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Langmuir 1990,6, 82-87
The Langmuir Lectures Translational and Collision-Induced Activation of CH4 on Ni( 11 1): Phenomena Connecting Ultra-High-Vacuum Surface Science to High-pressure Heterogeneous Catalysis S. T.Ceyert Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received October 16, 1989 This paper summarizes three recently observed phenomena, translational activation, collisioninduced activation, and collision-induced desorption, that are either explanations or plausible explanations for the different surface chemistry at high and low reactant pressure. It also briefly describes how the high-pressure requirement can be bypassed so that surface reactions which normally occur only under high pressures of gaseous reactants can be carried out in low-pressure, UHV environments.
Introduction It was Irving Langmuir who first suggested that the increased concentration of some substance near the surface of a solid was actually due to chemical bond formation between the substance and the solid rather than a physical attraction, almost like a gravitational attraction, as held by the prevailing theory of adsorption a t that time.’ In a series of papers around 1915,, Langmuir wrote that the surface of a solid could be viewed as a “checkerboard containing a definite number of spaces, each space capable of holding one atom or molecule” of a substance and that the substance was “chemically held by the atoms of the solid so as to form a continuation of the space lattice of the solid”. The mechanism for adsorption that Langmuir was envisioning is one where a sufficiently close approach of a molecule to a surface immediately results in formation of a chemical bond. Indeed, for a large number of molecules incident on many surfaces, this simple picture represents the essence of the adsorption process. But as we now know, the formation of this chemical bond is difficult for some molecules incident on some surfaces, because the molecule must attain a special configuration over a special small area of the surface. It is this kind of molecule-surface interaction that is the subject of this paper. It is suggested that the difficulty of attaining the special conditions necessary for molecule-surface bond formation often lies at the heart of the difference in surface chemistry observed under high reactant pressure conditions, such as those present in a commercial, heterogeneous catalytic reaction and under low-pressure conditions, such as those present in an ultrahigh-vacuum surface science experiment. This phenomenon is known loosely as the pressure gap in the reactivity in c a t a l y s i ~ . ~Specifically, .~ the lack of reactivity at low pressures is of concern here. An understanding of the origin of this pressure gap is important because without it the lack of reactivity at the low pressures where ~
+ A. P . Sloan Foundation Fellow, Camille and Henry Dreyfus Teacher-Scholar. (1) Rosenfeld, A. The Quintessence of Irurng Langmuir; Pergamon: London, 1966. (2) Langmuir, I. Phys. Reu. 1915, 6, 79. (3) Somorjai, G. A. Chemistry i n Two Dimensions;Cornel1 University Press: Ithaca, 1981. (4) Stoltze, P.; Norskov, J. K. Phys. Reu. Lett. 1985, 55, 2502.
0743-7463/90/2406-0082$02.50/0
ultra-high-vacuum (UHV) surface science techniques are operable certainly casts doubt on the relevance of UHV surface science to high-pressure processes such as catalysis, chemical vapor deposition, and etching reactions. The purpose of this paper is to summarize recent experiments which have demonstrated three physical phenomena, translational activation, collision-induced activation, and collision-induced desorption, which either are responsible for or are plausible explanations for the different surface chemistries at low pressure and high pressure. It will also be shown how knowledge of the microscopic origins of the pressure gap has allowed the development of a scheme t o bypass t h e high-pressure requirement so that high-pressure surface reactions can be carried out in low-pressure, ultra-high-vacuum environments.
Translational Activation The steam reforming of methane, which is the reaction of CH, and H,O to form CO and H,, is an example of a reaction that appears to proceed only at high pressure. This reaction is the commercial process for hydrogen production and is carried out over a supported Ni catalyst at about 30 atm-pressure of CH, and H,O and at 1000 K.5 No products from this reaction attempted at pressures readily attainable in an ultra-high-vacuum environment (lo-, Torr) are observed, despite favorable thermodyamics at these low pressures. A closer examination of this reaction reveals that the effect of pressure is manifested in a t least the first step of this reaction, which is the dissociation of methane. Dissociative chemisorption of at least one of the reactants is necessary to produce a more reactive adsorbate if the steam reforming reaction is to proceed. Methane is observed not to adsorb dissociatively when the methane pressure above the Ni surface is below Torr, whereas dissociation of methane is readily observed at pressures above 1 T ~ r r . A~ possible ,~ explanation for the effect of pressure (5) Rostrup-Nielsen, J. R. In Catalysis-Science; Anderson, J. R., Boudart, M., Eds.; Springer-Verlag: Berlin, 1984; Vol. 5. (6) Schouten, F. C.; Gijzeman, 0. L. J.; Bootsma, G . A. Bull. SOC. Chim. Belg. 1979, 88, 541; Surf.Sci. 1979, 87, 1. (7) Frennet, A,; Lienard, G. Catal. Reu.-Sci. Eng. 1974, 10, 37.
