Sonication Assisted Growth of Fluorophosphate Films on Alumina

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Sonication Assisted Growth of Fluorophosphate Films on Alumina Surfaces J. S. McNatt, J. M. Morgan, N. Farkas, and R. D. Ramsier* Departments of Physics, Chemistry, and Chemical Engineering, The University of Akron, Akron, Ohio 44325-4001

T. L. Young, J. Rapp-Cross, and M. P. Espe Department of Chemistry, The University of Akron, Akron, Ohio 44325-3601

T. R. Robinson and L. Y. Nelson Korry Electronics Company, 901 Dexter Avenue North, Seattle, Washington 98109 Received September 23, 2002. In Final Form: November 27, 2002 We present a spectroscopic and morphological study of the deposition of mono-, di-, and triester fluorophosphate films on alumina surfaces. Solid-state NMR and Fourier transform infrared characterization of the films show that species capable of forming three P-O-Al linkages are favored over mono- and bidentate binding moieties during the adsorption process. Our results indicate that film formation in conjunction with local sonication greatly enhances the resulting film density and homogeneity by removing weakly bound species and overcoming the influence of the autophobic properties of the adsorbates. This in turn leads to uniform films of tridentate bonded species that exhibit remarkable environmental stability in the presence of refluxing water.

Introduction Alumina surfaces have the propensity to form hydroxyl groups upon exposure to ambient conditions, and these hydroxyl groups assume an electric charge when subjected to a polar solution.1,2 In aqueous solutions of phosphatebased acids, alumina surfaces will be positively charged and highly attractive toward negatively charged (partially deprotonated) phosphate end-groups. Therefore, functional phosphoric acids are spontaneously attracted by ionic interaction3 to alumina surfaces upon submersion in an aqueous solution. On the other hand, evidence has also been presented to suggest that phosphoric acids form bidentate and tridentate chemical bonds with hydroxylated alumina via condensation reactions.4 It is thus possible that such chelated structures may be responsible for the strong adhesion properties found in these systems. We do not see a contradiction between these two adsorption models. It is likely that phosphonic acids are spontaneously attracted to alumina surfaces by ionic interactions and that once they are within close proximity to the appropriate adsorption sites, a condensation reaction occurs forming a covalently bonded durable thin film. Figure 1 depicts the combined process, where the ligands, R1 and R2, can be nonreactive alkyl and fluoroalkyl chains or may contain reactive substituents such as hydroxyl or vinyl groups. Beyond the fundamental interest in adsorption mechanisms on alumina surfaces, many industrial applications * To whom correspondence should be addressed. E-mail: rex@ uakron.edu. (1) Hass, K.; Schneider, W.; Curioni, A.; Andreoni, W. Science 1998, 282, 265. (2) Berg, J. C. In Wettability; Berg, J. C., Ed.; Surfactant Science Series Vol. 49; Marcel Dekker: New York, 1993; Chapter 2. (3) Bolger, J. C.; Michaels, A. S. In Interface Conversion; Weiss, P., Cheevers, D., Eds.; Elsevier: New York, 1969; Chapter 1. (4) Ramsier, R. D.; Henriksen, P. N.; Gent, A. N. Surf. Sci. 1988, 203, 72.

Figure 1. Ionic attraction and covalent bonding modes for phosphate species near hydroxylated alumina surfaces.

use hydrophobic phosphate-derived thin films where the film density, environmental resistance, and mechanical durability are of utmost importance. The deposition of dense hydrophobic films by self-assembly methods is often limited by the autophobic properties of the water/ phosphate solution. As the film begins to condense, the resulting hydrophobic properties inhibit water from wetting the surface and delivering phosphates to the substrate. While the film growth may be spontaneous, the autophobicity can lead to surprisingly low adsorption rates resulting in porous (patchy) film structures. Many commercial processes use ultrasonic agitation for cleaning immersed components. The sound waves create

10.1021/la0265979 CCC: $25.00 © 2003 American Chemical Society Published on Web 01/25/2003

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Figure 2. Structures of phosphate esters present in Zonyl UR.

