2774
J. Phys. Chem. 1980, 84, 2774-2779
molar entropy for Hg, So used In the calculation was that for the gaseous metal (174.8 J Kq mor'), the appropriate value being that for the HquM metal (76.0 J K-' mor'). As a result, the value of dAq5ddTrepated in ref 7 Is high by 1.02 mV K-' md-', the conect value being -0.40 mV K-' mot'. A similar error was made in ref 8 where the gasqhase molar entropy of Ag (182.9 J K-' md-')was used instead of that for the sdkl state (42.55 J K-' mol-'). The corrected estimate of dAq5 dd Tfor fwmamkle on the basis of the data discussed is 1.0 mV K-I mol-' S. Levine, G. M. MI, and A. L. s m k , J. phys. C b m . , 73,3534 (1969). Although a more general case was consklered In the odginal paper of L e h e , Bell, and ~mlth,*'the version presented hare assumes that the fn@tude of the dipole moments in the two orientations are qUei. J. R. Macdonald, J . chem. phys., 22, 1857 (1954).
(30) Nguyen Huu Cuong, A. Jenard, and H. D. Hurwltz, J . Electroanel. Chem., 103, 399 (1979). (31) W. R. Fawcett, 6. M. Ikeda, and J. 6. Sellan, Can. J. Chem., 67, 2268 (1979). (32) M. J. Aroney, R. J. W. LeFBvre, and A. N. Singh, J. Chem. Soc., 3179 (1965). (33) 6. B. Damaskh and A. N. Frurw1,€&ctmdh. Acta, 19, 173 (1974); 6. B. Damaskin, J . Electrosnal. Chem., 75, 359 (1977). (34) W. R. Fawcett, S. Levine, R. M. delrloklge, and A. C. McDonald, J. Ekfroenal. Chem., in press. (35) 2.Borkowska and W. R. Fawcett, EleMrokhlmiye, in press. (36) H.J. M. NedermeiJer-DeNessenand C. L. deligny, J. E l e c f r a e ~ l . Chem., 59, 1 (1975). (37) S. Trasatti, J . Ekfroansl. Chem., 33, 351 (1971). (38) S. Trasatti, J. Electroenel. Chem., 82, 391 (1977). (39) J. C. Rivlbe, Sol@ Sfate Surf. Scl., 1, 180 (1969).
Ultrahigh-Vacuum Techniques in the Measurement of Contact Angles. 5. LEED Study of the Effect of Structure on the Wettability of Graphite'** Malcolm E. Schrader David W. T a y b Naval Sh@ Research and Devebpmnf Center, Annapolis, Merylend 21402 (Received: November 5, 1979; In Final F m : June 16, 1980)
It was previously found that bakeout and ultrahigh evacuation of the (OOO1) plane of oriented graphite produced a clean surface (as determined by AES) which yielded a water contact angle of -35". Ion bombardment of that evacuated surface reduced the water contact angle to 0". The present work seeks to determine whether the ion bombardment removed undetectable (to AES) residual contamination or merely disordered an already clean surface. The (OOO1) plane of ZYE3 oriented graphite was examined by LEED after vacuum heating to -800 "C, and the water contact angle measured in situ. The surface was then put through several cycles of ion bombardment, LEED analysis, and water contact angle measurement. The original heated surface showed LEED patterns characteristic of clean graphite (OOO1) and yielded water contact angles of 38 f 3". The LEED patterns gradually disappeared with increasing ion bombardment, accompanied by decreasing water contact angles. The water contact angle did not reach zero before the LEED pattern had completely disappeared. It is concluded that the contact angle of 38 f 3" represents a clean (OOO1) surface of !ZYB oriented graphite while the 0" contact angle results from formation on the surface of a disordered (amorphous) layer. The value of 38 f 3" found on ordered ZYB is not necessarily that for a perfect (OOO1) surface. The contact angle of the latter is estimated to be in the range of 42 f 7". The results are discussed in terms of values reported in the literature for the surface energy of graphite (Ooal), and the wettability of surface carbon in general.
