Langmuir 1987, 3, 99-104
99
Adsorption of Alcohols on Titanium Dioxide (Rutile) Surface Yasuharu Suds,? Tetsuo Morimoto,t and Mahiko Nagao* Research Laboratory for Surface Science and Department of Chemistry, Faculty of Science, Okayama University, Tsushima, Okayama 700, Japan Received July 21, 1986. In Final Form: September 25, 1986 Adsorption isotherms of MeOH, EtOH, and 1-PrOH were measured at 25 "C on TiOz (rutile) samples having different surface hydroxyl contents. As the carbon chain of alcohols increased, the total amount of adsorption decreased and the tendency toward the formation of multilayer lowered. An irreversible adsorption of alcohols occurred on the hydroxylated surface as well as on the dehydroxylated surface, the extent being greater for the latter surface. On the dehydroxylated surface, alcohol molecules were found to be adsorbed dissociatively to produce surface hydroxyl groups and alkoxy groups. From the infrared spectroscopic observation and the gas chromatographic analysis, it was evident that a part of surface hydroxyls thus formed can undergo an esterificationreaction with alcohol. On the other hand, three types of adsorption modes could be possible for the adsorption of alcohols on the hydroxylated surface: rapid replacement for molecular water remaining on the surface, esterification with acidic surface hydroxyls, and reversible adsorption on the surface hydroxyls through hydrogen bonding. It was also revealed that when the rutile sample with preadsorbed alcohol is in contact with saturated water vapor at 25 "C, the substitution reaction of water for alcohol does not occur, in contrast to the case of the ZnO-alcohol-water system reported previously. These facts substantiate a stronger interaction of the rutile surface with alcohol molecules than with water molecules. Introduction The surface of metal oxide is usually covered with chemisorbed water, i.e., surface hydroxyl groups, on which further water molecules are physisorbed through hydrogen bonding. Since the surface hydroxyls are held tenaciously on the surface even after evacuation at higher temperatures, they should affect significantly the surface properties of such a solid as metal oxide. Titanium dioxide, one of the typical metal oxides, is widely used as a white pigment and as a photocatalyst. The use of this material in fine ceramics has increased remarkably with the recent rapid development of new materials with useful functions. Carrizosa e t al.'-3 have studied the interaction of aliphatic alcohols from C2to C5 with anatase TiO, surfaces, and they have found that the decomposition of the adsorbed phase can occur with all these alcohols. The acidity and basicity of the surface hydroxyl groups on Ti02,mainly on anatase, were examined by Boehm and his colleagues, who carried out a comprehensive series of experiments using a variety of reagents and techniques!$ However, relatively little work in which the role of surface hydroxyls is taken into consideration in the adsorption of the molecules other than water has been done so far. In our previous work on the adsorption of alcohols on ZnO surfaces with different hydroxyl contents: we demonstrated that the dissociative chemisorption of alcohols occurs on the dehydroxylated ZnO surface to produce both alkoxy and hydroxyl groups, but on the fully hydroxylated surface reversible physisorption takes place predominantly by hydrogen bonding. In the present paper, we report the effects of the surface hydroxyl groups on the interaction of rutile surfaces with the normal aliphatic alcohols, MeOH, EtOH, and 1-PrOH. Experimental Section The original TiOzsample (rutile), supplied by Teikoku-kako Co., was first treated with 0.1 N nitric acid and then with 0.1 N ammonia to remove basic and acidic impurities and, finally,was thoroughly washed with distilled water. After degassing at 600
* To whom correspondence should be addressed. Department of Chemistry.