small, highly energetic bubbles, which, upon implosive collapse (cavitation), generate intense microjets, shock waves, and locally high temperatures that remove unwanted species from the surface. The energetic bubbles also scavenge surfactants from the solution and transport these reactive materials to the surface.5 Upon reaching the solid/liquid interface, the high-speed jets of liquid are driven at velocities of hundreds of meters per second,6 resulting in a dense coverage of the surfactant material on the substrate. In the case of phosphate-based surfactants, the combination of two factors, (a) strong ionic adsorption and covalent bonding mechanisms and (b) the energetic delivery of phosphoric acid species, can produce a durable and dense surface coating. Understanding the film growth mechanism associated with ultrasonic assisted deposition (USAD) is the focus of this study. Experimental Section Chemicals. Zonyl UR (DuPont) is a mixture of anionic fluorosurfactants with the chemical formula CF3(CF2CF2)z(CH2CH2O)xPO(OH)y where x ) 1, 2, or 3, x + y ) 3, and z ranges from 1 to 7. Figure 2 depicts the species comprising Zonyl UR. Spectroscopy grade 2-propanol (Aldrich) is used as a solvent. All materials are used as received without further purification. A clear solution comprised of 5 g of Zonyl UR, 300 mL of 2-propanol, and 200 mL of water is obtained with magnetic stirring and mild heating for 2 h. These solutions are used to treat oxidized aluminum substrates and alumina powders using procedures described below. Substrate Preparation. (a) Bulk Aluminum Substrates. Pure aluminum (ESPI, 99.999%) sheet material is cut into rectangular sample coupons (nominally 20 mm × 48 mm) and polished in stages using successively finer diamond pastes to achieve a mirror finish. The coupons are then degreased, rinsed in deionized water, and dried in air. A thin native oxide forms on these substrates, so we refer to them as oxidized aluminum or alumina surfaces. (b) Thin Film Aluminum Substrates. Pure Al (Target Materials, 99.999%) is sputtered onto clean microscope slides in a highvacuum bell jar system in an atmosphere of Ar. The base pressure of the turbo-pumped chamber is 10-7 Torr, and the films are grown to a thickness of about 250 nm as determined by a quartz crystal thickness monitor. The surfaces are oxidized by venting the system with dry nitrogen and exposing the films to air, or by a dc oxygen glow discharge. (c) Aluminum Oxide Powders. Aluminum oxide powder (0.6 micron R-Al2O3) is purchased from Baikowski International Corp. and used as received. Phosphate Film Deposition. The aluminum substrates and alumina powders are treated with a liquid processor sonicator (Misonix) equipped with a sapphire tip. The stationary sonicator horn is immersed in a shallow water-alcohol bath containing the Zonyl UR material. In turn, the water-alcohol bath container is mounted on a computer-controlled x-y translation system. For USAD of the powders, a beaker (held stationary in this case) is charged with 2 g of alumina powder and 80 mL of the treatment liquid. The stationary sonicator horn is lowered into the liquid to a depth of approximately 13 mm, and the mixture is sonicated for 5 min. A comparative powder sample is coated with Zonyl UR by stirring the powder in the aforementioned solution for 15 min. These two types of powders will be referred to as USAD and stirred, respectively. Following Zonyl UR adsorption, the treated (5) Stefan, R. L.; Szeri, A. J. J. Colloid Interface Sci. 1999, 212, 1. (6) Suslick, K. S. Annu. Rev. Mater. Sci. 1999, 29, 295.

