The Journal of
Physical Chemistry
0 Copyright, 1987, by the American Chemical Society
VOLUME 91, NUMBER (1 APRIL 9,1987
LETTERS Surface- Induced Homeotropic and Homogeneous Alignments of Lyomesophases of Cylindrical Micelles Tsuneo Yoshino* and Miyo Suzuki Basic Research Laboratories, Toray Industries Inc., Tebiro, Kamakura 248, Japan (Received: December 10, 1985: In Final Form: February 11, 1987) Lyomesophases of cylindrical micelles with water-outside and water-inside structures both can be aligned with their optic axes in the z direction perpendicular to the substrate surface or in a specified direction in the xy plane, depending on the surface structure. Alignment in the xy plane is not necessarily along the direction x of the parallel grooves on the substrate surface, but it is in the x or y direction depending on the structure of the grooved surface. Alignment in the .z direction is realized on a surface rough in both the x and y directions. Nematic lyomesophases of rodlike or cylindrical micelles, which are known as the precursor to the normal hexagonal phase consisting of cylindrical micelles with water-outside structure, are considered to tend to orient their optic axes in the xy plane parallel to the substrate surface under the influence of the surface.’-’ This is also supported by the observed persistence of an initial flow alignment with the optic axis in the x y plane and the observed collapse with time evolution of an initial magnetic alignment with the optic axis in the z d i r e ~ t i o n . Surface-induced ~~~ alignment in a specified direction, however, has not been reported on any lyomesophase of cylindrical micelles, in contrast to the vast amount of reports on lyomesophases and on surface-induced alignments of thermotropic me so phase^.^ We report here that the normal and the reversed (water-inside structure) lyomesophase of cylindrical micelles both can be aligned with their optic axes in the z direction (homeotropic alignment) or in a specified direction in the xy plane (homogeneous align(1) Holmes, M. C.; Boden, N.; Radley, K. Mol. Cryst. Liq. Cryst. 1983, 100, 93-102.
(2) Boden, N.; Radley, K.; Holmes, M. C. Mol. Phys. 1981,42,493-496. (3) Forrest, B. J.; Reeves, L. W. Chem. Rev. 1981, 81, 1-14. (4) Yu, L. J.; Saupe, A. J. Am. Chem. SOC.1980, 102, 4819-4883. (5) Kelker, H.; Hatz, R.Handbook ofLipuid Crystals; Verlag Chemie: Weinhelm, 1980; pp 512-555 and 605-625, and papers cited there.
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ment), depending on the condition of the substrate surface. The methods of realizing the alignments are rather simple and easily applicable to laboratory works and practical purposes, and the relations found here between the surface structures and the alignments may be usable to interpret phenomena concerned with a natural or an artificial micellar aggregate and a solid surface. In the present investigation, alignment of a lyomesophase was confirmed by orthoscopic and conoscopic measurement on a 0.1 -mm layer (unless otherwise mentioned) inserted between surface-treated glass plates, which was cooled from a temperature about 20 O C above the isotropic transition temperature. The surface structures of the substrates used were estimated from their scanning electron micrographs. The lyomesophases employed were as follows: [I] poly(ethy1ene glycol) monooleyl ether ( n = 6)/ waterln-dodecane in 1/0.25/0.39 weight ratio; reversed structure; transitions to and from the isotropic phase occur in the ranges of 38-43 and 40-39 O C , respectively; the texture changes in the range down to 36 O C ; [11] poly(ethy1ene glycol) mono-p-nonylphenyl ether ( n = lO)/water in 1/1 weight ratio; normal structure?’ [111] potassium oleate/water/ 1-decanol in 1/0.6/1.6 weight (6) Friberg, S.;Mandell, L.; Fontell, K. Acta Chem. Scand. 1969, 23, 1055-1057.