"C under a pressure of 1 X N m-2 for 4 h, the sample was exposed to saturated water vapor at room temperature for 15 h. By heating this fully hydroxylated sample at various temperatures between 25 and 600 "C under vacuum, we obtained samples with different numbers of surface hydroxyl groups. The specific surface area obtained by applying the BET method to the nitrogen adsorption data was found to be 9.18 m2g-' for the sample pretreated at 600 O C . The number of hydroxyl groups remaining on the surface of each sample was determined by the successive-ignition-lossmethod in a manner similar to that described elsewhere.s The adsorbates used here were three kinds of normal aliphatic alcohols, methanol (MeOH), ethanol (EtOH), and l-propanol (1-PrOH),all of which are of guaranteed reagent grade. These alcohols were further purified by distillation,followed by drying with 4A molecular sieves previously activated at 500 "C for 5 h. The adsorption isotherms of alcohol vapor were measured at 25 "C with a conventional volumetric adsorption apparatus equipped with greaseleas stopcocks. The f i t adsorption isotherm was measured on each sample having a definite number of surface hydroxyls. After this the sample was degassed at 25 "C,and then the second adsorption isotherm was measured. Infrared spectroscopic investigatio5of the adsorbed alcohols was carried out in the same manner as in the previous work.6 Infrared spectra were recorded on a Nippon-BunkoModel A302 diffraction grating spectrophotometer at ambient temperature. Gas chromatographicanalysis was applied to the gas phase in equilibrium with the adsorbed phase of EtOH. The glass-tube column was packed with Porapack-N and kept at 120 "C during measurement. The thermal conductivity detector was used at a current of 125 mA, and the flow rate of carrier gas (He) was 40 cm3min-'. Results and Discussion The surface hydroxyl concentration of the. rutile is plotted as a function of the degassing temperature in Figure 1. The surface hydroxyl concentration is 7.88 OH groups per nm2 for the fully hydroxylated surface, and it (1)Carrizosa, I.; Munuera, G. J. Catal. 1977, 49, 174. (2) Carrizosa, I.; Munuera, G. J . Catal. 1977, 49, 189. (3) Carrizosa, I.; Munuera, G.; Castanar, S. J. Catal. 1977, 49, 265. (4) Boehm, H. P.; Herrmann, M. 2.Anorg. Allg. Chem. 1967,352,156. (5) Boehm, H. P. Discuss. Faraday SOC.1971,52, 264. (6) Nagao, M.; Morimoto, T. J. Phys. Chem. 1980, 84, 2054.
0743-7463I8712403-0099$01.50/0 0 1987 American Chemical Society
100 Langmuir, Vol. 3, No. 1, 1987
Suda et al. 0.3.
h
PP
*
0.2
0
0
200
400
600
Degassing temperature ,
0.6
0.8
Figure 3. Adsorption isotherms of EtOH on a rutile sample evacuated at various temperatures after complete hydroxylation. Symbols are the same as those in Figure 2.
1000
800 'C
0.4
Relative pressure(PIPo)
Figure 1. Surface hydroxyl content of a rutile sample evacuated at various temperatures.
o
.
3
F
I
J
0.6
0.8
1.0
r
E
c cE!
I
9
0
0.2
0.4
Relative pressure (P/P@)
I 0
0.2
0.4
0.6
Relative pressure (PIPo)
0.8
1.0
Figure 2. Adsorption isotherms of MeOH on a rutile sample evacuated at various temperatures after complete hydroxylation: (V)25 "c;(0) 100 "c;(A) 150 "c;(0) 200 "c; (0) 300 "c;(a) 600 "C. Open and filled marks represent the first and second adsorption, respectively.
decreases with increasing degassing temperature, particularly steeply in the range 25-300 "C. It can be seen that the surface hydroxyl concentration for the sample treated a t 600 "C is extremely small (0.132 OH groups per nm2), and hence such surface will hereafter be referred to as the dehydroxylated surface. The first and second adsorption isotherms of MeOH, EtOH, and 1-PrOH on the rutile samples with different concentration of surface hydroxyl groups are shown in Figures 2,3, and 4, respectively. These isotherms have a distinct knee in the lower relative pressure region (PIP, 0.05). The shape of the isotherm changes from the BET type I1 for MeOH to the Langmuir type for 1-PrOH as the carbon chain of alcohol molecules becomes longer. Such a gradual decrease in tendency toward multilayer formation with increasing chain length of alcohols has also been observed by Barto et al. in the case of alcohol adsorption on alumina surface.' They introduced a concept of gas(7) Barto, J.; Durham, J. L.; Baston, V. F.; Wade, W. H.J. Colloid Interface Sci. 1966, 22, 491.