Langmuir, Vol. 19, No. 4, 2003 1149 powder is collected via filtration and dried under a vacuum at 80 °C for 12 h. For USAD on the aluminum substrates, these are translated beneath the stationary horn (tip diameter of approximately 13 mm) at a rate of approximately 6 mm/s using the x-y stage. The tip is first positioned about 6 mm above the surface of the substrate which is lying in the water-alcohol-phosphate bath. The entire container is then scanned in a linear fashion along one edge of the aluminum substrate. Upon reaching the end of the substrate, the translation stage displaces the container by 6 mm in a direction orthogonal to the scanning direction and the horn then returns along a path adjacent to the first. This rastering pattern is repeated until the entire aluminum surface is directly exposed in a line of sight geometry to the ultrasonic horn. The ultrasonic power delivered to the solution beneath the horn is approximately 100 W, and because of the small tip diameter and proximity to the substrates the areal power density is estimated to be 75 W/cm2. Following the ultrasonic treatment, the aluminum substrates are removed from the solution, rinsed with water, and placed in a drying oven to remove traces of water. The dried Zonyl UR treated surfaces are highly hydrophobic as evidenced by water contact angles exceeding 90°. Aluminum substrates coated with Zonyl UR by stirring are handled similarly except that the USAD procedure is replaced by simple stirring. Environmental Stability. These studies are performed by inserting the phosphate-covered aluminum oxide surfaces (thin film or bulk Al) into a reflux distillation column containing 100 °C water and steam, for various lengths of time. The coupons are removed from the water environment periodically and IR scans are collected, and then the samples are placed back into the same water for further degradation. Fresh water is used when a new sample is inserted. Atomic Force Microscopy (AFM). Imaging with AFM is performed in constant force contact mode under ambient conditions using a Digital Instruments NanoScopeII. The AFM samples are prepared identically to those used for the IR studies, but are smaller (nominally 8 mm × 8 mm). We use triangular 200 µm silicon (MikroMasch ULTRASHARP) and silicon nitride (Digital Instruments) cantilevers and record multiple images with different scan sizes from various locations on each sample. Statistical analysis of the images is accomplished with commercial software. Infrared Spectroscopy. Infrared spectroscopy is performed with a Mattson 7020 spectrometer in the range of 700-4000 cm-1 at 4 cm-1 resolution. The entire bench is dry-air purged, and a liquid-nitrogen-cooled HgCdTe detector is used. Transmission IR spectra of neat reagents and coated powders are collected using a micro cell sampling fixture with AgCl windows. Specular reflection spectroscopy is performed on the thin film and bulk Al substrates with a fixed-angle sampling attachment containing gold mirrors and a polarizer. Solid-State Nuclear Magnetic Resonance (SSNMR). Solid-state NMR spectra are obtained using a Varian Unityplus200 (4.7 T) spectrometer with a Doty Scientific Magic-AngleSpinning (VTMAS) probe. The samples are packed into 7 mm silicon nitride rotors with Kel-F end caps. Spectra are acquired using a single π/2 excitation pulse (Bloch decay) with sample spinning speeds of 3 kHz and proton decoupling. All 31P chemical shifts are referenced to 85% H3PO4 at 0 ppm.

Results Atomic Force Microscopy. Figure 3 presents a good representation of our AFM results concerning the morphology of the two types of Zonyl UR films formed on aluminum substrates in this study. The AFM images of the films produced by USAD consistently show that the surface is comprised of large areas of relatively uniform surface coverage. In contrast, the films generated by stirring are less homogeneous in nature. The 5 µm × 5 µm images of Figure 3 are reproducible in different regions of the same films and for films grown in the same manner on different aluminum oxide substrates. On the basis of the AFM results, the films made by USAD are referred to as “dense” and those by stirring as “patchy” in this

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Figure 4. Specular reflectance IR absorption data from dense (USAD) and patchy (stirred) Zonyl UR thin films on oxidized aluminum substrates. Note the reproducibility of surface coverage and the similarity between the spectra for both deposition methods.

