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Martins MAR, Carvalho PJ, Alves D, Dariva C, Costa MC, Ferreira RAS, André PS, Morgado P, Pinho SP, Filipe EJM, Coutinho JAP. Surface crystallization of ionic liquid crystals. Phys Chem Chem Phys 2019; 21:17792-17800. [PMID: 31372606 DOI: 10.1039/c9cp03947c] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
The evidence for surface crystallization in ionic liquids is scarce. The existing reports seem to be contradictory as for its driving forces, since in the two compounds investigated in the literature, the contribution of coloumbic and van der Waals forces is very different. In this work 1-dodecyl-3-methylimidazolium tetrafluoroborate was studied and its surface crystallization characterized by surface tension, ellipsometry and optical microscopy. The results obtained seem to reconcile previous observations, and it was further shown, using the same techniques, that this phenomenon is prevalent in other ionic liquids. MD simulation results illustrate the different possibilities of organization, providing reasonable models to rationalize the experimental observations.
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de las Heras D, Martínez-Ratón Y, Velasco E. Surface and smectic layering transitions in binary mixtures of parallel hard rods. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2010; 81:021706. [PMID: 20365581 DOI: 10.1103/physreve.81.021706] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/13/2009] [Indexed: 05/29/2023]
Abstract
The surface phase behavior of binary mixtures of colloidal hard rods in contact with a solid substrate (hard wall) is studied, with special emphasis on the region of the phase diagram that includes the smectic A phase. The colloidal rods are modeled as hard cylinders of the same diameter and different lengths, in the approximation of perfect alignment. A fundamental-measure density functional is used to obtain equilibrium density profiles and thermodynamic properties such as surface tensions and adsorption coefficients. The bulk phase diagram exhibits nematic-smectic and smectic-smectic demixing, with smectic phases having different compositions; in some cases they are microfractionated. The calculated surface phase diagram of the wall-nematic interface shows a very rich phase behavior, including layering transitions and complete wetting at high pressures, whereby an infinitely thick smectic film grows at the wall via an infinite sequence of stepwise first-order layering transitions. For lower pressures complete wetting also obtains, but here the smectic film grows in a continuous fashion. Finally, at very low pressures, the wall-nematic interface exhibits critical adsorption by the smectic phase, due to the second-order character of the bulk nematic-smectic transition.
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Affiliation(s)
- Daniel de las Heras
- Departamento de Física Teórica de Materia Condensada, Universidad Autónoma de Madrid, E-28049 Madrid, Spain.
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Affiliation(s)
- Xuewu Liu
- a Department of Physics , Kent State University , Kent , OH , 44242 , USA
| | - Daniele Finotello
- a Department of Physics , Kent State University , Kent , OH , 44242 , USA
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Lau YGJ, Richardson RM, Cubitt R. Smectic order induced at homeotropically aligned nematic surfaces: A neutron reflection study. J Chem Phys 2006; 124:234910. [PMID: 16821956 DOI: 10.1063/1.2203067] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
Neutron reflection was used to measure the buildup of layers at a solid surface as the smectic phase is approached from higher temperatures in a nematic liquid crystal. The liquid crystal was 4-octyl-4'-cyanobiphenyl (8CB), and the solid was silicon with one of five different surface treatments that induce homeotropic alignment: (i) silicon oxide; (ii) a cetyltrimethylammonium bromide coating; (iii) an octadecyltrichlorosilane monolayer; (iv) an n-n-dimethyl-n-octadecyl-3- aminopropyltrimethyloxysilyl chloride monolayer; and (v) a lecithin coating. The development of surface smectic layers in the nematic phase of 8CB was followed by measuring specular reflectivity and monitoring the pseudo-Bragg peak from the layers. The scattering data were processed to remove the scattering from short-ranged smecticlike fluctuations in the bulk nematic phase from the specular reflection. The pseudo-Bragg peak at scattering vector Q approximately 0.2 A(-1) therefore corresponded to the formation of long-range smectic layers at the surface. The amplitude of the smectic density wave decayed with increasing distance from the surface, and the characteristic thickness of this smectic region diverged as the transition temperature was approached. It was found that the characteristic thickness for some of the surface treatments was greater than the correlation length in the bulk nematic. The different surfaces gave different values of the smectic order parameter at the surface. This suggests that the interaction with the surface is significantly different from a "hard wall" which would give the same values of the smectic order parameter and penetration depths similar to the bulk correlation length. Comparison of the different surfaces also suggested that the strength and range of the surface smectic ordering may be varied independently.
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Affiliation(s)
- Y G J Lau
- Institut Laue-Langevin, 6 Rue Jules Horowitz, BP 156-38042 Grenoble, Cedex 9, France
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Abstract
The structure of the fragile liquid-crystalline phases has a strong impact on the forces between bodies immersed in a liquid crystal (LC). We have equipped an atomic force microscope with a precise temperature control and measured various liquid-crystalline structural forces at temperatures close to the phase transitions. The observed forces agree well with predictions of Landau--de Gennes phenomenological theory of LCs, even at a nanoscale length. In addition to this, we have observed a molecular layer, adsorbed on the surfactant-covered glass surface, and determined its thickness and elastic properties.