Figure 4. Adsorption isotherms of 1-PrOH on a rutile sample evacuated at various temperatures after complete hydroxylation. Symbols are the same as those in Figure 2.
phase autophobicity in which the multilayer formation is restricted by a strongly oriented first adsorbed layer to explain this trend. Similar phenomena have been observed in the system ZnO-alcohol.6 The molecular cross-sectional areas of the adsorbed alcohol evaluated from the monolayer capacities are 0.210 and 0.273 nm2 for MeOH, 0.267 and 0.305 nm2 for EtOH, and 0.270 and 0.351 nm2 for 1-PrOH, the first and second values for each adsorbate being on the dehydroxylatedand hydroxylated surfaces, respectively. These values are in fair agreement with the calculated values, 0.20-0.30 nm2,7*8 based on the assumption that alcohol molecules are oriented vertically on the solid surface. Thus, alcohol molecules can be adsorbed in such a way that on completion of the monolayer, the adsorbed molecules are oriented perpendicularly to the surface, and hence, the molecules in the second and subsequent layers become gradually isolated from the solid substrate. This orientation effect is more significant for alcohols with longer carbon chains. Therefore, it may be concluded that the gas-phase autophobicity takes place also in the present system of the rutile surface and normal alcohols, in analogy with the cases of alumina7 and zinc oxide.6 (8)Hollabaugh, C. M.; Chessick, J. J. J. Phys. Chem. 1961, 65, 109.
Langmuir, Vol. 3, No. 1, 1987 101
Adsorption of Alcohols on Titanium Dioxide
11
I
38 36
38
36
34
32
30
28 " 17 16 15 14
Wavemumber, 102cm-'
Figure 5. Infrared spectra of MeOH adsorbed on the dehydroxylated (a-d) and hydroxylated (e-h) rutile surfaces: (a) background spectrum of rutile dehydroxylated at 600 O C ; (b) adsorption of MeOH at equilibrium pressure of 1 torr; (c) adsorption of MeOH at equilibrium pressure of 50 torr; (d) after evacuation of MeOH vapor at ambient temperature; (e) background spectrum of the hydroxylated rutile; (f) adsorption of MeOH at equilibrium pressure of 1torr; (g) adsorption of MeOH at equilibrium pressure of 60 torr; (h) after evacuation of MeOH vapor at ambient temperature. The amount of alcohol adsorbed on the rutile samples, which were pretreated at the same temperature and hence have the same hydroxyl content, decreases in the order MeOH > EtOH > 1-PrOH for both the first and second adsorption, as can be seen in Figures 2-4. For every adsorbate, the amount adsorbed in the first measurement increases with decreasing surface hydroxyl content of the sample, whereas those in the second one, especially for the sample pretreated a t lower temperature than 200 OC, do not vary appreciably with the hydroxyl content. The isotherms of the first and second adsorptions are parallel to each other, and the difference in the adsorbed amount between them appears evidently on the ordinate a t pressure zero. The gap a t pressure zero may be regarded as the amount of irreversible adsorption not desorbed by evacuation a t 25 "C. This amount is the largest for the dehydroxylated surface and decreases with an increase in the number of hydroxyls remaining on the sample. It is noteworthy that the fully hydroxylated rutile surface gives an appreciable amount of irreversible adsorption. The IR spectra of MeOH, EtOH, and 1-PrOH adsorbed on the dehydroxylated (600 OC treated) and hydroxylated (25 "C treated) rutile surfaces are shown in Figures 5 , 6 , and 7, respectively. When MeOH vapor is adsorbed on the dehydroxylated surface a t equilibrium pressure 1torr (torr = 1.33 X lo2 N m-2), two distinct absorption bands appear a t 2925 and 2820 cm-', which are highly characteristic of the methoxy group? In addition, the bands due to the asymmetric and symmetric CH bending vibrations can be observed a t 1440 and 1380 cm-' (shoulder), respectively. A small absorption band observed at 3655 cm-' may be due to the free OH groups on the rutile surface.1° It has been shown that on the dehydroxylated rutile sur(9) Bellamy,L. J. The Infra-red Spectra of Complex Molecules; Methuen: London, 1964. (10) Griffiths, D. M.; Rochester, C. H. J.Chem. SOC.,Faraday Trans. I 1977, 73,1510.
