Heat of Immersion of Ti02 in Organic Liquids. Effect of Chemisorbed H

The effect of chemisorbed HzO on the electrostatic field strength, F, of TiOz (rutile) was investigated by measuring the heat of immersion into linear...
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Langmuir 1987,3,1119-1123

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Heat of Immersion of Ti02 in Organic Liquids. Effect of Chemisorbed H 2 0 on the Electrostatic Field Strength Yasuharu Suda Department of Chemistry, Faculty of Science, Okayama University, Tsushima, Okayama 700, Japan Received March 31, 1987. I n Final Form: June 16, 1987 The effect of chemisorbed HzO on the electrostatic field strength, F, of TiOz (rutile) was investigated by measuring the heat of immersion into linear aliphatic organic liquids with different dipole moments. The surface properties of TiOz are differently affected by two kinds of chemisorbed HzO: dissociatively adsorbed HzO, i.e., surface hydroxyls, on the (110) surface and molecularly adsorbed HzO on the (100) and (101) surfaces. When the molecularly adsorbed Hz0 is desorbed by evacuating the sample at temperatures from 25 to 150 OC,the net heat of adsorption of organic molecules increases linearly and slightly, and when surface hydroxyls are removed by heating the sample from 150 to 600 "C,the heat value increases shar ly. The average F value of TiOz surface calculated from the heat-of-immersion data is largest, 1.82 X 10 statvolt cm-l, on the bare surface and decreases with increasing amount of chemisorbed HzO, sharply at the initial stage and slowly at the final stage. The final F value is 1.58 X lo4 statvolt cm-' on the fully hydrated surface. Though the curve of F against the coverage of chemisorbed HzO indicates the heterogeneity of rutile surface as a whole, further analysis of the F value shows that the two kinds of surfaces on TiOz are homo eneous and have individual F values: 3.26 X lo6 statvolt cm-' on the bare (110) surface and 4.47 x 109 statvolt cm-' on the bare (100) + (101) surface.

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Introduction It has been reported that the T i 0 2 (rutile) surface evacuated at room temperature carries molecularly adsorbed HzO as well as surface hydroxyls.'" Both kinds of chemisorbed H 2 0 play an important role individually A number of reon the surface properties of searchers have measured the heat of immersion of solids into organic liquids with different dipole moments to estimate the electrostatic field strength, F, on the solid surfaces.+l6 The F value of rutile was also measured by this method,*12J6 but no researcher has studied the effect of chemisorbed HzO on the F value of the solid. In a previous paper,le the F value of the ZnO surface was measured as a function of the amount of surface hydroxyls, and it was found that the F value was strongly affected by the presence of surface hydroxyls or decreased linearly with increasing amounts of hydroxyls. The present work has been attempted to investigate the effect of chemisorbed HzO on the F value of Ti02 (rutile) by measuring the heat of immersion of the samples, which have different amounts of chemisorbed HzO, into various organic liquids and to compare the data with those of ZnO. (1)Jones, P.; Hockey, J. A. Trans. Faraday SOC. 1971,67,2679. (2)Munuera, G.;Stone, F. S. Diacuss. Faraday Soc. 1971,52, 205. (3)Jackson, P.;Parftt, G. D. Tram. Faraday SOC.1971,67,2469. (4)Parfitt, G.D. h o g . Surf. Membr. Sci. 1976,11,189. (5)Griffitha, D.M.; Rochester, C. H. J. Chem. SOC., Faraday Trans. 1 1977,73,1510. (6)Suda, Y.; Morimoto, T.; Nagao, M. Langmuir 1987,3,99. (7)Zettlemover. A. C.: Ivencrer. R. D.: Scheidt. P. J. Colloid Interface Sci. 1966,22,li2.' (8)Suda, Y.; Nagao, M. J. Chem. SOC.,Faraday Trans. 1 1987,83, 1739. (9) Healey, F.H.; Chessick, J. J.; Zettlemoyer, A. C.; Young, G. J. Can. J. Chem. 1958,62,489. (10)Chessick, J. J.; Zettlemoyer, A. C.; Healey, F. H.; Young, G. J. Can. J. Chem. 1955,33,251. (11)Zettlemoyer, A. C.; Chessick, J. J.; Hollabaugh, C. M. J. Phys. Chem. 1958,62,489. (12)Romo, L. A. J. Colloid Sci. 1961,16,139. (13)Dear, D. J. A.; Eley, D. D.; Johnson, B. C. Trans. Faraday Soc. 1963,59,713. (14)Cochran, H.; Rudham, R. Trans. Faraday SOC.1965,61,2246. (15)Lavell, J. A.; Zettlemoyer, A. C. J. Phys. Chem. 1967,71,414. (16)Morimoto, T.;Suda, Y. Langmuir 1986,I, 239. I