Introduction The surfaces of carbon adsorbents have been regarded as high energy because of their efficacy as adsorbents for gases and vapors. Until recently, however, all reported contact angles for water on the surface of nominally elementary carbon have been greater than 0". Values for the graphite (OOO1) surface have generally ranged from 60 to 85" with considerable hystere~is.~-~ In recent work on oriented graphite, air-cleaved (O001) surfaces were cleaned by heating in ultrahigh vacuum.2 Auger electron spectroscopic (AES)measurements indicated that the surfaces were free of contaminants with the possible exception of hydrocarbons or chemisorbed hydrogen. The contact angle of water on this surface, measured in situ, was found to be ~ 3 5 When ~ . the surface was ion bombarded, however, water spread upon it spontaneously with a 0" contact angle. Evidence was presented that the high contact angles (60to 85") obtained when these measurements are made in air are due to carbonaceous contamination. It was not clear, however, whether the vacuum-baked surface yielding the 35" contact angle was clean, ordered graphite (OOO1) or a surface containing residual hydrocarbons (or chemisorbed hydrogen). If the former were true, the reduction in contact angle after ion bombardment would be the result of disordering of the (O001) structure, whereas if the latter
were true, the Oo contact angle obtained after ion bombardment could be the value characteristic of the clean, ordered (OOO1) surface. In this work, various types of graphite surface prepared in vacuum were examined by LEED (in addition to AES) for evidence as to which represented clean, ordered graphite (OOO1). Contact angles of water were measured in situ on each of the structures thus characterized.
Experimental Section Grade ZYB of a high-purity pyrolytic graphite called oriented graphite (Union Carbide) was used in these experiments. The sample was cleaved in air and mounted on a sample manipulator in a bell jar with porta for AES, electron bombardment heating, and ion bombardment as described previously? A stainless steel cold finger was wed to condense water vapor for in situ sessile drop contact angle measurements as described previously? In addition, a port with LEED capability was utilized in the present experiments. Results Effect of Ion Bombardment on LEED and Contact Angle (in situ), A ZYB oriented graphite surface which, after cleavage, had been tamped down with Kimwipe tissue
This article not subject to US. Copyright. Publkhed 1980 by the American Chemical Society
Wettability of Graphite
The Journal of Physlcel Chemistry, Vol. 84, No. 21, 1980 2775
TABLE I : Contact Angles of Water in Situ on Graphite (OOOl), Surface I contact angle, droD degree (advancing)
TABLE 11: Contact Angles of Water in Situ on Graphite (0001), Surface I1
a. After Electron-Bombardment HeatingQ 1 40 2 42.5 3 39 4 39 5 39 av 40 b. After Minimal Ion Bombardmentb 26 1 2 29, 26 3 25 4 33 av 28 c. After More Ion Bombardment 1 30 2 22.5 3 23 4 28.5 5 25 6 22.5 av 25 See Figure l a for LEED of this surface. See Figure l b for LEED of this surface.
a. After Electron-Bombardment Heatinp 35.75 1 2 35.0 3 37.0 av 36 b. After Ion Bombardmentb 1 7.5 2 12.0 3 0-3 4 6 5 7.5 av 7 See Figure a See Figure 2a for LEED of this surface. 2b for LEED of this surface.
and rinsed in acetone was inserted in the AES/LEED ultrahigh-vacuum apparatus. After evacuation and bakeout, the surface was heated by electron bombardment to -800 "C,and the LEED pattern observed (Figure la). The figure shows the LEED pattern at normal incidence and an electron energy of 65 eV. All LEED patterns are for normal incidence and electron energies in the range of 56-71 eV. If the sample were a single crystal, a hexagon of diffraction spots would appear, corresponding to the positions of the first-order (i.e., [lo] and [ll]) reflections from the hexagonal (OOO1) face of graphite with a surface interrow spacing of 1.23 A. Since the sample of oriented graphite consists of many crystallites oriented in the same plane but rotated with considerable randomness with respect to one another, many hexagons of diffraction spots with different rotational orientation appear, to form a continuous ring of intensity. Occasional bright spots in the ring indicate that there are some preferred rotational orientations. The width of the ring, as opposed to its diameter, is a measure of the average size of the crystallite3 and their disorientation out of the plane on the surface. A narrow ring indicates large crystallites lying flat in the surface, whereas a wide ring indicates either small crystallites or crystallites with large out-of-plane disorientation. In situ measurement of the contact angle yielded an average value of 40" (Table Ia). After evacuation of the water vapor and bakeout, the LEED pattern was reproduced. Ion bombardment of this surface for a short period of time yielded a fuzzy LEED pattern (Figure lb). It can be seen that the ring of intensity is much less well-defined, indicating that the ion bombardment has caused considerable surface damage, decreasing the size of the crystallites and increasing their disorientation. In situ measurements of the water contact angle on this surface yielded an average value of 28" (Table Ib). Evacuation, bakeout, and another short period of ion bombardment yielded an almost completely obliterated LEED pattern, indicating a further increase in structural randomness on the surface. The average contact angle measured on this surface in situ was 2 5 O (Table IC). In another run, the graphite surface was renewed by cleavage while in the sample holder, which was then installed in the vacuum system without tamping or rinsing
drop
contact angle, degree (advancing)
TABLE 111: Contact Angles of Water in Situ on Graphite (00011. Surface 111 contact angle, drop degree (advancing) a. After Room-Temperature Evacuation" 1 43.5 2 70 3 72 4 76 av 65 b. After Evacuation with Bakeoutb 40.5 1 2 58.5 3 40.5 4 46.5 5 43 6 59.5 7 63 8 68.5 9 60 10 68 av 65 See Figure 3a for LEED of this surface. See Figure 3b for LEED of this surface.