I
I
34
)
1
32
I
I
v-,
I
,,
,
I
30
28" 17 Wave number, 102cm-'
, 15
,
13
Figure 6. Infrared spectra of EtOH adsorbed on the dehydroxylated (a-d) and hydroxylated (e-h) rutile surfaces: (a) background spectrum of rutile dehydroxylated at 600 O C ; (b) adsorption of EtOH at equilibrium pressure of 1 torr; (c) adsorption of EtOH at equilibrium pressure of 20 torr; (d) after evacuation of EtOH vapor at ambient temperature;(e) background spectrum of the hydroxylated rutile; (f) adsorption of EtOH at equilibrium pressure of 1torr; (g) adsorption of EtOH at equilibrium pressure of 25 torr; (h) after evacuation of EtOH vapor at ambient temperature.
Wave number,
lo2cm-'
Figure 7. Infrared spectra of 1-PrOH adsorbed on the dehydroxylated (a-d) and hydroxylated (e-h) rutile surfaces: (a) background spectrum of rutile dehydroxylated at 600 O C ; (b) adsorption of 1-PrOH at equilibrium pressure of 0.5 torr; (c) adsorption of 1-PrOH at equilibrium pressure of 12 torr;(d) after evacuation of 1-PrOHvapor at ambient temperature; (e) background spectrum of the hydroxylated rutile; (f) adsorption of 1-PrOH at equilibrium pressure of 0.5 torr; (g) adsorption of 1-PrOH at equilibrium pressure of 14 torr; (h) after evacuation of 1-PrOH vapor at ambient temperature. face, alcohol molecules can be adsorbed dissociatively to produce both surface alkoxide and surface hydroxyl groups a t room t e m p e r a t ~ r e . ~ J l - 'The ~ spectrum b in Figure 5 representing the presence of OH groups and methoxy groups substantiates the dissociative adsorption of MeOH on the dehydroxylated surface. This spectrum also includes a broad band centered a t about 3400 cm-'. Ac(11) Isirikyan, A. A.; Kiselev, A. V.; Vskakova, E. V. Kolloid-Z. 1963, 25,125. (12) Jackson, P.; Parfitt, G. D. Trans. Faraday SOC.1971,67, 2469. (13) Jackson, P.;Parfitt, G. D. J. Chem. SOC.,Faraday Trans. 1 1972, 68, 1443. (14) Parfitt, G. D.B o g . Surf. Membr. Sci. 1976, 11, 189.
102 Langmuir, Vol. 3, No. 1, 1987 cording to the reports by Kiselev and Uvarov,15the oxide vacancies on the highly dehydroxylated surface of rutile can interact with adsorbate molecules having free electron pairs through the formation of a coordinate bond. In addition, Knozinger has revealed that ethanol molecules adsorbed on such oxide vacancies form a coordinate bond by utilizing a free electron pair of the oxygen atom in the OH group of alcohol, giving rise to a broad absorption band near 3450 cm-' assignable to the OH stretching vibration in IR spectra.16 Therefore, from the present results showing a broad absorption band near 3400 cm-' (spectrum b), it may be assumed that MeOH molecules are adsorbed on the oxide vacancies, i.e., surface Ti4+ions, through the formation of coordinate bonds. Furthermore, Knozinger has suggested the formation of an intermediate structure between the dissociative adsorption and the coordinatebond-type adsorption.16 In such intermediate, the hydrogen atom of OH group in alcohol is assumed to link to the adjacent surface oxygen ion through hydrogen bonding rather than complete rupture of the 0-H bond in the alcohol molecule. It is, therefore, rather reasonable to conclude that the broad band a t 3400 cm-' is due to the OH groups of adsorbed alcohol molecules, of which the oxygen atom is coordinated to the surface Ti4+ion and the hydrogen atom is bound strongly to the surface oxide ion through hydrogen bonding. In the spectrum d taken after outgassing MeOH vapor at ambient temperature, the absorption band assigned to the methoxy groups as well as broad band around 3400 cm-' due to the OH groups can be observed, while the band due to the free OH groups at 3655 cm-' disappears. It is possible to assume that the OH groups newly formed by dissociative adsorption of MeOH can also interact with MeOH vapor, as will be discussed later. It has been shown that molecular water is present on the fully hydroxylated rutile surface even after evacuation at room temperature.l7-lg This molecular water is bonded to the unsaturated surface titanium ions by a coordinate bond rather than to the surface hydroxyl groups by hydrogen b~nding.'