J. Phys. Chem. 1982, 86, 4188-4193
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However, the classical expression fails for small clusters of ammonia, pyridine, acetonitrile, and methanol about ions. This suggests that general application of the Thomson formulation to non-water systems will not be valid, at least at small cluster sizes.
Acknowledgment. Support of the U.S. Army Research Office under Grant No. DAAG 29-79-C-0133and the Atmospheric Sciences Section of the National Science Foundation under Grant No. AtM 79-13801 is gratefully acknowledged.
Interaction of Organic Molecules with the Surface of Zinc Oxide Mahlko Nagao, Kazuyukl Matsuoka,t Hltoko Hlrai,t and Tetsuo Morlmotot Research Laboratoty for Surface Sclence, and Department of Chemistry, Faculty of Sclence, Okayama University, Tsushima, Okayama 700, Japan (Received: December 21, 1981; I n Final Form: March 19, 1982)
Adsorption isotherms of organic molecules, n-BuOH,n-BuC1, n-C7H16,and CH3N02,on ZnO surfacm hydroxylated to varying degrees were measured at 25 "C, which allowed an estimation of the amounts of irreversibly and reversibly adsorbed molecules. Irrespective of the differences in polarity and functional group of the organic molecules, the amount of irreversibly adsorbed molecules decreased linearly, while that of reversibly adsorbed ones increased, with increasing surface hydroxyl content of the sample. This implies that hydroxyl groups on the metal oxide surface inhibit the chemisorption of organic molecules but act as effective sites for reversible physisorption of them. The infrared spectroscopic method was applied to identify the adsorbed species on surfaces of both strongly dehydroxylated and fully hydroxylated ZnO samples. n-BuOH molecules were found to be chemisorbed dissociatively to form surface alkoxyl and hydroxyl groups on the dehydroxylated surface, which resulted in the formation of an autophobic surface layer. It seemed likely that there was a strong interaction, which might be regarded as chemisorption, between n-BuC1 molecules and the dehydroxylated ZnO surface. Nonpolar, n-C7H16molecules with no functional group, however, were held on the surface simply by physical interaction. For CH3N02,surface hydroxyl groups were produced on the dehydroxylated ZnO sample, which was interpreted in terms of the electron-attractive effect of nitro groups in this molecule.
Introduction A study of the interaction between metal oxide surfaces and organic molecules is a fundamental subject in the field of catalyst chemistry as well as an attractive one in the surface chemistry of solids. The surface properties of a solid can be best appreciated by studying the interaction with organic molecules which have different properties such as molecular size, polarity, and kind of functional groups. At this time, special regard should be paid to the role of surface hydroxyl groups on metal oxides, because they are held tenaciously on the surface even after degassing the sample at higher temperatures.'" Actually, some cases are known where the presence of optimum amounts of water enhance a catalytic activity of metal oxide^.^?^ Therefore, it will be useful in the characterization of solid surfaces to understand the adsorption of organic molecules on metal oxide surfaces as a function of the surface hydroxyl content of the sample. In the previous work,' the adsorption of a series of normal aliphatic alcohols (C1-C3)on ZnO with a controlled number of surface hydroxyls has been measured to investigate the effect of hydroxyl groups on the adsorption of alcohol molecules. The results showed that the dissociative chemisorption of alcohols occurred on the dehydroxylated ZnO surface to produce both alkoxyl and hydroxyl groups, but on the fully hydroxylated surface reversible physisorption took place predominantly through hydrogen bonding. I t was also revealed that the amount of irreversibly adsorbed alcohol molecules decreases, while that of the reversibly adsorbed ones increases, with increasing hydroxyl content of the sample.
