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Mechanical and Surface Chemical Properties of Some Solid-Phase Hydrous Aluminum Oxide/Tannic Acid Particles Investigated Using Scanning Probe Methods Anselm Omoike and J. Hugh Horton* Department of Chemistry, Queen’s University, Kingston, Ontario, Canada K7L 3N6 Received July 9, 1999. In Final Form: September 20, 1999 Hydrous aluminum oxide particles precipitated from simulated wastewater were dispersed on mica substrates and examined using atomic force microscopy, adhesion force measurements, interfacial force microscopy (IFM), and zeta potential methods in order to understand their structural and coating properties. Results on IFM of the mica substrate are also reported. Three types of particles were examined: aluminum oxides coprecipitated by adding alum in the presence of a solution of phosphates and tannic acid; postprecipitated particles formed by adding phosphates and tannic acid to already formed aluminum oxides; a control case consisting of particles precipitated in the absence of either phosphate or organic component. In all cases, the particles all had similar values for the reduced modulus of the tip-sample system, although in the case of the postprecipitated particles there was evidence for the presence of a compliant organic coating. The adhesive force and zeta potential measurements were also consistent with this observation. The results are discussed in the context of the relative effectiveness of these aluminum oxides in removing phosphates and other contaminants from aqueous systems.
Introduction Aluminum oxides are an important component of many important systems, including soils,1,2 ore processing, and water filtration systems.3 In the latter case, hydrous aluminum oxides are added to the water to perform two functions: to remove phosphorus via the formation of insoluble hydroxy aluminum phosphates; to more rapidly effect the sedimentation of organic components by the formation of large flocs of aluminum hydroxides which can entrap solid components in the wastewate and allow them to more rapidly settle to the bottom of the sedimentation tank. Clearly, the nature of these particle’s morphology and their chemical properties depends very much on the conditions under which they are formed and the chemical species found on the particle surface.4 We have recently reported5 on a study hydrous aluminum oxide particles precipitated from alkaline solutions of alum (Al2(SO4)3‚nH2O) in the presence of phosphates and tannic acid (which simulates the humic organic compounds found wastewater).1,6 This work was carried out using tapping mode atomic force microscopy (AFM) in order to determine the morphology of these particles and to elucidate the chemical identity of their surface coating. The particles were dispersed on a mica substrate for ease of imaging. We found that so-called “postprecipitated” particles, that is, hydrous aluminum oxides precipitated from solution and subsequently exposed to a mixture of tannic acid and phosphate ion, demonstrated large changes in contrast with the mica substrate in the phase imaging mode as a function of set point and tip oscillation amplitude. These changes in the phase images were not observed either with control particles of aluminum oxide or with “coprecipitated” particles in which * To whom correspondence should be addressed: Tel (613)-5332379. FAX (613)-533-6669. E-mail:
[email protected]. (1) Goh, T. B.; Violante, A.; Huang, P. M. Soil Sci. Soc. Am. J. 1986, 50, 820. (2) Goh, T. B.; Huang, P. M. Clays Clay Miner. 1986, 34, 37. (3) Bowker, R. P. G.; Stensel, H. D. Phosphorus Removal From Wastewater; Noyes Data Corp.: New Jersey, 1990.
