Diffuse Reflectance Fourier Transform Infrared Study of the Adsorption

Any of the oleic acid dimer, the monomer, or the anion may be adsorbed, depending on the oxide ... has meant that the adsorption of oleate/oleic acid ...
3 downloads 0 Views 74KB Size
Download PDF
Langmuir 2000, 16, 4993-4998

4993

Diffuse Reflectance Fourier Transform Infrared Study of the Adsorption of Oleate/Oleic Acid onto Titania P. J. Thistlethwaite* and M. S. Hook Chemistry School, Melbourne University, Parkville, Victoria 3010, Australia Received November 19, 1999. In Final Form: March 2, 2000 Fourier transform infrared spectroscopy in diffuse reflectance mode is used to study the nature of the adsorbed species when rutile form titania is exposed to aqueous oleate/oleic acid over a range of pH values. Any of the oleic acid dimer, the monomer, or the anion may be adsorbed, depending on the oxide sample, in particular the incidence of surface Lewis acid sites. The results suggest that oleic acid monomer can be adsorbed by coordination as a σ-bonded ligand to Ti4+ Lewis acid sites, via its carbonyl group oxygen. The oleic acid dimer appears to simultaneously coordinate to two adjacent Lewis sites via the hydroxyl oxygens. At pH 3 there is evidence that the adsorbed amphiphiles make two-point contact with the oxide surface via an interaction of the CHdCH group with surface OH2+ groups.

Introduction The importance of oleic acid (HOl) as a flotation agent has meant that the adsorption of oleate/oleic acid onto a variety of mineral and oxide substrates has been widely studied. Despite this, the contribution of different possible interactions to the adsorption process is still unclear. The very low water solubility of oleic acid poses difficulties. If the goal is to understand the adsorption process, then much early work was compromised by the use of high concentrations of oleate such that, at lower pH, bulk precipitation of oleic acid from solution could be expected to occur alongside true adsorption.1,2 Further confusion can arise if genuine adsorption is confused with the bulk precipitation of metal oleate phases.3-7 Nevertheless, from experiments where bulk precipitation of oleic acid or metal oleate can be ruled out, there is evidence that oleate is specifically adsorbed to a number of oxides. There is flotation recovery data for oleate on goethite and haematite8,9 and electrokinetic data for oleate on R-Fe2O3,10 the latter demonstrating that oleate is able to increase the negative surface potential at pH’s above the oxide IEP. There is further evidence that both hydrophobic interactions and the presence of the CdC double bond might be significant factors in the adsorption. A study of the flotation of goethite8 indicated greater specific adsorption for oleate compared to the shorter chain laurate. For the same system, log plots of adsorption density (Γ) versus concentration (C) showed slopes close to 2, for pH’s of 8, * Author for correspondence. (1) Howe, T. M.; Pope, M. I. Powder Technol. 1971, 4, 338. (2) Purcell, G.; Sun, S. C. Trans. AIME 1963, 226, 6. (3) Peck, A. S. Report No. 6202; U.S. Bureau of Mines: 1963. (4) Somasundaran, P. J. Colloid Interface Sci. 1969, 31, 557. (5) Sherwood, A. F.; Rybicka, S. M. J. Oil Colour Chem.’ Assoc. 1966, 49, 648. (6) Peck, A. S.; Raby, L. H.; Wadsworth, M. E. Trans. AIME 1966, 235, 301. (7) Thistlethwaite, P. J.; Gee, M. L.; Wilson, D. Langmuir 1996, 12, 6487. (8) Iwasaki, I.; Cooke, S. R. B.; Colombo, A. F. Investigation No. 5593; U.S. Bureau of Mines: 1960. (9) Iwasaki, I.; Cooke, S. R. B.; Choi, H. S. Trans. AIME 1960, 217, 237. (10) Han, K. N.; Healy, T. W.; Fuerstenau, D. W. J. Colloid Interface Sci. 1973, 44, 407.