* Research Laboratory for Surface Science. 'Department of Chemistry. 0022-3654/82/2086-4188$01.25/0
In the present work, as an extension of the previous study, the interaction of ZnO surfaces with four kinds of organic molecules which have different functional groups was investigated in connection with the effect of surface hydroxyls on the adsorption of these organic molecules. Experimental Section Materials and Pretreatment. The original ZnO sample used in this study was Kadox 15 produced by New Jersey Zinc Co., being the same as that used in the previous work.' The sample was first degassed at 600 OC for 4 h under vacuum at 1 X N m-2 in order to remove surface contaminations which might be present. This sample was then kept for 15 h at room temperature in contact with saturated water vapor to ensure complete hydroxylation. We obtained ZnO samples covered with a different number of surface hydroxyls by evacuating the fully hydroxylated sample at various temperatures between 25 and 600 O C under vacuum at 1 X N m-2. The organic compounds used as adsorbated were l-butanol (n-BuOH), 1-chlorobutane (n-BuCl), heptane (nC7H16),and nitromethane (CH3N02),all of which were guaranteed grade reagents of Nakarai Chemicals. These adsorbates, except CH3N02,were purified by distillation in the usual way. CH3N02was dried over molecular sieves 4A dehydrated at 350 "C, since it was liable to be decom(1) Peri, J. B.; Hannan, R. B. J. Phys. Chem. 1960, 64, 1527. (2) Hockey, J. A.; Pethica, B. A. Trans. Faraday SOC.1961,57,2247. (3) Morimoto, T.; Nagao, M.; Tokuda, F. Bull. Chem. SOC. J p n . 1968, 41, 1533. (4) Morimoto, T.; Nagao, M.; Tokuda, F. J.Phys. Chem. 1969,73,243. (5) Roca, F. F.; Mourgues, L. De.; Trambouze, Y. J. Catal. 1969,14, 107. ( 6 ) Metcalfe, A.; Shanker,S. U. J. Chem. Soc., Faraday Trans. 1 1978, 74, 1945. (7) Nagao, M.; Morimoto, T. J. Phys. Chem. 1980,84, 2054.
0 1982 American Chemical Society
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Interaction of Organic Molecule with ZnO
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Figure 1. Adsorption isotherms of nBuOH on ZnO samples evacuated at various temperatures after complete hydroxylation: (V)25; (0) 200 (A)300;(0) 350;(0)600 "C. Open and filled marks represent the first and second adsorption, respectively.
posed by prolonged heating near the boiling point. Nitromethane-d3 (CD3N02)from Commissariat al' Energie Atomique (CEA) was also dried over molecular sieves. These organic liquids were allowed to evaporate into gas reservoirs equipped with a trapping tube and a greaseless stopcock, and subjected to several freezepump-thaw cycles before use. Determination of Surface Hydroxyl Content and Surface Area of the Sample. The surface hydroxyl content of the sample, which is the number of hydroxyl groups on the surface after evacuating the fully hydroxylated sample at given temperatures, was determined by the successive-ignition-loss method in a manner similar to that described previ~usly.~ The surface hydroxyl content was 11.00,9.14, 6.12,3.02, and 0.96 hydroxyl groups per nm2, for the samples evacuated at 25,200,300,350, and 600 "C, respectively. The specific surface area of the sample degassed at 600 "C, which was determined by the BET method based on N2 adsorption, was found to be 6.80 m2 g-l. Measurement of Adsorption Isotherms of Organic Molecules. The first adsorption isotherm of organic molecules was measured at 25 "C for the sample having a definite number of hydroxyls. The sample was then exposed to the saturated vapor of the same adsorbate for 10 h to ensure complete adsorption. After evacuating this N m-' for 4 h, sample at 25 "C under vacuum at 1 X the second adsorption isotherm was determined at the same temperature as before. The apparatus and procedures applied have been described el~ewhere.~ Measurements of IR Spectra. Infrared spectra of adsorbed species were measyed by the transmission method for surfaces of both strongly dehydroxylated (600 "C evacuated) and fully hydroxylated (25 "C evacuated) samples. A self-supporting sample disk of 2 cm diameter (ca. 200 mg) was placed in an in-situ cell fitted with fluorite windows. A reference cell having the same light-path length and windows as those in the sample cell was connected to the vacuum manifold, which permitted for compensation of the absorption due to vapor itself. After the sample was degassed at 600 "C, oxygen treatment was carried out in order to restore the original transmittance.8 Infrared spectra were measured by using a Nippon Bunko Model A302 diffraction grating spectrophotometer having a resolution of 0.4 cm-l in the region above 1300 cm-'. (8) Morimoto, T.; Yanai, H.; Nagao, M. J. Phys. Chem. 1976, BO, 471.
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Flgure 2. Adsorption isotherms of n-BuCI on ZnO samples evacuated at various temperatures after complete hydroxylation. Symbols are the same as those in Figure 1.
