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Influence of magnetic field on the molecular orientation in epitropic mesophase of nitrobenzene
Advances in Colloid and Interface Science 104 (2003) 239–243
Influence of magnetic field on the molecular orientation in epitropic mesophase of nitrobenzene B.A. Altoiza,*, T.V. Naroditskayaa, Yu.M. Popovskiib a Odessa National University, Dvoryanskaya 2, Odessa 65026, Ukraine Odessa State Maritime Academy, Didrikhsona 8, Odessa 65100, Ukraine
b
Abstract The influence of magnetic field on the molecular orientation type in epitropic liquid crystalline (ELC) phase of nitrobenzene was investigated by transmitted light ellipsometry method in gap optical lightguide with variable thickness. The outcomes of the experiment allow us to suggest the structure reorganization in wall-adjacent ELC layer of nitrobenzene, similar to Freederick’s transition in thermotropic LC. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Epitropic liquid crystalline (ELC) phase; Magnetic field action; Wall-adjacent layer; Nitrobenzene; Gap optical lightguide
An action of surface forces field induces an orientation ordering in wall-adjacent layers of series of non-mesogenic organic liquids with anisometric molecules on a surface of lyophilic solid substrate. Such layers with homogeneous properties and sharp boundary with the bulk are considered as special epitropic liquid crystalline phase (ELC). Earlier the optical and thermodynamic properties of such objects were investigated in a series of experiments w1,2x. The equilibrium thickness of ELC layers on dielectric surfaces (SiO2 , Al2O3 ) are approximately 50–60 nm, and on conducting surface it is much higher—approximately 3 mm. Typically anisometric (oblong) molecules of liquid in ELC phase have homeotropic type of orientation, but, as well as in thermotropic LC, microstructure of *Corresponding author. 0001-8686/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0001-8686Ž03.00044-7
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B.A. Altoiz et al. / Advances in Colloid and Interface Science 104 (2003) 239–243
substrate and the chemical properties of molecules can exert influence on the type of orientation in ELC layer. The purpose of our work was the investigation of magnetic field influence on the arrangement parameters in ELC phase of nitrobenzene in layers, formed on a conductive substrate, and to establish a possibility of structure reorientation, caused by field action, in such layers, similar to Freederick’s transition in thermotropic LC. In the preliminary experiment, it was established that molecules of nitrobenzene in ELC phase had homeotropic orientation (director L is perpendicular to a surface of substrate) on the conducting steel surface in absence of magnetic field. In the magnetic field (when B is perpendicular to a surface) it is expected that a transition of nitrobenzene molecules of ELC phase from homeotropic to planar orientation, because of superconductivity of collective p-electrons in the benzene ring w3x, and such orientation is energetically preferential. In order to organize the planar structure with given orientation the microrelief of the substrate was made by polishing, determining, as well as in LC, the easy axis of molecule orientation and, therefore, the direction of the main optical axis in ELC phase. For measurements of birefringence in ELC phase the method of transmitted light ellipsometry in the gap optical lightguide with variable thickness was used w4x. The measurements were carried out on the setup, shown in Fig. 1. The gap optical lightguide with metallical walls was placed in a diagonal position between crossed polarizer P and analyzer A and the extinction azimuth w was determined by rotation of the compensator C. The value of extinction azimuth allowed us to find the value of birefringence from expression: dsarctg2sin2w The calculation of birefringence Dnsn0yne was carried out in the approach of geometrical optics under the following scheme (see Fig. 2): The way of light in the ELC phase for single reflection: ls2d0 ysinqi. The total way: l*szØls2d0Lydcosqi, where z is a number of reflections. When q-58, l*f2d0Lyd. Phase shift on the lightguide outlet, caused by 2p birefringence in ELC phase: ds l*ØDNnM, where DNnMsNn0MyNneM. So: l ds
4pL 1 Žd0DNnM. l d
It is necessary to note, that linearly polarized light after the reflection from metallical surfaces transforms into the elliptically polarized one, that also gives the contribution to the measured value of d. Within the framework of geometrical optics the value of dref can be evaluated by the formula: drefsd0 yn2,
B.A. Altoiz et al. / Advances in Colloid and Interface Science 104 (2003) 239–243
241
Fig. 1. Scheme of setup for birefrigence measurement: (A) analyzer; (L) gap lightguide; (C) compensator; (P) polaryzer.