Figure 3. Atomic force microscope images of Zonyl UR films deposited on aluminum oxide substrates. The images are 5 µm × 5 µm with a vertical gray scale of 127 nm for the dense USAD film and 225 nm for the patchy film formed by stirring.

report. To investigate the effect of surface roughness on surfactant adsorption, we perform AFM studies of Zonyl UR adsorbed on alumina films sputter-deposited on float glass as well as on the mechanically polished aluminum surfaces and thin films grown on microscope slides. In all cases (data not shown), the USAD films are more homogeneous and denser than the patchy films deposited by stirring. We do not observe any evidence for disruption of the adsorbed Zonyl UR films due to AFM imaging. Infrared Spectroscopy. Figure 4 presents a set of IR absorbance spectra recorded in specular reflectance mode from thin films of Zonyl UR deposited on oxidized aluminum substrates. These films are less than 50 nm thick as estimated by spectroscopic ellipsometry. The spectral range shown includes the P-O and C-F absorption regions, and no baseline subtraction is performed. The only modification made to these spectra is to uniformly shift the baselines of the USAD data for clarity. These data from six different substrates demonstrate our reproducible control over the formation of the two types of coatings. It is obvious from Figure 4 that the USAD method deposits much more Zonyl UR material on the surfaces than the stirring method. However, the same spectral features (1255, 1220, 1155, and 1095 cm-1) appear

in both cases, indicating that the same species are present. These results are consistent with those from the AFM studies which indicate a greater surface coverage when USAD is used. The IR spectral features observed in this study are consistent with the literature relevant to the adsorption of phosphate and fluoroalkyl species on various surfaces4,7-13 and with data from other similar systems we have characterized (data not shown). The 1255, 1220, 1155, and 1095 cm-1 vibrations from Zonyl UR adsorbed on alumina substrates arise from the fluorocarbon chains and P-O modes, although exact assignments are complicated by overlapping spectral regions. The spectral signatures of the PdO and P-OH linkages of the neat material change upon adsorption, consistent with the surface condensation reactions depicted in Figure 1. We have verified by NMR that the USAD process does not cause any changes in the Zonyl UR while in solution. In addition to surface coverage and film morphology studies, the adsorption and film formation mechanism is also investigated. To facilitate comparison with NMR, Zonyl UR films are also generated on alumina powders. As shown in Figure 5, IR spectra of USAD Zonyl UR films on powder substrates (Figure 5B) contain the same salient features as those of films formed on oxidized aluminum substrates (Figure 5A). The only differences between such (7) Gao, W.; Dickinson, L.; Grozinger, C.; Morin, F. G.; Reven, L. Langmuir 1996, 12, 6429. (8) Textor, M.; Ruiz, L.; Hofer, R.; Rossi, A.; Feldman, K.; Ha¨hner, G.; Spencer, N. D. Langmuir 2000, 16, 3257. (9) Van Alsten, J. G. Langmuir 1999, 15, 7605. (10) Yee, C.; Kataby, G.; Ulman, A.; Prozorov, T.; White, H.; King, A.; Rafailovich, M.; Sokolov, J.; Gedanken, A. Langmuir 1999, 15, 7111. (11) To, X. H.; Pebere, N.; Pelaprat, N.; Boutevin, B.; Hervaud, Y. Corros. Sci. 1997, 39, 1925. (12) Walder, F. T.; Vidrine, D. W.; Hansen, G. C. Appl. Spectrosc. 1984, 38, 782. (13) Maege, I.; Jaehne, E.; Henke, A.; Adler, H.-J. P.; Bram, C.; Jung, C.; Stratmann, M. Prog. Org. Coat. 1998, 34, 1.

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Figure 5. Comparison of the infrared spectra of Zonyl UR films grown on oxidized aluminum substrates by USAD (A) with those grown on powders by both USAD (B) and stirring (C). The relative intensity of spectrum C is 5 times larger than those of spectra A and B.

Figure 6. 31P solid-state NMR spectra from (A) Zonyl UR, (B) alumina powder coated with Zonyl UR by sonication, and (C) alumina powder coated with Zonyl UR by stirring.