Results Adsorption Isotherm of n-BuOH. Figure 1 shows the first and second adsorption isotherms of n-BuOH on ZnO samples with various hydroxyl contents. The first adsorption isotherm, especially for the sample with a lesser hydroxyl content, is of the Langmuir type, having a sharp knee in the range of lower relative pressures. As is evident from Figure 1, a steep rise cannot be observed in the isotherm near the saturated vapor pressure, which indicates the absence of multilayers. This result may be regarded as an extension of the previous results which showed a decrease in the tendency for multilayer formation with increasing chain length from MeOH to TZ-P~OH.~ A similar phenomenon has been observed by Barto et aL9 in the adsorption of normal alcohols on alumina surface. These results obviously indicate an increased autophobicity of the surface which leads to the formation of restricted adsorption layers over the first layer owing to the orientation 1 also of alkoxy1 groups normal to the s u r f a ~ e . ~Figure J~ shows that the adsorbed amount of n-BuOH in the first adsorption increases with decreasing surface hydroxyl content, while that in the second one decreases. Adsorption Isotherm of n-BuC1. Adsorption isotherms of n-BuC1 are shown in Figure 2. The shape of the isotherm of n-BuC1 is quite different from that of n-BuOH, and is close to type I1 in Brunauer's classification, which suggests different adsorbed states between these two adsorbates. It can also be found that the adsorbed amount of n-BuC1 in the second adsorption is larger than that of n-BuOH. Hollabaugh and Chessick'l have also described the multilayer adsorption of n-BuC1 on Ti02 (rutile). The dependence of the amount of adsorbed n-BuC1 on the surface hydroxyl content of the sample is the same as that in the case of n-BuOH described above, that is, the amount of first adsorption increases and that of second one decreases with decreasing hydroxyl content of the sample. Adsorption Isotherm of n-C7H16. The adsorption of aliphatic hydrocarbon, n-C7HI6,with no functional groups, have an isotherm shape similar to that for n-BuC1 (Figure 3). However, the difference between the adsorbed amounts in the first and second adsorption is not so large for this adsorbate. Particularly, on the sample evacuated at 25 "C, the second adsorption isotherm quite agrees with (9) Barto, J.; Durham, J. L.; Baaton, V. F.; Wade, W. H. J . Colloid Interface Sci. 1966, 22, 491. (10) Blake, T.D.; Wade, W. H. J. Phys. Chem. 1971, 75, 1887. (11) Hollabaugh, C. M.; Chessick, J. J. J.Phys. Chem. 1961, 65, 109.
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Figwe 3. Adsorption isotherms of nC,H,, on ZnO samples evacuated at various temperatures after complete hydroxylation. Symbols are the same as those in Figure 1.
2
Flgure 5. Relationship between the monolayer capacity of adsorbed organic molecules and the suface hydroxyl content of the ZnO sample: ( 0 , O )n-BuOH; ( 0 ,B) n-BuCI; (V,V)nC,H,,; (A,A)CHSNOP. Open and fllled marks represent Vml and Vm2,respectively.
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F w e 4. Adsorption isotherms of CH3N02on ZnO samples evacuated at various temperatures after complete hydroxylation. Symbols are the same as those in Figure 1.
the first one, indicating an entirely reversible adsorption on the fully hydroxylated surface. A small amount of irreversible adsorption of n-C7H16,however, can be recognized on the sample pretreated at higher temperatures. The higher the pretreatment temperature of the sample, the larger the amount of the first adsorption and the lesser the amount of the second one, as for n-BuOH and n-BuC1. This fact truly tells that the presence of hydroxyl groups largely affects the adsorption of organic molecules, even if the molecules had a perfectly nonpolar nature. Adsorption Isotherm of CH&O2. The adsorption isotherms of CH3N02 are illustrated in Figure 4. The amount of CH3N02adsorbed is much larger than those of other three adsorbates used here. The shape of adsorption isotherms exhibits type I1 behavior representing the formation of multilayer. Here, we can see the characteristic feature that the amount of first adsorption varies considerably with the surface hydroxyl content, but that of the second one is affected very slightly by the latter. Such a situation is close to that found in metal oxide-water systems.2J2 From measurement of the adsorption isotherms of CH3NOzon a silica surface, Curthoys et al.I3 have reported that the amount of adsorption is significantly larger (12) Jurinak, J. J. J. Colloid Sci. 1964, 19,477. (13)Curthoys, G.; Davydov, V. Ya.; Kiselev, A. V.; Kiselev, S. A.; Kuznetsov, B. V. J. Colloid Interface Sci. 1974, 48, 58.
