Langmuir 1996, 12, 6429-6435
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Self-Assembled Monolayers of Alkylphosphonic Acids on Metal Oxides Wei Gao, Lucy Dickinson, Christina Grozinger, Frederick G. Morin, and Linda Reven* Department of Chemistry, McGill University, 801 Sherbrooke Street West, Montreal, Quebec H3A 2K6, Canada Received August 1, 1996X Dense, highly ordered monolayers can be prepared by the adsorption of octadecylphosphonic acid (ODPA) onto nonporous ZrO2, TiO2, and zirconated silica powders. ODPA reacts strongly with Al2O3 to form a bulk (aluminoalkyl)phosphonate. The infrared spectra of the ODPA monolayers indicate a degree of conformational order comparable to self-assembled monolayers on planar substrates. From the solid-state 31P and 13C NMR spectra, the strength of the surface interaction and the degree of conformational order decrease in the following order: zirconated silica > ZrO2 > TiO2. Although the inner methylenes are primarily in an all-trans conformation, the chain termina are highly disordered. In the case of zirconated silica, there is evidence for the formation of some bulk zirconium alkylphosphonate. Proton line width measurements from 2D WISE NMR experiments reveal that considerable chain mobility is present in the ODPA monolayers on ZrO2 and TiO2, in contrast to bulk crystalline phases.
Introduction In recent publications, we have employed solid-state NMR as a technique for characterizing the surface bond and chain dynamics of the most commonly studied selfassembled monolayers: alkylsilanes, thiols, and metal phosphonates.1a,2 In order to obtain sufficient sensitivity for an NMR study, these monolayers must be deposited on high surface area substrates such as metal and metal oxide colloids. One difference between SAMs deposited on high surface area versus planar substrates is that of the surface curvature, which may affect the efficiency of chain packing or cross-linking at the interfacial region. In the case of alkylthiol-capped gold colloids, where the colloid diameter is on the order of an extended octadecyl chain length, intercalation of domains of chains between neighboring colloids allows close packing of the chains. This system has been described as a “three dimensional self-assembled monolayer”.3 Monolayers deposited on larger diameter metal oxide particles are a closer model for 2D SAMs since intercalation is not possible and socalled planar metal oxide surfaces are rough on a microscopic level. In our studies of trichlorosilane selfassembled monolayers on nonporous silica,1a we could not be certain whether a high degree of coverage is obtainable with only horizontal, and no vertical, polymerization occurring. At the low deposition temperatures where an optimal degree of conformational order of the alkyl chains is obtained, no surface Si-O bonds are formed,4 and thus, 29Si NMR is not useful for distinguishing between a bulk or a surface-attached siloxane network. In addition, the assembly of alkylsilane monolayers is highly sensitive to solvent, temperature, and trace amounts of water. In view of these difficulties, we have focused on monolayers of X Abstract published in Advance ACS Abstracts, December 1, 1996.
(1) (a) Gao, W.; Reven, L. Langmuir 1995, 11, 1860. (b) Neff, G. A.; Page, C. J.; Meintjes, E.; Tsuda, T.; Pilgrim, W. C.; Roberts, N.; Warren, W. W. Langmuir 1996, 12, 238. (2) Badia, A.; Gao, W.; Singh, S.; Demers, L.; Cuccia, L.; Reven, L. Langmuir 1996, 12, 1262. (3) (a) Badia, A.; Singh, S.; Demers, L.; Cuccia, L.; Brown, G. R.; Lennox, R. B. Chem. Eur. J. 1996, 2, 359. (b) Terril, R. H.; Postlethwaite, T. A.; Chen, C. H; Poon, C. D.; Terzis, A.; Chen, A.; Hutchison, J. E.; Clark, M. R.; Wignall, G.; Londono, J. D.; Superfine, R.; Falvo, M.; Johnson, C. S.; Samulski, E. T.; Murray, R. W. J. Am. Chem. Soc. 1995, 117, 12537. (4) Tripp, C. P.; Hair, M. L. Langmuir 1992, 8, 1120.