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Affiliation(s)
- Klemen Kocevar
- J. Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia
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Kocevar K, Musevic I. Forces in the isotropic phase of a confined nematic liquid crystal 5CB. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2001; 64:051711. [PMID: 11735949 DOI: 10.1103/physreve.64.051711] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/15/2001] [Indexed: 05/23/2023]
Abstract
Using a temperature controlled atomic force microscope, we have measured the temperature dependence of the force between a flat silanated glass surface and a silanated glass microsphere, immersed in the isotropic phase of the nematic liquid crystal 5CB (4'-n-pentyl 4-cyanobiphenyl). At separations of several nanometers, we observed a weak, short range attractive force of the order of 100 pN, which was increased by decreasing the temperature. The temperature dependence of the amplitude and the range of this attractive force can be described by a combination of van der Waals and a mean-field prenematic force due to the surface-induced nematic order. This is supported by ellipsometric study and allows for the determination of the surface coupling energy of 5CB on a silanated glass surface.
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Affiliation(s)
- K Kocevar
- J. Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia
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Moses T. Surface ordering above the isotropic-smectic-A transition at a silane-treated substrate. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2001; 64:010702. [PMID: 11461211 DOI: 10.1103/physreve.64.010702] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/17/2001] [Indexed: 05/23/2023]
Abstract
Surface ordering in a homologous series of alkyl cyanobiphenyl (nCB) liquid crystals having a direct isotropic-smectic-A (I-A) transition was investigated using evanescent-wave ellipsometry. The liquid crystal was bounded by a solid substrate treated with a silane surfactant which induced homeotropic (perpendicular) ordering of the liquid crystal molecules in the smectic-A phase. In the isotropic phase, one of the liquid crystals (10CB) partially wet the interface with an orientationally ordered, homeotropically aligned layer. The ordered interfacial layer grew without layering transitions but remained finite in thickness as the bulk I-A transition was approached. The interfacial layer has significantly lower orientational order than is observed in the smectic phase, indicating the possibility that the surface region of 10CB may be in a surface-induced, nonspontaneous nematic phase. The other liquid crystals (11 and 12CB) showed no surface ordering behavior whatsoever. Models describing the ordered surface layer of 10CB are presented. The results can be interpreted as a sharp transition in the surface ordering behavior as the chain length of the liquid crystal is varied, at the I-A transition of a liquid crystal.
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Affiliation(s)
- T Moses
- Department of Physics, Knox College, Galesburg, Illinois 61401, USA
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Lucht R, Marczuk P, Bahr C, Findenegg GH. X-ray reflectivity study of smectic wetting and prewetting at the free surface of isotropic liquid crystals. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2001; 63:041704. [PMID: 11308864 DOI: 10.1103/physreve.63.041704] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/15/2000] [Indexed: 05/23/2023]
Abstract
We study the structures of free-surface-wetting layers above the isotropic to smectic-A transition of three liquid-crystal compounds that show different kinds of growth of the wetting film as the bulk transition is approached: layer-by-layer, continuous, and continuous with prewetting. The smectic-A surface phase of the layer-by-layer compound consists of well-defined layers and possesses a sharp boundary to the isotropic bulk phase, whereas in the two continuous compounds sinusoidal density oscillations with a continuously decaying amplitude are found. In the continuous case with prewetting, the wetting film below the prewetting transition does not show an essential difference to the continuous case without prewetting.
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Affiliation(s)
- R Lucht
- Institute of Physical Chemistry, University Marburg, D-35032 Marburg, Germany
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Braun FN. Disclination renormalization of the disjoining pressure between walls bounding a nematic liquid crystal. J Chem Phys 2001. [DOI: 10.1063/1.1344615] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
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Lucht R, Bahr C, Heppke G. Wetting behavior above the liquid-crystal-isotropic transition in a homologous series. PHYSICAL REVIEW. E, STATISTICAL PHYSICS, PLASMAS, FLUIDS, AND RELATED INTERDISCIPLINARY TOPICS 2000; 62:2324-2333. [PMID: 11088700 DOI: 10.1103/physreve.62.2324] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/17/2000] [Indexed: 05/23/2023]
Abstract
An ellipsometric study of the wetting behavior at the free surface above the isotropic to nematic or isotropic to smectic-A transition of nine homologous compounds with even alkyl chain lengths n in the range from four to twenty carbon atoms is presented. All compounds show a pretransitional increase of the nematic or smectic surface coverage as the bulk isotropic to liquid-crystal transition is approached from above. The behavior of the nematic compounds (n=4 to 10) can be interpreted, within the framework of a Landau model, as complete wetting. In short nematic homologs the divergence of the nematic coverage is strongly reduced by a decrease of the nematic susceptibility of the isotropic phase. The elastic coefficient L of the Landau model shows a pronounced increase with increasing n, resulting in the occurrence of a discontinuous prewetting transition in the shortest smectic homolog (n=12) that is still describable by the nematic Landau model. In the longer smectic homologs (n=14 to 20), layering steps appear in the pretransitional increase of the coverage. The results indicate probable partial wetting for the longest homolog, whereas for the other smectic compounds the distinction between complete and partial wetting is difficult on the basis of ellipsometry.
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Affiliation(s)
- R Lucht
- Institute of Physical Chemistry, University Marburg, D-35032 Marburg, Germany
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Sakamoto N, Sakai K, Takagi K. Layering transition at the free surface of 12CB observed by scanning angle reflectometry. J Chem Phys 2000. [DOI: 10.1063/1.480620] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
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Lucht R, Bahr C, Heppke G. Layering Transitions at the Free Surface of a Smectic Liquid Crystal. J Phys Chem B 1998. [DOI: 10.1021/jp981832a] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- R. Lucht
- Institute of Physical Chemistry, University Marburg, D-35032 Marburg, Germany
| | - Ch. Bahr
- Institute of Physical Chemistry, University Marburg, D-35032 Marburg, Germany
| | - G. Heppke
- I.-N.-Stranski-Institut of Physical and Theoretical Chemistry, Technical University Berlin, D-10623 Berlin, Germany
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