~-~l Primet et al. have demonstrated the removal of such molecular water from the rutile surface by evacuation at 150 0C.20,21In the IR spectrum for the fully hydroxylated rutile surface (spectrum e in Figure 5), three distinct absorption bands can be observed a t 3655, 3530, and 3400 cm-l, which are respectively due to the free surface OH groups,1othe surface OH groups perturbed by molecular water through hydrogen bonding,1° and both species of adsorbed water molecules'0~22and hydrogenbonded OH group^.'^ In addition, the band at 1625 cm-' is assigned to the bending vibration of molecular ~ a t e r . ~ ~ When J ~ , ~ MeOH ~ , ~vapor ~ is adsorbed on this hydroxylated surface at equilibrium pressure of 1torr, the bands ascribed to the methoxy group appear at 2925,2820, and 1440 cm-l, as in the case of adsorption on the dehydroxylated surface (spectrum f). With the adsorption of MeOH vapor, the intensities of both bands due to the free OH groups at 3655 cm-' and molecular water at 1625 cm-' decrease markedly. When the vapor pressure of MeOH is increased to 60 torr, the band due to the free OH groups (15) Kiselev, A. V.; Uvarov, A. V. Surf. Sci. 1967, 6, 399. (16) Knozinger, H. 2.Phys. Chem. 1970, 69, 108. (17) Jones, P.; Hockey, J. A. Trans. Faraday SOC.1971, 67, 2669. (18) Jones, P.; Hockey, J. A. Trans. Faraday SOC.1971, 67, 2679. (19) Jaycock, M. J.; Waldsax, J. C.R. J. Chem. Soc., Faraday Trans. 1 1974, 70, 1501. (20) Primet, M.; Pichat, P.; Mathieu, M.-V. J . Phys. Chem. 1971, 75, 1216. (21) Primet, M.; Pichat, P.; Mathieu, M.-V. J . Phys. Chem. 1971, 75, 1221. (22) Munuera, G.; Stone, F. S. Discuss. Faraday SOC.1971, 52, 215.
Suda et al. disappears because of the formation of hydrogen bonding with MeOH molecules in the gas phase, leaving a broad band centered at 3400 cm-l. In spectrum h, taken after outgassing MeOH vapor, the absorption bands due to the methoxy groups remain tenaciously and the free OH band at 3655 cm-' is restored, but its intensity is decreased remarkably compared with the background spectrum e. Furthermore, the 1625-cm-' band assigned to the bending vibrations disappears completely by this treatment. The IR spectra for EtOH and 1-PrOH shown in Figures 6 and 7 have the same features as in the case of MeOH adsorption. The absorption bands characteristic of the CH stretching vibrations of alkyl groups are distinct when these alcohols are adsorbed on the dehydroxylated rutile surface (spectrum b and c). Two pairs of bands a t 2965 and 2885 and a t 2940 and 2855 cm-' (in case of EtOH, three bands a t 2970 and 2870 and at 2935 cm-') can be assigned to the asymmetric and symmetric stretching vibrations of the methyl and methylene groups, respectively. The pattern of the spectra for EtOH resembles that for 1-PrOH, though the band due to the methylene groups is not so clear as for the latter because of the shorter carbon chains. In the region of the CH bending vibrations, the CH, symmetric and CH, asymmetric vibration bands appear at 1375 and 1440 cm-l for EtOH and 1380 and 1460 cm-' for 1-PrOH, respectively (spectrum b and c). These bands, due to the alkyl groups, still remain even after the evacuation of alcohol vapor (spectrum d). The 3655-cm-' band assigned to the free OH groups can be seen also in the adsorption of EtOH and 1-PrOH when the vapor pressure of alcohol is low (spectrum b), which is indicative of the dissociative adsorption as stated above for the case of MeOH adsorption. The broad band around 3400 cm-' (spectrum d) suggests the presence of alcohols bonded to the coordinatively unsaturated surface Ti4+ ion^.'^,'^ The features of IR spectra for EtOH and 1-PrOH adsorbed on the hydroxylated rutile surface are the same as those in the case of MeOH adsorption (spectrum e-h in Figures 6 and 7). Gas chromatographic analysis demonstrated the presence of water vapor in the gas phase in equilibrium with the adsorbed phase when a monolayer of EtOH molecules was adsorbed. The more the surface hydroxyl content of the sample, the more the amount of water expelled: 1.75, 0.40, and 0.051 water molecules per nm2 for the sample treated at 25, 150, and 600 "C, respectively. From the results of IR spectra and gas chromatography, three types of mechanisms may be postulated for the adsorption of alcohols on the hydroxylated rutile surface. The first possibility is a replacement of molecular water present on the rutile surface by alcohol molecule^.'