where d0 is the value obtained for the empty optical lighteguide, and n is the refractive index of medium, filling in the lightguide. The thicknesses of the optical lightguide were varied within the range of 5–50 mcm with accuracy "1 mcm, and the values of extinction azimuth w were measured with accuracy 0.2–0.58. The measurements for the empty optical lightguide in order to find d0 were previously carried out, then the lightguide was filled with nitrobenzene. To escape the meniscus effect the top and bottom ends of the lightguide were covered by flat glass plates and the measurements of extinction azimuth were carried out in the absence of a magnetic field. Then the optical lightguide was placed into magnetic field (Bf0.4 T), and the measurements were repeated. The shift of extinction azimuth values indicates the reorientation of molecules in the nitrobenzene wall-adjacent layers under the action of the magnetic field. An optical anisotropy of ELC layers of C6H5NO2, existing on the lateral metal surfaces of the lightguide were measured by this method for different thicknesses of liquid interlayer (5–40 mm). The calculation of an optical anisotropy–birefringence in the wall-adjacent layers of the lightguide was conducted under geometrical optics approaching w5x. The
242
B.A. Altoiz et al. / Advances in Colloid and Interface Science 104 (2003) 239–243
Fig. 2. The illustration for the scheme of birefringence calculation under the geometrical optics approach.
scheme of calculation of phase shift value d between orthogonally related light wave components (Ep and Es), elapsed through multilayer structure in the flat optical lightguide is shown in Fig. 2. Experimental results of an optical anisotropy of nitrobenzene in magnetic field in different experiment geometries are shown in Fig. 3. According to Fig. 3a, the dependence of birefringence on the reciprocal optical lightguide thickness (1yd) changes under the implication of field and it is different for different experimental geometry. It indicates the reorientation of nitrobenzene molecules in ELC phase under the action of magnetic field from homeotropic to planar structure. When the easy axis was directed parallel to light rays, the values of extinction azimuth w and d decreased (Fig. 3). It was because of the fact that the main optical axis was directed parallel to light raises in this case. However, because of insufficient orientation of nitrobenzene molecules, the value of birefringence did not decrease to zero. When the direction of easy axis was perpendicular to the light rays, the values of extinction azimuth w and the birefringence d, measured in a magnetic field increased
B.A. Altoiz et al. / Advances in Colloid and Interface Science 104 (2003) 239–243
243
Fig. 3. Experimental dependence of birefringence on the reciprocal lightguide thickness (1yd) in the magnetic field (1) and without it (2) for different experiment geometries: (a) easy axis is parallel to the light rays; (b) easy axis is perpendicular to the light rays.
(Fig. 3b) because the birefringence value is higher for planar orientation of molecules, than for the homeotropic one. References w1x B.A. Altoiz, Yu.M. Popovskii, Physics of Wall-Adjacent Layers, Astroprint, Odessa, 1995. w2x Derjagin B.V., Popovskii Yu.M., Altoiz B.A., Research of a Liquid–Crystal State Arising Under Action of Surface Forces, DAN USSR, 1982, Vol. 262, N4. w3x O.Ja. Naland, Organic Chemistry, Vyshaya shcola, Moscow, 1990. w4x Derjagin B.V., Popovskii Yu.M., Silenko G.P., DAN, 1978, Vol. 236. w5x B.V Derjaguin, B.A. Altoiz, I.I. Nikitenko, Epitropic liquid crystal layers of nonmesogens on qiartz substrate, J. Colloid Interface Sci. 145 (1991) 2.
