Langmuir 1985,1,135-137 Figure 6. These ideas are speculative but seem consistent with some recent displacement experiments wherein SERS signals from adsorbed citrate were seen to decrease after addition of pyridine and TTF to colloidal suspensions previously aggregated with sodium perchlorate.20 No such decrease was seen when TCNQ was added to the sol, although strong SERS signals characteristic of TCNQ'.Owere readily visible. Surface Charge-Transfer Reaction. The roughly linear regions in the kinetic data of Figure 5 suggest a process whereby converts to TTF1.O+,rather than independent adsorption or desorption of the TTF species. Such a surface charge-transfer reaction could be rationalized by assuming that TTF migrates or diffuses from Lewis base sites (adsorbed as 0.3+ state) to Lewis acid sites (as 1.0+ state). Experiments using electron donors related to TTF offer some support for this mechanism.20 DBTTF likewise adsorbs onto gold in two oxidations states but displays no time-dependent conversion to the 1+surface oxidation state. Also, when TCNQ was added to the colloidal suspension, TCNQ'.& bands appear as before but we observed no concurrent change in the relative intensity of the electron donor bands, as found with TCNQ addition after TTF adsorption. TMTSF adsorbs onto the colloid in only the 1+ oxidation state, and agains subsequent addition of TCNQ gives TCNQl- but no change in the 1+ population. We attribute this behavior to the larger molecular size of these two TTF analogues.21 Being larger they have somewhat lower ionization potentials and are more polarizable, so adsorb more strongly than TTF.This makes it more difficult to displace them by other adsorbates and slows down the migration process from Lewis base to Lewis acid sites. It seems likely that the rate-limiting step in the formation of TTF1.O+from TTF0.3+is the displacement of the latter from the surface. If we assume an Arrhenius preexponential factor of 1013s-l, the observed first-order (20) Sandroff, C. J.; Weitz, D. A.; Lin, M. Y.,unpublished results.
(21)Both compounds studied, dibenzyltetrathiafulvalene(DBTTF) and tetramethyltetraaelanafulvalene (TMTSF), gave good SERS spectra. The most intense band in the SERS spectrum of the latter was at 1391 cm-*, characteristic of TMTSF'.@+. The two bands observed for DBlTF in the 500-cm-' region were similar to those of TTF and ita tetramethyl analogue. For spectra, see: Menenghetti, M.; Bozio, R.; Zanon, I.; Pecile, C.; Ricotta, C.; Zanetti, M. J. Chem.Phys. 1984,80, 6210.
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decay of TTF0.3+implies a desorption barrier of 23 kcal/mol. This number is in rough accord with the 28 kcal/mol barrier inferred from recent studies of organic disulfides on gold.22 In Figure 3, we noted that after the addition of TCNQ, the TTF1.O+signals increased at the same time that TTF0.3+signals decreased. This is the expected behavior if Lewis acid and base sites exist near one another on the surface in roughly comparable proportions. Thus, as soon as TTF is displaced from the Lewis base site to which it is bound as TTF0.3+, it can rapidly readsorb to a Lewis acid site where it becomes fully oxidized. This readsorption process is probably facilitated by the low solubility of TTF in the aqueous medium surrounding the gold colloidal particle. In conclusion, we find that recognition of the Lewis acid-base character of Au+/Auo adsorption sites on gold colloidal surfaces provides reasonable interpretations for several experiments involving electron donor or acceptor molecules as adsorbates. Charge-transfer compounds such as T T F and TCNQ should find useful applications in studies of other complex interfacial environments, where detailed knowledge of the electronic nature of a metal surface is difficult to obtain. The ability to place charge at a colloidal surface should be especially useful in gaining insight into the nature of colloidal particle interactions. This could lead to a better understanding of the mechanism of colloidal aggregation and offers the possibility of connecting colloidal surface chemistry to flocculation kinetics and the fractal structure of colloidal aggregates.20Bp24 Finally, the fact that chemically diverse sites can give rise to SERS should be taken into account in any theoretical treatments of the mechanism of surface-enhanced Raman scattering.
Acknowledgment. We thank L. V. Chiang for providing us with samples of DBTTF and TMTSF and J. A. Creighton for educating us about pyridine complexes. We benefited enormously from conversions with R. G. Nuzzo and his critical reading of the manuscript. Registry No. Au, 7440-57-5;TCNQ, 1518-16-7;TTF, 3136625-3; pyridine, 110-86-1. (22)Nuzzo, R. G.;Allara, D. L. unpublished results. (23)Weitz, D. A.; Oliviera, M. Phys. Rev. Lett. 1984,52, 1433. (24)Weitz, D. A.;Huang, J. S. unpublished results.