0 1990 American Chemical Society
Langmuir, Vol. 6, No. l , 1990 83
The Langmuir Lectures is the presence of a barrier along the dissociative reaction coordinate which can be overcome only by those molecules incident on the surface with energies greater than the energy of the barrier. Since it is the translational or internal energy of the incident molecule that is important in surmounting the barrier and not the surface temperature, the rate of reaction is limited by the flux of energetic incident molecules. If the barrier is sufficiently high such that a very small fraction of molecules incident with thermally distributed energies has the requisite energy, then the rate of dissociative chemisorption will be very slow. An increase in the pressure simply increases the absolute number but not the fraction of molecules with energies sufficient to overcome the barrier, thereby increasing the rate and allowing the products of dissociative chemisorption to be readily observed. We have recently demonstrated the validity of this hypothesis by monitoring the extent of the dissociative chemisorption of CH, on Ni(ll1) as a function of the incident energy of the methane molecules.899 These measurements show that there is a barrier to dissociation and that the translational and vibrational energies of the CH, are effective in overcoming it. The correlation of these low-pressure dissociation probability measurements with the high-pressure rates is shown to be excellent. The experiment is carried out in an experimental arrangement combining molecular beam techniques with ultra-high-vacuum surface electron spectroscopies.'*'' The molecular beam is formed by an adiabatic expansion of a high pressure of gas into a vacuum which results in a fairly monoenergetic source of CH, ( M / E< 12%). In combination with seeding technique^'^ and a variabletemperature nozzle, the energy of the CH, can be varied over a wide range from 0.3 to 20 kcal mol-'. The beam passes through two stages of differential pumping before entering into the ultra-high-vacuum chamber with a base pressure of 4 X lo-'' Torr, where it is incident on the Ni(ll1) surface. The crystal is mounted on a liquid He cooled manipulator14 which enables the rotation around an axis parallel to the crystal surface so that the angle of incidence of the beam on the crystal is variable. The temperature of the crystal is adjustable from 8 K to its melting point. The crystal can be removed from the beam path so that the beam is directly in line of sight of the quadrupole mass spectrometer. The translational temperature and the collision energy of the beam are measured by a time-of-flight technique that utilizes a slotted disk chopper in the second differential stage. An electronic shutter mounted in the first differential chamber controls the exposure time of the surface to the beam. In these experiments, the key measurement is the probability of dissociative chemisorption. This probability is the ratio of the number of molecules dissociatively chemisorbed to the total number of molecules incident on the surface. The latter is evaluated from the absolute beam flux. The number of dissociatively chemisorbed molecules is determined from the absolute coverage mea(8) Lee, M. B.; Yang, Q. Y.; Tang, S. L.; Ceyer, S. T. J . Chem. Phys. 1986,85, 1693. (9) Lee, M.B.; Yang, Q. Y.; Ceyer, S. T. J . Chem. Phys. 1987, 87, Ann A
2124.
(10) Ceyer, S. T.; Beckerle, J. D.; Lee, M. B.; Tang, S. L.; Yang, Q. Y.; Hinea, M. A. J. Vac. Sci. Technol. A 1987,5, 501. (11) Ceyer, S.T.Annu. Reu. Phys. Chem. 1988,39,479. (12) Ceyer, S.T.; Gladstone, D. J.; McGonigal, M.; Schulberg, M. T. In Physical Methods of Chemistry, 2nd ed.; Rossiter, B., Hamilton, J. F., Baetzold, R. C., Eds.; Wiley: New York, 1990. (13) Abauf, N.; Anderson, J.; Andrea, R.; Fenn, J.; Mardsen, D. Science 1967. 155. 917. (14) Beckerle, J. D.; Yang, Q. Y.; Johnson, A. D.; Ceyer, S. T. Surf. Sci. 1988, 195, 77.