Influence of magnetic field on the molecular orientation in epitropic mesophase of nitrobenzene B.A. Altoiza,*, T.V. Naroditskayaa, Yu.M. Popovskiib a Odessa National University, Dvoryanskaya 2, Odessa 65026, Ukraine Odessa State Maritime Academy, Didrikhsona 8, Odessa 65100, Ukraine
b
Abstract The influence of magnetic field on the molecular orientation type in epitropic liquid crystalline (ELC) phase of nitrobenzene was investigated by transmitted light ellipsometry method in gap optical lightguide with variable thickness. The outcomes of the experiment allow us to suggest the structure reorganization in wall-adjacent ELC layer of nitrobenzene, similar to Freederick’s transition in thermotropic LC. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Epitropic liquid crystalline (ELC) phase; Magnetic field action; Wall-adjacent layer; Nitrobenzene; Gap optical lightguide
An action of surface forces field induces an orientation ordering in wall-adjacent layers of series of non-mesogenic organic liquids with anisometric molecules on a surface of lyophilic solid substrate. Such layers with homogeneous properties and sharp boundary with the bulk are considered as special epitropic liquid crystalline phase (ELC). Earlier the optical and thermodynamic properties of such objects were investigated in a series of experiments w1,2x. The equilibrium thickness of ELC layers on dielectric surfaces (SiO2 , Al2O3 ) are approximately 50–60 nm, and on conducting surface it is much higher—approximately 3 mm. Typically anisometric (oblong) molecules of liquid in ELC phase have homeotropic type of orientation, but, as well as in thermotropic LC, microstructure of *Corresponding author. 0001-8686/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0001-8686Ž03.00044-7
240
B.A. Altoiz et al. / Advances in Colloid and Interface Science 104 (2003) 239–243
substrate and the chemical properties of molecules can exert influence on the type of orientation in ELC layer. The purpose of our work was the investigation of magnetic field influence on the arrangement parameters in ELC phase of nitrobenzene in layers, formed on a conductive substrate, and to establish a possibility of structure reorientation, caused by field action, in such layers, similar to Freederick’s transition in thermotropic LC. In the preliminary experiment, it was established that molecules of nitrobenzene in ELC phase had homeotropic orientation (director L is perpendicular to a surface of substrate) on the conducting steel surface in absence of magnetic field. In the magnetic field (when B is perpendicular to a surface) it is expected that a transition of nitrobenzene molecules of ELC phase from homeotropic to planar orientation, because of superconductivity of collective p-electrons in the benzene ring w3x, and such orientation is energetically preferential. In order to organize the planar structure with given orientation the microrelief of the substrate was made by polishing, determining, as well as in LC, the easy axis of molecule orientation and, therefore, the direction of the main optical axis in ELC phase. For measurements of birefringence in ELC phase the method of transmitted light ellipsometry in the gap optical lightguide with variable thickness was used w4x. The measurements were carried out on the setup, shown in Fig. 1. The gap optical lightguide with metallical walls was placed in a diagonal position between crossed polarizer P and analyzer A and the extinction azimuth w was determined by rotation of the compensator C. The value of extinction azimuth allowed us to find the value of birefringence from expression: dsarctg2sin2w The calculation of birefringence Dnsn0yne was carried out in the approach of geometrical optics under the following scheme (see Fig. 2): The way of light in the ELC phase for single reflection: ls2d0 ysinqi. The total way: l*szØls2d0Lydcosqi, where z is a number of reflections. When q-58, l*f2d0Lyd. Phase shift on the lightguide outlet, caused by 2p birefringence in ELC phase: ds l*ØDNnM, where DNnMsNn0MyNneM. So: l ds
4pL 1 Žd0DNnM. l d
It is necessary to note, that linearly polarized light after the reflection from metallical surfaces transforms into the elliptically polarized one, that also gives the contribution to the measured value of d. Within the framework of geometrical optics the value of dref can be evaluated by the formula: drefsd0 yn2,
B.A. Altoiz et al. / Advances in Colloid and Interface Science 104 (2003) 239–243
241
Fig. 1. Scheme of setup for birefrigence measurement: (A) analyzer; (L) gap lightguide; (C) compensator; (P) polaryzer.