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Table I. Number of Adsorbed Molecules, Molecules nm-* 25 O c a 100°C 150°C 200 'C 300 "C 600 "C 1-BuOH 2.27 2.93 3.08 3.15 3.24 3.27 1.92 2.02 1-BuCl 1.77 2.08 2.14 2.16 n-C7HI6

1.30

1.34

1.38

1.39

1.41

1.42

Degassing temperature.

Experimental Section The TiOz (rutile) sample used in this study was supplied by Teikoku-kako Co. and purified in the same way as was reported previously? The sample was first degassed at 600 O C for 4 h under a vacuum of 1.33 mN m-2in order to remove surface contaminants, followed by equilibration with saturated H20vapor for 15 h at room temperature to establish complete surface hydration. The sample was then evacuated at different temperatures between 25 and 600 O C under a vacuum of 1.33 mN m-2 to obtain the differently hydrated samples. The content of chemisorbed HzO remaining on the surface was measured by the successiveignition loss method." The specific surface area of the sample first degassed at 600 "C was determined by the Nz-BETmethod and was found to be 22.6 m2g-', being unchanged by further treatment at temperatures lower than 600 O C . Immersional liquids used were heptane (n-C,H,,), 1-butanol (1-BuOH), and 1-chlorobutane (1-BuCl), all of which were guaranteed grade reagents of Nakarai Chemicals. These liquids were dried by the use of a suitable desiccant, distilled, and stored in contact with molecular sieves 4A activated at 500 "C. The heat of immersion was measured at 28 0.1 "C by using an adiabatic calorimeter equipped with a thermistor of 10-kQ resistance as a temperature-sensing element.'* Absence of HzO was carefully confirmed by repetition of the heat-of-immersion measurement by using the same rutile sample; conclusively,every measurement was carried out in the presence of activated molecular sieves 4A.

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Results The HzO content of rutile sample, which is the amount of chemisorbed HzO remaining on the surface after evacuating the fully hydrated sample at a given temperature, is shown in Figure 1as a function of evacuation temper(17)Morimoto, T.;Shiomi, K.; Tanaka, H. Bull. Chem. Soc. Jpn. 1964, 37,392. (18)Nagao, M.; Morimoto, T. J. Phys. Chem. 1969,73,3809.

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200 400 600 Degassing temperature, 'C

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Figure 1. Surface H 2 0 content of rutile sample. 60C r

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40C

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200 400 600 Degassing temperatureC '.