S0743-7463(96)00762-7 CCC: $12.00
long chain alkylphosphonic acids, which, like alkanethiols, self-assemble in an epitaxial fashion without polymerization. Another advantage of the alkylphosphonates is that the surface bonding can be easily characterized by 31P NMR.1 Due to their strong chelation properties, phosphonic acids are employed as corrosion inhibitors,5 for metal extraction,6 and in biochemical applications7 such as the prevention of calcification. Phosphonic acids react with a wide range of alkali and transition metals to form lamellar metal phosphonates that have been used for ion exchange, selective sorption, and catalysis.8 These layered compounds have also been deposited as self-assembled8,9 or Langmuir-Blodgett multilayers10 on a variety of substrates. Mallouk and co-workers have synthesized metal phosphonate multilayers on high surface area fumed silica.11 The primer layer for these multilayers consists of either Zr cations or PO3H2 functional groups attached to the silica via a siloxane bond. In our initial solid-state NMR study of a zirconium alkylphosphonate monolayer on silica,1a in which the primer layer consisted of a zirconated silica surface, we found that the degree of coverage was limited by the extent of zirconation. To overcome this limitation, as well as eliminate the possibility of forming bulk zirconium phosphonates, we have adsorbed long chain phosphonic acids directly onto the metal oxides. Methyl- and phenylphosphonic acids will react directly with silica surfaces. A 29Si and 31P solid state NMR study of the reaction of these acids with silica gel shows that covalent attachment occurs via the formation of a single (5) Duprat, M.; Shiri, A.; Derbali, Y.; Pebere, N. In Electrochemical Methods in Corrosion Research, Materials in Corrosion Research, Materials Science Forum; Duprat, M., Ed.; Trans Tech: Aedermannsdorf, 1986; Vol. 8, p 267. (6) Aguilar, M; Miralles, N.; Sastre, A. M. Rev. Inorg. Chem. 1989, 10, 93. (7) Francis, M. D.; Russell, R. G. G.; Fleisch, H. Science 1969, 165, 1264. (8) Cao, G.; Hong, H.-G.; Mallouk, T. E. Acc. Chem. Res. 1992, 25, 420. (9) Katz, H. E. Chem. Mater. 1994, 6, 2227. (10) (a) Byrd, H.; Pike, J. K.; Talham, D. R. Chem. Mater. 1993, 5, 709. (b) Byrd, H.; Whipps, S.; Pike, J. K.; Ma, J.; Nagler, S. E.; Talham, D. R. J. Am. Chem. Soc. 1994, 116, 295. (c) Byrd, H.; Pike, J. K.; Talham, D. R. J. Am. Chem. Soc. 1994, 116, 7903. (11) Hong, H. G.; Sackett, D. D.; Mallouk, T. E. Chem. Mater. 1991, 3, 521.
© 1996 American Chemical Society
6430 Langmuir, Vol. 12, No. 26, 1996
P-O-Si ester linkage.12 We have found that long chain phosphonic acids, which have much larger pKa’s, will not condense with the surface silanols, necessitating a primer layer of Zr cations or phosphonic acid groups. However, phosphonic acids do bind strongly to ZrO2 and TiO2. Modification of TiO2 and ZrO2 powders with phosphoric acid and short chain alkylphosphonic acids for separations in biotechnology have been recently investigated.13 Whereas phosphonate ions can be partly removed by water washing, alkylphosphonates form a permanent hydrophobic layer on TiO2 and ZrO2.13b Phosphonic acid functional groups have also been used to modify ITO electrodes14 and to anchor metal complexes to TiO2 films.15 Contact angles of long chain hydroxamic, carboxylic, and phosphonic acids adsorbed onto the native oxide surfaces of a variety of metals have been measured.16 The surface bonding and the chain conformation of the phosphonic acids on metal oxides were not characterized in this study. However, on the basis of contact angle measurements, the hydroxamic acid monolayers were concluded to be superior to those of phosphonic acids with the exception of titanium dioxide.16 We also attempted to study carboxylic acids on basic metal oxides such as Al2O3 but found that the surface attachment is so weak that most of the surfactant is removed during the washing steps. In this study, the surface bonding and conformational order of long-chain phosphonic acids adsorbed onto Al2O3, TiO2, ZrO2, and zirconated silica is characterized by solidstate NMR and vibrational spectroscopy. Octadecylphosphonic acid (ODPA) binds more strongly and is better oriented on ZrO2 compared to TiO2. Although the ODPA monolayer on TiO2 is not removed by washing, the 31P chemical shift indicates ODPA is not fully deprotonated whereas attachment through POZr linkages occurs on ZrO2. ODPA monolayers on zirconated silica display a high degree of conformational order. However, the appearance