where d0 is the value obtained for the empty optical lighteguide, and n is the refractive index of medium, filling in the lightguide. The thicknesses of the optical lightguide were varied within the range of 5–50 mcm with accuracy "1 mcm, and the values of extinction azimuth w were measured with accuracy 0.2–0.58. The measurements for the empty optical lightguide in order to find d0 were previously carried out, then the lightguide was filled with nitrobenzene. To escape the meniscus effect the top and bottom ends of the lightguide were covered by flat glass plates and the measurements of extinction azimuth were carried out in the absence of a magnetic field. Then the optical lightguide was placed into magnetic field (Bf0.4 T), and the measurements were repeated. The shift of extinction azimuth values indicates the reorientation of molecules in the nitrobenzene wall-adjacent layers under the action of the magnetic field. An optical anisotropy of ELC layers of C6H5NO2, existing on the lateral metal surfaces of the lightguide were measured by this method for different thicknesses of liquid interlayer (5–40 mm). The calculation of an optical anisotropy–birefringence in the wall-adjacent layers of the lightguide was conducted under geometrical optics approaching w5x. The
242
B.A. Altoiz et al. / Advances in Colloid and Interface Science 104 (2003) 239–243
Fig. 2. The illustration for the scheme of birefringence calculation under the geometrical optics approach.
scheme of calculation of phase shift value d between orthogonally related light wave components (Ep and Es), elapsed through multilayer structure in the flat optical lightguide is shown in Fig. 2. Experimental results of an optical anisotropy of nitrobenzene in magnetic field in different experiment geometries are shown in Fig. 3. According to Fig. 3a, the dependence of birefringence on the reciprocal optical lightguide thickness (1yd) changes under the implication of field and it is different for different experimental geometry. It indicates the reorientation of nitrobenzene molecules in ELC phase under the action of magnetic field from homeotropic to planar structure. When the easy axis was directed parallel to light rays, the values of extinction azimuth w and d decreased (Fig. 3). It was because of the fact that the main optical axis was directed parallel to light raises in this case. However, because of insufficient orientation of nitrobenzene molecules, the value of birefringence did not decrease to zero. When the direction of easy axis was perpendicular to the light rays, the values of extinction azimuth w and the birefringence d, measured in a magnetic field increased
B.A. Altoiz et al. / Advances in Colloid and Interface Science 104 (2003) 239–243
243
Fig. 3. Experimental dependence of birefringence on the reciprocal lightguide thickness (1yd) in the magnetic field (1) and without it (2) for different experiment geometries: (a) easy axis is parallel to the light rays; (b) easy axis is perpendicular to the light rays.
(Fig. 3b) because the birefringence value is higher for planar orientation of molecules, than for the homeotropic one. References w1x B.A. Altoiz, Yu.M. Popovskii, Physics of Wall-Adjacent Layers, Astroprint, Odessa, 1995. w2x Derjagin B.V., Popovskii Yu.M., Altoiz B.A., Research of a Liquid–Crystal State Arising Under Action of Surface Forces, DAN USSR, 1982, Vol. 262, N4. w3x O.Ja. Naland, Organic Chemistry, Vyshaya shcola, Moscow, 1990. w4x Derjagin B.V., Popovskii Yu.M., Silenko G.P., DAN, 1978, Vol. 236. w5x B.V Derjaguin, B.A. Altoiz, I.I. Nikitenko, Epitropic liquid crystal layers of nonmesogens on qiartz substrate, J. Colloid Interface Sci. 145 (1991) 2.