aluminum oxides were precipitated out of solution in the presence of phosphates and tannic acid. We could tentatively conclude that the postprecipitated particles demonstrated large changes in their surface viscoelastic properties which must be associated with the presence of a compliant organic coating on the particle surface that is absent in the case of the coprecipitated particles. The present paper undertakes to quantify the nature of this change in the viscoelastic properties of the surface by using a combination of scanning probe techniques. These include tapping-mode AFM,7 measurements of adhesive forces between tip and sample as a function of pH,8 and interfacial force microscopy9 (IFM) for measuring the contact modulus between tip and sample, essentially a measure of the hardness of the particles. In addition, zeta potential measurements10 on these particles are presented. Using these methods, we show that the presence of a tannic acid layer bound to the aluminum oxide particles has important effects on both the morphology and the chemical reactivity of these materials. Experimental Section Full details of the chemical synthesis of the hydrous aluminum oxide particles used in this study have been described elsewhere.5,11 Briefly, particles of Al(OH)3 were formed by addition of alum to alkaline solutions of NaHCO3. Control particles were precipitated directly from this solution. Postprecipitated particles were allowed to age for 5 min in solution followed by addition of tannic acid and phosphates to the mixture. Coprecipitated particles were precipitated directly from alkaline solutions (4) Schofield, R. K.; Taylor, A. W. J. Chem. Soc. 1954, 4445. (5) Omoike, A.; Chen, G.; Van Loon, G.; Horton, J. H. Langmuir 1998, 14, 4731. (6) Dempsey, B. A.; Ganho, R. M.; O’Melia, C. R. J. AWWA 1984, 76, 141. (7) Putman, C. A. J.; van der Werf, K. O.; de Grooth, B. G.; Van Hulst, N. K.; Greve, J. Appl. Phys. Lett. 1994, 64, 2454. (8) van der Werf, K. O.; Putman, C. A. J.; de Grooth, B. G.; Greve, J. Appl. Phys. Lett. 1994, 65, 1195. (9) Houston, J. E.; Michalske, T. A. Nature 1996, 356, 266. (10) Shaw, D. J. Introduction to Colloid and Surface Chemistry; Butterworth-Heinemann, Ltd.: Oxford, U.K., 1992. (11) Omoike, A. I.; VanLoon, G. W. Water Res., in press.
10.1021/la990896p CCC: $19.00 © 2000 American Chemical Society Published on Web 12/15/1999
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containing phosphate and tannic acid and allowed to age for a further 5 min. The particles were then filtered from solution and allowed to air-dry. All the particles were deposited on to freshly cleaved mica substrates approximately 1 cm × 1 cm square. The particles were first dispersed into distilled water by placement in an ultrasonic bath for 60 min. The concentration of the dispersion was 0.35 g L-1, and 30-100 µL of the dispersion was placed on the mica substrate, spin-coated at 4000 rpm for 60 s, and subsequently dried in air for 1 h. The amounts of dispersion used were chosen in order to obtain an even distribution of particles of approximately 50% coverage on the substrate for the imaging experiments. Larger volumes of the solution, resulting in 100% coverage of the surface with particles, were required when preparing samples on which force curve data was acquired. AFM data were acquired using a PicoSPM operated in MAC mode (Molecular Imaging, Tempe, AZ), using a Nanoscope IIE controller (Digital Instruments, Santa Barbara, CA). The MAC mode is essentially the same as tapping mode, except that the cantilever is magnetically coated and is driven by an external oscillating magnetic field.12 The cantilevers had a force constant of ∼1.0 N m-1 and a resonance frequency of ∼100 kHz. Images were acquired under ambient conditions, as well as under solutions of various pH, at the fundamental resonance frequency of the Si cantilevers. The solution-damped frequency of the cantilevers was ∼30 kHz. Height and phase shift data were all recorded simultaneously, as a function of both cantilever oscillation amplitude (Ao) and set point ratio rsp) Asp/Ao. Images were recorded at scan rates of 1-2 lines/s using a 30 µm × 30 µm scanner. Force-distance curves for measurement of adhesion forces between tip and sample8 were obtained using the same apparatus, in this case employing a tip with force constant of 0.1 N m-1. Curves were acquired under freshly prepared unbuffered NaOH or HCl solutions of pH ranging from 3 to 10. Unbuffered solutions were used in order to prevent any unwanted interactions between the surfaces and ions in solution. Some 300-500 curves were obtained for each sample and were analyzed by finding the difference between the value of the maximum tip deflection at the bottom of the adhesive well and the zero of tip deflection at large tip-sample separations.13 The reported values of the adhesive interaction are an average of all the force curves obtained while the reported errors reflect the standard deviation of the