7, and 6, implying the adsorption of a dimer type species, possibly (HOl)2-.11 In some cases of oleate adsorption onto minerals, there is a change in slope of the log Γ versus log C plot at a coverage of 55 A2 per molecule, postulated to correspond to a switch from two-point to one-point contact.12,13 Ottewil and Tiffany14 reported two-step isotherms for oleate adsorption from heptane onto titania, which pointed to adsorption initially involving two-point contact corresponding to an area per molecule of approximately 50 A2 molecule-1. At higher adsorption density, the adsorbate was thought to stand vertical at an area of approximately 25 A2 molecule-1. There is some support for this concept in the form of IR spectroscopic evidence suggesting that when oleic acid is adsorbed from benzene onto titania, there is an interaction of the oxide with both the CH2 chain and the CdC linkage.5 Neither of the two foregoing studies, both involving nonaqueous solvents, may be relevant to the aqueous situation. Although they carry some implications, none of the foregoing studies establish the nature of the adsorbed species at the oxide surface or the nature of the adsorbatesubstrate binding. It has been suggested that a possible driving force for adsorption is the attachment of adsorbates as σ-bonded ligands to incompletely coordinated metal atoms (Lewis acid sites).15-17 In an earlier study7 we used diffuse reflectance, infrared, and Fourier transform (DRIFT) spectroscopies to identify the adsorbed species when ZrO2 is exposed to aqueous oleate/oleic acid. At pH 9 both oleate anions and the oleic acid dimer are adsorbed. The adsorption of the dimer seemed to provide some support for the above mechanism. Accordingly, we have been interested in investigating the nature of the adsorbed species when TiO2 is exposed to aqueous oleate. Like ZrO2, TiO2 is a particularly suitable oxide for a DRIFT study of (11) Jung, R. Ph.D. Thesis, Melbourne University, 1976. (12) Read, A. D.; Manser, R. M. Trans.sInst. Min. Metall., Sect. C 1972, 81, 69. (13) Paterson, J. G.; Salman, T. Trans.sInst. Min. Metall., Sect. C 1970, 79, 91. (14) Ottewil, R. H.; Tiffany, J. M. J. Oil Colour Chem.’ Assoc. 1967, 50, 844. (15) Buckland, A. D.; Rochester, C. H.; Topham, S. A. J. Chem. Soc., Faraday Trans. 1 1980, 76, 302. (16) Jones, P.; Hockey, J. A. Trans. Faraday Soc. 1971, 67, 2669. (17) Jones, P.; Hockey, J. A. Trans. Faraday Soc. 1971, 67, 2679.

10.1021/la991514i CCC: $19.00 © 2000 American Chemical Society Published on Web 05/03/2000

4994

Langmuir, Vol. 16, No. 11, 2000

Figure 1. DRIFT spectrum following exposure of sample A TiO2 to 5 × 10-4 M aqueous oleate, at pH 9: spectrum a (filled circles), sample; spectrum b (open circles), pH 9 blank (see text).

adsorption because it itself shows no IR bands above 1000 cm-1. Our study has revealed an interesting variability of behavior which we believe provides insight into the adsorption process and provides evidence for the role of Lewis acid sites. Experimental Section Two different TiO2 samples were studied. Both were obtained from Tioxide Ltd, U.K., and had been produced by the hydrolysis of TiCl4. Both samples had been calcined at 250 °C. Sample A had subsequently been extensively Soxhlet washed. Both samples had previously been shown, by X-ray diffraction, to be of the rutile form, and this was confirmed by Raman spectroscopy, the Raman spectrum showing no evidence of the anatase crystal form. Sample B was stated to contain 7 wt % chloride ion. Because it was subjected to Soxhlet washing, sample A was expected to contain substantially less chloride. Elemental analysis showed the chloride content to be 0.13 wt %. Transmission electron microscopy showed that the two samples had similar morphology, both consisting of ellipsoidal particles: in the case of sample A of length approximately 275 nm and width 50 nm, and in the case of sample B of length 315 nm and width 80 nm. The isoelectric points determined with a Matec Acoustosizer were 5.8 and 6.6, for samples A and B, respectively. Oleic acid was from Aldrich. Stock sodium oleate solutions of concentration 10-3 and 10-6 M were prepared shortly before use by saponification of oleic acid. All solutions were prepared with Milli-Q water. The samples for DRIFT spectroscopy were prepared as follows. For the pH 9 samples, 5 mg of TiO2 was added to 50 mL of water, the solution pH was adjusted to 9, and the particles were dispersed by sonication. The solution volume was brought to 100 mL using 0.001 M sodium oleate to achieve a final oleate concentration of 5 × 10-4 M. The solution pH was then readjusted to 9, and the sample was stirred for 24 h. The titania suspension was filtered (Millipore, 0.8 µm filter), washed with four 250-mL aliquots of pH 9 water to remove physisorbed oleate, and air-dried. For the pH 3 experiments, an oleic acid concentration of 10-8 M was used to ensure that the solubility of oleic acid was not exceeded. This very low concentration necessitated the use of large volumes of oleic acid solution for the adsorption experiments. With 10 L of solution the number of oleic acid molecules initially in solution is about one-quarter of the number required for monolayer coverage of 5 mg of oxide. The titania suspension was filtered, washed with pH 3 water, and dried as above. The DRIFT spectra were run using oxide mixed with spectroscopic grade KBr in an approximately 1:50 ratio. The instrument used was a Digilab FTS-65A fitted with a Praying Mantis DRIFT accessory.