of two methyl signals indicates that some bulk zirconium octadecylphosphonate (ZrODPA) is formed. Although the study by Folkers et al. reported no etching of the native metal oxide surfaces by any of the long-chain organic acids studied, including phosphonic acid,16 elemental analysis and powder X-ray diffraction show that ODPA reacts strongly with Al2O3 to form a bulk (aluminoalkyl)phosphonate. Experimental Section Sample Preparation. Commercial nonporous fumed silica, (Cab-O-Sil, BET surface area 100 m2/g, Cabot Corp.), ZrO2 (monoclinic, VP zirconium oxide, BET surface area 40 m2/g, DeGussa AG), TiO2 (70% anatase/30% rutile, titanium dioxide P25, BET surface area 50 m2/g, DeGussa AG), and a nonporous γ-Al2O3 (aluminum oxide C, BET surface area 100 m2/g, DeGussa AG) were used as received. The n-alkylphosphonic acids were synthesized by the Michaelis-Arbuzov reaction17 of the n-alkyl bromides and triethyl phosphite. The ODPA-Zr/SiO2 was prepared as reported previously.1a In a typical preparation of alkylphosphonate monolayers on the metal oxides, a 5-fold excess of alkylphosphonic acid relative to the moles for a full surface coverage on the metal oxide was dissolved in 1000 mL of 3:1 (12) Lukes, I.; Borbaruah, M.; Quin, L. D. J. Am. Chem. Soc. 1994, 116, 1737. (13) (a) Schafer, W. A.; Carr, P. W.; Funkenbusch, E. F.; Parson, K. A. J. Chromatogr. 1991, 587, 137. (b) Randon, J.; Blanc, P.; Paterson, R. J. Membrane Sci. 1995, 98, 119. (c) Nawrocki, J.; Dunlap, C. J.; Carr, P. W.; Blackwell, J. A. Biotechnol. Prog. 1994, 10, 561. (14) Gardner, T. J.; Frisbie, C. D. ; Wrighton, M. S. J. Am. Chem. Soc. 1995, 117, 6927. (15) Pe´chy, P.; Rotzinger, F. P; Nazeeruddin, M. K.; Kohle, O.; Zakeeruddin, S. M.; Humphry-Baker, R.; Gra¨tzel, M. J. Chem. Soc., Chem. Commun. 1995, 65. (16) Folkers, J. P.; Gorman, C. B.; Laibinis, P. E.; Buchholz, S.; Whitesides, G. M.; Nuzzo, R. G. Langmuir 1995, 11, 813. (17) Bhattacharya, A. K.; Thyagarajan, G. Chem. Rev. 1981, 81, 415.
Gao et al. Table 1. Characterization by Elemental Analysis of ODPA Adsorbed onto Metal Oxides metal oxide
elem anal. surface areab (m2/g)
%C
calcd (%) surface coveragea
Zr/SiO2 ZrO2 TiO2 Al2O3
100 40 50 100
4.0 3.99 4.10 28.05
26 73 61 230
a Coverages were estimated by assuming an area of 24 Å2 for each bound ODPA molecule. b Surface area of the Cab-O-Sil silica is from a N2 BET measurement. The surface areas of the other oxides were taken from the Technical Bulletin No. 56, Degussa AG, Frankfurt, Germany.
methanol-water mixed solvent. A suspension of 2 g of metal oxide in 200 mL of deionized water was added dropwise to the acid solution. The resulting suspension was held at 100 °C for 3 days with stirring. The solid was washed and centrifuged with 200 mL of methanol seven times to remove any physisorbed alkylphosphonic acid and then dried under vacuum at room temperature. A portion of the ODPA-Al2O3 sample was annealed for several hours at 100 °C under vacuum (10-5 Torr). Elemental analyses were carried out by Galbraith Laboratories, Knoxville, TN. FT-IR. For acquisition of the infrared spectra, the samples were prepared by dispersing the modified metal oxide into a KBr pellet. The FT-IR data were collected on a Bruker IFS-48 Fourier transform spectrometer equipped with an A590 microscope and operated at 0.5 cm-1 nominal resolution. NMR. The solid-state 67.92-MHz 13C and 109-MHz 31P CP MAS spectra were run on a Chemagnetics CMX-270 NMR spectrometer with a 7 mm double-tuned fast-MAS Doty probe. For 1H-13C cross polarization, the carbon and proton power levels were adjusted to achieve the Hartmann-Hahn match at 60 kHz. The 1H 90° pulse widths were between 3.5 and 4.5 µs, and contact times of 3 ms were used. The samples were typically spun at 3.5-5.5 kHz and an average of 1000-4000 scans taken with delay times of 3 s. For the variable-temperature CP MAS experiments, the sample temperature was controlled to within (2 °C by a Chemagnetics temperature controller. In the 2D wide-line separation pulse sequence (WISE),18 a 1H 90° pulse was followed by a proton evolution period, t1, consisting of 32 increments of 2 µs. After each t1 period, cross polarization, followed by carbon detection with proton decoupling, gives a carbon spectrum that is modulated as a function of t1 by the free induction decay of the associated protons. A second Fourier transform gives a 2D spectrum with high-resolution 13C CP MAS spectra along the first dimension and the wide-line proton spectra associated with each carbon along the second axis.