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Figure 1. Absolute dissociation probability of CH, and CD, as a function of the normal component of translational energy. The lines are linear least-squares fits to the data, and the error bars are 95% confidence limits of a series of six to eight measurements for CH, and of a series of three measurements for
CD,. sured by Auger electron spectroscopy. The Auger signal is calibrated for absolute coverage against the Auger signal a t a known coverage determined from a low-energy electron diffraction experiment. The probability for dissociative chemisorption of CH, on Ni(ll1) as a function of the translational energy of the incident CH, molecule is shown in Figure 1. Below 12 kcal mol-', there is no dissociative chemisorption above the sensitivity limit for carbon detection, but as the translational energy is increased to 17 kcal mol-', the dissociation probability increases exponentially by 2 orders of magnitude. The dissociation probabilities shown in Figure 1are measured with the molecular beam incident a t the normal angle to the crystal. Dissociation probabilities measured at other angles of incidence and plotted versus the translational energy in the normal direction fall on the straight line in Figure 1. Therefore, the dissociation probability correlates not with the total energy of the incident methane molecule but with the kinetic energy only in the normal direction. Measurements for CD, exhibit similar trends except that the absolute magnitudes of the dissociation probabilities are about 1order of magnitude smaller than those of CH,, suggesting that tunneling may also play a role in the dissociation step. The effectiveness of vibrational energy in surmounting the barrier was also investigated. Measurements of the dissociation probability as a function of nozzle temperature for a fixed translational energy indicate that vibrational energy, most of it concentrated in v, (umbrella mode) and v2 (bending mode), is at least as effective as translational energy if not slightly more effective. The temperature of the surface has no effect on the dissociation probability. These results show that there is a barrier to the direct dissociative chemisorption of CH, which translational energy only in the normal direction and vibrational energy are effective in overcoming. These observations are assimilated into a coherent model for the physical origin of the barrier to dissociation that explains how translational and vibrational excitation allow the molecule to overcome it. The telling clue is the equivalent effective-
84 Langmuir, Vol. 6, No. 1, 1990 ness of the normal component of the translational energy and the vibrational energy, which suggests that translational and vibrational excitation lead to the same motion of the nuclei along the reaction coordinate and over the barrier. This is possible if the role of translational energy is distortion or deformation of the CH,. The fact that the methane molecule must be distorted from its tetrahedral configuration in order for dissociation to occur is reasonable upon consideration of the energetic requirements for C-H bond cleavage. The C-H bond energy in methane is about 100 kcal mol-'. In order for there to be sufficient energy release to break the C-H bond, a Ni-H bond, which is worth about 63 kcal mol-', and a Ni-C bond, worth approximately 40 kcal mol-', must be formed. To a flat Ni surface with no protrusions, the methane molecule appears spherical; that is, the interaction between the flat surface and the methane molecule is isotropic. The hydrogen atoms effectively bury the carbon atom, preventing a strong attractive interaction between the Ni atoms of the surface and the carbon atom. A slowly moving methane molecule incident on the surface interacts primarily through the shielding hydrogen atoms. No C-H bond cleavage occurs, because the carbon atom cannot move close enough to the Ni surface to interact strongly. However, as the translational energy of the incident molecule is increased, the methane molecule begins to suffer substantial deformation upon collision with the repulsive wall of the surface due to its increasing impact. This deformation serves to push the hydrogen atoms out from between the surface and the carbon atom in the same way as vibrational excitation of the v, and v2 modes, thereby exposing the carbon atom to the Ni surface. In this configuration, both a Ni-C bond and a Ni-H bond can be formed, breaking a C-H bond. However, the molecule does not need to be deformed completely, because at some distance the attractive part of the potential, through interaction with the antibonding orbitals, tempers the barrier so that the barrier becomes sufficiently narrow for the light hydrogen atom to tunnel through to the product regime. These detailed measurements have provided not only a microscopic picture of the CH,-Ni interaction but also a means to understand the macroscopic phenomenon of the pressure gap. They have shown that a barrier to dissociation which arises from the energy required to deform CH, is present because Ni-C bond formation is not possible in the undistorted configuration. In turn, the barrier to dissociation is the physical reason behind the lack of reactivity a t low pressures, and this is corroborated in the following way.15 The probabilities for dissociative chemisorption as a function of the energy of the incident CH, are convoluted with a Maxwell-Boltzmann distribution function at some temperature to yield a calculated value for the thermal rate constant for CH, decomposition over a Ni(ll1) crystal. These calculated rate constants are then compared to those measured for CH, decomposition on a Ni(ll1) crystal under high-pressure conditions as a function of the temperature of the system.15 The temperature of the system refers to the temperature a t which both the gas and the surface are held. Excellent agreement is found to within a factor of 2-3 between the rate constants measured at high pressure and those calculated from the low-pressure dissociation probability measurements. This result clearly establishes the barrier to the dissociative chemisorption of CH, as an origin of the pressure (15) Beebe, T. P., Jr.; Goodman, D. W.; Kay, B. D.; Yates, J. T., Jr.