the surface. After evacuation and bakeout, the surface was heated rapidly to -800 O C by electron bombardment. Observation of the LEED pattern indicated that the crystallites were now partially disordered through outof-plane disorientation. After standing for 1 day, the crystallites settled into their original plane but retained considerable in-plane rotational disorientation, as shown by a circular LEED pattern. Periodic heating and annealing by electron bombardment continued to yield the circular LEED pattern (Figure 2a). Note that the ring is narrower and more uniform than in Figure 1,indicating that the electron-bombardment heating and annealing has now decreased the out-of-plane disorientation but increased the in-plane rotational randomness. The average in situ contact angle of water on this surface was 36" (Table IIa). After evacuation of the water vapor and bakeout, the LEED pattern was reproduced. A short period of ion bombardment obliterated the LEED pattern (Figure 2b). In situ measurements of the contact angle of water on the resulting surface yielded an average value of 7" (Table IIb). Effect of Evacuation and Bakeout on LEED and Contact Angle (in situ). In an attempt to follow the decontamination of a graphite (Oool) surface exposed to air and hydrocarbonaceous material, a cleaved sample was tamped with Kimwipe tissue, rinsed in acetone, and installed in the vacuum apparatus. The system was evacuated at room temperature, and the LEED pattern observed (Figure 3a).
27?6
The .bum/ of ~ y s k echermsby, l Vd. 84, No. 21, 1980
a
a
b
b
1. LEED panern of graphte (0001).surface I: (a)after eiec-
trorrbombardmentheating; (b) after minimal ion bombardment. shadow on the panern is due to the sample holder. TABLE IV: Rapid Measurements in Air of Contact Angle of Water on Freshly Cleaved Graphite (0001) contact angle,
surface
degree (advancing)
a. ZYD Surfaces 1 2 3 4 5 6
7 8 9 10
16 17 18 19 av
37.5 45 33.5 46.5 45.5 46 44 42 49 49.5
45 46.5 39.5 43 44t 5 ~~~
b. ZYASurfaces 1 2 3
45 48.5, 48.0 46.5
The
M2. LEED panern of graphite (0001). surface 11: (a) after e!.%lrm&mbardment heating; (b) after ion bombardment.