Results and Discussion Elemental Analysis. The elemental analyses and calculated coverages, listed in Table 1, show that the amount of adsorbed ODPA on ZrO2 and TiO2 is higher than on zirconated silica. Whereas essentially all surface sites of TiO2 and ZrO2 should be available for adsorption of phosphonic acid, on silica the initial Zr primer layer determines the final coverage. Only a small amount of physisorbed ODPA is detected if the reaction is carried out on silica without the Zr primer layer. Other groups have used this method to deposit metal phosphonate multilayers on silica and silicon dioxide surfaces. Mallouk reported that multilayer films grown on zirconated silica were slightly more uniform than if a primer layer of phosphonic acid terminated silanes was used.11 Corn and co-workers found that while the adsorption of Zr4+ onto OH groups is weak, the film appears to “self-anneal” as more layers are added by sequential adsorption of alky(18) (a) Schmidt-Rohr, K.; Clauss, J.; Spiess, H. W. Macromolecules 1992, 25, 3237. (b) Clauss, J.; Schmidt-Rohr, K.; Adam, A.; Boeffel, C.; Spiess, H. W. Macromolecules 1992, 25, 5208.
Self-Assembled Monolayers of Alkylphosphonic Acids
lbisphosphonic acid and ZrOCl2.19 Katz and co-workers report the same average film thicknesses for multilayers using a zirconated silicon oxide surface or a phosphonic acid surface.20 Multilayers based on hafnium treated silicon oxide surfaces give similar results.21,22 A recent AFM study of the film growth of zirconium bis(phosphonate) mono- and multilayers on a zirconated silicon substrate show an island growth of the film.23 Apparently, in our monolayer synthesis, some of the Zr that is weakly adsorbed onto the surface hydroxyls is removed during the extensive washing and centrifuging that is carried out to remove any physisorbed species. Since only one layer is deposited, none of the so-called self-annealing takes place, leading to a patchy film on the fumed silica. The coverages on ZrO2 and TiO2 are somewhat less than estimated for a complete monolayer. Studies of the reaction of phosphoric acid and short chain phosphonic acids, CnH2n+1PO3H2, n ) 1-4, with ZrO2 and TiO2 powders prepared by a sol-gel process, found that a complete monolayer is formed.13b However, the fumed metal oxides used in our study consist of agglomerated particles and, as pointed out by Mallouk and co-workers,11 not all of the BET surface area is accessible to the long-chain phosphonic acids. ODPA reacts strongly with γ-Al2O3 (aluminum oxide C) to give a bulk aluminophosphonate, and the elemental analysis listed in Table 1 indicates that at least two layers are formed. A series of sharp diffraction peaks are observed in a powder X-ray diffraction spectrum (available as Supporting Information), confirming the formation of a bulk compound. Aluminum oxide C does not have strong reflections in the low angle range (2θ < 20°). After adsorption of ODPA, several strong even reflection lines are observed in the low angle region (2θ < 15°), similar to the XRD patterns observed for lamellar long-chain alkylphosphonate metal salts.24 Analysis of the pattern, assumed to be due to the 0k0 progression, yielded a value of 45.2 Å for the interlayer distance between adjacent inorganic layers. This value is close that observed for the interlayer spacings in Zr and Mn octadecylphosphonate multilayer films.10 Although strong reflections from the Al2O3 in the high angle region are still present, there appears to be more that two layers of aluminophosphonate since the XRD reflections in the small angle region are stronger than observed for Zr phosphonate multilayers on fumed silica.11 Adsorption of ODPA onto Al2O3 under milder conditions (no heating, shorter reaction time) still resulted in the formation of some bulk (aluminoalkyl)phosphonate. Although a multilayer compound results from the reaction of ODPA with Al2O3, the NMR and infrared spectra of this sample are also shown for comparison with the monolayers formed on the other metal oxides. Vibrational Spectroscopy. Figure 1 shows the methylene stretching region of the FTIR spectra of ODPA on the four metal oxides. The extent of alkyl chain ordering compared to bulk crystalline samples can be estimated from the frequencies and peak widths of the methylene asymmetric (νa(CH2)) and symmetric (νs(CH2)) stretching (19) Frey, B. L.; Hanken, D. G.; Corn, R. M. Langmuir 1993, 9, 1815. (20) Putvinski, T. M.; Schilling, M. L.; Katz, H. E.; Chidsey, C. E. D.; Mujsce, A. M.; Emerson, A. B. Langmuir 1990, 6, 1567. (21) O’Brien, J. T.; Zeppenfeld, A. C.; Richmond, G. L.; Page, C. J. Langmuir 1994, 10, 4657. (22) Zeppenfeld, A. C.; Fiddler, S. L.; Ham, W. K.; Klopfenstein, B. J.; Page, C. J. J. Am. Chem. Soc. 1994, 116, 9158. (23) Byrd, H.; Snover, J. L.; Thompson, M. E. Langmuir 1995, 11, 4449. (24) (a) Lynch, V. M.; Mallouk, T. E. Inorg. Chem. 1988, 27, 2781. (b) Cao, G; Lee, H.; Lynch, V. M.; Mallouk, T. E. Solid State Ionics 1988, 26, 63.