data. Nanomechanical data on the particles was acquired using an interfacial force microscope (IFM).14 This differs from a conventional AFM in that the cantilever is replaced by a differential capacitance force sensor. The instrument used here has been previously described15 and allows imaging of the sample surface (albeit at lower resolution than the tapping mode AFM), selection of the features of interest, and then measurement of their mechanical properties with nanonewton force resolution. The force sensor was calibrated using a thiol-covered gold standard. The force-distance curves were acquired by bringing a tungsten tip into contact with the sample up to a preset repulsive load, followed by withdrawal from the surface. The repulsive interaction can be analyzed according to Hertzian theory to yield a value for the reduced modulus, E*, for this system. The relationship between the applied force on the tip, F, and the tip displacement, D, is given by
F)
4 E*R1/2D3/2 3
(1)
where R is the reduced radius of curvature (essentially equal to the tip radius of curvature in this case). R for the tips used in this study was 80-100 nm, as determined from electron microscope images. (12) Han, W.; Lindsay, S. M.; Jing, T. Appl. Phys. Lett. 1996, 69, 4111. (13) Han, T.; Williams, J. M.; Beebe, T. P. Anal. Chim. Acta 1995, 307, 365. (14) Joyce, S. A.; Houston, J. E. Rev. Sci. Instrum. 1991, 62, 710. (15) Warren, O. L.; Graham, J. F.; Norton, P. R. Rev. Sci. Instrum. 1997, 68, 4124.
Figure 1. Force-distance curves obtained using the interfacial force microscope on a mica surface in air for 20 min (dashed line) and 3 h (solid line) after cleaving the sample. Approach and retraction portions of the curves are indicated using arrows. The thicker line on the second curve indicates the curve fitting of eq 1 to the data as described in the text. Zeta potential measurements on the particles were carried out using a Laser Zee electrophoresis cell. Measurements were taken again at a pH range of 3-10, with a cell potential of 150 V.
Results To act as a standard, IFM data were obtained for mica surfaces of the type used as a substrate for our dispersed particles as shown in Figure 1. The graph shows the restoring force applied to the sensor assembly as a function of tip displacement: larger values of the displacement indicate a closer approach of the tip toward the surface. Two curves are shown: the first (dashed line) obtained about 15 min after the mica sample had been cleaved and placed in the IFM; the second (solid line) obtained several hours later. The curves are offset slightly from one another for clarity. In both cases a complete cycle of tip approach to the surface followed by withdrawal is indicated by the arrows. Far from the surface, there is zero restoring force on the IFM sensor, indicating that there is no surface-tip interaction. In the first case, as we approach the surface we note a strong repulsive (positive) interaction as the tip is embedded in the surface. Just before the repulsive interaction there might be a very slight (50%) on the surface.
The possibility of particles moving during indentation must be taken more seriously. However, the only system in which we saw evidence in the imaging of particles having moved during the indentation process was with the coprecipitated particles. In this case not only was the reduced modulus of 18 ( 2 GPa the highest observed, but we also saw indentations within the particles themselves. Also, the adhesion characteristics of the four systems were very different, which suggests we are observing different surfaces. It is also possible of course that if the particles are significantly harder than the substrate, then the tip will push them into the substrate surface, and the measured quantity will be the substrate modulus. The reduced modulus measurement then suggests that the particles themselves are fairly similar to one another in terms of their bulk mechanical properties and are quite a bit softer than corundum. In all probability the particles have been extensively hydrolyzed to Al2O3‚2H2O or Al(OH)3. While Young’s modulus and Poisson ratio data for Al(OH)3 do not appear to have been published, presumably the modulus of this material must be lower than that of corundum, leading to the much lower reduced modulus value observed here. A significant observation is the shallow slope seen on the approach curve on the postprecipitated particles in Figure 4b. In this case, we have hypothesized that the system consists of a thin overlayer of tannic acid adsorbed on the particle surface. Other similar systems which have been examined include stearic acid monolayers on alumina using AFM21,22 and IFM studies of alkanethiol monolayers adsorbed on a gold film23-25 In this latter case, force-
(19) Cotrell, A. H. The Mechanical Properties of Matter; John Wiley & Sons: New York, 1964. (20) Grigoriev, I. S.; Meilikhov, E. Z. Handbook of Physical Quantities; CRC Press: Boca Raton, FL, 1997.