Surface hydroxyl content , OH groupshm'
Flgure 8. Relationship between the amount of irreversible adsorption and the surface hydroxyl content of the ZnO sample.
on the hydroxylated surface than on the strongly dehydroxylated surface, which is in contrast to the present results for the ZnO-CH3N02 system. They used a silica sample dehydrated at a very high temperature, 1000 "C, which might cause surface stabilization and hence result in the reduction of its adsorbability, as has been recognized in water adsorption on silica s ~ r f a c e s . ' ~ J ~
Discussion Relation between the Amount of Adsorbed Molecules and the Surface Hydroxyl Content of the Sample. Figure 5 illustrates the relationship between the monolayer capacity of organic molecules and the surface hydroxyl content of the ZnO sample, the former being calculated by Langmuir plots for n-BuOH and by BET plots for other three adsorbates. Here, V,, and V,, are the monolayer capacities based on the first and second adsorption isotherms, respectively. For the four kinds of adsorbate molecules, V,, is in the order CH,NO, > n-BuOH > nBuCl > n-C,H16 and V,, in the order CH3N02> n-BuC1 > n-C7HI6> n-BuOH. As is shown in Figure 5, an excellent linear relationship is found between the surface hydroxyl content of the sample and the amount of adsorbed molecules, regardless of the nature of the organic molecules. The V,, value which involves both chemi(14)Young, G. J. J. Colloid Sci. 1958, 13, 67. (15) Young, G. J.; Bursh, T. P. J. Colloid Sci. 1960, 15, 361.
Interaction of Organic Molecule with ZnO
sorption and physisorption decreases linearly with increasing surface hydroxyl content, while the V,, value involving only reversible physisorption increases linearly. A similar relationship has also been established for the systems ZnO-normal aliphatic alcohol^.^ The difference between V,, and V,,, being denoted as V,, is also plotted against the surface hydroxyl content of the sample in Figure 6. Here, V , refers to the amount of adsorbed molecules remaining on the surface after evacuation at 25 "C under vacuum at 1 X N m-2, the majority of which may be due to chemisorption. It is quite evident from Figure 6 that the amount of irreversibly adsorbed organic molecules decreases linearly with an increase in the surface hydroxyl content. The dependence of V , and V , upon the surface hydroxyl content suggests that surface hydroxyls on ZnO inhibit the irreversible adsorption but act as effective sites for the reversible adsorption of organic molecules, irrespective of the difference in molecular size, polarity, or kind of functional groups. It can hardly be thought, therefore, that chemical reactions such as esterification by a l c ~ h o l s ' ~or J ~ reaction with chlorosilanes,'&20in which surface hydroxyl groups participate directly, may also occur in the present systems. Adsorbed State of Organic Molecules. ( a ) n-BuOH. The chemisorption mode of alcohols expected to occur on metal oxide surfaces is esterification with surface hydroxyls16J7or dissociative adsorption of alcohol molec u l e ~ . ~ ' -If~ ~esterification occurs, the amount of irreversibly adsorbed species (Vi,,) should increase with increasing surface hydroxyl content of the sample. The present results shown in Figure 6 are completely antithetic to those derived from the above assumption. On the other hand, if we assume dissociative adsorption for n-BuOH, the present result seems to follow without contradiction. In other words, if n-BuOH molecules are chemisorbed dissociatively to form surface hydroxyls and butoxyl groups on the dehydroxylated ZnO surface, it would result in the formation of an autophobic surface layer on which further adsorption is difficult. This is supported by the fact that the amount of the second adsorption is extremely small on the strongly dehydroxylated surface which has the largest amount of irreversible adsorption. The average mea occupied by irreversibly adsorbed n-BuOH molecule is estimated