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Figure 1. Methylene stretching region (3100 to 2700 cm-1) of the FT-IR spectra of ODPA on (a) Zr/Cab-O-Sil, (b) ZrO2, (c) TiO2, (d) Al2O3, and (e) Al2O3 (annealed at 100 °C). Table 2. Peak Positions and Line Widths for the Methylene Stretching Region metal oxide
νa(CH2)
fwhh
νs(CH2)
fwhh
Zr/SiO2 ZrO2 TiO2 Al2O3 Al2O3 (annealed)
2918.6 2918.8 2918.2 2918.8 2915.0
17.4 18.4 19.9 19.3 30
2849.3 2850.6 2850.4 2850.2 2848.0
9.9 10.4 12.0 10.2 12.0
modes that are listed in Table 2. The frequencies of all the monolayer samples are similar, with values of 2918 and 2850 cm-1, respectively, for the νa(CH2) and νs(CH2) stretching modes. More variation is observed in the line widths, with ODPA-Zr/SiO2 displaying the smallest value (fwhh ) 17 cm-1) and ODPA-TiO2 displaying the largest value (fwhh ) 20 cm-1) for the asymmetric stretch νa(CH2). In regard to the methylene stretching region in the multilayer compound, ODPA-Al2O3, heat treatment of this sample produced a broad, asymmetric line shape for νa(CH2) with the low frequency edge moving to a lower wavenumber of 2915 cm-1, indicating that annealing increases the crystallinity in part of the sample. If annealing merely resulted in an overall increase in chain disorder, the line should broaden and move to a higher rather than lower wavenumber. Frequencies and line widths as low as 2917 and 15 cm-1 have been reported for highly ordered octadecylsilane monolayers. However, the frequencies and line widths listed in Table 2 are comparable to those reported for an ODPA monolayer on a silicon wafer, prepared by combining Langmuir-Blodgett and self-assembly techniques.10 Such values are typically taken as evidence that the self-assembled monolayer can be described as a dense, well-ordered, “crystalline” layer versus values of νa(CH2) ) 2925 cm-1 and fwhh > 35 cm-1, for a highly disordered “liquid like” film. As discussed below, the NMR chemical shifts and line widths are much more informative in regard to the variations in chain packing for the ODPA monolayers on the different substrates. The P-O stretching region between 1300 and 800 cm-1 could only be examined for ZrO2, which does not have a strong adsorption band in this part of the infrared spectrum. In Figure 2, the FTIR spectra for octadecylphosphonic acid (ODPA) and ODPA-ZrO2 are compared. The broadness and complexity of the peaks in this region make it difficult to interpret, but the large changes observed show that a strong interaction of the phosphonate headgroup with the ZrO2 surface is present. The P-O stretches of ODPA at 1228 and 950 cm-1 in ODPA are not present in ODPA-ZrO2. Also, the PO3 stretching bands
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Gao et al.
