Langmuir 1987, 3, 1169-1170 Flotation 0-8 Diffraction. The flotation study was made by usingthe solution X-ray diffractometa, which measured diffracted
X-ray beams from particles at the liquid surface. The minerals are first floated by stirring in an air-supplied flotation beaker mounted in the diffractometer. Diffraction studies are then made on the stationary liquid surface to which the mineral has been floated. The beam is adjusted to give diffraction from the boundary layer between the air and the floated mineral. The crystallographic planes, which are parallel to the surface, are responsible for the diffraction pattem. This information can thus be used to interpret which cleavage planes are active in the flotation process. The principles for getting the orientation effect are described in Figure 2. Due to the ability of the X-ray beam to pass through matter, not only the surface but also the bulk structure is included in the X-ray spectra. If the particles were polycrystalline, then crystallographic planes not representing active cleavage planes would contribute. The background spectra due to the water structure are subtracted in Figure Id. Flotation of 10 mg of sphalerite powder (37-62 pm) was made in a 25-mL flotation beaker with a 1.0 mM solution of ethyl xanthate as collector. The pH value was adjusted to 6.0 with HCl/KOH. The results in Figure Id show that the (l,l,O) plane is almost totally dominant for floated sphalerite, although a microscopic study of the powder does not show high symmetry of the crystallites with dominant planes. The orientation effect can be observed by comparing the result from the flotation step shown in Figure Id with the diffraction pattem from the randomly oriented mineral powder shown in Figure IC.
Discussion The reported liquid diffraction method with in situ flotation provides a neat technique to measure how different minerals are orientated at the surface. (11) Scharizer, R.Z.Kristallogr. Mineral 1920,55, 440-443. (12) Von Georg, W. Phys. Z. 1920,21, 718-720. (13) Huggins, M. L.Am. J. Sci. 1923, 5,303-313. Fr.Mineral. Cristallogr. 1959,82,158-163. (14) Hartman, P.Bull. SOC.
1169
At this early stage the results cannot be used to decide if the surface orientation is an effect of chemical activation or simply an effect of the natural cleavage in sphalerite. Probably it is a combination of both effects. The surface can be naturally hydrophobic or hydrophobic due to a surface adsorption of the collector added. This uncertainty is partly due to the fact that some sulfide ores are hydrophobic by nature, which makes it difficult to isolate the chemical effects of the collector used. If differences in orientation effects are observed by varying the flotation conditions (e.g., by using different collectors), then information about selectivity of the binding of the collector to different planes may be obtained. This information can then be used as a starting point for modeling the chemical interaction between the collector and the preferred crystallographic planes of a mineral. All structural types more complicated than cubic have the advantage of exposing different cleavage planes of comparable size and frequency. This may be even more informative concerning the relationship between the collector and the surface structure. Flotation 6-0 diffraction supplies a simple tool for getting quick information about differences in mineral-collector affinities for different types of active groups in various collectors. If the colledor could be prepared to fit a specific surface, then, theoretically, a total selectivity between different minerals would be possible in flotation. Besides the described application, including mineral structure verification, this method will be used as an information base for surface structure studies on single crystals with reflectance FTIR spectroscopy and for collector adsorption modeling. Acknowledgment. This work forms part of a program financially and scientifically supported by the Swedish National Board for Technical Development.
Electron Microscopic Evidence for the Layered Structure of a Conducting Polypyrrole Langmuir-Blodgett Film Tomokazu Iyoda, Masanori Ando, Takehira Kaneko, Akira Ohtani, Takeo Shimidzu,* and Kenichi Honda Division of Molecular Engineering, Graduate School of Engineering, Kyoto University, Kyoto 606, Japan Received May 4, 1987. I n Final Form: July 22, 1987 A transmission electron micrograph of a section of a conducting polypyrrole Langmuir-Blodgett (LB) film prepared by the electropolymerization of the LB film of octadecyl4-methylpyrrole-3-carboxylate and octadecane in a 2:l ratio is presented. A clear image of the layered structure of conducting polypyrrole and insulating alkyl chain layers is observed over the entire cross section.
Introduction The Langmuir-Blodgett (LB) technique is widely accepted as a powerful means of preparing highly ordered molecular assemblies.12 There is much current emphasis on preparing highly integrated functional multilayers, manipulating them, and analyzing their structure. There has been much interest, in this connection, with thermal (1) Blodgett, K. B.; Langmuir, I. Phys. Reu. 1937,51,964. (2) Fukuda, K.;Sugi, M.; Sasabe, H. Langmuil-Blodgett Film and Electronics; CMC: Japan, 1986.
and photochemical polymerization of LB film^.^!^ Such polymerizations give considerable mechanical strength to the otherwise delicate LB f i i and have led to ow interest in topochemical polymerization. A goal has been to obtain highly anisotropic conducting polymer thin films, and we have shown that some amphiphilic pyrrole derivatives such as octadecyl 4-methylpyrrole-3-carboxylatecan be pre(3) Bubeck, C.; Weiss, K.; Tieke, B. Thin Solid Films 1983, 99, 103. (4) Fukuda, K.;Shibasaki, Y.; Nakahara, H. Thin Solid Films 1983, 99, 87.
0 1987 American Chemical Society
Letters
1170 Langmuir. Vol. 3,No. 6,1987
..