0 1987 American Chemical Society
2010 The Journal of Physical Chemistry, Vol. 91, No.8, 1987
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Figure 1. (a, b; left, middle) Orthoscopic micrographs of aligned 0.1-m layers of I between aluminum-coated glass surfaces (xy plane) rubbed with paper along x (horizontal direction in the micrographs) lightly (a) and repeatedly (b). Crossed polarizers with polarizations parallel to x and y were used. (c, right) Conoscopic photograph of a 0.1-mm layer of I between indium-tin oxide coated glass surfaces. The optic axes of (a), (b), and (c) are parallel toy, x, and z, respectively. The dim lines along x in (b) are due to groups of aluminum fine lines. The sides of (a) and (b) correspond to 0.6 and 0.35 mm, respectively.
ratio; reversed structure? These three lyomesophases are optically negative and their optic axes tend to lie along a smooth glass surface, as is usually expected for mesophases of cylindrical mice1les. Glass plates coated with an aluminum evaporated film (7 Q/o, 10-20 nm thick) were rubbed with paper in the x direction. Half of the aluminum film was erased to leave abrasions along the x direction. When a warm layer of I between the glass plates was left to cool (about 1 OC/min), a texture of parallel stripes of about 10 pm width appeared. The same cooling method was employed in the following experiments giving planar alignments. The stripes were found to be parallel to the rubbing direction x, and the optic axes of the stripes were found in they direction (Figure 1). The alignment found in this experiment was excellent, although slight color differences were discernible between the stripes by using a sensitive color plate. Color change along a single stripe was found to be much smaller than the color differences between neighboring stripes. Though not as excellent as that mentioned above, alignments of the stripes in the x direction and the optic axes in they direction were found in layers of I and 111 between glass surfaces rubbed with paper covered with polishing powder, and also in a layer of I between glass surfaces with photoetched parallel grooves, along the x direction, of 10 pm width, 10 pm separation, and 10 nm depth. We see, therefore, that the alignment with the optic axis in the y direction and the stripes in the x direction does not necessarily require an aluminum film nor rubbing an aluminum or a glass surface. The aluminum-coated glass plates stated above were now rubbed with paper many times in the x direction. The aluminum films were mostly removed from the glass surfaces. Fine lines of about 10 nm width and series of islands of about 100 nm in diameter, aligned in the x direction, were found on the surfaces. A layer of I between the glass plates showed excellent parallel stripes of about 10 pm width. The stripes were found to be parallel to they direction and the optic axes of the stripes were found in the rubbing direction x. These observations are different from those described before wherein the stripes and optic axes are in the x and y directions, respectively. Though not as excellent as the alignment given above, alignments with the optic axis in the x direction and the stripes in the y direction were found for a I layer between glass plates which had been coated with a silver evaporated film (10 nm thick) and rubbed with paper many times in the x direction, and also for a I1 layer between glass plates rubbed with paper strongly in the
x direction beforehand. Therefore, aluminum coating in advance is not necessarily required for alignment of this type, though it makes easier the appearance of a surface suitable for the alignment. The observations described above show that a lyomesophase of cylindrical micelles can be aligned with its optic axis and stripes in specified directions, at right angle to each other, in the xy plane. However, the direction of the optic axis, which is parallel to the long axes of the cylindrical micelles, does not necessarily agree with the direction x parallel to the grooves and ridges on the substrate surface. The optic axis is in the x or y direction but not at an intermediate angle between the x and y directions. This coincidence of the optic axis to the x or y direction shows that the