^^^^ As stated above, there exists molecular water on the rutile surface treated at room temperature."-21 When alcohol molecules are adsorbed on the hydroxylated surface, the absorption band at 1625 cm-' due to the bending vibration of water molecules disappears, as is seen from Figures 5-7. This band cannot be restored even after evacuating the alcohol vapor, and the broad band near 3400 cm-' due to the OH groups is virtually invariant, compared with the background spectrum (spectrum e in Figures 5-7). The number of water molecules expelled from the hydroxylated (25 "C treated) surface, 1.75 H 2 0 molecules per nm2, is much larger than that from 150 "C treated surface on which molecular water has been removed in practice.20,21 By heat treatment between 25 and 150 "C, 2.07 water molecules per nm2has been liberated, as can be seen from (23) Day, R. E.; Parfitt, G. D.; Peacock, J. Discuss. Faraday SOC.1971, 52, 215.
Langmuir, Vol. 3, No. 1, 1987 103
Adsorption of Alcohols on Titanium Dioxide
f
51
E.
4
$1
I
S I 0
2
4
6
Water content, OHgroups nm-?
8
Figure 8. Relationship between the monolayer capacity of adsorbed alcohol and surface hydroxyl content of rutile sample: (0) MeOH; (0) EtOH; (A)1-PrOH. Open and filled marks represent V,, and V,,, respectively.
the variation of water content with temperature shown in Figure 1. Second, an esterification reaction could be postulated.11J3J6 Boehm and Herrmann4 have suggested that two different types of OH groups exist on the anatase surface, namely, one is a terminal OH group bound to one Ti4+site and a bridged OH group bound to two such sites, and that the latter group should be strongly polarized by the cations and therefore be acidic in character. Furthermore, it has been established by them that approximately half of the hydroxyls present on the surface are strongly acidic with pK, of 2.9 and the remainder are weakly acidic with pK, of 12.7.4 Based on the model of a rutile (110) plane proposed by Jones and Hockey,la we assume that both types of surface OH groups suggested by Boehm and Herrmann for the anatase sample can exist also on the rutile surface. After adsorption on the hydroxylated surface and succeeding outgassing of the alcohol vapor, the absorption band due to the free OH groups diminishes remarkably (spectrum h in Figures 5-7). In the case of alcohol adsorption on the 150 "C treated surface on which water bound in molecular form should be almost completely removed, the liberation of water from the sample was proved by gas chromatographic analysis of the gas phase in equilibrium with the adsorbed phase. From these facta it is likely that an esterification reaction can take place between acidic OH groups on rutile surface and some of adsorbed alcohol molecules. A small quantity of water was detected in the gas phase also in the case of the adsorption on the 600 "C treated sample with the surface highly dehydroxylated. This fact can be interpreted as follows. The dissociative chemisorption of alcohols takes place first on the dehydroxylated rutile surface, by which the surface hydroxyl groups are produced and react with additional alcohol molecules by esterification, resulting in an evolution of water. Owing to this reaction the OH band initially observed disappears by further adsorption of alcohols, as can be seen in the IR spectra for the dehydroxylated surface. The formation of hydrogen bonding between alcohol molecules and the surface hydroxyl groups may be regarded as the third possible adsorption mode.6p8*22 In the IR spectra, when the alcohol vapor is present in the gas phase, the free OH band can not be observed (spectrum g in Figures 5-7)) but it reappears after outgassing the alcohol vapor (spectrum h in Figures 5-7), which indicates a reversible adsorption of alcohol molecules through the formation of hydrogen bonding with surface hydroxyl groups.