Results Figure 1 shows the DRIFT spectrum when sample A TiO2 is exposed to 5 × 10-4 M aqueous oleate at pH 9. The

Thistlethwaite and Hook

pH of 9 is substantially above the precipitation edge of oleic acid at the oleate concentration used. Figure 1 also shows a pH 9 “blank” spectrum, for which the TiO2 sample was treated in exactly the same way as that for the “sample” spectrum but with no oleate present. The blank spectrum exhibits a weak broad absorption with a maximum at approximately 3420 cm-1 that can be attributed to hydroxyl groups of the hydrated oxide surface and of adsorbed water. The intensities of the blank and sample spectra are shown as found. The intensity of the blank spectrum is negligible in the regions where oleic acid bands appear in the sample spectrum, and thus no subtraction has been done. By contrast, in the sample spectrum, where adsorbed oleate species are present, the oleic acid peaks prominent near 2900 cm-1 are superimposed on an underlying surface hydroxyl-adsorbed water band which seems to be centered at approximately 3000 cm-1. The sample spectrum of Figure 1 is essentially identical to that of liquid oleic acid, which consists predominantly of dimer. In the CH stretching region, the spectrum shows strong bands at 2856 and 2928 cm-1, assignable respectively to the symmetric and asymmetric CH2 stretches of the hydrocarbon moiety, a shoulder at 2960 cm-1, due to the asymmetric stretch of the terminal CH3 group, and a weak but definite band at 3007 cm-1, attributable to the olefinic CH stretch. In the carboxyl group region, a strong band is seen at 1714 cm-1 which can be associated with the CdO group of an undissociated carboxyl group (νCdO). The value of 1714 cm-1 (cf. 1775 cm-1 for monomer oleic acid in the vapor phase) indicates that this CdO group is involved in H-bonding and coincides with the value found for the dimer. The remaining bands at 1463, 1413, and 1285 cm-1 can be associated with the CH2 deformation (δCH2), the CsOsH in-plane bend (δCsOsH), and the Cs OH stretch (νCsOH). There is no sign of the OsH stretch of the acid nor any evidence for adsorbed anion. It should be noted that the sample for Figure 1 was extensively washed with pH 9 water to remove the possibility of physisorbed oleic acid. Figure 2a shows superimposed blank and sample spectra when sample B TiO2 is exposed to 5 × 10-4 M aqueous oleate at pH 9. In this case the adsorption of oleate species is again evident in the strong C-H stretch bands around 2900 cm-1, although these bands are weaker than those for sample A. It is noteworthy that in this case the surface-hydroxyl-adsorbed water bands in the 3300cm-1 region are closely similar for the sample and the blank, with the band maximum being at approximately 3306 cm-1. The blank also displays an absorption around 1600 cm-1 attributable to the bending mode of adsorbed water. The blank and sample spectra are shown as obtained, without any scaling of intensity. The spectrum resulting from the subtraction of the Figure 2a blank from the Figure 2a sample spectrum is shown in Figure 2b. The difference spectrum of Figure 2b is dominated by a band at approximately 1509 cm-1, which can be attributed to the asymmetric stretching frequency of the carboxylate group (νas,CO2-) and is definitive for the oleate anion. Also evident is a broad complex band showing a maximum at its high-frequency end at 1459 cm-1. This maximum can be assigned to the CH2 deformation (δCH2). The absorption around 1427 cm-1 (two peaks at 1436 and 1420 cm-1) can be attributed to the symmetric stretch of the carboxylate group (νs,CO2-). There is also evidence of weak absorption in the region of 1700 cm-1, suggesting that the oleic acid dimer is again present, but in much lower concentration. If that is so, it would be expected that the absorption in the vicinity of 1420 cm-1 would have a contribution from

Adsorption of Oleate/Oleic Acid onto Titania

Langmuir, Vol. 16, No. 11, 2000 4995

Figure 2. (a) DRIFT spectrum following exposure of sample B TiO2 to 5 × 10-4 M aqueous oleate, at pH 9: spectrum a (filled circles), sample; spectrum b (open circles), pH 9 blank. (b) Difference spectrum: sample spectrum of part a minus blank spectrum of part a.