J. Chem. Phys. 1987.87, 2305.
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Figure 2. Cross section for dissociation of CH, physisorbed on Ni(ll1) at 47 K induced by the impact of Ar atoms vs the kinetic energy of the Ar atom in the normal direction. Methane coverage is 0.3 monolayer. Each point at the same total energy represents a different incident angle from 0' to 5 5 O : (top circle) 51.8, (diamond) 47.1, (triangle) 42.3, (square) 37.2, (X) 32.5, (bottom circle) 27.8 kcal mol-' total energy. The solid lines are the result of the model calculation discussed in the
text.
gap in the reactivity. The presence of this barrier along the dissociative reaction coordinate establishes a link between experiments carried out in ultra-high-vacuum environments, where the adsorbate pressures are d where Ei is the total energy, Oi is the incident angle of the Ar atom, b is the impact parameter, 4 is the azimuthal angle of the collision, mA and m, are the masses of Ar and CH,, and d is the hard-sphere collision diameter. Once the methane has acquired the energy E,, the second step in the mechanism is dissociation. The probability that the methane molecule dissociates is given as a function of E, by the previous measurements of the dissociation probability, P, versus the normal component of the kinetic energy of the incident methane molecule. These data, which are shown in Figure l, are described by the functional form
where a , V, and A are treated as fitting parameters. This expression is combined with that derived from the hardsphere collision model for the energy transferred to CH, to yield the probability for collision-induced dissociative chemisorption, PCID, as a function of the incident energy and angle of the Ar atom and the impact parameter and azimuthal angle of the collision PcID(Ei,Oi,b,+)= P[En(Ei,Oi,b,4)1 Although Ei and Oi are fixed in the experimental measurements, the impact geometry is not. Therefore, it is necessary to integrate PcIDover all values of b and 4 to yield a calculated value for the collision-induced cross section for dissociation, Z
The results of this calculation are shown as the solid lines in Figure 2. The agreement between the model and data is excellent. The model closely reproduces the complicated dependence of the cross section on the normal kinetic energy of the incident Ar atoms. The excellent agreement strongly suggests that the breakdown in normal kinetic energy scaling in the Ar energy is the result of the range of impact parameters that contribute to the cross section and that a hard-sphere collision model accurately describes the energy transfer. The physical picture that emerges is straightforward. The Ar atom collides with the methane molecule, trans-
fers some fraction of its energy, depending on the kinematics of the collision, and then reflects from the surface. The subsequent methane-surface collision is the same as in translational activation. The methane molecule, with its newly acquired energy, is accelerated into the surface, deforms, and dissociates. Because the translational activation results can be mapped onto the cross sections for collision-induced dissociation, it is concluded that translational activation and collisioninduced activation are completely consistent with each other. They are simply different methods for providing energy to deform the molecule, but the mechanism for dissociation is the same. In competition with dissociation by the impact of the Ar atom, collision-induced desorption also occurs.1G18That is, once the Ar atom transfers energy to the methane, the methane molecule collides with the surface and can rebound into the gas phase if the site on which it is physisorbed or if its orientation is not energetically favorable for dissociation. Desorption induced by collisions is roughly 1-2 orders of magnitude more probable than collision-induced dissociation. The dynamics of collisioninduced desorption are studied by measuring the desorption cross section as a function of the energy and incident angle of the Ar atoms. The mechanism for desorption is determined to involve a direct and impulsive, bimolecular collision between Ar and CH,. Molecular dynamics simulations show that the complicated energy and incident angle dependence of the desorption cross section is the consequence of the competition between the decrease in the energy transferred in the normal direction and the increase in the collision cross section as the incident angle increases.” But perhaps more important than the physics behind these processes is the fact that they have been observed. Knowledge of their existence is very significant for understanding the complex environment of high-pressure heterogeneous catalysis, because collision-induced chemistry and desorption likely play important roles in highpressure catalytic reactions. This is because under these conditions a catalyst is covered with adsorbate, and the adsorbate-covered catalyst is continually bombarded by gas-phase molecules. With the observation of these processes, no mechanism for a high-pressure reaction can now be considered complete without an assessment of the role of collision-induced chemistry and desorption as potential major steps. These observations are now cause for reexamination of the mechanisms of catalytic reactions in which inert gas effects on reaction rates have been noted.lg Collision-induced chemistry and desorption are additional contributors to the pressure gap in the reactivity of heterogeneous catalysis. They are additional reasons why surface chemistry at high pressures is often very different from surface chemistry at low pressures.