alignment depends on the surface structure prepared by rubbing or photoetching. As for selection from the x and y directions, it may be said that on a substrate surface with numerous fine grooves and ridges parallel to the x direction, the optic axis and stripes are in the x and y directions, respectively, that on a surface with coarse grooves and ridges parallel to the x direction, the optic axis and stripes are in the y and x directions, respectively, and that on a surface with fine and coarse grooves and ridges parallel to the x direction, one of these alignments, which is superior to the other in stability, takes place. The selection described above agrees with the idea that micelles and stripes tend to form along the fine and coarse grooves (and/or ridges), respectively, which are similar in size to the micelles and stripes, respectively. In other words, the micelles and stripes tend to avoid to lie across the fine and coarse grooves mentioned above, respectively, and the micelles and stripes both tend to lie along the substrate surface. Let us suppose that the observed alignment is due to the additional energy of distortion of cylindrical micelles or stripes lying across the grooves and ridges, on the analogy of thermotropic liquid crystals.$" Then a hypothetical planar alignment on a surface rough in both the x and y directions will have additional energy contributed from distortion of the micelles and stripes lying across the hills and basins of the surface, compared with that on a smooth surface of the same material. The additional energy due to distortion of the micelles would be prominent, if the horizontal and the vertical sizes of the hills and basins are nearly the same as the width and the vertical sizes of the ridges and grooves which hinder the micelles to lie across. Vertical alignment of cylindrical micelles along the z direction on such a rough surface would require a slight deformation of the micellar ends where arrangement of amphiphilic molecules is
(7) Bronw, G. H.; Labes,M. M.Liquid Crystals 3, part I ; Gordon and Beach: London, 1972; pp 41 5-440. (8) Ekwall. P.; Mandell, L.; Fontell, K. Acta Chem. Scund. 1968, 22, 373-375.
(9) Berreman, D. W. Phys. Reo. Leu. 1972, 28, 1683-1686. (10) Kahn, F. J.; Taylor, G. N.; Schonhorn, H. Proc. IEEE 1973, 61, 823-829. ( 1 I ) Haller, 1. Appl. Phys. Lerr. 1974. 24, 349-351.
J. Phys. Chem. 1987,91, 2011-2015
surfaces.' Glass plates coated with a thicker indium tin oxide film (50 Q/n,60 nm thick), the surface structure of which is slightly different from that of a (In),were found to give a vertical alignment of I inferior to that given by the glass plates coated with a (In). On the other hand, when a thin gold evaporated film was superimposed on the (In),vertical alignment of I between these new surfaces was as excellent as that of I between the (In) surfaces. Since most of the (In)on the twice coated glass plate is considered to be covered with gold, despite of its small quantity, it seems that the vertical alignment depends on the structure rather than on the material of the surface. The vertical alignments on various rough surfaces and of different micellar phases found here seem to indicate that the alignments are primarily based on geometrical factors. This contrasts with the fact that thermotropic liquid crystals have not been vertically aligned by surface roughness.I2 The reasons why a glass surface coated with a (In) affords an excellent vertical alignment are probably to be found in such conditions of the hills and basins in the (In] as similarity in size, homogeneous distribution, sufficient steepness, and a planar size suitable for preventing micelles of the lyomesopases employed here to lie upon, although there is not sufficient evidence at present. A uniform alignment as those observed here may form through a transition to a hexagonal phase from a nematic or an isotropic phase, orientation in a nematic phase, or some combination of them, after nucleation of the alignment at the substrate surface. A nematic phase might be in a narrow temperature range between those of a hexagonal and an isotropic phase. Accordingly, it is difficult at present to specify the phase or phase boundary where the observed alignment took place.