Figure 2. FT-IR spectra of the P-O stretching region (1800800 cm-1) of (a) ODPA-ZrO2 and (b) ODPA.
at 1077 and 1006 cm-1 are broadened and shifted to 11451090 cm-1. The disappearance of the peaks at ∼1200 and 950 cm-1 was also observed in a study of phenylphosphonic acid on ZrO2.13b The absence of these two bands, which the authors assigned to the PdO and P-O-H groups, was interpreted as evidence for a tridentate bonding mode. However, since the ranges for the different P-O stretching peaks greatly overlap and depend on the degree of hydrogen bonding or metal binding, the definite assignment of these bands is difficult. 31P Solid-State NMR. The 31P solid-state NMR spectra, shown in Figure 3, are more useful for revealing the nature of interaction of the phosphonic acid headgroup with the different metal oxides. In the bulk compounds, the 31P chemical shifts of alkyl phosphonate salts decrease with increasing valency and electronegativity of the cation, ranging from 26 to 0 ppm.1,25 The spectrum of ODPA on Zr-SiO2, as reported previously1a and shown again in Figure 3A, displays an isotropic chemical shift of 7 ppm and a small set of spinning sidebands that is identical to that of bulk zirconium octadecylphosphonate with an additional broad peak at ∼16 ppm. On TiO2, the 31P isotropic peak at 28 ppm is only slightly shifted from the chemical shift for the pure phosphonic acid at 31.5 ppm. The small shift indicates a relatively weak interaction with the TiO2 surface and perhaps incomplete deprotonation of the phosphonic acid headgroup. A larger change is observed for ODPA-ZrO2 where the phosphorous resonance is shifted further upfield to 25 ppm and is inhomogeneously broadened. 31P chemical shifts are sensitive to variations in the O-P-O bond angles as well as the number and electronegativity of nearest neighbor atoms. The line width of the 31P resonance of ODPA-ZrO2, which is not averaged under MAS, reflects a distribution of chemical shifts due to different types of surface bonds and/or bonding sites on the ZrO2 surface. The surface of ZrO2 is complex, with the presence of two types of surface hydroxyls that may be protonated or deprotonated, as well as coordinately unsaturated zirconium(IV) ion sites that are very strong Lewis acid sites.13a From the chemical shift, we propose (25) (a) Harris, R. K.; Merwin, L. H.; Ha¨gele, G. Z. Naturforsch. 1989, 44b, 1407. (b) Burwell, D. A.; Valentine, K. G.; Thompson, M. E. J. Magn. Reson. 1992, 97, 498. (c) Burwell, D. A.; Valentine, K. G.; Timmermans, J .H.; Thompson, M. E. J. Am. Chem. Soc. 1992, 114, 4144.
Figure 3. 31P CPMAS NMR spectra of (a) Zr/Cab-O-Sil, (b) ZrO2, (c) TiO2, and (d) Al2O3 (annealed). *,• indicate spinning sidebands. Arrows indicate peaks of the small amount of bulk metal alkylphosphonate that is formed under prolonged reaction times.
that the binding of ODPA is primarily through esterification with the surface hydroxyls, as occurs in the surface modification of porous ZrO2 with phosphoric acid.13a In the reaction of methylphosphonic acid with silica, it was determined from the 31P chemical shift that the surface bond consists of a single ester linkage giving an upfield shift of 5 ppm from the free acid. From comparison with model silyl compounds, attachment via two ester linkages to the silica surface was ruled out since such a species would be expected to show an upfield shift of 14 ppm.12 The chemical shift of bulk ZrODPA, where the three phosphonate oxygens are each coordinated to Zr cations, is shifted upfield by 24 ppm from ODPA. Assuming a trend similar to that observed for silica, the mono- and bidentate species on ZrO2 could be expected to have upfield shifts ranging between 5 and 15 ppm from the free acid. From the average upfield shift of 6.5 ppm from the octadecylphosphonic acid, the primary mode of attachment to ZrO2 is probably monodentate rather than tridentate as concluded from the changes in the P-O stretching region of the infrared spectrum.13b Adsorption of ODPA at the strong Lewis acid sites may also be contributing to the inhomogeneous line width. In the surface modification of zirconia with phosphoric acid, the phosphate initially adsorbs at the coordinatively unbound zirconium Lewis acid sites and the esterification with surface hydroxyls only takes place after longer reaction times.13a If aggressive conditions of low pH, high phosphoric acid concentration, high temperature (100 °C), and long reaction times are used, some dissolution of the bulk ZrO2 matrix occurs. The zirconium phosphates that reprecipitate on the surface can be detected by 31P NMR.13a As indicated in Figure 3, small peaks at ∼7 ppm, corresponding to bulk metal alkylphosphonate, can be detected in ZrO2 and TiO2 samples prepared at a low pH (