Molecular Beams as Synthetic Tools: Synthesis of Adsorbed Methyl Radicals and Synthesis of Benzene from Methane Having established this link between high-pressure catalysis and UHV surface science, we now know how to bypass the high-pressure requirement simply by raising the energy (16) Beckerle, J. D.; Yang, Q. Y.; Johnson, A. D.; Ceyer, S. T. J.Chem. Phys. 1987,86,7236. (17) Beckerle, J. D.; Johnson, A. D.; Yang, Q. Y.; Ceyer, S. T. J.Chem. Phys. 1989,91,5756. (18)Beckerle, J. D.; Johnson, A. D.; Ceyer, S. T. Phys. Reu. Lett. 1989,62,685. (19)Hudgins, R. R.;Silveston, P. L. Catal. Reu.-Sci. Eng. 1975,II, 167.
86 Langmuir, Vol. 6, No. 1, 1990 Lu
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of the incident molecule (translational activation) or collisionally inducing dissociation (collision-induced activation). We have used both methods to synthesize and identify spectroscopically, by high-resolution electron energy loss spectroscopy, an adsorbed CH, radical for the first time under low-pressure, ultra-high-vacuum conditions. This was accomplished originally by measuring the vibrational spectrum of methane after deposition on the surface a t 140 K with a translational energy of 17 kcal/ mol.' The crystal temperature is maintained at a low value in order to trap the nascent product of the dissociative chemisorption event rather than a species produced by thermal decomposition of the nascent product. The products of the dissociative chemisorption event after translational or collision-induced activation are identified as an adsorbed methyl radical and adsorbed H atom. A recent, higher resolution and higher sensitivity highresolution electron energy loss spectrum is shown in Figure 3. The mode assignments, given in the figure caption, were made by comparison to the frequencies for metal alkyls and verified by isotopic substitution. In conjunction with spectra of CH,D and CD,, a Fermi resonance between the overtone of the asymmetric deformation mode and the low frequency or "soft" C-H symmetric stretch has been confirmed. These spectra have also allowed a symmetry analysis to be carried out that is sufficient to establish that the CH, species is adsorbed with C,, symmetry in a threefold hollow site with the hydrogens either eclipsed over or staggered between the surrounding Ni atoms." Adsorbed methyl radicals have long been invoked as reaction intermediates in a wide variety of hydrocarbonsurface reactions carried out both in an ultra-high-vacuum environment and under high-pressure conditions. Despite their importance as proposed reaction intermediates, this result represents the first time that adsorbed methyl radicals have been produced cleanly on a single
crystal metal surface and unambiguously identified by vibrational spectroscopy. Adsorbed methyl radicals have not been produced previously because there is no simple way to synthesize them. The commonly studied adsorbates, C,H, and C,H,, which dissociate readily under UHV conditions, have not been observed to produce methyl radicals on metallic surfaces. Methane and ethane, natural candidates for the clean production of methyl radicals, are completely unreactive with most metal surfaces under low-pressure conditions of the adsorbing