Langmuir 1986,2, 599-602 to 220.24 These observations do not indicate that all or nearly all ions have a cubiclike or some other special shape but only that there is some peaking in the distributions a t several values of n. Some evidence exists, which indicates that alkali halide aerosol particles formed from compressed-air nebulizers are more or less rectangular in shape.8 We have studied the CsN03 aerosol particles from our aerosol generator in the size range 75-300 nm by electron microscopy and have found only spherical particles. 6. Conclusions The results found for interaction between alkali salt particles in aerosol form and a hot surface show that each particle partially melts on the surface and that the adsorbed (melted) alkali salt gives alkali ions via surface ionization on the hot surface. The signal current varies (24) Katakuse, I.; Nakabushi, H.; Ichihara, T.; Sakurai, T.; Matsuo, T.; Matsuda, H. Int. J. Mass Spectrom. Ion Processes 1984, 57, 239.
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slower than linearly with solution concentration (particle size). At moderate voltages, 100-500 V, the signal current passes through a maximum and then decreases almost linearly with applied voltage. At low voltages, the signal is independent of particle size and varies as the square of the applied voltage. This last variation is interpreted as being due to the diffusion and collection processes of the charged aerosol. A publication in preparation interprets the other experimental results more in detail.lg
Acknowledgment. We thank Paivi Paananen for performing most of the final measurements. Our thanks also go to G. Wahlberg and G. Dunlop a t the Physics Department, Chalmers University of Technology, for help with the electron microscopy study. The work presented here is part of the results of a project supported by the National Swedish Board for Technical Development (STU), No. 80-3726. The project was also partly supported by the Swedish Natural Science Research Council. Registry No. &NO3, 7789-18-6; NaCl, 7647-14-5.
Exploitation of Lateral Effects: The Possibility of Induced Surface Ensembles and Template Formation Steven S. Miller U.S. Department of Energy, Morgantown Energy Technology Center, Morgantown, West Virginia 26507-0880 Received February 13, 1986. I n Final Form: May 27, 1986 The possibility of inducing reactive surface ensembles via the exploitation of lateral effects is introduced. Implications of ligand-exchange kinetics for mononuclear systems are introduced in the context of nearest-neighboreffects on catglytic metal surfaces, and recent work concerning the observation of these types of interactions is cited. A dynamic model of the hypothesis was tested by using Monte Carlo methods. The result of this modeling indicates that relatively small lateral effects on nearest-neighbor sites will significantly influence the configuration of surface adsorbates. It is inferred that preconfiguration of the adsorbates will exert a major influence on product distributions. The use of catalytic processes in chemical reactions serves several purposes. The most basic of these purposes is the lowering of barriers to a reaction (activation energy) so as to allow the reaction to proceed under relatively mild conditions. It follows from this initial statement that further advantage may be obtained from a catalytic process by lowering the activation barrier to a particular reaction pathway. This second mode of operation can be interpreted as a shift in the dynamics of the approach to equilibrium and could allow one to increase the yield of a specific set of products which are kinetically disfavored under “normal” process (noncatalytic) conditions. Once a catalytic system is implemented, it may be fine-tuned by the use of promoters and inhibitors.’ These adspecies, which increase of decrease the rate of reaction, are commonly used in heterogeneous systems to further modify product distributions in a favorable manner. Of particular interest to this author are those inhibitor systems which achieve their effects by the blocking of reactive sites critical to the formation of the precursor ensembles for certain reaction pathways. If the precursor complex cannot form, one has effectively eliminated a (1) Norskov, J. K.; Holloway, S; Lang, N. D. Surf. Sci. 1984, 137,
65-78. (2) Dwyer, D. J.; Hardenbergh,J. H. Appl. Surf. Sci. 1984 29, 14-27.
wmteful side reaction. Promoters may sometimes be used to recover reactivity lost through electronic and steric interactions of the inhibitor. The activity of an inhibitor, though desirable, is a poisoning effect. Perhaps it could be possible to manifest both of these effects, inhibition and promotion, in a single species. For the most part, the effects of promoters and poisons are described as long The effect of these species on adsorption of the reactants may be likened to a modification of the surface-reactant bond order. Secondary effects on the adsorbates (usually) include complementary modification of the bond orders within the adsorbate itself, if it is a polynuclear species. A commonly cited example is the attenuation of the C-0 bond weakening of CO adsorbed on a metal surface with decreasing ability of the surface to back-bond to the destabilizing a* orbital^.^ Such an effect is commonly observed for sulfided metal surfaces. The reverse effect can be seen by using alkali metal promoters to add electron density to the catalyst’s surface.lv4 These effects are generally global in nature but need not be so. Recent experimental and theoretical (3) Goodman, D. W . Appl. Surf. Sci. 1984,19, 1-13. (4) Houston, J. E.; Rogers, J. W., Jr.; Goodman, D. W.; Belton, D. N. J. Vac. Sci. Technol. A 1984,2, 882-883. (5) Goodman, D. W.; Kiskinova, M. Surf. Sci. 1981, 105, L265-L270.
