo Park, CA 94025
| | - Harald Reichert
- European Synchrotron Radiation Facility, The European Synchrotron, 38000 Grenoble, France
| | - Benjamin M Ocko
- National Synchrotron Light Source II (NSLS-II), Brookhaven National Laboratory, Upton, NY 11973
| | - Moshe Deutsch
- Physics Department, Bar-Ilan University, Ramat Gan 5290002, Israel;
- Institute of Nanotechnology and Advanced Materials, Bar-Ilan University, Ramat Gan 5290002, Israel
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Sloutskin E, Sapir Z, Bain CD, Lei Q, Wilkinson KM, Tamam L, Deutsch M, Ocko BM. Wetting, mixing, and phase transitions in Langmuir-Gibbs films. PHYSICAL REVIEW LETTERS 2007; 99:136102. [PMID: 17930612 DOI: 10.1103/physrevlett.99.136102] [Citation(s) in RCA: 30] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/03/2007] [Indexed: 05/25/2023]
Abstract
Millimolar bulk concentrations of the surfactant cetyltrimethylammonium bromide (CTAB) induce spreading of alkanes, H(CH(2))(n)H (denoted C(n)) 12< or =n< or =21, on the water surface, which is not otherwise wet by these alkanes. The novel Langmuir-Gibbs film (LGF) formed is a liquidlike monolayer comprising both alkanes and CTAB tails. Upon cooling, an ordering transition occurs, yielding a hexagonally packed, quasi-2D crystal. For 11< or =n< or =17 this surface-frozen LGF is a crystalline monolayer. For 18< or =n< or =21 the LGF is a bilayer with a crystalline, pure-alkane, upper monolayer, and a liquidlike lower monolayer. The phase diagram and film structure were determined by x-ray, ellipsometry, and surface tension measurements. A thermodynamic theory accounts quantitatively for the observations.
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Affiliation(s)
- E Sloutskin
- Department of Physics and Institute of Nanotechnology and Advanced Materials, Bar-Ilan University, Ramat-Gan 52900, Israel
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Sloutskin E, Solutskin E, Ocko BM, Tamam L, Taman L, Kuzmenko I, Gog T, Deutsch M. Surface Layering in Ionic Liquids: An X-ray Reflectivity Study. J Am Chem Soc 2005; 127:7796-804. [PMID: 15913369 DOI: 10.1021/ja0509679] [Citation(s) in RCA: 241] [Impact Index Per Article: 12.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
The surface structure and thermodynamics of two ionic liquids, based on the 1-alkyl-3-methylimidazolium cations, were studied by X-ray reflectivity and surface tensiometry. A molecular layer of a density approximately 18% higher than that of the bulk is found to form at the free surface of these liquids. In common with surface layering in liquid metals and surface freezing in melts of organic chain molecules, this effect is induced by the lower dimensionality of the surface. The concentrations of the oppositely charged ions within the surface layer are determined by chemical substitution of the anion. The temperature-dependent surface tension measurements reveal a normal, negative-slope temperature dependence. The different possible molecular arrangements within the enhanced-density surface layer are discussed.
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Affiliation(s)
- Eli Sloutskin
- Department of Physics, Bar-Ilan University, Ramat-Gan 52900, Israel
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Sloutskin E, Bain CD, Ocko BM, Deutsch M. Surface freezing of chain molecules at the liquid–liquid and liquid–air interfaces. Faraday Discuss 2005; 129:339-52; discussion 353-66. [PMID: 15715317 DOI: 10.1039/b405969g] [Citation(s) in RCA: 57] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Surface freezing (SF) is the formation of a crystalline monolayer at the free surface of a melt at a temperature Ts, a few degrees above the bulk freezing temperature, Tb. This effect, i.e. Ts > Tb, common to many chain molecules, is in a marked contrast with the surface melting effect, i.e. Ts < or = Tb, shown by almost all other materials. Depending on chain length, n, the SF layer shows a variety of phases, in some cases tuneable by bulk additives. The SF behaviour of binary mixtures of different-length alkanes and alcohols is governed by the relative chain length mismatch, /delta n/n/2, yielding a quasi-"universal" behaviour for the freezing of both bulk and surface. While SF at the liquid air interface was studied rather extensively, Lei and Bain (Phys. Rev. Lett., 2004, 94, 176103) have shown only very recently that interfacial freezing (IF) can be induced also at the water: tetradecane interface by adding the ionic surfactant CTAB to the water phase. We present measurements of the interfacial tension of the water: hexadecane interface, as a function of temperature and the ionic surfactant STAB, revealing IF at a STAB-concentration-dependent temperature Ti > Tb. The measurements indicate that a single frozen monolayer is formed, with a temperature-existence range of up to 10 degrees C, much larger than the 1.2 degrees C found for SF at the free surface of the melt. We also find a new effect, where the IF allows tuning of the interfacial tension between the two bulk phases to zero for a range of temperatures, deltaT = Tmix - Tb < or = Ti - Tb by cooling the system below Ti. We discuss qualitatively the factors stabilizing the frozen layer and their variation from the liquid-air to the liquid-liquid interfaces. The surfactant concentration dependence of Ti is also discussed and a tentative theoretical explanation is suggested.
