Langmuir 1987,3, 851-853
I High Dispersion Rhenium (For Methyl Ether)
I Low
Dispersion Rhenium
(For Hydrocarbons)
0-TI?
I
(For Alcohols)
-:-a 0-TI-0 0
-TI - O
a
Figure 8. Schematic representation of the Re species present on the Re/TiO, catalysts and their role in CO hydrogenation. adsorption sites and the above catalytic rhenium species is not known. Due to the increasing sample optical density and thus decreasing signal-to-noise ratio with increasing Re loading, the behavior of these two weak CO absorption peaks as a function of Re loading was not obtained. Finally, the changes of the chain growth probability CY for the hydrocarbon products with the Re loading are noted (Figure 2). Such changes are commonly associated with the changes in the particle size of the dispersed metal catalyst.23 If this is also true for the present system, the step change of CY a t the Re loading of ca. 1 wt % may (23) Bartholomew, C. B.; Pannell, R. B.; Butler, 3. L. J. Catal. 1980, 65, 335.
851
indicate the drastic change in the particle size of the lowdispersion Re species. Then the steep increase in the formation rates of hydrocarbons and alcohols a t about 1 wt % Re loading found in Figure la,b suggests that the turnover frequencies for these products are also dependent on the particle size of the low-dispersion Re species. In conclusion, the above discussion is schematically exemplified in Figure 8. There are two Re species on the present Re/Ti02: the low-dispersion Re and the highdispersion Re. Over the high-dispersion Re methyl ether is formed, as well as stable rhenium tricarbonyl. The role of rhenium tricarbonyl in CO hydrogenation was not established. Over the low-dispersion Re hydrocarbons and alcohols are formed, while for alcohol formation partial reduction of titania to expose the Ti atom is necessary. With such a model, mixed reports on the formation of oxygenates in CO hydrogenation over dispersed rhenium may be understood. Apparently, for the production of alcohols a kind of additive, viz., oxophilic ions, is necessary. In certain cases, with appropriate support and/or reduction conditions, these oxophilic ions are formed on the rhenium surface or a t the interface of the rhenium aggregate and the support surface. When this happens, the catalytic system will produce oxygenates along with hydrocarbons, while if it does not, it produces only hydrocarbons. As for the methyl ether formation, it seems that very high dispersion of Re is essential. The formation of such species would also be dependent on the supports used as well as on the preparation and pretreatment conditions.
Letters Photographic Records of Ordered and Disordered Structures in Polymer Latex Dispersions Tsuyoshi Yoshiyama and Ikuo S. Sogami* Department of Physics, Kyoto Sangyo University, Kyoto 603, Japan Received October 2, 1986. I n Final Form: February 12, 1987 The Lang method devised in X-ray topography was applied to investigate the global internal structure of aqueous solutions of highly charged polymer particles. The bright and dark patterns recorded in the traverse photograph reveal the stable coexistence of the ordered and disordered states of particles in solutions at low ionic strength. It is pointed out that the observed stability of isolated crystallites and the reduction of interparticle distances in ordered mays lead necessarily to the existence of the weak long-range attraction, in addition to the strong short-range repulsion, between colloidal particles in dilute solutions. Aqueous suspensions of monodisperse charged particles are known to form crystalline orderings a t the low level of foreign salt.' Studies on such ordered structures were made intensively for polymer latex particles by the optical diffraction method2i3and by means of ultramicroscopes.H (1) Luck, W.; Klier, M.; Wesslau, H. Ber Bunseges. Phys. Chem. 1963, 67, 15. (2) Hiltner, P. A.; Krieger, I. M. J. Phys. Chem. 1967,73, 2386. Ackerson, B. J.; Clark, N. A. Phys. Rev. Lett. 1981,46, 123. Pieranski, P. Contemp. Phys. 1983,24, 25. (3) Yoshiyama, T.; Sogami, I.; Ise, N. Phys. Rev. Lett. 1984,53,2153. Yoshiyama, T.; Sogami, I. S. Phys. Rev. Lett. 1986, 56, 1609.
While the latter technique established by Hachisu4 is effective for direct observation of the local and instantaneous features of the array of solute particles, the former is suitable for investigation of the average aspects. Complementary applications of these methods provide substantially sufficient information on the structures of col(4)Kose, A.; Ozaki, M.; Takano, K.; Hachisu, S. J. Colloid Interface Sci. 1970, 44, 330. (5) Ise, N.; Okubo, T.; Sugiura, M.; Ito, K.; Nolte, H. J. J. Chem. Phys. 1983, 73, 536. (6) Ito, K.; Nakamura, H.; Ise, N.
J. Chem. Phys.
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1986, 85, 6136.