to be 0.308 nm2 from the maximum value of V,, i.e., 3.25 molecules per nm2 when the surface hydroxyl content is extrapolated to zero in Figure 6. It agrees well with the values of 0.25-0.30 nm2 obtained by assuming that a straight-chain molecule is adsorbed perpendicularly to the solid s u r f a ~ e . ~ ! ~ * Figure 7 shows the IR spectra of n-BuOH adsorbed on dehydroxylated and hydroxylated surfaces. For n-BuOH adsorbed on a dehydroxylated sample, four distinct absorption bands appear at 2970,2855,2932, and 2871 cm-', the former two being assigned to asymmetric and symmetric CH stretching vibrations of the methyl group, respectively, and the latter two being assignable to asymmetric and symmetric CH stretching vibrations of the methylene group, re~pectively.~~ The absorption bands (16) Sidorov, A. N.Zh. Fiz. Khim. 1956, 30, 995. (17) McDonald, R.S. J . Phys. Chem. 1958, 62, 1175. (18) Armistead, C. G.;Hockey, J. A. Trans. Faraday SOC.1967, 63, 2549. -_
(19) Hertl, W.J. Phys. Chem. 1968, 72, 1248. (20) Hair, M.L.;Hertl, W. J. Phys. Chem. 1969, 73, 2372. (21) Borello, E.;Zecchina, A.; Morterra, C. J.Phys. Chem. 1967, 71, 2938, 2945. (22) Thornton, E.W.;Harrison, P. G. J. Chem. SOC.,Faraday Trans. 1 1975, 71, 2468. (23) Beliakova, L.D.; Kiselev, A. V. Zh. Fiz. Khim. 1959, 33, 1534. (24) McClellan, A. L.;Harnsberger, H. F. J . Colloid Interface Sci. 1967, 23, 577.
The Journal of Physical Chemistry, Vol. 86, No. 2 1, 1982 4101
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Flgure 7. Infrared spectra of adsorbed n-BuOH on dehydroxylated (a, b) and hydroxylated (c, d) ZnO surfaces. Broken and solid lines represent the spectra recorded in the presence of vapor and after evacuation at room temperature, respectively.
assignable to the bending vibrations were also observed at 1458 and 1380 cm-' for the methyl group, and those at 1465 cm-' for the methylene group. In addition to the CH bands, new absorption bands can also be recognized in the OH stretching region (around 3580 and 3400 cm-'), which substantiates the dissociative adsorption of n-BuOH. The previous report concerning the dissociative adsorption of C143 alcohols on the dehydroxylated ZnO surface showed that the absorption bands due to the OH stretching vibration become obscure although the CH bands are greatly enhanced with increasing carbon number from MeOH to n-PrOHa7 It seems reasonable, therefore, to adopt the mechanism of dissociative adsorption also for n-BuOH. The vague appearance of the OH bands may be ascribed to the formation of stronger hydrogen bonding between hydroxyl groups and neighboring alkoxy1 groups produced by the dissociative adsorption of the alcohol. The difference in the spectra obtained when alcohol vapor is present or absent can hardly be discernible, which supports the fact that reversible physisorption of n-BuOH is difficult on prechemisorbed species, corresponding to the low V,, value shown in Figure 5. On the other hand, when n-BuOH is adsorbed on the fully hydroxylated ZnO surface, the absorption bands due to the CH stretching vibrations are observed at the same frequencies as in the case of the dehydroxylated surface (Figure 7c,d). The bands from the free hydroxyl groups at higher frequencies (3695 and 3675 cm-'; cf. Figure 9d) were strongly perturbed when n-BuOH vapor was introduced, and they were not restored even after evacuating the vapor at room temperature. The more intense bands at 3580 and 3400 cm-' may involve absorption bands of these perturbed hydroxyl groups. It can be considered from these observations that n-BuOH molecules are physisorbed on the free hydroxyl groups through fairly strong hydrogen bonding. ( b ) n-BuC1. Unlike the adsorption of n-BuOH, n-BuC1 molecules have a tendency to adsorb as multilayers on the solid surface, as can be understood from the shape of isotherms in Figure 2. In this case, therefore, the alkyl (25) Bellamy, L. J. "The Infrared Spectra of Complex Molecules"; Wiley: New York, 1958; 2nd ed.