originally singular even on a smooth surface. The additional energy due to the vertical alignment on a rough surface would be, therefore, much smaller than that of a hypothetical planar alignment on the same rough surface. Occurrence of the vertical alignment is considered to depend primarily on whether or not the additional energy difference between the planar and vertical alignments mentioned above is larger than the free energy difference between these two alignments on a smooth surface of the same material, which probably favors the planar alignment. Glass plates coated with an indium tin oxide evaporated film (250 Q/o,25 nm thick, 50-100 nm separation between hill centers; abbreviated as (In)) were assembled into a cell. A layer of I in the cell was warmed and then left to cool slowly (about 1 OC/h) in a thermos flask containing water to give an excellent vertical alignment. The same slow cooling method was adopted also in producing the vertical alignments described below. A more careful treatment was generally required to realize a vertical alignment compared with a planar alignment along a specified direction in the xy plane. Vertical alignments were also found in the cases of I1 and I11 in the same indium tin oxide cell, though less excellent than that mentioned above. Vertical alignment of I took place even when the spacing of the indium tin oxide cell was 0.5 mm. A 0.1-mm layer of I between glass plates with a CF4 plasmaetched surface was found to show a vertical alignment, though the alignment was not excellent. We see, therefore, that the vertical alignment does not necessarily require an indium tin oxide film. When the plasma-etched glass plate was further treated with dimethyldichlorosilane, the resulting water repellent surface gave a vertical alignment of I of equal grade to that given by the untreated plasma-etched surface. The independence of the vertical alignment from affinity to water of the substrate surfaces suggests that the vertical alignment is scarcely related with the chemical nature of the surfaces, and that either or both of the water repellent and nonrepellent surfaces might be covered by a layer or layers of amphiphile molecules reducing the difference between the
An EXAFS Study of the Co-Mo/y-Al,O,
2011
Acknowledgment. We thank Mr. H. Iguchi for taking the electron micrographs. (12) Berreman, D. W. Mol. Cryst. Liq. Cryst. 1973, 23, 215-231.
Hydrodesulfurizatlon Catalystt
G. Sankar, S. Vasudevan, and C. N. R. Rao* Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 01 2, India (Received: August 29, 1986; In Final Form: December 8, 1986)
While Mo in the Co-Mo/y-A1203 hydrodesulfurization catalyst is present as a sulfidic species similar to MoS2, Co shows two types of coordination, one with six sulfurs (but not a bulk sulfide) and the other with four oxygens. The significance of such species is discussed. In addition to an additive relation of the EXAFS function and the residual spectra, the ratio of amplitude terms of the catalyst and the model system has been employed in the analysis.
Introduction Cobalt-molybdenum catalysts are used extensively for the hydrodesulfurization (HDS) of crude feed stocks in petroleum industry. The catalyst generally has the nominal composition of 12-15% molybdenum oxide promoted by cobalt oxide (3-5%) and is usually supported on 7-A1203. Under reactor conditions, the catalyst is sulfided and it is the sulfided surface that is responsible for the catalytic In spite of several physical and chemical studies, the role of Co in the HDS catalyst remains ambiguous; none of the models proposed are capable of fully accounting for all the known fact^.^,^ Based on MBssbauer studies, Topme et aL4 have proposed that the activity of the catalysts +Contribution No. 377 from the Solid State and Structural Chemistry Unit. * T o whom correspondence should be addressed. .
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resides in a "Co-Mo-S" phase on the 7-A1203. The presence of such species has been supported by X-ray photoelectron spectroscopy (XPS) and other s t ~ d i e sbut , ~ the exact nature of the Co and Mo species has remained elusive. Extended X-ray absorption fine structure (EXAFS) studies6,' seem to indicate that Mo in the HDS catalyst is present as small crystallites of MoS2 (10-20 A), but the nature of the Co species has not been unra(1) Ratnaswamy, P.; Sivasankar, S. Catal. Reu. Sci. Eng. 1981, 22, 401. (2) Chianelli, R. R. Carol. Reu. Sci. Eng. 1984, 26, 361. (3) Topsore, H.; Clausen, B. S. Catal. Rev. Sci. Eng. 1984, 26, 395. (4) Candia, R.; Clausen, B. S.; Topsae, H. J . Catal. 1982, 77, 564. (5) Sankar, G.; Sarode, P. R.; Srinivasan, A.; Rao, C. N. R.; Vasudevan, S.; Thomas, J. M. Proc. Indian Acad. Sci., Chem. Sci. 1984, 93, 321. (6) Clausen, B. S.; Topsae, H.; Candia, R.; Villadsen, J.; Lengellar, B. J. Phys. Chem. 1981,85, 3868. (7) Parham, T. G.; Merill, R. P. J. Catal. 1984, 85, 295.
0 1987 American Chemical Society