spectra are slight shifts in the vibrational frequencies (about 15 cm-1 lower) and changes in relative intensity in the Zonyl UR-powder data. These changes are attributable in part to the broad underlying background due to the alumina powder. The lower two spectra of Figure 5 indicate that the same type of species are dominant on the aluminum and powder surfaces following USAD. It is obvious, however, that significant differences exist between these samples and powders stirred in the Zonyl UR solution (Figure 5C). The broad features exhibited by the stirred powders indicate that an inhomogeneous mixture of species is trapped on the powder without sonication. The fact that stirred powders yield a larger overall IR absorbance than sonicated powders (about a factor of 5) is not considered significant, since our powder sampling method does not strictly control the amount of material in the path of the IR beam. Solid-State Nuclear Magnetic Resonance. The 31P spectrum of neat Zonyl UR (Figure 6A) consists of three well-resolved peaks at +1, -0.5, and -2 ppm, assigned to the mono-, di-, and triester phosphate species present

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in the material, respectively.14 While proton decoupling is used during data acquisition, the 31P signal is not simultaneously decoupled from the fluorine present in the sample. The relatively narrow line widths indicate that dipolar coupling to the fluorine is weak, a result of substantial dynamics of the alkyl chains. The data are collected using a Bloch decay under fully relaxed conditions, so the relative peak intensities correspond to the relative intensities of the three different phosphoric acids present in Zonyl UR. Upon adsorption of Zonyl UR on the alumina powders by the USAD method, the 31P spectrum (Figure 6B) contains a new peak at -16 ppm with a line width of only 2.5 ppm. As phosphate groups connected to four aluminate units typically have chemical shifts in the range from -23 to -34 ppm,15 the phosphate group of Zonyl UR is unlikely to be interacting with alumina through all four oxygens. This is consistent with the fact that the monoester of Zonyl can only react with alumina through three oxygens. Comparison with 31P data from other metals and nonmetals reacted with phosphates suggests that the interaction of Zonyl UR with alumina is through a tridentate interaction. For example, the formation of the mono- and disilyl-phosphate species from methyl phosphonic acid and tert-butyldimethylsilanol results in upfield shifts of the 31P peak of 5 and 15 ppm, respectively.16 The reaction of octadecylphosphate (ODPA) with Zr on nonporous silica, through the three oxygens of ODPA, causes a 24 ppm upfield shift of the 31P peak relative to that of the free acid.17 Also, after exposure of hafnium-functionalized CabO-Sil silica with a phosphonic acid, the 31P chemical shift of the acid is again observed to shift upfield, in this case by 18 ppm.18 The shift of the 31P peak observed for the Zonyl UR/alumina complex formed in our USAD studies is 17 ppm upfield relative to that of the free monoester acid, consistent with the formation of three P-O-Al bonds between the phosphate group of Zonyl UR and the alumina powder surface. As the sonication of Zonyl UR does not result in the loss of fluorocarbon chains, only the monoester form of Zonyl UR gives rise to the peak at -16 ppm in the 31P NMR spectrum. Previous 13C solid-state NMR characterization of Zr/ ODPA indicated that the alkyl chains of the ODPA monolayer are well-ordered, with the chains almost completely adopting an all-trans conformation. The ordering is occurring not on a flat substrate but rather on Cab-O-Sil particles. Structural homogeneity is also present at the Zr/ODPA interface, giving rise to a narrow (∼3 ppm) peak in the 31P spectrum.17 While a similar 13C NMR characterization is not possible for Zonyl UR adsorbed on powders due to the fluorine atoms, the peak at -16 ppm in the 31P spectrum is also quite narrow at 2.5 ppm. These results suggest that some regions of Zonyl UR deposited by USAD on the powder surfaces are also structurally homogeneous. The NMR spectrum from Zonyl UR sonicated onto alumina powder (Figure 6B) also contains a second, broad (10 ppm) peak at -4 ppm with a shoulder extending out to near -14 ppm. Similar spectra have been obtained when ODPA is adsorbed onto ZrO2, where the 31P peak is shifted upfield by 6.5 ppm and has a line width of ∼12 ppm. This (14) Gorenstein, D. G. In Progress in Nuclear Magnetic Resonance Spectroscopy; Emsley, J. W., Feeney, J., Sutcliffe, L. H., Ed.; Pergamon Press: New York, 1984; Vol. 16, p 1. (15) Akporiaye, D.; Stocker, M. Zeolites 1992, 12, 351. (16) Lukes, I.; Borbaruah, M.; Quin, L. D. J. Am. Chem. Soc. 1994, 116, 1737. (17) Gao, W.; Reven, L. Langmuir 1995, 11, 1860. (18) Neff, G. A.; Page, C. J.; Meintjes, E.; Tsuda, T.; Pilgrim, W.-C.; Roberts, N.; Warren, W. W. Langmuir 1996, 12, 238.