Figure 2. Dependence of heat of immersion of rutile in organic 1-BuCl,(0) liquids on degassing temperature: ( 0 )1-BuOH,(0) n-C7HIG.

ature. The content is expressed in the number of hydroxyls per unit area by assuming that a H20 molecule liberated comes from the condensation dehydration from two surface hydroxyls. I t is seen from Figure 1that the amount of chemisorbed H 2 0 gives the highest value (6.90 OH groups nm-2) on the fully hydrated surface evacuated at 25 "C and decreases with rising evacuation temperature, especially steeply between 25 and 300 "C, and that it is almost nil on the 600 "C evacuated sample. The heat of immersion of rutile in organic liquids is plotted against the evacuation temperature as illustrated in Figure 2. As shown from Figure 2, the heat-of-immersion value increases with rising evacuation temperature, i.e., with decreasing amount of chemisorbed H20;the heat value increases sharply in the temperature range up to 300 "C and then very slowly up to 600 OC. The heat value on a surface treated at a given temperature becomes greater in the order, 1-BuOH > 1-BuCl > n-C7H16. In a previous paper: the number of organic molecules adsorbed on the unit area of rutile was calculated from the monolayer capacity in the first adsorption isotherm measured just after evacuation treatment of the hydrated rutile sample. The results obtained are cited in Table I. It is evident from Table I that the density of adsorbed molecules increases on every adsorbate with rising evacuation temperature of the sample, and it is greatest on 1-BuOH and smallest on n-C7H16,the latter being almost unchanged by the evacuation temperature. Furthermore, it was clarifieds that 1-BuOH is dissociatively adsorbed on the bare surface of r ~ t i l e ~ Jand + ~also ~ that it is chemisorbed

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2 4 6 Amount of chemisorbed H20, OH groups nm-2

Figure 3. Relation between net heat of adsorption of organic molecules and surface H20 content of rutile. Symbols are the same as those in Figure 2.

through esterification with acidic surface hydroxyls22on the hydrated surface of rutile. However, 1-BuC1and n-C7H16can only be physisorbed on the rutile surface, regardless of the degree of surface dehydration.s The difference in the adsorption mechanism between 1BuOH and the other two kinds of molecules, 1-BuC1and n-C7H16,will account for the abnormally high heat values of 1-BuOH compared with those expected from the magnitude of dipole moments of the molecules: n-C7H16(0) < 1-BuOH (1.75) < 1-BuC1 (1.90 D).23 By subtracting the surface enthalpy of the immersional liquid from the heat-of-immersion value, one can obtain the net heat of adsorption per square nanometer, which permits further calculation of the net heat of adsorption per molecule by introducing the density of adsorbed molecules in Table I. The heat value thus calculated is illustrated in Figure 3 as a function of the amount of chemisorbed H20. It is found from Figure 3 that the net heat of adsorption is largest on the bare surface and decreases steeply with increasing amount of chemisorbed H20, until the H 2 0 content amounts to 2.42 OH groups nm-2, and then gradually. The shape of curves in Figure 3 seems to suggest that the actual surface of the present sample is composed of two types of surfaces which have different properties on the interaction with organic molecules. The average electrostatic field strength F of rutile surface can be calculated from the slope of the linear part of the curve illustrating the net heat of adsorption per molecule against the dipole moment of organic molec u l e ~ . ~Lave11 - ~ ~ and Zettlemoyer16found that the heat values of rutile in every immersional liquid gave a collinear relationship against the dipole moment of the liquid, contrary to the present result that the heat of 1-BuOH is extraordinarily large. Therefore, the heat values of 1BuOH were not adopted in the present calculation of the F value. The F value thus calculated is shown in Figure 4 as a function of the amount of chemisorbed H20. Figure 4 shows that the F value of the rutile surface is largest, i.e., (19) Jackson, P.; Parfitt, D. G. J. Chem. SOC., Faraday Trans.I 1972, 68,1443. (20) Isirikyan, A. A.;Kiselev, A. V.; Uskakova, E. V. Kolloidn. Zh. 1963,25, 1255.

(21) Primet, M.; Pichat, P.; Mathieu, M. V. J . Phys. Chem. 1971, 75, 1221. (22) Boehm, H.Discuss.-FaradaySOC. 1971,52, 264. (23) Kagaku-binran Kisohen 2, Nihonkagakukai ed.; Maruzen: Tokyo, 1984; Vol. 3.