Figure 3. (a) DRIFT spectrum following exposure of sample A TiO2 to 10-8 M aqueous oleate at pH 3, case 1: spectrum a (filled circles), sample; spectrum b (open circles), pH 3 blank. (b) Difference spectrum: sample spectrum of part a minus blank spectrum of part a.

the CsOsH in-plane bend (δCsOsH). There is also a weak band around 1650 cm-1. This is too high a frequency for the asymmetric stretch of a carboxylate group, and the band must be attributed to the CdO stretch of undissociated monomer oleic acid. The assignment of this band is best deferred until later. Study of the adsorption at pH 3 is complicated by the need to work with a very low oleic acid concentration in order not to exceed the oleic acid solubility. In this case, two different instances of adsorption onto sample A at pH 3 are shown. When sample A TiO2 is exposed to 10-8 M oleic acid at pH 3, the spectrum shown in Figure 3a is obtained. The pH 3 blank spectrum has been superimposed on the pH 3 sample spectrum with the blank spectrum scaled so as to cause the Kubelka-Munk values to be equal at the surface-hydroxyl-adsorbed water band maximum in the vicinity of 3400 cm-1. (The spectra in Figure 3 all exhibit an uncancelled CO2 band at 2350 cm-1.) In this case, the profiles of the surface-hydroxyl-adsorbed water bands are the same in the sample and blank spectra, with the maximum lying at 3420 cm-1. The blank spectrum again shows an absorption around 1600 cm-1 attributable to the bending mode of adsorbed water. Figure 3b shows the difference spectrum. As expected from the much lower aqueous oleic acid concentration, the intensity of the IR bands is now much lower. However, the signal-to-noise ratio is more than adequate to allow comparison with the pH 9 spectra. Oleate species adsorption is still clearly evident in the form of the CH2 stretch bands between 2800 and 3000 cm-1, but the intensity of these bands is

now about 100 times weaker (0.0022 compared to 0.2). The signal-to-noise ratio of the CH2 stretch bands is high enough to suggest that the olefinic CH stretch band, if present, should be seen. There is, however, no sign of it. At 1517 cm-1 is a well-defined band attributable to νas,CO2and definitive for the oleate anion. The band at 1413 cm-1 can be assigned to the νs,CO2- band of oleate anion but probably also contains a contribution from the δCsOsH band of undissociated oleic acid. This band has a shoulder at 1451 due to δCH2. A strong band at 1644 cm-1 shows a shoulder at 1714 cm-1. This shoulder indicates the presence of a small amount of oleic acid dimer. The band at 1644 cm-1 must be assigned, as in the case of a similar band seen in Figure 2b, to the CdO stretch of undissociated monomer oleic acid. This assignment will be further discussed at a later point. Figure 4a shows blank and sample spectra for a different pH 3 experiment on sample A TiO2, while Figure 4b shows the difference spectrum. The spectra in this case are affected by the presence of uncanceled water vapor peaks, which is often difficult to prevent in spectra of low Kubelka-Munk value (ca. 0.02). Despite this interference, the difference spectrum of Figure 4b shows obvious similarities to that of Figure 3b with respect to bands in the vicinity of 1405, 1505, 1640, and 1714 cm-1. At the same time, there are significant differences. In this case, the underlying surface hydroxyl-adsorbed water band in the 3000-3500-cm-1 region has shifted to lower frequency on oleate adsorption. Following oleate adsorption, the surface hydroxyl-adsorbed water band maximum is at approximately 3180 cm-1. There is now more evidence for

4996

Langmuir, Vol. 16, No. 11, 2000

Figure 4. (a) DRIFT spectrum following exposure of sample A TiO2 to 10-8 M aqueous oleate at pH 3, case 2: spectrum a (filled circles), sample; spectrum b (open circles), pH 3 blank. (b) Difference spectrum: sample spectrum of part a minus blank spectrum of part a.