Collapsed Monolayers of Egg Lecithin Herman E. Ries, Jr.,*t Genevisve Albrecht, and Lisbeth Ter-Minassian-Saraga* Physico-Chimie des Surfaces et des Membranes, Equipe de Recherche du CNRS associCe ci 1'UniversitC Paris V , UER BiomCdicale, 75270 Paris Cedex 06, France Received September 12, 1984. I n Final Form: October 22, 1984 Electron microscope examinationof collapsed monolayers of egg lecithin has revealed many large flattened oblate spheroids as well as elongated monolayer islandlike structures and multilayer disks. The spheroids and elongated island structures are in general oriented normal to the direction of compression. Superposed spheroids on island structures have also been observed. Such collapse structures are strikingly different from the ridges, ribbons, platelets, and fibers of earlier studies of collapsed solid-type monolayers. The liquid nature of the collapsing film of egg lecithin is thus assessed by electron microscopy.
Introduction Little is known in detail about the thin-film structure of lipids in membranes that control many biological prot Present address: Department of Biology, The University of Chicago, Chicago, IL 60637.
cesses. Egg lecithin represents the unsaturated lipid components of biomembranes, and monolayers are the ultimate thin-film models of such membranes. Electron microscope studies of collapsed monolayers of egg lecithin have now revealed structures entirely different from those formed by related materials. Large flattened oblate
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136 Langmuir, Vol. 1. No. 1. I985
Figure 1. Electron micrograph of a collapsed monolayer of egg
lecithin showing flattened oblate spheroids and monolayer isindicatesthe directionof shad. landlike fragments.The ow-casting and shadows appear as light areas. The scale bar is 1 Irm.
spheroids are observed in the micrographs of the collapsing f h of egg lecithin whereas heretofore long ridges, narrow ribbons, flat platelets, and fiberlike structures have been found with collapsing solid-type monolayers of lipids and related compounds.14 The fluid nature of the egg lecithin film is thus clearly indicated. Experimental Section The egg lecithin used was high-purity Grade 1 of Lipid products. Pressure-areaand tranafer experiments were performed on a Lauda automated film halance of the horizontal-floattype. The substrate water was triply distilled, once from KMnO,. Merck spectroscopic grade cyclohexane was the volatile solvent used for the dilute solutions of lecithin. Spreading was performed with an all-glass Agla microsyringe. A modified LangmuirBlodgett technique was used to transfer samples from the water surface to Parlodion-coated electron microscope grid@ at 24-25 "C. Transfer was performed at 44 mN m-' and 0.45 nm2 per molecule; collapse began at 43 mN m-' and 0.53 nmz per molecule. A t collapse the grid plate was raised at a rate of 14 cm2per min. The transferred samples were shadow-cast with platinum-palladium at an angle of 1l0 and in the direction of film compression." The shadow-cast films were then examined in a JEM 100 CX I1 electron microscope.
Results and Discussion Pressure-area isotherms for egg lecithin have been reported earlier!.? The isotherm bends over gradually t o (I) Ries, H.E..Jr.; Walker, D. C. J . Colloid Sei. 1961, 16, 361. (2) Ries. H. E., Jr.; Matsumoto, M.; Uyeda, N.; Suito. E. In "Monolayers":Gcddard. E. D.,Ed.; American Chemical Society: Washington, DC, 1975: p 286. (3) Ries. H. E.. Jr.; Swift, H.J. Colloid Interface Sei. 1978,64,111. (4) Ries. H.E.. Jr. Notum (London1 1979.281.287. (5)Blodgett, K. B.; Langmuir, I.,Phys.Reo. 1937,51,964 ( 6 ) de Bernard. L. Bull. Soe. Chtm. B i d . 1958, 40, 161.
Ries, Albrecht, and Ter-Minassian-Saraga
Figure 2. Electron micrograph of a colbsed monolayer of egg lecithin showing some disklike structures. The arrow indicates the dimtion of shadow-casting and shadows appear as light =e=. The scale bar is 1 pm. give a collapse plateau at 4 3 4 5 mN m-'. A bending over of this type without a sharp falloff in pressure is generally considered a liquid-type collapse.' A collapse pressure equal to the equilibrium spreading pressure7 is another criterion. Representative collapse structures of various sizes and shapes are seen in the micrographs of Figures 1 and 2. Most remarkable and unique are the structures that approach a flattened oblate-spheroid geometry (Figure 1). These are oriented normal to the direction of compression (direction of shadow-casting)as are the ridges, ribbons, and fibers of earlier This orientation provides supporting evidence that these structures are formed during compression. Dimensions of the collapsed structures vary considerably: length 100-500 nm, width 60-300 nm. Many of these structures are somewhat flattened and have thickness values of 2-20 nm. The latter are calculated directly from shadow widths and the angle of shadowcasting. Visible also in Figure 1are elongated islandlike structures oriented normal to the direction of compression. These are irregular in size and shape and appear to be 2 nm, or monomolecular, in thickness. They apparently rest on a monolayer that covers the surface quite homogeneously. Such monomolecular island fragments are probably forced up onto the continuous monolayer during the collapse process and form bimolecular thick regions. Some of the flattened oblate spheroids of Figure 1are contiguous t o or rest on these island structures. Others approach a more circular or multilayered disk geometry as seen in Figure 2. Close examination of some of the disks reveals an edge structure, 4-6 nm thick, with surface microasperities about 2 nm high. As with the island structures, the disks and the flattened oblate spheroids are undoubtedly formed from the monolayer during collapse. (7) Hendrikx, Y. J. Chim. Phys. 1973. 70, 1727.