(21) Burnham, N. A.; Dominguez, D. D.; Mowery, R.L.; Colton, R. J. Phys. Rev. Lett. 1990, 64, 1931. (22) Burnham, N. A.; Colton, R. J.; Pollock, H. M. J. Vac. Sci. Technol., A 1991, 9, 2548.
The reduced moduli observed for the three different particles as well as the mica surface are essentially the same within experimental error. The Young’s modulus of Al2O3 (corundum) has been reported variously as 250280 GPa.16,19 The reduced modulus (E*) of the tip sample system used here is given as
E* )
(
)
1 - υ2s 1 - υ2t + Es Et
-1
(2)
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Figure 7. Zeta potentials of the various particles as a function of pH. The particle studied is indicated on each graph. The curves are to be used as a guide to the eye.
surface overlaying the aluminum hydroxide particle beneath it. There are some differences, however, with the alkanethiol system in that here there appears to be a discontinuity between the hard and soft regions, as well as a small adhesive interaction upon retraction. This suggests that perhaps the tip has “punched through” the tannic acid layer to encounter the aluminum oxide particle beneath. Certainly the adhesive interaction suggests some kind of hydrophilic surface is present, either the aluminum oxide surface itself or hydrophilic phenolic or ester linkages on the tannic acid molecule. Whether adhesion arises due to a true interaction between tip and sample or because of a capillary effect due to water adsorption cannot, however, be discerned from the IFM data for reasons described below. IFM measurements were carried out in air using a tungsten tip. As such, a major contribution to the attractive interactions observed in these data must arise from capillary forces between tip and sample. For example, in the case of the mica sample we see in Figure 1 that immediately after cleaving we see little adhesive interaction. After longer exposure times to air, the adhesion increases. After several hours at even the relatively low (20%) humidity in which experiments were conducted, mica adsorbs several monolayers of water.26 Any adhesive interactions observed in the IFM data on the particle samples must have considerable contribution due to capillary effects,27 and thus these observations are not particularly instructive in trying to understand the chemical functionality of the surface. We thus consider further the data summarized in Figure 6, in which adhesive forces were measured under more controlled conditions under solution of varying pH. In this case, the probe consisted of a Si3N4 tip. Such tips have been used to examine tip-sample interactions between a variety of different substrates, including Si3N4,28 mica,29 corundum (Al2O3),30 and silica,31 as a function of pH. Silicon nitride undergoes hydrolysis upon exposure to aqueous solution to form various silanol and silylamine surface groups. This tip is thus amphoteric with an isoelectronic point being reported variously as pH 5.5730 or 6-8.5.28 We start by looking at the results for the control (Al(OH)3) particles as seen in the upper left graph in Figure 6. We observe that on approach, there is an attractive region between pH 5.2 and 6.7 (as indicated by the A and dashed line boundaries on the figure), while at other pH values the approach curve shows a repulsive interaction. This is consistent with previous observations on a corundum (Al2O3) surface and Si3N4 tip and can be explained on the basis of surface charge effects alone.30,31 The isoelectronic point of a corundum surface is at pH ) 9.32 Thus at pH values above 9, the surface will be negatively charged while below pH ) 9 the surface is
distance curves showed a fairly shallow repulsive interaction followed by a stiffer interaction as the indentation proceeded. In accordance with eq 1, the slope of the curve should decrease as the value of the reduced modulus of the system decreases, hence indicating that the shallow region corresponds to a soft overlayer. No adhesive or attractive interaction was seen. The lack of these interactions was expected since in this case the monolayer consisted of a hydrophobic alkyl layer. In our case, we also see this shallow repulsive interaction before the onset of a much stiffer interaction with the particle itself. This is a good indication that what we see here is a soft layer of tannic acid a few nanometers thick adsorbed on the