adsorbed dimer in the form of the greater prominence of the band in the vicinity of 1714 cm-1. The band at 1640 cm-1 is relatively weaker than that in Figure 3b. Again, although the CH2 stretch bands are quite intense, there is no sign of the olefinic CH stretch at 3007 cm-1. For sample B TiO2, no adsorption of oleate species could be detected at pH 3. Discussion Figure 1 shows that for sample A at pH 9 the dimer is strongly and apparently exclusively adsorbed. Although the attachment of the oleic acid dimer to the titania surface by H-bonding to surface hydroxyl groups is conceivable, such a mode of attachment would lead to weak binding which would not survive the extensive washing with pH 9 water used in these experiments. It is difficult to see what mechanism for chemisorption other than coordination of electron donor carboxyl groups to surface Lewis acid sites can account for the adsorption of oleic acid dimer. We believe that the observation of strong dimer adsorption for the Soxhlet-washed sample, the different tendencies to adsorb dimer by the two TiO2 samples, and the shift in the surface-hydroxyl-adsorbed water band that seems to be associated with adsorption of the oleic acid dimer are all explicable in terms of dimer adsorption depending on the attachment of oleic acid as a σ-bonded ligand at Ti4+ Lewis acid sites. Jones and Hockey16,17 have previously used IR spectroscopy of adsorbed pyridine to confirm the existence of such sites on rutile TiO2. Their studies suggest that, for the 101 and 100 planes of TiO2, octahedral coordination

Thistlethwaite and Hook

of surface titanium atoms, that is, those not completely coordinated by oxide or hydroxide ions, can be completed by each titanium acquiring a water molecule as a σ-bonded ligand. Jones and Hockey attributed parts of the broad surface-hydroxyl-adsorbed water absorption in the region 2900-3700 cm-1 to the OH stretch of liganded water molecules. We believe our observations are broadly in line with their proposals. We suggest that the shift in the surface-hydroxyladsorbed water maximum from approximately 3420 to approximately 3100 cm-1, that is apparent in Figure 1, is associated with the displacement of water molecules from Lewis acid sites by oleic acid dimer. Moreover, it is suggested that the different tendencies of the two TiO2 samples to adsorb dimer are associated with different incidences of Lewis sites in the two cases. Where the incidence of Lewis sites is low, the surface-hydroxyladsorbed water absorption lies at lower frequency, the dimer is only slightly adsorbed, as in Figure 2, and little shift in the hydroxyl-adsorbed water band occurs on adsorption. Adsorption is then mainly of the anion and involves a ligand replacement of OH- by oleate, as suggested previously.15 An explanation for a lower concentration of Lewis sites in sample B plausibly lies in the different preparative histories. Jones and Hockey16 have suggested that TiO2 prepared by the hydrolysis of TiCl4, and which retains surface Cl-, can be less adsorbing, because Cl- tends to complete the coordination of Ti4+ at the oxide surface. Our results accord with this suggestion given the known substantial Cl- concentration in sample B and the much lower concentration in sample A, and are consistent with the proposal that the Soxhlet-washed sample contains a much higher concentration of Lewis sites. Moreover, this proposal is compatible with the measured IEP values, the Soxhlet-washed sample having an IEP of 5.8, versus 6.6 for sample B. Several questions have to be addressed. How is it that at pH 9, where the solution species is the anion, it is neutral oleic acid in the form of the dimer that is sometimes adsorbed, and why can adsorption of the dimer sometimes be favored over that of monomer oleic acid? Equally, it can be asked why at pH 3, where the stable solution species is undissociated oleic acid, the anion can be adsorbed as well as the monomer. The first and third questions can be answered by reference to the special conditions prevailing at the solidsolution interface. Although the bulk solution concentration of hydrogen ion is very low at pH 9, a much higher hydrogen ion concentration can be anticipated at the interface due to attraction of hydrogen ions as counterions to the negatively charged oxide surface. (It can equally be argued that at pH 3, although the solution concentration of OH- ions is low, the situation near the interface is rather different.) At pH 9, protonation of oleate anions near the surface allows the formation of the dimer, and this will confer added stability provided it does not interfere with coordination of oleic acid carboxyl groups to Ti4+ Lewis sites. Earlier adsorption isotherm studies in fact support the idea of adsorption of a dimeric species.11 With regard to the preference for dimer adsorption, at least on some samples, we note an interesting possibility. Two of the common crystal planes expected to be exposed at the surface of rutile crystals are the 100 and 110 planes. These planes, and in fact any planes containing the rutile c axis, display a Ti-Ti distance of 3.57 Å.18 This distance coincides (18) Wyckoff, R. W. G. Crystal Structures; John Wiley and Sons: 1963; Vol. 1, p 250.