.:
Y
E Y
. I ,/I
i
?.
4
r-:
L-
Figure 1. Transmission electron micrograph of the crosn emtion of the polypyrrole LB film. pared as LB films and then polymerized by an electrochemical method.5 The unit is shown below.
The subject of thii letter is the structure of polymerized polypyrrole films, particularly as obtained by the electropolymerization method. Both diffraction experiments and electron microscopy are found to give structural information. X-ray diffraction experiments indicated a layered structure. The present investigation was undertaken to examine the dimensions of the ordered structure; for this electron microscopy is indispensable. Thus, many X-ray diffraction experiments with LB films, including those of our previous report? have suggested that the expected layered structure is present. Though the imperfect crosa section of the multifold ridge was reported! there has been no such success in direct observation of the structure of the cross section with an electron microscope in the present paper. We report here a direct observation of the layered structure in a crosa section of a conducting polypymole LB film, using transmission electron microscopy. The s u m of this technique should encourage the study of ways of manipulating molecular assemblies in LB films.
Experimental Seetion Spread monolayera were prepared by using a benzene solution of the amphiphilic pyrrole monomer ( I ) and &decane (2) with a 2:l mole ratio and a sum of concentrations of I and 2 of 5 X lCrJM. -decane makes the &decy1 group of the amphiphilic pyrrole monomer pack compactly in a monolayer. In a 2 1 ratio, the molecular packing of the hydrophobic part ofthe monomer is considered tn he closest from the *-A isotherm profile and the CPK model. The solution was spread on a pure water subphase at 22 'C and the spread film compressed. LR films were then prepared on an ITO-deposited polyester substrate (ITO-PE)? The mixed monolayers were transferred as Y-type films (120 (5) Iyoda. T.; Ando, M.; Kancko. T.: Ohtani. A,; Shimidm. T.; Honda. K. Tetrahedron Lprt. 1986,27,5633. (6) b a u d , k, Lcloup. J.; Maire. P.: Ruaudcl-TaWsr. A. Thin Solid Film 1985. 13.9. 133. (7) CELEC-K EC, Daisel Chem. Ind., Ltd. IT0 is indium-tin mids with 1W500 R I D ofthe e h n t re8ittanc-a.
dation of the monomeric LB film, a broad absorption band a p peared at around 1500 nm. characteristicof an ordinally doped layers) with a transfer ratio of 0.9-1.1 and at a film pressure of 35 mN/m and a dipping speed of 50 mm/min. The pyrrole monomer in the LB film so prepared on the I M P E substrate was electropolymerized in CH&N c o n ~ i n g LiClO, (0.1 M) under potenticatatic conditions (>1V va SCE); polymerization was verified spectroecopically. After electrooxipolypyrrole, and an ATR-IR absorption band at 780 cm-l disappeared. This last is assigned to C-H out-of-planebending at the 2- and 5-position of the pyrrole ring. These observations confirm that the resulting compound is a polypyrrole polymerized at the 2- and 5-position of the pyrrole ring. X-ray diffraction showed a layered Structure for the monomeric LB film, with a bilayer d-spacing of 64 A. This layered structure was preserved in the polymerized LB film, with a bilayer d-spacing of 66 A. These spacings are about 8 A smaller than the ones reported in our previous paper: where the transfer was at 30 mN/m f h pressure and a silated ITO-depositedglass substrate was used. The layered structure in the cross &ion of the polymerized LB film was directly observed with the use of a JEM-1200EX transmission electron microscope (JEOL, Ltd.). Specimens were prepared as follows. The polymerized LB film on the ITO-PE substrate was first treated with RuO, vapor for 30 min. and the stained LB film was then embedded in epoxy resin (12 h a t 60 "C). The embedded film, together with the ITO-PE substrate, was sectioned vertically to the plane of the film, using a diamond cutter. The direct magnification was 1M)ooOX. and the accelerating voltage was 120 kV.
Results and Discussion Figure 1shows one of our enlarged electron micrographs of the cross section of the polymerized LB film. All sections studied showed similar electron micrographs. We consider the dark regions to correspond to polypprole units r e n d with RuO,, while the light regions correspond to the alkyl chains. The striped pattem of dark and light lines thus demonstrates the alternating layered structure of the film. The spacing of the stripes shown in Figure 1corresponds to a bilayer d-spacing of 55-62 A, a value close to that obtained from the X-ray diffraction measurements. We believe that i t is the increased mechanical strength of the LB film, due to its polymerization, that allows us to obtain undistorted, clear images of the film structure at the molecular level. While the layered structure is observed over the entire depth of the sample, some dislocations are visible. One possibility is that these last are artifacts resulting from the embedding and/or sectioning processes! We d o believe, however, that the highly anisotropic de conductivity of the polymerized LB film, uI = l(r' and ul = l(r" S/cm?is a consequence of its layered structure.
Acknowledgment. We are grateful to Mr. Aita,JEOL, Ltd., for the electron microscopy. Also, this investigation was supported by a Grant-in-Aid from the Ministry of Education of Japan. Registry No. I. 106176-10-7; 2, 593-45-3. (E) The distortion mads the TEM picture ambiguous because the thiehaa of the apecimen wm W 5 0 0 A, which was much larger than the observed bilayer d-spacing. (9) Preliminarily, de conductivity of the polymerized LB film was measured (ref 5).