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Heat of Immersion of TiOz in Organic Liquids

b Amount of chemisorbed H20, OHgroups nni2

Figure 4. Relation between electrostatic field strength of rutile and surface H20 content.

1.82 X lo5 statvolt cm-l, on the bare surface and decreases with increasing surface concentration of chemisorbed H20, sharply in the initial stage and then slowly, giving a convex curve against the abscissa. Lave11 and Zettlemoyer16reported the F value of a rutile sample having an unknown amount of surface hydroxyls to be 2.72 X lo5 statvolt cm-', which was larger than the value (1.82 X lo5 statvolt cm-') of the bare surface in the present work. However, it should be noted from the present data that the amount of chemisorbed H20 alters the F value of rutile. When the surface concentration exceeds 2.42 OH groups nm-2, the F value decreases gently and almost linearly until the surface is fully hydrated. On the fully hydrated surface, the F value is smallest, i.e., 1.58 X lo4statvolt cm-l, which is less than one-tenth of the value on the bare surface.

Discussion It has been reported that the surface of powdered rutile sample is mainly composed of three kinds of crystal planes: (1101, (loo), and (lOl).l*N Furthermore, it is pointed out that the (110) surface adsorbs HzO dissociatively to form surface hydroxyl~,l-~* whereas the (100) and (101) surfaces can adsorb HzO molecularly onto the surface Ti4+ions.ls4 In a previous a possible model for these two kinds of chemisorbed HzO was proposed. It was experimentally found that the molecularly adsorbed HzO was primarily desorbed by evacuating at temperatures below 150 "C and that the surface hydroxyls could be removed at temperatures above 150 0C.26 When the surface is treated at 150 "C, it carries chemisorbed HzO of 2.42 OH groups nm-2. It may be reasonable to consider that the 1-BuC1molecule is adsorbed on the bare surface of TiOz by directing the negative pole, i.e., C1 atom, of the molecule to the surface Ti4+ion, keeping the dipole axis perpendicular to the surface as in the case on the bare ZnO surface.16 This type of adsorption will be true on the (110) plane as illustrated in Figure 5a, which results in a large interaction through the electrostatic attractive force between the C1 atom and the Ti4+ion. On the (100) plane, the C1 atom cannot touch directly on the Ti4+ion but can touch the 02-ions, because the geometry of the (100) plane does not permit the C1 atom to come in contact with the Ti4+ions, as shown in Figure 5b. Admittedly, the electrostatic attractive effect should be extremely small in this case. The same feature appears also when the 1-BuC1 molecule is adsorbed on the (101) plane, though the model is not illustrated. These models account for the data in Figure 3 that the heat of adsorption of 1-BuCl is larger on the (110) surface than on the (100) and (101) surfaces. On the (24) Rutley's Elements of Mineralogy; Read, H. H., ed.; George M e n & Unwin: London, 1962. (25) Jones, P.; Hockey, J. A. Trans. Faraday SOC.1971, 67, 2669. (26) Suda, Y.; Morimoto, T. Langmuir 1987,3, 786.

0

Ti

Oo

a

Figure 5. Adsorption models of 1-BuClmolecule on rutile surface: (a) (110); (b) (100)plane.