(23) Joyce, S. A.; Houston, J. E.; Michalske, T. A. Appl. Phys. Lett. 1992, 60, 1175. (24) Joyce, S. A.; Thomas, R. C.; Houston, J. E.; Michalske, T. A.; Crooks, R. M. Phys. Rev. Lett. 1992, 68, 2790. (25) Thomas, R. C.; Houston, J. E.; Michalske, T. A.; Crooks, R. M. Science 1993, 259, 1883. (26) Hu, J.; Xiao, X.-D.; Ogeltree D. F.; Salmeron, M. Surf. Sci. 1995, 327, 358. (27) Weisenhorn, A. L.; Maivald, P.; Butt, H. J.; Hansma, P. K. Appl. Phys. Lett. 1989, 54, 2651. (28) Senden, T. J.; Drummond, C. J. Colloids Surf., A 1995, 94, 29. (29) Weisenhorn, A. L.; Maivald, P.; Butt, H.-J.; Hansma, H.-J. Phys. Rev. B 1992, 45, 11226. (30) Feldman, K.; Fritz, M.; Hahner, G.; Marti A.; Spencer, N. D. Tribol. Int. 1998, 31, 99. (31) Marti, A.; Hahner, G.; Spencer, N. D. Langmuir 1995, 11, 4632. (32) Hahner, G.; Marti, A.; Spencer, N. D. Tribol. Lett. 1997, 3, 359.
Surface Properties of Aluminum Oxide Particles
positively charged. Similarily, the tip will be negatively charged at pH values above its isoelectronic point and positively charged below. The net result is that the surface and tip will have opposite charges and thus attract at pH values intermediate to their respective isoelectronic points and have like charges and a repulsive interaction will occur at pH outside these values. The data in Figure 6 suggest then an isoelectronic point for the tip of about 6.7, consistent with the range of values quoted for Si3N4 in the literature and an isoelectronic point of about pH ) 5.2 for the control particles. This latter value is roughly consistent with the zeta potential measurements on the control particles seen in Figure 7 which show that the zeta potential becomes zero, and hence the isoelectronic point is reached at a pH of 4.7. This is quite different from the isoelectronic point of pH ) 9 reported for the corundum surface. It suggests that, as might be expected, the control particles are hydrolyzed to a considerable extent and are probably closer to Al(OH)3 which, since the surface charge must arise from protonation/deprotonation of dangling Al-O- species, will be expected to have quite a different isoelectronic point from that of corundum. The result is also, of course, consistent with the lower values than would be expected for Al2O3 obtained in the reduced modulus measurement on these particles. While the isoelectronic point was not the same, the adhesive interaction observed on the control Al(OH)3 particles is qualitatively consistent with previous observations on a corundum surface. In that case, both the adhesive interaction and the lateral force were found to reach at maximum at a pH intermediate to the two isoelectronic points. We see a similar trend in the adhesion data in Figure 6, with the adhesive interaction reaching a maximum at pH 5.7. The mica surface is known to exhibit a net negative charge under all pH conditions,33 although charge reversal is known to occur under conditions of high ionic strength. On the mica surface, we note an attractive potential upon tip approach at pH values of less than 5, as seen in Figure 6. This is roughly consistent with the observations on the control particles which indicated that the isoelectronic point of the tip is near a pH of 6.7. Below this point, the tip should be positively charged, and hence we observe an attractive interaction on approach. In this case, there does not appear to be any dramatic change in the tip-sample adhesive interaction as a function of pH. No interaction exceeded 5 nN. This is consistent with previous observations on this system28 although in that case no attempt was made to quantify the adhesive interaction. It is important to note that in the case of the experiments on the particles themselves, the adhesive interaction behavior as a function of pH differs considerably from that of the mica substrate, demonstrating that our force distance curves are truly being acquired on particle sites on the surface rather than on the substrate. The adhesive interaction between tip and sample differs considerably as a function of pH on the postprecipitated and coprecipitated samples, and indeed from the control case. The postprecipitated particles show a broad maximum of about 5 nN in the adhesive interaction at pH values ranging from 5 to 8. The coprecipitated particles on the other hand, show a large increase in the adhesive interaction between tip and sample above pH ) 8, with the interaction becoming as high as 20 nN, much larger than that observed on any other particles. Clearly the surface chemistry of these two particles must be quite different. (33) Kekicheff, P.; Marcelija, S.; Senden, T. J.; Shubin, V. E. J. Chem. Phys. 1993, 99, 6098.