This article not subject to US.Copyright. Published 1986 by the American Chemical Society
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studies indicate that some short-range effects are also evident.1,6 It is proposed here that, with the introduction of the proper species to the catalyst system (a “guest poison”), one may induce surface ensembles on sites in the neighborhood of those occupied by the added species. This induced ensemble (a reaction template) should be able to kinetically control the product distribution for a surface reaction by dynamically ordering the surface configuration of the reactants (or intermediates) prior to final product formation and subsequent desorption. In the case of the Fischer-Tropsch reaction, for example, one might avoid the usual statistical distribution resulting in the competition between chain propagation and desorption. If a class of such substances could be found, one could conceive of tailor-made products (or, at the least, product distributions) for many catalytic processes, based upon the concept of induced surface ensembles. With that aspect of catalysis in mind, we may consider an interesting example from the metal carbonyl chemistry. Kinetic studies of ligand substitution reactions were conducted on vanadium h e x a c a r b ~ n y l . ~It, ~was found, following an initial phosphine substitution reaction on the hexacarbonyl species, that there was a decrease in rate for subsequent substitution reactions of 3-5 orders of magnitude. The effect was investigated further and found to be electronic (rather than steric). All five of the remaining carbonyl ligands were stabilized against further substitution, and thus, even if the so-called “trans effect” is called into play, it is not crucial to the interaction. Despite the risk involved in making analogies from monatomic systems to bulk metal, this information offers a tantalizing prospect for improving catalytic systems. It was considered desirable for the ideal ”guest poison” to have certain properties. The ability to be easily derivatized, so as to allow subtle control over product morphology via electronic and/or steric interactions, would greatly enhance the utility of the process. Reversible, nondissociative, adsorption would also be a key asset. A system could then be flushed of one species and new reaction products obtained from the same catalyst bed with relative ease. Prior to setting up an experimental regimen to screen species for lateral effects indicative of induced ensembles, a computer simulation was undertaken. The objective of the simulation was to see if such a phenomenon could exist under idealized conditions. Reactant properties could be defined in a manner presumed necessary for successful experimental results. If no ensemble formation could be simulated under these conditions, then it was deemed unlikely that the effect could be observed in the laboratory. If, however, the ensembles are formed, then the magnitude of interaction required for a nonnegligible effect could be determined. The setup of the model is as follows. The adsorbent surface will be defined as a 90 X 34 array of points which are to correspond to surface sites. This size was chosen in order to allow viewing of the developing surface state on a Digital VTlOO terminal with advanced video features without edge effects on the 80 X 24 matrix area actually observed (the matrix is 8% edge). The remaining area of the terminal was reserved for viewing (1)the total number of impacts on the surface and the equivalent value in (6) Trenary, M.; Uram, K. J.; Yates, J. T., Jr. Surf. Sci. 1985, 157, 512-538. (7) Shi, Q. 2.;Richmond, T. G.; Trogler, W. C.; Basolo, F. J . Am. Chem. Sot. 1984.106. 71-76. (8) Richmond; T. G.; Shi, &,-Z.; Trogler, W. C . ; Basolo, F. J . Am. Chem. Soc. 1984, 106, 76-80.
Miller
Figure 1. Portion of the computer-generated array and the defined interaction regions. (P) site occupied by poison; (E) site affected by nearest-neighbor interactions; (0) unaffected site.