hydroxylated surface, the 1-BuC1molecule will be adsorbed by directing the C1 atom to the H atom of a surface OH group and by arranging its carbon chain parallel to the surface, as described in a previous paper: which results in a depression of the adsorption energy. As can be understood from the data in Table I, the n-C7H16molecule will be adsorbed with the hydrocarbon chain parallel to the surface even when the degree of surface hydration varies.* As a matter of course, two kinds of surfaces on rutile will behave differently on the adsorption of ?z-C~H,~. On the (110) surface, the CH3 (or CH2) group of the molecule can touch directly Ti4+ions as well as 02-ions, though the van der Waals radius of the CH, group (0.20 nm)23is somewhat larger than the radius of the C1 atom (0.18 nm).23 On account of the nonpolar nature of n-C7H16,the interaction of this molecule with the (110) surface should involve the contribution due to the induced dipole-electrostatic field interaction in addition to the dispersion force, in contrast to the 1-BuC1molecule which displays an additional effect, the permanent dipole-electrostatic field interaction, besides the two kinds of effects. Thus, the heat of adsorption of n-C7H16increases more gently than that of 1-BuC1when the (110) surface is dehydroxylated, as shown in the coverages smaller than 2.42 OH groups nm-2 in Figure 3. On the (100) and (101) surfaces, the density of both Ti4+ and 02-ions is smaller than that on the (110) surface,1-26 and in addition, direct contact of the CH3and CH2groups with the Ti4+ions is impossible, as in the case of the C1 atom. Thus, the n-C7H16molecule adsorbed on the (100) and (101) surfaces will cause only a small interaction due to the induced dipole, compared with that on the (110) surface. This model can account for the data in Figure 3 that only a very small heat evolves when n-C7H16is adsorbed on the surface dehydrated at temperatures below 150 "C. In a previous work,16the F value was measured on a ZnO sample with a well-developed (1010) plane by means of the same method as in the present study. I t was found that the F value of ZnO was highest, 3.38 X lo5 statvolt cm-', on the dehydroxylated surface, decreased linearly with

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Figure 6. Separation of heat of adsorption of organic molecules into two kinds of surfaces: (0)1-BuCl, (0)n-C,Hl,; (A) hydroxylated (110),(B) partidy dehydrated (100) + (101),(C) fully dehydrated (100) + (101), (D)partially dehydroxylated (110). increasing amounts of surface hydroxyls, and finally reached the lowest value, 3.68 X lo3 statvolt cm-l, on the fully hydroxylated surface. In other words, the additivity holds between the two F values of the bare surface and of the fully hydroxylated one of ZnO. This indicates that the surface of the ZnO sample used is energetically homogeneous, as demonstrated by the measurement of the differential heat of formation of surface hydroxylsaZs Figure 4 reveals the change in the average F value of the rutile surface with the surface H 2 0 content; the rutile surface as a whole can be understood to be heterogeneous from the shape of the F vs H 2 0 content curve. However, the F value decreases almost linearly in the HzO content region exceeding 2.42 OH groups nm-z, which suggests the molecular-HzO-adsorbing surface to be homogeneous, in reference to the linearity of the F vs HzO content curve on ZnO. This stimulates us to separate the average F value into two parts, i.e., the F value on the hydroxyl-adsorbing surface (110) as well as that on the molecular-HzO-adsorbing surface, (100) + (101). For this purpose, the following two assumptions are required (1)the heat of adsorption of both the 1-BuCl and n-C,H16 molecules is the same on two kinds of fully hydrated surfaces; (2) the surface density of each adsorbate is the same on two kinds of fully hydrated surfaces. First, the heat of adsorption on the fully hydrated surface can be allotted to two kinds of surfaces in the following way. The total amount of chemisorbed HzO is 6.90 groups nm-2 on the fully hydrated surface (Figure l), and the amount of molecularly adsorbed H20 on the (100) + (101) surface was experimentally found to be 1.61 HzO nm-2 on the present sample?s Therefore, the amount of hydroxyls on the (110) surface can be calculated to be 3.68 OH groups nm-2. On the basis of the structural modelz6of the hydrated rutile surface, the HzO content on every cleavage plane of the crystal can be evaluated to be 10.2 OH groups, 3.95 H20, and 3.70 HzOnm-z for the (110), (loo), and (101) planes, respectively. From these values of surface HzO content, we can calculate the ratio of the (110) and (100) + (101) surfaces to be 46% and 54%, respectively. In this calculation, it is assumed that the (100) and (101) surfaces exist in equal p r o p o r t i ~ n . ' ,On ~ ~ the basis of this ratio of the actual surface, the heat-of-adsorption data in Figure 3 are separated graphically into two kinds of surfaces, as illustrated in Figure 6. Since the surface density of 1-BuCl (27) Chessick, J. J. J. Phys. Chem. 1962, 66, 762. (28) Nagao, M.; Yunoki, K.; Muraishi, H.; Morimoto, T. J. Phys. Chem. 1978,82, 1032.