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The coprecipitated particles contain a number of different -OH terminated sites, all of which will tend to be ionized at higher pH. Presumably this large concentration of ionized sites leads to the adhesive interaction at high pH. This is also indicated by the zeta potential measurements, which show a more negative zeta potential than that of the control particles at any given pH. Presumably this is due to the higher number of ionizable sites present arising from phosphates or carboxylic and phenolic sites (from tannic acid) present in the coprecipitated particles in addition to, of course, Al-OH sites available on both particles. We finally note that on the postprecipitated particles, there is little or no adhesive interaction, suggesting a fairly low density of hydroxyl sites on the surface, at least compared to the coprecipitated particles. Since the zeta potential of these particles is also less negative, this is also an indication that there are fewer hydroxyl sites. Indeed, the zeta potential of the postprecipitated particles plateaus around a pH of 4.5 and does not begin to drop again until a pH of 8. This indicates that unlike the other two cases, there are no sites on the postprecipitated particle’s surface which can be ionized over this pH range. Once a pH of 8 is reached, however, the zeta potential drops. This is the pH condition at which we would expect ionization of phenoxy OH groups from the tannic acid to begin, as the pKa of the phenoxy group is in the range of 8-9. This observation, then, is consistent with a surface coating of tannic acid on the postprecipitated particles. Conclusions A series of aluminum hydroxyl-based particles dispersed on a mica substrate have been examined by scanning probe methods. Zeta potential measurements have also been made on the particles in solution as a function of pH. These particles are important as synthetic versions of the colloidal particles present during wastewater treatment processes. Tapping mode AFM images of aluminum hydroxide particles precipitated from alkaline solutions of Al2(SO4)3 showed that the particles were spherical and several hundred nanometers in diameter. IFM measurements on the particles gave a reduced modulus of the system of 12 ( 3 GPa, while force microscopy and zeta potential measurements showed the particles had an isoelectronic point at a pH of about 5, consistent with the main surface features being Al-OH sites. When similar particles were coprecipitated in the presence of phosphates and tannic acid, the morphology and elastic properties of the system were little changed, although the zeta potential of the system was lower, consistent with more ionizable OH sites on the surface arising from phosphate and carboxcylic acid groups. Particles precipitated from solution and then exposed to tannic acid (so-called postprecipitated) showed very different morphological characteristics. AFM images show much smaller particles, as well as very high contrast with the mica substrate in the phase-imaging mode. IFM and zeta potential measurements were consistent with the particles being coated in a layer of tannic acid. This model for the particles explains the relative differences in chemical reactivity of the postprecipitated and coprecipitated aluminum oxides toward the removal of phosphates and other contaminants from aqueous systems. Acknowledgment. We acknowledge the Natural Sciences and Engineering Research Council of Canada for financial support. We also acknowledge John Graham for his help in the acquisition of the IFM data. LA990896P