Langmuir 1985, 1, 137-141 On the water surface, the egg lecithin in all these structures is fully hydrated and in the liauid-crystalline phase.8 The dehydration which takes piace during the high-vacuum degassing that precedes the shadow-casting may account for some of these observations. Electron microscopy has thus disclosed unusual structures in the collapsed film of egg lecithin. Studies of this (8) Reiss-Husson, F.; Luzzati, V. Adv. Biol. Med. Phys. 1967,11,87.
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type should ultimately shed light on some aspects of biomembrane structure.
Acknowledgment. We thank M.-F. Alfonsi, N. Marchi, J. M. Le Pecq, and A. Terquem for valuable technical assistance and the Electron Microscopy Department of the University of Paris V for the use of their shadow-casting apparatus. We are also grateful to the “Fondation Dour la Recherche MBdicale”f o r support of this cooperative research.
Potential Functions for Diffusive Motion in Carbon Molecular Sieves M. B. Rao and R. G. Jenkins* Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802
W.A. Steele Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802 Received August 28, 1984 Gas-solid interaction potentials are calculated for a number of simple gases within the micropores of carbon molecular sieves. Effects of nonspherical molecular shape and the periodically varying potential which produces barriers to surface diffusion are both explicitly included. The critical dimensions for the onset of activated diffusion in a slit-shaped pore are determined and compared with experiment for each gaseous molecule. Barriers for surface diffusion in the (1011) and (1121) crystallographic directions are estimated. The role of the energy barrier to entry into a micropore in determining diffusion rates is discussed, and it is shown that this becomes the critical factor when the pore size is smaller than the critical dimension for a given gas.
Introduction Molecular sieve materials are used in large-scale industrial processes for gas separation. These materials have pores of molecular dimensions that give rise to selective adsorption. The selectivity is a result of the different rates of activated diffusion into the pore structure. It is believed that the diffusing molecule experiences a net repulsive interaction upon entering these very narrow pores and thus must pass over an energy barrier to gain admittance to the adsorption volume. The differences in magnitudes of these energy barriers for various species gives rise to different diffusion rates and thus causes separation. A fundamentally important parameter in the study of molecular sieves for separation processes is the critical pore dimension below which diffusion for a particular species becomes activated. This pore dimension is defined to be that for which the minimum potential for a diffusing species in the pore is zero. Previous attempb at calculating this critical pore size from the interaction potentials have been limited to simple gases (He, Ar, Ne, Kr);l estimates of the critical pore dimensions for more complex molecules have not been considered. In this paper, the critical pore dimensions and the diffusion activation energies are calculated for a number of nonspherical molecules in carbon molecular sieves. The micropores of porous carbon materials are generally considered to be slit shaped. Thus, kinetic measurements
of the adsorption of organic solvents show that planar molecules have easier access to these pores than spherical molecules.2 Furthermore, measurements of the heat of adsorption of various gases seem to agree with a slit shaped rather than cylindrical pore modeL3s4 Walker and coworkers’ have calculated the interaction potential for noble gases passing between two parallel graphite basal planes and found that diffusion becomes activated when the spacing between the pore walls becomes less than the sum of the kinetic diameter of the diffusing species and 1.6 A. They reasoned that this factor of 1.6 8, arises from the n-clouds emanating from the carbon basal plane; thus each basal plane contributes 0.8 A. The difficulties in using this technique to estimate the critical pore dimension are 2fold. First, the value for the thickness of the r-clouds of the basal planes need not be the same as that for nonnoble gases. Second, estimates of the kinetic diameter of nonspherical diffusing species obtained from the gas are generally not applicable to the calculation of the critical pore dimension. The kinetic diameter in the gas phase is proportional to the cube root of the effective volume of the molecule when it rotates freely in three dimensions, whereas diffusion of nonspherical molecules through narrow pores involves a t least some loss of rotational freedom since it is likely that the molecule enters the pore along its minimum dimension. The kinetic diameter as determined from gas-phase data is, therefore, unsatisfac-
(1) Walker, P. L., Jr.; Austin, L. G.; Nandi, S. P. In ‘Chemistry and Physics of Carbon”;Marcel Dekker: New York, 1966, Vol. 2, pp 257-371.
(2) Dacey, J. R.; Thomas, D. G. Trans. Faraday SOC.1954, 50, 740. (3) Perret, A.; Stoeckli, F. Helu. Chim. Acta 1975, 58, 2318. (4) Stoeckli, F. Helv. Chim.Acta 1974,57, 2192.
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