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Flgure 8. Infrared spectra of adsorbed n-BuCl on dehydroxylated (a, b) and hydroxylated (c, d) ZnO surfaces. Broken and solid lines have the same meanings as in Figure 7.
groups in the molecules are considered to be oriented horizontally rather than perpendicularly to the surface. The average molecular area evaluated from the maximum number of adsorbed molecules (1.9 molecules per nmz, Figure 6) is 0.526 nm2,which is fairly larger than the value expected for vertical orientation. Assuming a rodlike molecule and a cross-sectional area of 0.20 nm2 for nBuCl," we can get a rather smaller area of 0.438 nm2 per molecule for the flatwise adsorption from the liquid density. The IR spectra of n-BuC1 adsorbed on dehydroxylated and hydroxylated samples are given in Figure 8. When n-BuC1 vapor was present in equilibrium with the adsorbed phase, the spectra changed somewhat depending on its vapor pressure, in particular, in the region of the CH stretching vibrations. At a lower equilibrium pressure (1.3 X lo2 N m-2),the bands at 2975 and 2860 cm-I due to the methyl groups and those at 2930 and 2850 cm-' due to the methylene groups appeared clearly. The most intense band at 2975 cm-' became smaller and broader as the pressure was raised, whereas lower-frequency bands were enhanced. Besides the deformation bands of both methyl and methylene groups (1450 and 1380, and 1465 cm-', respectively),the following bands appeared: the symmetric deformation and out-of-plane (wagging) vibration bands of the CH2 group attached to the chlorine atom at 1438 and 1240 cm-l, respectively, and the overtones of the C-Cl stretching vibration mode at 1315 and 1290 cm-1.25These four bands are distinct when the vapor is present, but disappear after evacuation at room temperature. On the dehydroxylated surface, the absorption bands remaining after evacuation in the region of CH stretching vibrations are very similar to those of the butoxyl groups produced by dissociative adsorption of n-BuOH. It can therefore be expected that on the dehydroxylated ZnO surface n-BuC1 molecules are strongly adsorbed, though it is not decided whether complete dissociation of n-BuC1 has occurred or not. Hollabaugh and Chessick" have reported that on an activated TiOz(rutile) surface a small amount of n-BuC1 reacts with the most active Ti-O-Ti sites to produce T i 4 1 and Ti-OR groups. If such a complete dissociative adsorption occurred in the present system, the absorption band due to the Zn-C1 stretching vibration should appear in the frequency region of 350-225 cm-1.26 Unfortunately, it could not be confirmed because (26) Nakamoto, K. 'Infrared Spectra of Inorganic and Coordination Compounds";Wiley: New York, 1970.
Wavenumber , io'cm-'
Flgwe 9. Infrared spectra of adsorbed nC,H,6 on dehydroxylated (a, b) and hydroxylated (c, d) ZnO surfaces. Broken and solid lines have the same meanings as in Figure 7.
of frequencies which were too low, beyond the detectable region of the spectrophotometer used in the present study. Here, we propose eq 1 as the adsorption mode from the above observations. H,_/C3H7
As is seen from Figure 8, the IR spectrum of n-BuC1 adsorbed on the fully hydroxylated ZnO surface seems to be similar to that on the dehydroxylated surface when vapor is present. Most of the absorption bands except the OH bands disappear by evacuation at room temperture, which suggests that reversible physisorption is the main process occurring on this hydroxylated surface. However, the absorption bands in the region of the CH stretching vibrations remain slightly after evacuation, indicating irreversible adsorption even on the hydroxylated surface, in agreement with the result shown in Figure 6. In view of the appearance of the bands of perturbed hydroxyl groups, n-BuC1 molecules seem to be adsorbed through hydrogen bonding (Cl-- -H-0), directing its C1 atom to the free hydroxyl groups on the fully hydroxylated ZnO surface. ( c ) n-CTH16. Figure 9 shows the IR spectra of n-C7HIG adsorbed on the surface of ZnO. On the fully hydroxylated surface the spectrum is restored to its original pattern after evacuation at room temperature, in agreement with the fact that the two isotherms, the first and second ones in Figure 3, are just the same. It can also be found that at equilibrium with vapor the OH bands are little affected by the adsorbed species. These results suggest a weak interaction between the solid surface covered with hydroxyls and adsorbate molecules, probably through a dispersion force. On the dehydroxylated surface, however, the residual CH bands are just observable even after evacuating the vapor. It is particularly surprising in view of the fact that the n-C7H16molecule has neither permanent dipole moments nor reactive functional groups. If we take into account the fact that the dehydroxylated ZnO surface has an enhanced electrostatic field,27the irre-
J. Phys. Chem. 1982, 86, 4193-4196
4193
surface hydroxyl groups on the adsorption of CH,N02. It may be due to the electron-attractive nature of the nitro group, where the hydrogen atom attached to the carbon atom (a position) adjacent to the nitro group is easily activated to leave as a proton. CH,NO2 is a compound exhibiting tautomerism (eq 2), in which the aci-nitro form
+
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nitro form
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aci-nitro form
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Figure 10. Infrared spectra of adsorbed CH,NO, on dehydroxylated (a, b) and hyckoxylated (c, d) ZnO surfaces. Broken and sold lines have the same meanings as In Figure 7.