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Affiliation(s)
- Eli Sloutskin
- Physics Department, Bar-Ilan University, Ramat-Gan 52900, Israel
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Sloutskin E, Wu XZ, Peterson TB, Gang O, Ocko BM, Sirota EB, Deutsch M. Surface freezing in binary mixtures of chain molecules. I. Alkane mixtures. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2003; 68:031605. [PMID: 14524779 DOI: 10.1103/physreve.68.031605] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/16/2002] [Revised: 05/05/2003] [Indexed: 05/24/2023]
Abstract
X-ray surface scattering and surface tension measurements are used to study surface freezing in molten mixtures of alkanes. These binary mixtures consist of protonated and deuterated alkanes, as well as of alkanes of different lengths. As for pure alkanes, a crystalline monolayer is formed at the surface a few degrees above the bulk freezing temperature. The structure of the monolayer has been determined on an angstrom scale. A simple theoretical approach is used to account for the thermodynamical observations at the surface and in the bulk. The model is based on a competition between entropic mixing and a repulsive interaction due to chain-length mismatch. The surface and bulk liquid phases are treated as ideal mixtures, while the solid phases are treated as regular mixtures. The theory is found to account well for all the mixtures studied, both hydrogenated-hydrogenated and hydrogenated-deuterated. The repulsive interaction and its dependence on the chain lengths of the components are determined from fits to the measured data.
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Affiliation(s)
- E Sloutskin
- Physics Department, Bar Ilan University, Ramat Gan 52900, Israel
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Sloutskin E, Gang O, Kraack H, Doerr A, Sirota EB, Ocko BM, Deutsch M. Surface freezing in binary mixtures of chain molecules. II. Dry and hydrated alcohol mixtures. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2003; 68:031606. [PMID: 14524780 DOI: 10.1103/physreve.68.031606] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/16/2002] [Revised: 05/05/2003] [Indexed: 05/24/2023]
Abstract
Surface freezing is studied in dry and hydrated alcohol mixtures by surface x-ray scattering and surface tension measurements. A crystalline bilayer is formed at the surface a few degrees above the bulk freezing temperature. The packing is hexagonal, with molecules aligned along the surface normal in all cases. The in-plane lattice constant reveals a qualitatively different behavior with composition for hydrated and dry mixtures. The simple theoretical approach used successfully for alkane and deuterated alkane mixtures accounts well also for the alcohol mixtures. The repulsive length-mismatch term opposing the mixing entropy term in the free energy of the mixtures is shown to have a universal behavior for all mixtures studied: protonated alkanes, deuterated alkanes, and dry and wet alcohols. This universality is somewhat counterintuitive in view of the different interactions (e.g., hydrogen bonding in alcohols) in the different mixtures.
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Affiliation(s)
- E Sloutskin
- Physics Department, Bar Ilan University, Ramat Gan 52900, Israel
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Sloutskin E, Kraack H, Gang O, Ocko BM, Sirota EB, Deutsch M. A thin–thick transition in the surface-frozen layer of a binary alcohol mixture. J Chem Phys 2003. [DOI: 10.1063/1.1574313] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
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