The water contact angle was then measured in situ, yielding an average value of 65' (Table IIIa). The system was then evacuated and baked out, and the LEED pattern observed again (Figure 3b). In situ measurements of the water contact angle on this surface yielded an average value of 55O (Table IIIb). Rapid Contact Angle Measurements in Air. Rapid advancing contact angle measurements were made in the open air on freshly cleaved ZYD (lower quality than ZYB) and ZYA (higher quality than ZYB) surfaces. A drop of water was placed on each surface within 3 s of cleavage, and the readings were made in less than 30 s. Each reading listed in Table JV,a and b, was made on a different surface. The average value on ZYD was 44 f 5 O (Table IVa), and on ZYA 46 f 3" was obtained (Table IVb). Discussion Nature of Surface Yielding Zero Contact Angle. In the AES/LEED ultrahigh-vacuum system used in the preaent investigation, graphite (OOO1) surfaces yielding 38 f 3 O in situ contact angles were obtained reproducibly through electron-bombardment heating. These surfaces always yielded a representative graphite (OOO1) LEED pattern of some sort both before and after in situ measurements of the contact angle. Ion bombardment of this surface always resulted in at least partial obliteration of the LEED pattern, accompanied by a decrease in contact angle. To obtain a Oo contact angle, we had to continue ion bombardment past the point at which the LEED pattern was obliterated. These results, of course, strongly indicate that the '0 contact angle is a consequence of disordering of the surface during ion bombardment. It could be suggested
Wettabnny of Qraphite
a
The Journal
Of
m k a r chemlrby, Vd. 84, No. 21. 19.90 2221
structural change than were the LEED patterns, which could be completely obliterated while the contact angle remained nonzero. If one extrapolates from ZYB to the more ordered direction, it would not be expected that a refinement in the LEED patterns would be accompanied by a substantial increase in contact angle. This is s u p ported by the results of the open-air experiments, where contact angles of ZYD and ZYA were found to be nearly the m e . S i n c e those experiments were subject to possihle organic contamination, which would raise the contact angle, the open-air experiments on ZYA can reasonably be taken to provide an upper limit to the possible value of the water contact angle on perfect graphite (OOO1). In view of all these considerations, the experimental value of 38 3' found on clean structured ZYB is probably quite close to the true value for a perfect surface. In any event, reasonable upper and lower limits suggested by the experiments require that the true value be in the range of 42 70. Effect of Hydrocarbonaceous Contaminations. The experiments in which LEED and contact angle measurements were made on a cleaved surface after evacuation a t room temperature, and then after evacuation with bakeout, were performed in an attempt to find further support for the hypthesis that the high contact angles ( 6 ( t 8 5 O ) found for water on graphite (Oool) during normal measurements a t atmospheric pressure result from hydrocarbonaceous contamination? Although the observation of lower contact angles with increased heating during evacuation support that hypothesis, the LEED results were inconclusive, having failed to display a sharp difference between the contaminated and clean surfaces. This may be due to the known ability of LEED to seek out areas which yield a pattern, in this case the clean areas, without being hampered very much by those areas which do not contribute. The contact angle, on the other hand, is a macroscopic measurement which will tend to reflect average surface properties. There is another manifestation of the hypothesis that the high water contact angles observed on air-exposed graphite (OOO1) are due to hydrocarbonaceous contamination rather than to adsorption of molecular components of air. Substantial physical adsorption, if it occurs, of the small molecules of air should take place instantaneously, Le., within a fraction of a second. On the other hand, contamination by deposition of large organic molecules dispersed in air is a much slower process, which can take as long as a few minutes to go to completion. Consequently, if it is indeed these organic molecules which caw the high contact angles measured in air, it should be possible to minimize the effect by means of rapid determination of the contact angle after exposing a new surface through cleavage. The results of the rapid measurements on the ZYD and ZYA surfaces consequently support the organic contamination hypothesis, yielding contact angles Considerably lower than those obtained when making the open air measurements in the usual manner. Surface Energy of Graphite (OOOZ). The history of various approaches to a theory of wettability of smooth solid surfaces has recently been summarized! In brief, the critical surface tension concept of Zisman and his collaboratorss provided the first quantitative tool for studying wettahility as a function of solid-surface constitution. A theoretical approach to the subject was subsequently introduced by Girifalco and Goodlo in their treatment of general interfacial forces. The interaction term in the energy of adhesion was taken as proportional to the g e e metric mean of the surface tensions of the phases at the
*
b
npUn 3. LEED pattern of graphite (0001). surface 111: (a) after m t m p e r a t u r e evacuation: (b) after evacuation wiih bakeout.