852 Langmuir,Vol. 3, No. 5,1987
Letters
loser beam
lens
-cuvette
film
Figure 1. Schematic top view of the transmission arrangement of the Lang experiment. SIand S, are, respectively, the entrance and receiving slits. The specimen and the fh are synchronously
moved in parallel.
loidal erystals. A newly devised experiment must be used to invMtigate the more global charaderisticsof suspensions such as the coexistent distributions of ordered and disordered states of solute particles. In this paper we report an application of the Lang method' developed in X-ray topography that enables us to take photographic records of the order and disorder structurea in colloidal dispersions. Well-deionized solutions of monodisperse S03H-conW i n g polystyrene particles (SS32 diameter 1560 A and surface charge -3 X 10' e)LS were introduced into thin quartz cuvettes of dimensions 1 X 10 X 40 mm and remained motionless for several months until colloidal crystallites grew into large grains. Only those solutions in which high transparency was reached were used as the specimens to observe the global structures of particle distributions inside the solution. Using Ar (4880A) and H e N e (6238 A) laser beams and the Lang camera (Rigaku Denki type) devised for X-ray topographs, we have photographed the bright field images of the specimens. The schematic top view of the apparatus is shown in Figure 1. Parallel laser beams enlarged by a convex optical lens are incident on an entrance line slit SI with 30-mm height and 0.3-mm width. A receiving line slit S2with the Same opening as S, is placed just behind the cuvette so that only the beams transmitted straight through the solution are allowed to reach the film. The cuvette and the film are moved slowly and synchronously over a 7-mm range in parallel during the exposure while the thin collimated beams are incident normally on the cuvette surface. The laser beams entering into the disordered regions of colloidal particles are subject strongly to random scattering, which diminishes the light transmitted to the receiving slit S2. On the other hand, the beams incident to the ordered regions are less dispersed in the solution and pass through the slit S2without sizable loss of intensity. Consequently, the relative difference of scattering powers gives rise to the observable contrast between the traverse pattems for the ordered and disordered regions. Figure 2 shows an example of the traverse photograph thus taken by Ar beams for the specimen of 5.0 vol % particle concentration. Bright patchy patterns in this picture prove that the solution is filled with a large number of crystal grains with sue distribution ranging up t o a few millimeters. Dark hazy lines and cracks represent, respectively, the grain boundaries and the disordered regions where particles are either liquidlike or glassy. Figure 3 is a photograph of the backward diffraction taken from the same specimen. Such a clear Kossel image of diffraction was obtained only when the fine laser beams were pinpointed just at the spot on the specimen corresponding to ( I )JAW, A. R J. Appl. Phys. 196%,30,1148 h f f , L.V. Elements of X-my Crptollopzphy; McGrsw-Hill: New York, 1968.
5.0" Figure 2. Traverse photograph by Ar beams recording the ordered and disordered distributions of latex particles in the solution with 5.0 vol R particle concentration.
Fwre 3. Backward Kwsel image taken hy pinpointing colloidal crydites with the tine laser beams. The specimen waq the same that was uned fur Figure 2. The Kossel pattern shows the crystal has the fcc twin Xtriwture
the bright region in the traverse pinup, and no diffraction signal was gained from the spot corresponding to its dark region. These evidences confirm our interpretation that the bright and dark pattems in the traverse picture are the direct photographic records of the ordered and disordered structures in polymer latex dispersions. From the Kossel pattern in Figure 3 it is found that the crystal has fcc twin structure, and, after correction for the refractive index, the lattice constant is determined to be 4697 A. Notice that, if the latex particles fill the solution uniformly with a crystalline distribution of fcc type, the lattice constant is calculated to be 5413 A for the 5.0 vol % specimen. The observed value is smaller than the calculated value by 13%. The contraction of colloidal crystals of this kind in dilute and semidilute solutions was first discovered by Ise et al."" through a detailed inspection of microscopic data and confirmed later by the systematic analysis of the K o s s ~ lines? l This effect, which is concealed at high particle concentrations, is confirmed to be substantially enhanced at low particle concentrations! The reduction of interparticle spacings in ordered arrays, which must be compensated by an increme of mean interparticle distance in disordered regions, requires the existence of
Langmuir 1987, 3, 853-855 a weak long-range attraction in addition to the strong short-range repulsion between colloidal particles. The traverse pictures of the specimen that remained without disturbance over several weeks did not exhibit any detectable change from Figure 2. From this observation the semidilute solutions of highly charged particles are found to form the comparatively stable spatial structures of ordered and disordered states, the existence of which was demonstrated also by the cross correlation data of Clark et aL8 and by direct observation by means of ult r a m i c r o s ~ o p e ~As . ~ is widely accepted,l0 the ordering formation in the concentrated suspensions is well described by the Alder-Wainwright transition'l in which the interparticle repulsion and the wall effect are premised only. However, it is not possible for the repulsion-only assumption to explain the stable existence of isolated crys(8)Clark, N. A.; Ackerson, B. J.; Hurd, A. J. Phys. Rev. Lett. 1983,50, 1459. (9)be, N Okubo, T.; Ito, K.; Dosho, S.; Sogami, 1. Langmuir 1985,1, 176. JDn. 1972.32. 1147. (10)Wadati. M.:Toda. M. J.Phvs. SOC. (11) Alder, B. J.; Wainwright, T."W. J. Chkm. Phys. 1957,27,1208; 1959,31,459.