2 1
6
0
, 2
,
, 4
,
, 6
,
,I 8
Water content ,OHgroups nm-2
Figure 9. Relationship between the amount of irreversibly adsorbed alcohol and the surface hydroxyl content of rutile sample: (0)MeOH; (0) EtOH; (A)1-PrOH.
The dependence of the amount of adsorbed alcohols on the surface hydroxyl content of the sample is represented in Figure 8. Here, V,, and Vm3are monolayer capacities estimated by applying the B-point method to the first and second adsorption isotherms in Figures 2-4, respectively. For every alcohol, the value of V,, is a maximum on the dehydroxylated surface and decreases almost linearly with increasing surface hydroxyl content of the sample. However, the variation of the V,, value with hydroxyl content is not so large, especially for the longer chain alcohols. Alcohol molecules are strongly held on the dehydroxylated surface either through esterification with surface hydroxyls produced by dissociative adsorption or by the formation of coordinate bond to unsaturated surface Ti4+ions. These adsorbed species still remain on the surface even after evacuation a t ambient temperature. On the other hand, either the displacement of adsorbed water by alcohol or the esterification reaction with acidic surface OH groups can occur on the hydroxylated surface, and hence, it is reasonable to consider that the majority of the surface evacuated after first adsorption is covered with alkoxy groups. On the surface with an intermediate hydroxyl content, the adsorption of alcohols may occur through both adsorption mechanisms for the dehydroxylated surface as well as for the hydroxylated surface. The rutile surface prior to the second adsorption, therefore, is assumed to be almost covered with alkoxy groups to give similar surface states, which results in an almost constant value for V,, regardless of the hydroxyl content of the sample. Figure 9 shows the variation of the amount of irreversibly adsorbed species, Vi,,, with surface hydroxyl content of the rutile sample, the former value being obtained by subtracting V,, from V,, in Figure 8. Vi,, is maximum for the dehydroxylated surface, and decreases with increasing surface hydroxyl content of the sample. For the samples having appreciable hydroxyl contents (about 2-8 OH groups per nm2),the value of Vi, is greater for alcohols with longer carbon chains. This can be explained as follows. I t is generally known that the alkyl groups in organic molecules have an inductive effect by which an electron-donating character is greater for the molecules with larger number of carbon atoms.24 Therefore, the electron density of oxygen atoms in the alcohol molecules increases in the order MeOH < EtOH < 1-PrOH. Since the alcohol molecules are assumed to be adsorbed with their hydroxyl groups attached to the (24) E.g.: Fieser, L. F.; Fieser, M. Textbook of Organic Chemistry; Maruzen: Tokyo,1952.
104
Langmuir 1987, 3. 104-106
surface, it would be expected that the molecules with higher electron density in the oxygen atom interact more strongly with surface electron-accepting sites (in the case of hydroxylated surface, the hydrogen atom of surface hydroxyls).6 Therefore, the molecules having longer carbon chains form stronger hydrogen bonding with surface hydroxyls, which results in the increased amount of irreversible adsorption on the hydroxylated surface. On the other hand, on the surface with a lesser hydroxyl content, the value of Vir, is almost the same for three kinds of alcohols, i.e., about 3 molecules per nm2. Jones and Hockey have assumed that most of the external plane of rutile is composed of three planes, the (110) plane in 60% and (101) and (100) plane each in 20%.18 From their model, the number of fivefold Ti4+ ions interacting with alcohol molecules can be estimated to be 5.1,7.9, and 7.4 ions per nm2for the (110), (IOl), and (100) plane, respectively.18,22 Taking into account the steric hindrance by the larger size of alcohol molecules, only half of these ions should interact with alcohols. From the ratio of each cleavage plane and the number of Ti4+ions accessible for alcohol molecules in the plane, the value of 2.9 Ti4+ions per nm2 can be evaluated as the number of effective sites interacting with alcohols as a whole, which is
in good agreement with the value of about 3 molecules per nm2 for Vbr on the dehydroxylated surface. The possibility of substitutional reaction by water for adsorbed alcohol was examined for the present rutile sample, as in the case of the ZnO-alcohol-water system.6 The adsorption isotherm of alcohol was first determined on the dehydroxylated surface and then the sample was evacuated, followed by exposure to saturated water vapor a t 25 "C, during which there is a possibility of substitutional reaction of water with adsorbed alcohol. Next the sample was evacuated again and the second adsorption isotherm of alcohol was measured. This isotherm was found to be quite the same as that of the second adsorption on the sample with preadsorbed alcohol and without exposure to water vapor. This implies that the substitutional reaction of water for adsorbed alcohol does not occur on the rutile surface, which was also confirmed by infrared spectroscopic observation. Thus, it can be concluded that the interaction of the rutile surface with alcohol molecules is stronger than that with water, in contrast to the case of ZnO. Registry No. TiOz, 13463-67-7; MeOH, 67-56-1; EtOH, 64-17-5; PrOH, 71-23-8.