that the disordering is incidental and that the ion bombardment removed islands or rings of hydrocarbonaceow mntamination around the crystallites that did not interfere substantially with the LEED pattern. This is highly unlikely, however, since experience has shown that a far smaller dose of ion bombardment than that required to substantially lower the contact angle will remove this type of contamination from a solid surface. The results may, therefore, be regarded as conclusively showing that the Oo contact angle is characteristic of a disordered, i.e., amorphous, clean carbon surface. Value of Contact Angle for Ordered Graphite (OOOZ). The evidence obtained to the effect that disordering of a structured graphite (OOO1)surface lowers the contact angle points to a problem in interpreting the 38" contact angle found for clean structured ZYB surfaces. S i n c e the contact angle is found to decrease upon disordering the structured surface, the possibility exists that a more perfectly ordered surface would yield a still higher contact angle. In that m e , the exact value of the water contact angle would be, in principle, obtainable only on a defect-free (OOO1) plane of a single crystal. It would be quite difficult, if at all possihle, however, to obtain and prove the presence of structural perfection. Nevertheless, some clues can be obtained from the present work as to the likelihood of a substantial increase in contact angle on a more perfectly ordered surface. ZYA is the highest grade of oriented graphite produced by the manufacturer and is more oriented than ZYB. First, it is noted that in investigating surfaces less ordered than electron-bomhardmentheated ZYB, i.e., the ion-bombarded surfaces, it was found that the contact angles were considerably less sensitive to
*
2778
Schrader
The Journal of Physical Chmistry, Vol. 84, No. 21, 1980
TABLE V: Surface Energy of Graphite (0001). Comparison of Values Calculated from Ultrahigh-VacuumWater Contact Angles with Results Obtained by Other Methods group
type of surface energy reported
method
result, erg/cm
assumed y d , erg/cmz
1. Bryant et al." 2. BrennanI9
experimental : ultrahigh-vacuum cleavage theoretical
total total
875
875
514 35 _.
514
3. Girifalco and Ladzo 4. Crowell"
theoretical experimental : compressibility
total total
165 167 176
5. Fort et al."34
experimental: krypton and xenon adsorption
dispersion
151
6. Schrader
experimental: water contact angle on ZYB exDerimental: estimated water contact angle on ideal surface
dispersion dispersion
157 189 162-196
interface, while a, the proportionality constant, was calculated by summing the energy of interaction of pairs of molecules across the interface. Subsequently, Fowkes"J2 proposed separating the surface tension of each phase into additive components and utilized twice the geometric mean of the dispersion components of each phase as the interaction energy when it was reasonable to attribute the entire interaction to dispersion forces. Lifshitz,13in proposing a comprehensive macroscopic theory of long-range van der Waals forces, which utilized the continuum properties of the attracting bodies and the medium separating them, treated wettability as a problem in predicting the thickening or thinning of an existing film of liquid on a substrate.14 Subsequent progress in utilizing macroscopic theory for accurate calcu1ations'"" led to its application to wettability problems. Although it was originally hoped that a form of macroscopic theory alone could provide an adequate treatment of wettability data, it was subsequently found that the role of long-distance forces, treated by macroscopic theory in determining wettability, was strictly limited, and generally accounted for only a minor effect.'* The method of separation of forces" offers a convenient means of comparing water contact angles with other surface energy determinations and calculations on graphite (Oool). The comparisons, of course, must be confined to determinations which offer a reasonable probability of not being affected by hydrocarbonaceous contamination of the graphite surface. Table V lists a series of surface energies which fulfill this criterion. The results of Brennanlg and of Girifalco and Lad20 are theoretical, while Crowell's calculationsz1are based on compressibility measurements which do not involve exposed surfaces. Bryant el aLz2 measured the force of cleavage in ultrahigh vacuum. Fort and collaboratorsa% utilized ultrahigh vacuum valves, with two cold traps between their pumps and the manifold, for their measurements of krypton and xenon adsorption on graphitized carbon black, It is reasonable to suppose that the interaction of water with perfectly structured graphite (OOO1) consists mainly of dispersion forces, with the possible addition of a hydrogen-bonding component arising from interaction of water hydrogen with the ?r electron cloud of the graphite surface. This would be qualitatively similar to, for example, the situation thought to exist at the benzene-water interface. If one neglects any hydrogen-bond contribution to the interaction, the dispersion component of the graphite-surface free energy, y$, can be obtained from the contact angle of water on graphite by means of the equationl2 yL(c0se + 1) (ysd)'/2