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Figure 7. Infrared spectra indicating almost no change in the Zonyl UR films grown on oxidized aluminum surfaces by USAD even after exposure to 100 °C steam/water.

peak has been assigned to ODPA attached to the particle surface through a single bond.7 The lower field peaks in the Zonyl UR/alumina material probably also arise from Zonyl UR attached to the alumina through mono- and bidentate bonding. The broad line widths of these peaks indicate that these species reside in a structurally/ chemically diverse environment. This could include species in which the phosphate group is interacting with a number of different types of alumina sites. The NMR data can therefore discern between the structurally/chemically homogeneous regions of the Zonyl UR layer on the alumina surface and those domains that are quite heterogeneous. The 31P NMR spectrum from Zonyl UR on alumina formed by stirring the materials together in solution is shown in Figure 6C. We interpret these data in the same manner as for the sonicated samples discussed above. The signal intensity is lower for this sample when compared to the sonicated material, and the relative intensity of the two main peaks has changed significantly. The narrow peak at -16 ppm is much reduced in this sample relative to the broader peak at -4 ppm, indicating that a smaller fraction of the material binds to the alumina surface through a tridentate arrangement under these conditions. Environmental Stability. The stability of the phosphate films on oxidized aluminum surfaces is also investigated by IR spectroscopy. Figure 7 demonstrates the tenacious adhesion of the dense USAD films to alumina surfaces. Note that the relative intensities of the spectral features do not change, indicating that refluxing does not preferentially remove certain species and leave others. This observation holds for both bulk polished and vacuumdeposited thin film surfaces, except that refluxing eventually causes the thin films to spall off the glass microscope slides. The peak intensities in Figure 7 decrease about 30% after 5 h of refluxing, implying that some of the Zonyl UR material is removed by the refluxing process. Although not shown here, stirring of the substrates in room-

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Figure 8. Infrared spectra indicating significant changes in the patchy Zonyl UR films deposited by stirring on oxidized aluminum surfaces after exposure to 100 °C steam/water.

temperature water for extended periods of time has no significant effect on the IR spectra of the Zonyl UR films. For the case of the patchy Zonyl UR films formed by stirring, we see in Figure 8 that initially the refluxing makes little difference. However, after about 1 h, broad hydroxyl bands at 1600 and 3000-3600 cm-1 appear. In addition, a sharp feature at 1080 cm-1 and a large band near 800 cm-1 begin to dominate the spectra. None of these features appear during refluxing of the dense Zonyl UR films grown by USAD, and they are indicative of hydroxylation (1600 and 3000-3600 cm-1) and oxidation (800 and 1080 cm-1) of the substrate through voids and holes in the patchy films. The changing baseline in the 1200 cm-1 region of Figure 8 makes it difficult to quantitatively determine whether any of the Zonyl UR material is being removed by refluxing; however our qualitative interpretation is that little material is actually removed. Discussion Our results demonstrate that USAD is an ideal technique for the deposition of the fluoroalkylphosphate, Zonyl UR, onto alumina surfaces. This technique has the advantage that the material can be applied in a relatively short period of time, a few minutes, and there is no need for high-temperature treatments. These qualities are especially relevant to the coating of alumina with phosphonates/phosphates. Efforts to apply monolayer coatings of ODPA onto γ-Al2O3 by heating a suspension of ODPA and alumina to 100 °C result in dissolution of some of the alumina and the formation of bulk aluminooctadecylphosphonate.7 The bulk material is clearly evident in the 31P solid-state NMR spectrum with a pair of narrow peaks separated by 13 ppm. In our SSNMR studies of Zonyl UR/alumina, there are no peaks in the 31P spectra