Adsorption of Oleate/Oleic Acid onto Titania

closely with the diagonal O-O distance between the hydroxyl oxygens of the planar, six-membered ring of the carboxyl group dimer.19 This suggests that the carboxyl group dimer is peculiarly well adapted for simultaneous coordination to two adjacent Ti4+ Lewis sites, via the hydroxyl oxygens acting as lone-pair donors. The hybridization of the carbonyl oxygen is not compatible with two such oxygens coordinating simultaneously to two Ti4+ Lewis sites. In the pH 3 experiments, sample B, which exhibits a much lower tendency to adsorb dimer at pH 9, showed no detectable adsorption. At pH 3, sample A shows variable behavior. In Figures 3 and 4, anion is seen to be adsorbed along with monomer oleic acid, but with varying relative prominence. The oleic acid dimer is also adsorbed, but again to a varying degree. As earlier, there appears to be a correlation between a shift in the surface-hydroxyl-adsorbed water band and the adsorption of dimer, supporting the view that dimer adsorption is associated with a displacement of water molecules from their original sites to new sites. In the case of Figure 3, where oleate adsorption causes very little shift in the surface-hydroxyl-adsorbed water maximum, which remains at 3420 cm-1, the adsorption of dimer is weak and the 1640-cm-1 band attributed to the monomer is relatively more prominent. In Figure 4 the adsorption of oleate species leads to the surface-hydroxyl-adsorbed water maximum shifting from 3420 cm-1 to approximately 3180 cm-1 while a shoulder at 3420 cm-1 remains. In this case the CdO stretch band of the dimer is much more evident in the region of 1715 cm-1, while the 1640-cm-1 band attributed to the monomer is relatively weaker. The overall greater adsorption in Figure 4 can be attributed to the larger amount of oleic acid dimer in that case and is evident from the Kubelka-Munk values for the CH2 stretch bands: approximately 0.02 in Figure 4 compared to 0.002 in Figure 3. In both cases, at pH 3 the adsorption is much weaker than that at pH 9. This is readily understandable in terms of the much lower solution oleate concentration. That the nature of adsorbed water is different for different titania samples is evident in changes in spectra when samples are additionally hydrated. A DRIFT spectrum on sample A taken directly from the bottle showed a surface-hydroxyl-adsorbed water maximum at approximately 3260 cm-1. After exposure to pH 9 water (see spectrum of the blank in Figure 1), this maximum is at approximately 3420 cm-1. By contrast, a DRIFT spectrum on sample B taken directly from the bottle showed a surface-hydroxyl-adsorbed water maximum at approximately 3280 cm-1. After exposure to pH 9 water (see spectrum of the blank in Figure 2a), this maximum is at approximately 3306 cm-1; that is, there is comparatively little change in the surface-hydroxyladsorbed water maximum in the case of sample B. The band in the vicinity of 1645 cm-1 has been attributed to the CdO stretch of the adsorbed oleic acid monomer. The value of 1645 cm-1 (in comparison to the value of 1775 cm-1 for the free acid) might be taken to indicate that the CdO group is strongly H-bonded. However it is unlikely that H-bonding would alone account for the adsorption of the monomer. Two possibilities then present themselves. The first is that monomer oleic acid attaches as a σ-bonded ligand to a Ti4+ Lewis site via its hydroxyl oxygen, while its CdO group is simultaneously H-bonded to a neighboring OH or OH2+ group. This is somewhat unlikely, as the very low CdO stretch value of 1645 cm-1 implies an extremely strong H-bond. A second more (19) Derissen, J. L. J. Mol. Struct. 1971, 7, 67.