a53
tallites and the reduction of their lattice constants in dilute and semidilute solutions of highly charged particles. Some kind of cohesive force must exist between colloidal particles to give rise to these phenomena. It is not reasonable, however, to ascribe this cohesion mechanism exclusively to the van der Waals attraction k la DLV0,12because the DLVO theory fails to describe the melting of colloidal crystals by the added salt.13 Therefore, the results of the present experiment require us to reexamine the characteristics of interactions between highly charged particles in dilute s01utions.l~ Acknowledgment. This work was supported by financial aid from the Ministery of Education, Science, and Culture. We are grateful to Prof. Norio Ise for encouragement and discussion. (12)Derjaguin, B. V. Trans. Faraday SOC. 1940,36,203.Verwey, E.
J. W.; Overbeek, J. Th. G . Theory of the Stability of Lyophobic Colloid; Elsevier: Amsterdam, 1952. (13)Hachieu, S.;Kobayashi, Y.; Kose, A. J. Colloid Interface Sci. 1973.42. 342. (14)Sogami, I. Phys. Lett. 1983,A%, 199. Sogami, I.; Ise, N. J. Chem. Phys. 1984,81,6320.
Twisted Double-Layer Ribbons and the Mechanism for Monolayer Collapse Herman E. Ries, Jr.,* and Hewson Swift Department of Molecular Genetics and Cell Biology, The University of Chicago, Chicago, Illinois 60637 Received March 12, 1987 Electron microscope examination of collapsed monolayers of isoleucine-gramicidin B, a membrane channel-formingantibiotic, reveals twisted and folded double-layer ribbons detached from the underlying monolayer. This observation and related studies support a mechanism for monolayer collapse in which tall ridges form, bend, and break to give long narrow platelets, or ribbons, two molecules thick. Electron micrographs of a collapsing film of cerebronic acid and a collapsed film of cholesterol provide additional support for this mechanism. Introduction Because of the steadily growing interest in monolayer and bilayer structure and function in physics, chemistry, and biology, the mechanism for thin-film collapse is the focus of increasing attention.'-12 For many years the (1)Ries, H. E., Jr.; Kimball, W. A. Proceedings of the 2nd International Congress on Surface Activity; Butterworths: London, 1957;Vol. 1, p 75. (2)Neuman, R. D. J. Colloid Interface Sci. 1976,56,505. (3)Nakagaki, M.; Handa, T. Bull. Chem. Soc. Jpn. 1976,49, 880. (4)Gebrielli, G.; Guerni, G. G. T.; Bastianini, F. J.Colloid Interface Sci. 1979,69,352. (5)Takenaka, T.; Fukazaki, H. J. Raman Spectrosc. 1979,8, 151. (6)Smith, R. D.;Berg, J. C. J. Colloid Interface Sci. 1980, 74,273. (7)Ries, H. E., Jr.; Swift, H. J. Colloid Interface Sci. 1982,89,245. (8) Fischer, A.; Sackmann, E. J.Phys. (Les Ulis, Fr.) 1984,45, 517. Tamm, L. K.; Weis, R. M. Proc. Natl. Acad. Sei. (9)McConnell, H. M.; U.S.A. 1984,81,3249. (10) Ries, H. E., Jr.; Albrecht, G.; Ter-Minassian-Saraga, L. Langmuir 1985,1, 135. (11)Abraham, B. M.; Ketterson, J. B. Langmuir 1985,1, 461.
simplified schematic drawing of Figure lA'J3 has been used and widely accepted14J6as the mechanism for monolayer collapse. Recently two questions have been raised about details of this picture.16 These questions concern (a) the nature of the upper termination of the folded and bent structures and (b) whether the collapsed structures remain attached to the monolayer or break off to give independent ribbons or platelets. Experimental Section As in earlier studies, films were transferred under controlled conditions f r o m the water surface of the film balance t o Formvar-coated electron microscope grids by a modified Langmuir(12)Fereshtehkhou S,;Neuman, R. D.; Ovalle, R. J. Colloid Interface Sci. 1986,109,385. (13)Ries, H. E., Jr. Nature (London) 1979.281,287. (14)Gaines, G. L.,Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience: New York, 1966;p 149. (15)Adamson, A. W. Physical Chemistry of Surfaces, 4th ed.; Wiley: New York, 1982;p 134. (16)de Gennes, P.G., private communication, November 13, 1986.
0743-7463/87/2403-0853$01.50/00 1987 American Chemical Society