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consistent with the dissolution of alumina. In addition, comparison to the phosphate films generated by stirring a suspension of Zonyl UR and alumina shows that the surface coverage is higher with the use of sonication. While the autophobic characteristics of the alumina surface partially covered with Zonyl UR limit the extent of surface coverage during stirring, USAD can overcome this restriction and yield surfaces with higher coverages. The use of sonication to generate phosphate films appears to be a softer method for thin film formation, resulting in a lower occurrence of weakly bound material while also increasing the resulting surface coverage and film homogeneity. The use of sonication also affects the distribution of binding modes between the phosphates and alumina. The Fourier transform infrared (FTIR) and SSNMR data show that with simple stirring the alumina powder/phosphate interface is structurally and/or chemically heterogeneous. One mechanism for the heterogeneity is the presence of phosphate bonded to alumina through 1, 2, or 3 ester linkages. With the use of USAD, the heterogeneity is reduced and both the SSNMR and FTIR indicate that the principal phosphate species present on the surface is attached through three P-O-Al bonds. In this system, the energy supplied by sonication presumably results in the desorption of the more weakly interacting phosphates and an increase in surface migration. Upon arrival at binding sites with the proper geometry where three bonds can be formed, the phosphate strongly attaches to the alumina surface. The change in phosphate/alumina bonding with sonication indicates, as expected, that the formation of the three P-O-Al linkages is the thermodynamically favored configuration. The use of USAD then allows the system to achieve this optimal adsorption structure in a larger fraction of the potential binding sites. This occurs by keeping the coverage of monodentate and bidentate bonds low, which increases the sticking coefficient of the monoester species and provides sufficient energy for surface diffusion. If our model is correct, then the strongly bound tridentate phosphate should be the only species detected on alumina surfaces in all cases. This is consistent with our observations for USAD deposition on aluminum and alumina powder and for deposition on flat substrates by stirring, but not for the case of Zonyl UR adsorbed on alumina powders by stirring. Here, both FTIR and SSNMR indicate a heterogeneous population of the surface, involving species with different numbers of ester linkages

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presumably bound at different types of adsorption sites. We would argue that these results are entirely consistent with our physical picture of the adsorption process. The more weakly bound mono- and bidentate compounds are stabilized by the structural heterogeneity of the powder surface and are not removed by the rinsing and drying procedures employed following adsorption. Such heterogeneity does not exist on the oxidized aluminum substrates, therefore leading to the removal of weakly bound species by rinsing and drying. Finally, our Zonyl UR films on oxidized aluminum substrates show a tenacity for adhesion and a robust environmental stability. The former seems to hold with stirred and sonicated films, indicating that the monoesters bind very strongly in both cases. However the latter property is observed only for USAD films, which are much more homogeneous and continuous than those formed by stirring. The higher adsorbate coverage and film density provided by USAD protect the substrate from corrosive attack by hot water. USAD can therefore be used to quickly and reproducibly form environmentally passive Zonyl UR coatings on alumina surfaces. We have therefore connected the fundamentally rich chemistry of this phosphate/ surface system to a technologically relevant deposition method and demonstrated the synergy between the two in the formation of corrosion resistant films. Conclusions In this paper, we demonstrate that localized sonication results in higher coverage and structural homogeneity in thin films of Zonyl UR on alumina surfaces as compared to deposition by immersion. The reproducibility of this deposition technique and a description of the role of sonication in enhancing uniform film growth are presented. Our data suggest that only those species capable of forming three P-O-Al bonds are stable on these alumina surfaces and that these species exhibit tenacious adhesion properties and environmental stability when deposited in dense, homogeneous films. Acknowledgment. Partial support of this work by The University of Akron is greatly appreciated. We are also grateful to Mr. Matthew Shepard and Ms. Trocia Clasp for their help with the powder IR and NMR work, respectively. Finally, we thank Professor Bi-min Newby and Professor Sergei Lyuksyutov for access to some of the equipment used in this work. LA0265979