Langmuir-Blodgett Deposition of a Ring-Shaped Molecule (Valinomycin) J. B. Peng,+ B. M. Abraham, P. Dutta,* and J. B. Ketterson Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60201
H. Frank Gibbard Power Conversion Incorporated, Elmwood Park, New Jersey 07407 Received July 21, 1986. I n Final Form: October 3, 1986 Valinomycin, unlike most surfactants, is a ring-shaped molecule; its hydrophilic groups are evenly distributed around the ring. By the Langmuir-Blodgett (LB) technique, a single monolayer of valinomycin can be deposited on withdrawing a (hydrophilic)glass or mica substrate from the subphase, but the contact angle is not appreciably changed by the deposition of this flat molecule. The monolayer then peels off as the substrate is reintroduced into the water. However, if a thin film of silver is first evapoated on the substrate or three monolayers of lead stearate deposited on it, multiple layers of valinomycin can be built up. LB multilayers can also be deposited on poly(methy1 methacrylate) (PMMA). Monitoring the actual (dynamic)contact angle during the process of deposition suggests that, for successful LB deposition, the contact angle must be greater than 90° on immersion and less than 90" on withdrawal. For valinomycin this sequence occurs only with substrates whose contact angles are hysteretic even in clean water.
Introduction Valinomycin is a cyclic dodecadepsipetide. Three units, each consisting of D-valin, L-lactic acid, L-valine, and Dhydroxyisovaleric acid, are joined sequentially to form a ring. The study of Langmuir monolayers of this material,1-3 and of its Langmuir-Blodgett (LB) deposition characteristics, is particularly interesting because its molecular structure is quite different from that of typical film-forming compounds. The compound has also received attention because it selectively transports K+ across both natural and synthetic membra ne^.^^ Smith et al.' determined the structure of crystalline valinomycin; Neu'Permanent address: University of Science and Technology of China, Hefei, Anhui, People's Republic of China.
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pert-Laves et aL8 determined the structure of crystalline K+ complexed valinomycin. The results of the two determinations established a clear difference in the conformation, unit cell size, and space group between the complexed and uncomplexed crystalline compound. Some years ago, Gibbardg prepared thick films of valinomycin in an organic binder which was deposited on the surface (1) Ries, H.E.,Jr.; Swift, H. J. Colloid Interface Sci. 1978, 64, 111. (2) Abraham, B. M.; Ketterson, J. B. Langmuir 1985, I, 461. (3) Kemp, G.; Wenner, C. E.Biochim Biophys. Acta 1972, 282, 1. (4) Crisp, D. J. J. Colloid Sci. 1946, 1, 49. (5) Ries, H.E.,Jr.; Walker, D. C. J. Colloid Sci. 1961, 16, 361. (6) Blank, M.J. Phys. Chem. 1962, 66, 1911. (7) Smith, G.D.; Daux, W. L.; Langs,D. A.; DeTitta, G . T.;Edmonds, J. W.; Rohrer, D. C.; Weeks, C. M. J. Am. Chem. SOC.1975,97, 7242. (8) Neupert-Laves, K.;Dolber, M. Helu. Chim. Acta 1975, 58, 432. (9) Gibbard, H.F., unpublished results.
0 1987 American Chemical Society