gas. However, it is now clear both why methane is unreactive a t low pressures and how to activate it. The variability of the collision energy afforded by molecular beams makes them a tool with which novel adsorbates can be synthesized. With adsorbed CH, species conveniently and cleanly synthesized, their stability and reactivity have been probed by monitoring the vibrational spectrum as a function of surface temperature. The methyl radicals adsorbed on Ni(ll1) are stable below surface temperatures of 150 K. Above this temperature, the methyl radicals dissociate to form CH. A vibrational spectrum of CH adsorbed on Ni(ll1) along with the assignments is shown in Figure 4. These assignments are supported by the spectrum resulting from the thermal decomposition of CH,D in which the frequencies of the CH and CD stretch vibrations of this mixture of CH and CD species are identical with those of CH produced from CH, decomposition and CD produced from CD, decomposition.2 The ability to bypass the high-pressure requirement has allowed us to carry out a high-pressure reaction at low pressure: the synthesis of C,H, from CH,.21 In addition, because this reaction is carried out a t low pressure, we have been able to identify the adsorbed intermediates by high-resolution electron energy loss spectroscopy and to determine the mechanism of this reaction. The synthesis is effected by exposing a monolayer of CH, physisorbed on Ni(ll1) at 47 K to a beam of Kr atoms. The collision of the incident Kr with the physisorbed CH, distorts the CH, from its tetrahedral configuration, thereby lowering the barrier to dissociation into an adsorbed methyl radical and an adsorbed hydrogen atom. As the surface
(20) Yang, Q. Y.; Johnson, A. D.; Maynard, K. J.; Ceyer, S.T., to be published.
(21) Yang, Q. Y.; Johnson, A. D.; Maynard, K. J.; Ceyer, S. T. J. Am. Chem. Soc. 1989, 111,8748-8749.
Langmuir 1990, 6, 87-96 temperature is raised to 230 K, all the adsorbed CH, dissociates to CH and the CH recombines to form adsorbed C,H,. Some of the C,H, trimerizes to adsorbed C,H6 and at 410 and 425 K, respectively, the atomically adsorbed hydrogen desorbs as H,, and some of the chemisorbed C6H, desorbs. The gas-phase benzene is detected mass spectrometrically in a thermal desorption experiment. The benzene remaining on the surface dehydrogenates. Although the maximum desorption yield for C,H, is 1.5%, the gas-phase hydrocarbon selectivity of this synthesis for benzene production is 100%. This procedure represents the first synthesis of benzene from methane over a single catalyst and establishes molecular beams as a synthetic tool. These data also provide mechanistic infor-
a7
mation useful to the possible extrapolation of this synthesis from molecular beam-UHV environments to more practical conditions. Acknowledgment. The work summarized here is the result of the substantial efforts of several Ph.D. students, M. B. Lee, J. D. Beckerle, Q.Y. Yang, and A. D. Johnson. Financial support has been provided by the National Science Foundation (CHE-8508734);the donors of the Petroleum Research Fund, administered by the American Chemical Society; and the Energy Laboratory at MIT. Registry No. CH,, 74-82-8; Ni, 7440-02-0; CH,, 2229-07-4.