2(7Ld)*/Z
where yL is the surface tension of water, 0 the contact angle, and yLdthe dispersion component of the surface
35 165 167 176 151 157 189 162-196
tension of water, 21.8 erg/cmz. By substitution, this reduces to
(~s~= ) ' (COS / ~ 0 + 1)/0.13 From our estimate of 42 f 7" as the range for the contact angle of water on a perfect (0oO1)surface, ysd is calculated to be within the range 179 f 17 erg/cmz. When one takes each of the total energies reported in Table V a~ equal to ysd, it can be seen that our value, together with those of groups 3,4, and 5, form a cluster in the range of 150-200 erg/cm2. The fact that our value is toward the upper portion of this range may be due to neglect of the hydrogen-bonding contribution to the water/graphite (OOO1) interaction. Interaction of Water with the Ion-Bombarded Surface. The rationale for letting E 'v Y~~ for the (OOO1) plane (where E is the total energy) and also for assuming that the interaction of water with graphite (OOO1) is due mainly to dispersion forces is the fact that the interplane cohesive forces in graphite seem to be of the van der Waals type. The assumption does not apply of course, to the interaction of water with ion-bombarded graphite (0oO1). Disruption of the covalently bonded sheet forming the (0oO1) plane should lead to a high concentration of "dangling bonds" on the clean surface in ultrahigh vacuum. Dissociation of the water molecule to form functional groups on the carbon surface is possible, but questionable, in view of the ease of desorption on evacuation a t room temperature. Possibly, the unused chemical valencies promote formation of a weak chemisorbed species with the undissociated water molecules, forming a monolayer upon which liquid water is spread. Also possible is that the combination of size reduction and disorientation of the graphite crystallites produces a molecular d e roughness effect, which reduces the water contact angle to Oo. It should be noted, in this connection, that ion bombardments will not generally reduce the water contact angle of a typical hydrophobic surface, such as a hydrocarbon, to 0". If the molecular scale roughness effect is indeed responsible here, no doubt that is because the undisturbed graphite surface already is considerablyhydrophilic. This can be seen from cos 42" = 0.74, as compared to cos 0" = 1 for a surface which spreads water, Still another alternative, of course, to interaction with a monolayer of "dangling bonds" on the one hand and molecular scale roughness on the other is a combination of these two effects, with the water sinking into a matrix of dangling bonds in amorphous carbon a few layers deep. In any event, an essential point to be gleaned from the behavior of ion-bombarded graphite is that there is nothing hydrophobic per se about carbon itself. Rather, its hydrophobicity or lack thereof is a function of ita chemical and crystallographic state. Roughly speaking, its hydrophobicity is at a maximum when in a state of high
J. phys. Chem. 1980, 8 4 , 2779-2782
chemical saturation (of course moderated by the characteristics of the combining elements themselves, such as C1, Br, F, etc.) and at a minimum or nonexistant when chemically unsaturated. One more point requiring mention is the possible effect of the graphite substrate on an amorphous surface layer. That is, is the attraction to water which produces spreading, due to the sum of forces exerted on the water by the crystalline graphite substrate and the superficial amorphous layer, or to the amorphous layer alone. If the substrate does have an effect, it would, of necessity, be due to long-range forces. Previous calculations have indicated, however, that a layer such as that produced here by ion bombardment can be expected to completely screen the long-range forces of any substrate.18 The behavior of the amorphous carbon layer produced here by ion bombardment of graphite can therefore be generalized to hold true for all equivalent amorphous carbon layers regardless of the nature of the underlying bulk material.
Conclusions 1. Electron-bombardment heating in ultrahigh vacuum of ZYB oriented graphite (Union Carbide) yields a surface with a LEED pattern and a water contact angle of 38 f 3O. Disordering of this surface through controlled ion bombardment destroys the LEED pattern and gradually lowers the contact angle to 0' while retaining the mirrorlike appearance of the surface. 2. The contact angle of water on a clean, perfectly ordered graphite (O001) surface lies in the range of 42 f.'7 3. The contact angle of water on a clean, highly disordered, elemental carbon surface is 0'. 4. Comparison of the estimated contact angle of water on perfect graphite (0001) with theoretical and experimental determinations of the surface energy of the latter yields results consistent with the hypothesis that the
2779
graphite (0001)/H20 interaction is mainly through dispersion forces, with a possible small hydrogen-bonding contribution. 5. The hydrophobicity of surface carbon tends to increase with its covalent chemical saturation, e.g., hydrocarbons > graphite > clean ion-bombarded carbon. Acknowledgment. I thank Max. G. LagaUy for valuable discussions on interpretation of the LEED patterns.