langmuirs, (2) the number of each species adsorbed upon the surface and fractional monolayer coverage, and (3) the ratio of the two reactive species actually ”adsorbed on the surface. In this manner, it was possible to observe the results of the simulated experiment as they evolved through time (“exposure”). Surface sites were chosen a t random, using two independently seeded random number generators, to choose x and y coordinates on the 90 X 34 array. A third random number generator, also independently seeded, was used to make all decisions at each point by generating values between 0 and 1000, inclusive. A potential adsorbate was chosen a t random, corresponding to “guest poison”, P, in a 1:l mixture of binary reactant species A and B. All three species had a initial sticking probability (Le., on an empty matrix) of 0.90 on an unoccupied site. In the absence of mitigating effects, species A and B had a probability of 0.50 of being desorbed if “struck” by any of the other species. The probability of desorbing species P was 0.05 under all conditions. When the surface reached its equilibrium coverage, this process of removing a species from an occupied site is equivalent to a random desorption event. The lateral effects were designated to be nearest-neighbor effects as per Figure 1. The enhancement factor for these nearest neighbors was so defined as to identify a value of unity with no nearest-neighbor effects; i.e., the species P merely blocks the site it occupies and a dynamic equilibrium is reached on the surface between A, B, and P with the ratio A:B reflecting that defined in the imaginary gas phase above the surface. The enhancement factors for B, in the neighborhood of P, were chosen as 1,2,3,5, 10, 30, 100, and 1000.9 The effect of the enhancement factor is as follows: if, with an enhancement factor of 1, a species has a desorption probability of 0.5 when “struck” by an incoming species, then for a factor of 2 the desorption probability decreases to 0.25. For an enhancement factor of 10, the probability is reduced to 0.05, and so forth. Since the randomly generated values for determination of the fate of a species are in the range 0-1000, an enhancement factor greater than 500 causes adsorption to be permanent in the neighborhood of the poison. For the purposes of this model, it was assumed the effect of the enhancement of B was complemented by a deenhancement of A. Species P remains unaffected by its neighbors. No provision was made for surface migration. It was determined that migration effects would further enhance any nearest-neighbor (9) In context of this model, an enhancement factor of 1000 is equivalent to irreversible adsorption of B in the neighborhood of P. Since, P itself is desorbable (p = O.OB/impact),a mechanism for the removal of this ‘permanent” B does exist.
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Figure 2. Several simulations with an enhancement factor of (a) 1 and (b) 10. interaction by increasing the opportunities for B to enter the proscribed lateral range of P. Since the aim of the model was to show the trends (if any) of the interaction, the omission of migration would clearly not prejudice the model toward positive results.’O In brief, then, the actual constraints upon the system are minimal. Direct observation of the matrix shows what appears to be excellent random behavior, with the appearance of transitory domains and the ratio A/B oscillating about unity (Figure 2a). The random number generators produce the same sequence of numbers if given the same initial seed values. This allows for the repeating of identical “experiments” with variation of the enhancement factor. Similarly, one could perform different “experiments” by using different seed values for the initialization and determine if the model is consistent. In Figure 2b, five examples of an enhancement factor of 10 are given. A typical run of 200.0 langmuirs established a “steady-state” surface ratio (starting a t ca. 40 langmuirs and tested by extension to 500.0 langmuirs in several cases). Figure 3, shows the results of a series of experiments wherein the initial seed values of the random number generators were identical and the magnitude of the enhancement factor was varied in steps from 1 through (10) Following completion of this manuscript, a preliminary version of the program which includes surface migration has been devised to allow for adsorbate-adsorbate interaction studies. Trials on the current work confirm that the main affect would be an acceleration of the ordering effects of the ‘guest poison”. It ww decided that the trends observed in template formation, in the absence of migration,better show the disparity that may exist between surface ratio and the actual ordering phenomena desired. The version incorporating migration, however, should be of value in modeling defect-site/adsorbate and associative/dissociative mechanisms.
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Figure 4. Number of poisoned sites with four type B nearest neighbors as a function of enhancement factor and exposure. 1000. Since the only sites on the surface which exhibit a different affinity for A vs. B are in the defined neighborhood of P, the effect is somewhat greater than the surface ratio would appear to indicate. Investigation of the occupancy of the nearest-neighbor sites themselves revealed that the actual formation of ensembles was much less pronounced than indicated by the surface ratio (Figure 4). It is interesting to note that the initial effect of the localized affinity results in what would likely be observed as a global surface effect, and only stronger effects significantly configure the species. This suggests that the phenomenon of nearest-neighbor effects might be typical behavior for an adspecies. The elusiveness of its observation may stem from the fact that (apparent) limiting behavior is reached on a surface (surface populations change and reaction is modified or suppressed) at lower surface coverages than those a t which the effects of localized ordering would be apparent. The modeling experiments demonstrate that if even a modest degree of nearest-neighbor effects could be induced on a surface, then ensemble formation will occur. Theoretical studies of nearest-neighbor interactions have been conducted comparing chlorine, sulfur, and phosphorus.’Jl All three of the elements were considered in their elemental (11) (a) Feibelman, P. J.; Hamann, D. R. Surf. Sci. 1985,149, 48-66. (b) Kiskinova, M.; Goodman, D. W. Surf. Sci. 1981, 108, 64-76.