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2 L 6 Amount of Chemisorbed H20. OHgroups nm-2

Figure 7. Relation between net heat of adsorption of organic molecules and surface H20content on two kinds of surfaces: (110) surface, ( 0 )1-BuC1,(M) n-C7H16; (100)+ (101)surface, (0) 1-BuC1, (0) n-C7Hl,. is 1.77 molecules nm-2 and the heat of adsorption of the molecule is 76.3 erg cm-2 on the fully hydrated surface, both values on the hydroxylated (110) surface amount to 0.814 molecules nm-2 and 35.1 erg cm-2, respectively. A similar calculation for the adsorption of n-C7H16gives 0.589 molecules and 24.0 erg cm-z on the hydroxylated (110) surface. In the course of dehydration of the sample at temperatures from 25 to 150 "C, where the coverage decreases from 6.90 to 2.42 OH groups nmW2,the molecularly adsorbed H 2 0 on the (100) + (101) surface is simply desorbed, and the fully hydroxylated (110) surface remains unchanged. In this coverage region, therefore, the surface density and the heat-of-adsorption value of adsorbate on the (110) surface are also unchanged and equal to that on the 25 "C treated surface (Figure 6A), and the increases in the surface density and the heat of adsorption are exclusively attributed to those on the (100) + (101) surface (Figure 6B). On the other hand, at the higher evacuation temperatures from 150 to 600 "C, i.e., at the coverage region less than 2.42 OH groups nm-2, the surface density and the heat of adsorption are kept constant on the (100) + (101) surface (Figure 6C) and increase on the (110) surface in the course of dehydroxylation (Figure 6D). The heat of adsorption per molecule thus computed on each of the (110) and (100) + (101) surfaces is illustrated in Figure 7 as a function of the amount of chemisorbed HzO. The results in Figure 7 realize the adsorption models of the two kinds of organic molecules as demonstrated above. In other words, the heat of adsorption of organic molecules increases slowly on the (100) + (101) surfaces when the molecularly adsorbed HzO is desorbed and steeply on the (110) surface when surface hydroxyls are removed. In addition, the difference of the heat of adsorption of the two kinds of molecules, 1-BuCl and nC7HI6,is small on the (100) + (101) surface but larger on the (110) surface. From the heat-of-adsorption data in Figure 7, the F value on each of the surfaces, (110) and (100) + (1011, can be calculated and plotted against the amount of chemisorbed HzO as illustrated in Figure 8. As shown in Figure 8, the F value of the (100) + (101) surface, F(loo),is 1.58 X lo4 statvolt cm-' on the fully hydrated surface and increases linearly with decreasing amount of chemisorbed HzO, which suggests the (100) + (101) surface to be homogeneous. The p,,, value amounts to 4.47 X lo4 statvolt cm-l, when the surface is completely dehydrated. On the

Langmuir 1987, 3, 1123-1127

Amount of chemisorbed HzO, OH groups n d

Figure 8. Relation between electrostatic field strength and surface H20content on each of two kinds of surfaces: ( 0 )(110) surface; (0) (100) + (101) surface. other hand, the F value of the (110) plane, F(,,,), remains constant at 1.54 X lo4 statvolt cm-l at evacuation temperatures from 25 to 150 OC. The F(llo,value then increases almost linearly with decreasing amount of surface hydroxyls, which also impliesthe (110) surface to be homogeneous. Strictly speaking, on the extremely dehydroxylated surfaces the F(',,) curve deviates slightly to higher values from the straight line, probably because af the presence of active sites or various kinds of surface defeds. In conclusion, the two kinds of surfaces, (110) and (100) + (101), are found to be homogeneous, respectively.