Langmuir, Vol. 16, No. 11, 2000 4997

plausible possibility is that the low CdO stretch frequency of 1645 cm-1 indicates that the oleic acid monomer is coordinated to a Ti4+ Lewis site via the CdO oxygen. Complexes of transition metal cations (Mn2+, Fe2+, Co2+, Ni2+) with ligands containing the carbonyl group, for example, acetone, acetaldehyde, and acetophenone, have been extensively studied by Driessen and Groeneveld.20-22 In these complexes, in which the coordination was thought to be via the oxygen atom, the CdO stretch frequencies were found to be in the range 1621-1684 cm-1. The CdO stretch frequency was found to shift to lower values as the electronegativity of the cation increased. The observed value of 1645 cm-1 is well in accord with the above results. In our earlier study of the adsorption of oleate/oleic acid onto ZrO2 at pH 3, we observed a band at 1681 cm-1, which was assigned to the CdO stretch of adsorbed monomer oleic acid. It now seems likely that that band similarly reflects an oleic acid molecule coordinated via its carbonyl oxygen to a Zr4+ Lewis site. Figures 3b and 4b provide an interesting comparison. Figure 4b shows greater overall adsorption, as indicated by the 10-fold greater CH stretch intensity. In line with this is the relatively greater prominence in that case for the dimer and the lower prominence for the monomer, and the more obvious shift in the adsorbed water band. The variability in behavior seen at pH 3 is plausibly explained by the way the oleate/oleic acid interacts with the surface. It is noteworthy that at pH 3, where the oxide surface is positively charged, the olefinic CHdCH stretch is never seen. This supports the view that the CHdCH moiety can interact with the surface, as previously suggested.5 An interaction of the CHdCH group with the surface would be enhanced when the surface acquires OH2+ groups at pH 3. It is reasonable to suggest that a two-point interaction of oleic acid with the oxide surface would interfere with binding of the dimer if, as suggested earlier, the latter involves simultaneous coordination to two adjacent Ti4+ Lewis acid sites. It seems plausible that variation between samples in the distribution of Lewis acid sites and OH2+ groups will lead to variability in the species adsorbed and their conformation on the oxide surface. The bands attributable to the oleate anion in Figures 3 and 4 allow some speculation as to the mode of attachment of the anion to the oxide surface. It has been suggested15,23 that the difference, ∆ν ) (νas,CO2- - νs,CO2-), provides a guide to the mode of binding of carboxylate adsorbates to oxide surfaces. A value of ∆ν appreciably higher than that in the carboxylate anion in an ionic salt is thought to indicate unidentate attachment; a value appreciably lower, a chelating bidentate attachment; and a value comparable, a bridging bidentate attachment. For the oleate anion in sodium oleate, Peck et al.6 have reported νas,CO2- and νs,CO2- to be 1550 and 1410 cm-1, respectively, giving ∆ν ) 140 cm-1. Figures 3 and 4 do not allow νs,CO2to be located with absolute certainty because, as well as oleate anion, oleic acid is present and is expected to contribute to absorption in the region near 1400 cm-1. Despite this, an upper limit of (1517-1405) ) 112 cm-1 can be placed on ∆ν. This value might be taken to imply (20) Driessen, W. L.; Groeneveld, W. L. Recl. Trav. Chim. 1969, 88, 977. (21) Driessen, W. L.; Groeneveld, W. L. Recl. Trav. Chim. 1971, 90, 87. (22) Driessen, W. L.; Groeneveld, W. L. Recl. Trav. Chim. 1971, 111, 258. (23) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 5th ed.; John Wiley and Sons: New York, 1997; Part B, p 59.

4998

Langmuir, Vol. 16, No. 11, 2000

chelating bidentate attachment, but the bridged structure cannot be ruled out. Conclusion When rutile form TiO2 is exposed to oleic acid/oleate at pH 3 and 9, the adsorption behavior is variable depending on the nature of the particular oxide sample, in particular the incidence of Lewis acid sites. Any of the dimer, the monomer, or the anion may be adsorbed. At pH 9, and where there is a high incidence of Ti4+ Lewis acid sites, the dimer adsorbs by displacing water ligands and simultaneously coordinating, as a σ-bonded ligand (via the hydroxyl oxygens), to two adjacent Lewis sites. When the incidence of Lewis sites is lower, overall adsorption

Thistlethwaite and Hook

at pH 9 is weaker, and the anion is now the dominant species adsorbed, although monomer and dimer oleic acid are also weakly adsorbed. Again at pH 3 adsorption is variable. The oleate anion is always adsorbed while the dimer and monomer appear to compete, so that when dimer is more evident, monomer is less so. This variability can be understood in terms of a variation in the distribution of Lewis sites and OH2+ groups, and by dimer formation being interfered with by the monomer making a two-point interaction with the surface.

LA991514I