References and Notes (1) Presented In part at the 177th Natlonal Meeting of the American Chemlcal soclety,"onolukr, HI, Aprll 1979. (2) Part 4 M. E. Schradw, J. phys. chem.79, 2508 (1975). (3) F. M. Fowkes and W. D. Harkhs, J. Am. Chem. Soc., 62, 3377 (1940). (4) I. Morcos, J . cdkld Interface Scl., 34,469 (1970). (5) I. Morcos, J . Chem. phys. 57, 1801 (1972). (6) B. R. b y , presented at the 163rd Natknal Meeting of the Amerlcen chemical soclety, Aprll 1972. (7) M. E. Tadros, P. Hu,and A. W. A d a m , J . W I n t e r l e c e Scl., 49, 184 (1974). (8) M. E. Schradw, "Swface Contamlnatlon: Gene&$, Detectton, and Control", Vol. 2,K. L. M a l . Ed., Plenum, New York, 1979. (9) W. A. Z h n , A&. Chem. Ser., No. 43,1 (l963),and references threln. (10) L. A. Qmalco and I?.J. Goal, J. phys. Chem., 61,904 (1957). (11) F. M. Fowkes, J. phys. Chem., 0 ,382 (1962). (12) F. M. Fowkes, I d . €ng. Chem., 56, 40 (1984). (13) E. M. Ltfshltz, Zh. Eksp. Tew. Rz., 29,94 (1955);Sov. phys. J. (~ngl. 2, 73 (1956). (14) I. E. DzyakshlnskY, E. M. Ltfshltz, and L. P. Pttaevsktl, A&. phys.
mensr.),
10, 165 (1959). (15) V. A. Parseglan, Trans. Fera&y Soc..62,848 (1966). (16) V. A. Parwglan, Sclence, 156, 939 (1967). (17) V. A. and B. W. NFsram, &h(Lada?), 224,1197 (1999). (18) V. A. perseglan, Q. H. W e b , end M. E. schreder, J. W Z n t e i i & @ Scl., 61,356 (1977). (19) R. D. Brennan, J . chem. phys., 20, 40 (1952). (20) L. A. CYifalco and R. A. Lad, J . Chem. phys., 25, 593 (1956). (21) A. D. Crowd, J. Chem. phys., 29,446 (1958). (22) P. J. Bryant, P. L. Qutshal, and L. H. Taylor, Weer, 7, 118 (1984). (23) F. A. Pumam and 1. Fort, Jr., J. phys. chem., 79, 459 (1975). (24) 1.Fat, Jr., and V. P. Toan, presented at the 177th N a b 1 k h t h g of the American Chemical Society, AprR 1979.
Interaction between Hydroxides of Alkali Metals and Acid Centers on the Surface of Alumina R. Fledorow' and I. 0. Dalla Lana' Depertment of chemlcal Em-,
U n m of A m , Edmonton, Cam& T60 206 (Receki: ApU 7, 1080)
The interaction between lithium and sodium hydroxides and alumina surface centers was studied by using IR spectroscopy. The results indicate that the amount of ypicoline (a base capable of selective interaction with Lewis-acid centers) decreases significantly as a result of introduction of sodium cations and less so after the introduction of lithium cations. It is concluded that alkali metal ions can react not only with Bronsted-acid centers but also with Lewis-acid centers. Furthermore, the evidence suggests a greater than stoichiometric interaction, possibly from steric coverage effects, when the ionic radius of the alkali metal ion exceeds that of lithium ions.
Introduction The deposition of sodium hydroxide upon the surface of y-alumina changes both the surface properties and the catalytic activity'+ of the alumina. These changes have been attributed to ion exchange between the sodium ions and the acidic OH groups of the alumina surfa~e;~ however, the possibility of interaction between the alkali metal and
coordinatively unsaturated A13+ cations cannot be discounted. According to Pines and Haag,6 sodium hydroxide can react with Lewis-acid sites on the surface of alumina to form the following complex:
'Institute of Chemistry, A. Mickiewicz University, Poznan, Poland.
contrary to Roas and Bennett6who were unable to observe detectable amounts of such a complex. In addition, Da-
0022-3654/8O/2084-2779$0 1 .OO/O
-AI
I I
+
NaOH
-
0 1980 American Chemical Society
I I
[-AI-OH]
-
No.
i