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Furthermore, it is interesting to see that the F(llo) value on the bare (110) surface is 3.26 X lo5 statvolt cm-', being remarkably larger than the average F value of the actual surface (Figure 4). In the bulk of the rutile crystal, the coordination numbers of Ti4+and 02-are 6 and 3, respectively, so that the formal charges of Ti4+and 02-are +2/3eand -2/3e per bond, respectively, where e is the electronic charge. On the other hand, in the bulk of the wurtzite crystal the coordination numbers of both Zn2+and 02-are 4, so that the formal charges of Zn2+are 02-is + 1 / 2 eand -1/2eper bond, respectively. Thus it can be expected that the F value of the bare (110) surface of rutile is larger than that of the bare (10x0) plane of ZnO. However, the experimental F values of rutile and ZnO are found to be almost equal, Le., 3.26 X lo5 and 3.38 X lo5 statvolt cm-l, respectively. Upon the analysis procedure of heat of adsorption stated above, it is assumed that the heat values of the organic adsorbates on the hydroxylated (110) surface are identical with those on the hydrated (100) + (101) surface, but in practice the former may be larger than the latter. With this point of view taken into account, the true F value of the (110) surface of rutile may be larger than the F(',,, value shown in Figure 8.

Acknowledgment. I thank Professor Tohru Takenaka of Kyoto University for his kind advice and helpful suggestions and Professor Mahiko Nagao of Okayama University for much help in the heabof-immersionexperiments. I also thank Professor Tetsuo Morimoto of Okayama University for his helpful discussions and constant encouragement throughout this work. Registry No. H20, 7732-18-5;Ti02, 13463-67-7; 1-BuOH, 71-36-3; l-BuCI, 109-69-3;n-C,Hle, 142-82-5.

Interactions of Diatomic Molecules with Graphite Mary J. Bojan and William A. Steele* Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802 Received March 31, 1987. In Final Form: June 24, 1987 Low-coverage isotherm data for the adsorption of 02 on exfoliated graphite are reported and analyzed by using the virial adsorption isotherm. The molecule-solid and the adsorbed molecule-molecule second virial coefficients are obtained for this system over a range of temperature. These data are compared with theory, and best-fit parameters for the site-site moleculesolid interaction potential are given. In addition, a comparative study of the molecule-molecule interactions for 02,N2,and CO is presented, based on the molecule-molecule virial coefficients reported here (and elsewhere for N2and CO). It is concluded that the solid produces an alteration in the site-site well-depths for these systems in agreement with theoretical arguments. 1. Introduction The thermodynamics and phase equilibria of simple molecules adsorbed on the (nearly) homogeneous exposed basal plane of graphite have been a subject of great interest over the past decade or so. An understanding of the interaction of these molecules with the solid and with each other when in an adsorbed film is essential for these systems both to the development of an adequate theoretical understanding and to the elucidation of molecular behavior via computer simulation. The virial adsorption isotherm is a powerful tool for analysis of experimental data, since 0743-7463/87/2403-1123$01.50/0

it enables one to extract unambiguous information concerning single adsorbed molecules and pairs of such molecules from the measurements.l This approach has led to reasonably good interaction potentials for the rare gases on graphite, judged by the success of computer simulations in reproducing the phase behavior of these systems.2 Applications to nonspherical adsorbate molecules (1) Steele, W. A.; Haleey, G. D., Jr. J. Chem. Phys. 1954, 22, 979. Sams, J. R., Jr.; Constabaris, G.; Halsey, G. D., Jr. J.Phys. Chem. 1960, 64, 1689. Sams, J. R., Jr.; Constabaris, G.; Halsey, G. D., Jr. J. Chem. Phys. 1962,36, 1334.

0 1987 American Chemical Society