Wet Chemical Approaches to the Characterization of Organic Surfaces: Self-Assembled Monolayers, Wetting, and the Physical-Organic Chemistry of the Solid-Liquid Interface George M. Whitesides* and Paul E. Laibinis Department of Chemistry, Harvard University, Cambridge, Massachusetts 02138 Received November 29, 1989 Physical-organic methods are useful in studying the surface chemistry of organic solids. These methods complement the usual spectroscopic approaches in characterizing the solid-liquid interface. This paper focuses on two topics drawn from physical-organic surface chemistry: preparations of ordered organic surfaces by self-assembly of organic molecules on inorganic supports and uses of wetting in characterizing these and other surfaces. Monolayer films prepared by chemisorption of alkanethiols and dialkyl disulfides on gold are the best characterized and most widely studied of the self-assembled systems. Wetting is uniquely valuable in characterizing surfaces for its combination of high surface sensitivity and applicability to disordered surfaces. Introduction The interfacial chemistry of organic materials is an important but underdeveloped subfield of surface science.' The relevance of this subject ranges from technology (adhesion to polymers,' biocompatible mat e r i a l ~fiber-matrix ,~ interactions in composites4)to basic science (wetting of solids by cell-surface bio(1)Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, S.; Israelachvili, I.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987,3,932-950. (2)Zisman, W.A. In Handbook of Adhesives, 2nd ed.; Skeist, I., Ed.; Van Nostrand Reinhold: New York, 1977;Chapter 3. Schonhorn, H. In Polymer Surfaces; Clark, D. T., Feast, W. J., Eds.; Wiley: Chichester, 1978;Chapter 10. Leger, L. Ann. Chim. (Paris) 1987,12, 175184. (3)Ratner, B. D. In Biomaterials: Interfacial Phenomena and Applications; Cooper, S. L., Peppas, N. A,, Eds.; Advances in Chemistry 199;American Chemical Society: Washington, DC, 1982;Chapter 2, pp 9-23. Surfaces and Interfacial Aspects of Biomedical Polymers; Andrade, J. D., Ed.; Plenum: New York, 1985. Andrade, J. D.; Hlady, V. Adu. Polym. Sci. 1986, 79, 1-63. Durrani, A. A,; Chapman, D. In Polymer Surfaces and Interfaces; Feast, W. J., Munro, H. S., Eds.; Wiley: Chichester, 1987;Chapter 10. Lundstrom, I.; Ivarsson, B.; Jonsson, U.; Elwing, H. In Polymer Surfaces and Interfaces; Feast, W. J., Munro, H. S., Eds.; Wiley: Chichester, 1987;Chapter 11. (4)Delannay, F.; Froyen, L.; Deruyttere, A. J . Mater. Sci. 1987,22, 1-16. Schultz, J.; Lavielle, L. ACS Symp. Ser. 1989,391,185-202. (London) 1805,95,65-87. (5)Young, T.Philos. Trans. R. SOC. (6) Wenzel, R. N. Ind. Eng. Chem. 1936,28,988-994.Wenzel, R. N. J . Phyf. Chem. 1949,53,1466-1467.Bracke, M.; DeBisschop, F.; Joos, P. Prog. Colloid Polym. Sci. 1988,76,251-259.
0743-7463/90/2406-008~$02.50 JO
chemistry"). Organic surface science is less developed than corresponding subfields focused on inorganic surfaces (metals and metal oxides) for several reasons. First, organic surfaces are usually less ordered, less stable thermally, and less amenable to characterization by scattering and diffraction techniques than are crystalline inorganic surfaces. Second, many of the advances in surface science have relied on instrumental techniques derived from surface physics." Because organic materials may be sensitive to radiation damage, and because they and their damage products are often volatile, techniques requiring high vacuum are often not applicable to them. Finally, (7)Neumann, A. W. Adu. Colloid Interface Sci. 1974,4, 105-191. Adamson, A. W. In Physical Chemistry of Surfaces, 4th ed.; Wiley: New York; 1982. Joanny, J. F.; de Gennes, P . 4 . J . Chem. Phys. 1984,81, 552-562. de Gennes, P.-G. Reu. Mod. Phys. 1985,57,827-863.Schwartz, Garoff, S. Langmuir 1985,1,219-230. Schwartz, L. W.; Garoff, L. W.; S. J. Colloid Interface Sci. 1985, 106,422-437. Pomeau, Y.;Vannimenus, J. J. Colloid Interface Sci. 1985,104,477-488. (8) Neogi, P.; Miller, C. A. J. Colloid Interface Sci. 1983,92,338-49. (9).Joanny, J. F. J. Chem. Phys., Phys.-Chem. Biol. 1987,84,197-8. Neogi, P.; Miller, C. A. J . Colloid Interface Sci. 1982,86,525-38.Lopez, J.; Miller, C. A.; Rudenstein, F. J . Colloid Interface Sci. 1976,56,4608. (10)Albersheim, P.; Anderson-Prouty, A. J. Ann. Reu. Plant Physiol. 1975, 26, 31-52. Denburg, J. L. Adu. Comp. Physiol. Biochem. 1978,7,105-226. (11)Somorjai, G. A.; Bent, B. E. Prog. Colloid Polym. Sci. 1985,70, 38-56. Feldman, L. C.; Mayer, J. W. In Fundamentals of Surface and Thin Film Analysis; Elsevier Science: New York, 1986.
0 1990 American Chemical Society