versible adsorption of such a molecule might occur owing to a stronger interaction with induced dipoles of the molecules, in addition to the dispersion force. From the standpoint of the adsorption through the dispersion force, it may be reasonable to consider that horizontally oriented molecules are more stable than perpendicularly oriented ones on the solid surface. Actually, the molecular area occupied by adsorbed n-C7Hl, is calculated to be 0.625 nm2 based on the adsorption data (Figure 6), being very large compared with the value expected for the vertical orientation. (d) CHJVO,. The IR spectra of CH3N02adsorbed on the ZnO surface are illustrated in Figure 10. On the dehydroxylated surface, a broad and intense band centered at 3365 cm-l appears, in addition to the CH stretching bands in the range 2972-2852 cm-'. The band at 3365 cm-' should be assigned to the OH stretching vibrations, since the OD band was clearly observed around 2500 cm-' when deuterated nitromethane (CD3N02)was adsorbed on the same sample. These facts substantiate the formation of (27)Manuscript to be submitted for publication.
(pK, = 3.2) is predominant in the presence of a proton acceptor.28 On the dehydroxylated ZnO surface, the oxygen atom will attract this proton to produce hydroxyl groups, the remainder being linked to the surface zinc atom. The presence of C=N bonding is confirmed by the band at 1630 The formation of strong hydrogen bonding between adsorbed species will cause a perturbation of hydroxyl groups, resulting in a shift of the OH band toward lower frequencies. On the hydroxylated surface, the bands for the CH, (2972 cm-') and NOz groups (1555 and 1370 cm-l) are reduced by evacuation of the vapor, while the bands due to the CH2 (2920 cm-l) and C=N groups (1630 cm-') remain distinct (Figure 1Od). Furthermore, the free hydroxyl bands at 3695 and 3675 cm-' disappeared through the adsorption of CH3N02. From these observations, it is reasonable to infer that CH3N02molecules can be adsorbed in the aci-nitro form on the free hydroxyl groups. Acknowledgment. The present work was partly supported by a Grant-in-Aid for Scientific Research, No. 00547008, from the Ministry of Education, Science and Culture of Japanese Government. (28) Imoto, M. "Yuki-Denshiron-Kaisetsu";Tokyo Kagaku-Dojin: Tokyo, 1967.
Surface Tension of Lithium Fluoride and Beryllium Fluoride Binary Melt Kunlmltsu Yajlma, Hlrotake Morlyama, Jun Olshl, and Yasunobu Tomlnaga Department of Nuclear Engineering, Kyoto University, Kyoto 606, Japan (Received: December 14, 1981; In Final Form: June 28, 1982)
The surface tension of molten salt containing lithium fluoride and beryllium fluoride has been measured by the maximum bubble pressure method. The surface tension isotherm has a minimum at about 40 mol 70of beryllium fluoride, which might be due to the formation of complex ions. An empirical equation of the surface tension of molten halides is introduced, and the surface tension of lithium fluoride-beryllium fluoride binary melt is explained with this equation, taking the formation of tetrafluoroberyllate complex ion in a surface layer into consideration. Introduction Molten salts are useful materials from the viewpoint of their physical and chemical merits. In particular, LiFBeF2 binary melt is expected to be applied to numerous fields in nuclear engineering, for example, to the fuelsolvent of molten salt breeder reactors. The properties of LiF-BeF, binary melt have been measured vigorously for 0022-3654/82/2086-4 193$0 1.25/0
many years,lV2 but the interfacial properties such as the surface tension are known little in spite of their importance., The surface phenomena are also of interest with (1) K. A. Fbmberger, J. Braunstein, and R. E. Thoma, J.Phys. Chem., 76,1154 (1972). (2) F. Vaslow and A. H. Narten, J. Chem. Phys., 59, 4949 (1973).
0 1982 American Chemical Society