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form (oxidants with respect to the substrate), and variations in the distance over which their interactions were felt are noted. Very recent studies of the mechanism of sulfur’s poisoning effect on Ni (111)have confiied that significant localized perturbation in adsorption affinities does occur on metal surfaces. This particular study was directed toward the negative (poisoning) aspects of such an inter.action, but there remains no fundamental reason this type of interaction could not be attractive rather than remlsive. Similarly, work with potassium adsorbed on Ni(lb0) was probed via This study, indicated that localized interactions were present. As indicated by the I
(12) Mullins. D. R.; White, J. M.; Luftman, H. S. Surf. Sci. 1985,160, 70-74
model, this might prove to be typical adsorbate behavior. What remains to be determined is what set of materials will exhibit the effect in a positive manner.I3 We are currently in the process of experimentally testing some simple systems. The number of systems to be considered is obviously orders of magnitude larger than any single laboratory could examine, and experimental followups on this concept by other laboratories are heartily encouraged. (13) Uram, K. J.; Ng, L.; Folman, M.; Yates, J. T., Jr. J. Chem. Phys. 1986,84,2891-2895. The author is grateful to Prof. Yates for forwarding a remint of his article oublished while this manuscriDt was in the review process. The observation of the CO ensemble in the neighborhood of K(Ni(111) waa a delightful proof that the ensemble effect is probably quite general but difficult to detect without deliberate effort (Figure 3 vs. Figure 4).
Measured Elastic Constants of a Surface Gel Phase from a Dilute Aqueous Solution of Methylcellulose+ Bernard M. Abraham* and John B. Ketterson Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60201
Feredoon Behroozi Department of Physics, University of Wisconsin-Parkside, Kenosha, Wisconsin 53141 Received December 3, 1985. I n Final Form: May 28, 1986 Measurements of the time dependence of the surface tension of four dilute solutions (6 X 9x lo4 wt %) of methylcellulosedisclose a strong adsorption of the polymer into the surface. Concomitantly with the drop in surface tension, there is an increase in surface elasticity as shown by measurements on three of the solutions. The response of each surface to a shearing stress was modeled as a spring and dash pot in series (Maxwell liquid) for the 9 X wt % solution and as a spring and dash pot in parallel (Kelvin solid) for the 12 and 24 X wt % solutions. The surface of the 24 X wt % solution sustained a shearing stress for 3600 s with almost no relaxation, from which the static shear modulus, po, was calculated to be 9.5 dyn/cm. The observed phenomona are interpreted as arising from the formation of a two-dimensional gel phase. The surface of a second 24 X lo-‘ wt % solution was subjected to a small compressional strain in a horizontal trough fitted with a Langmuir balance. The resulting surface stress was sustained for 1h with no observable relaxation. The directly measured stress divided by the strain gives the unilateral compressional modulus, K’ = (A + 2p) = 29.5 dyn/cm, where X and p are the Lam6 constants. On combining the shear and compressional measurements, we obtain X = 10.5 dyn/cm; static shear modulus, po, = p = 9.5 dyn/cm; compressional modulus, K , = X + p = 20 dyn/cm; Young’s modulus, E , = 4 p K / K ’ = 25.8 dyn/cm; and Poisson ratio, u, = XJK‘ = 0.36. 12 X lo4 and 24 X
Introduction The surface of an aqueous solution of methylcellulose (0.10 wt % ) was shown recently to display a static shear modulus whereas a solution of poly(viny1 alcohol) (0.50 wt % ), which had similar bulk properties, was only viscoelastic.’ Detailed information about the polymers, such as the number average molecular weight and the fraction hydrolized, were lacking; nevertheless, the results were sufficiently unusual to warrant reporting preliminary measurements. We have now carried the investigation further with a single methylcellulose polymer and have obtained both the time- and the concentration-dependent surface tensions and viscoelastic moduli. The Lam6 coefficients and the Poisson ratio were evaluated from the surface response to shear and to unilateral stresses. Methylcellulose ether polymers are completely soluble in water but very rapidly develop into a gel as the conWork a t Northwestern University supported under DOE Grant DE-FG02-84ER45125 and a t Argonne National Laboratory under DOE Contract W-31-109-ENG-38 +
0743-7463/86/2402-0602$01.50/0
centration increases above ca. 5 w t % .2 Further, solutions as dilute as 1.5 wt % will transform reversibly from a highly viscous solution into a sol as the temperature is r a i ~ e d . The ~ solubilizing mechanism, which is assumed to be formation of hydrogen bonds with the water, is inhibited above ca. 30 O C and the gel transforms into a sol as it is heated. The methylcellulose ethers are only slightly surface active in comparison with, e.g., the alkanesulfonates, soaps, etc. It is well-kn0~1-1~ that a soluble surfactant that lowers the surface tension of the solvent must concentrate a t the surface, consequently, a concentration gradient will exist over a distance which is on the order of several molecular lengths. The surface of a solution of soluble polymer might (1)Abraham, B. M.; Miyano, K.; Ketterson, J. B. J. Colloid Interface Sci. 1986, 107, 264.
(2) Dow Chemical Co. METHOCEL Product Information Bulletin 192-684-78. (3) Heymann, E. Trans. Faraday SOC.1935, 31, 846. (4) Landau, L. D.; Lifshitz, E. M. Statistical Physics;Pergamon Press: London, 1958; p 465.
0 1986 American Chemical Society
