Interactions of Benzoic Acid and Phosphates with Iron Oxide Colloids

Department of Chemistry, Queen's University, Kingston, Ontario, Canada K7L 3N6 ... The surface pKa of 4-(12- mercaptododecyloxy)benzoic acid has been ...
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Langmuir 2005, 21, 10608-10614

Interactions of Benzoic Acid and Phosphates with Iron Oxide Colloids Using Chemical Force Titration Jana Liang and J. Hugh Horton* Department of Chemistry, Queen’s University, Kingston, Ontario, Canada K7L 3N6 Received July 6, 2005. In Final Form: August 15, 2005 Colloidal iron oxides are an important component in soil systems and in water treatment processes. Humic-based organic compounds, containing both phenol and benzoate functional groups, are often present in these systems and compete strongly with phosphate species for binding sites on the iron oxide surfaces. Here, we examine the interaction of benzoate and phenolic groups with various iron oxide colloids using atomic force microscopy (AFM) chemical force titration measurements. Self-assembled monolayers (SAMs) of 4-(12-mercaptododecyloxy)benzoic acid and 4-(12-mercaptododecyloxy)phenol were used to prepare chemically modified Au-coated AFM tips, and these were used to probe the surface chemistry of a series of iron oxide colloids. The SAMs formed were also characterized using scanning tunneling microscopy, reflection-absorption infrared spectroscopy, and X-ray photoelectron spectroscopy. The surface pKa of 4-(12- mercaptododecyloxy)benzoic acid has been determined to be 4.0 ( 0.5, and the interaction between the tip and the sample coated with a SAM of this species is dominated by hydrogen bonding. The chemical force titraton profile for an AFM probe coated with 4-(12- mercaptododecyloxy)benzoic acid and a bare iron oxide colloid demonstrates that the benzoic acid function group interacts with all three types of iron oxide sites present on the colloid surface over a wide pH range. Similar experiments were carried out on colloids precipitated in the presence of phosphoric, gallic, and tannic acids. The results are discussed in the context of the competitive binding interactions of solution species present in soils or in water treatment processes.

Introduction The adsorption of humic-type and phosphate-containing substances at aluminum oxide and iron oxide mineral surfaces have been studied extensively, as the distribution of these species in environmental systems is dominated by their surface-adsorbed forms.1 Phosphate and humicbased compounds are known to compete for similar binding sites on hydrous metal oxide particle surfaces, and their interactions can have important consequences for the sedimentation and movement of these species in such environments as agricultural and forest soils, freshwater and marine ecosystems, and wastewater treatment systems.2 A number of analytical methods have been applied to these systems, including ζ potential and titration methods focusing on the competitive adsorption of phosphate with various analogues of humic materials, including malic and oxalic acids in soils or on the iron oxide mineral goethite.3,4 Goethite, either in its crystalline form or in the form of amorphous colloids of approximately the same chemical composition, FeO(OH), is often used as an analogue in such studies.5 Infrared spectroscopy has also been employed to examine the structures of adsorbed phosphate, benzoic, and phenolic compounds on goethitetype colloids.6,7 Our previous work in this area has focused on using scanning probe techniques to explore the nature of * To whom correspondence should be addressed. Telephone: (613)-533-2379/(613)-533-6704. Fax (613)-533-6669. E-mail: [email protected]. (1) Lui, C.; Huang, P. M. Can. J. Soil Sci. 2000, 80, 445. (2) Lopez-Hernandez, D.; Siegert, G.; Rodriguez, J. V. Soil Sci. Soc. Am. J. 1986, 50 1460. (3) Liu, F.; He, J.; Colombo, C.; Violante, A. Soil Sci. 1999, 164, 180. (4) Hunter, K.; Carpenter, P. D.; Hawke, D. Environ. Sci. Technol. 1989, 23, 187. (5) Evanko, C. R.; Dzombak, D. A. Environ. Sci. Technol. 1998, 32, 2846. (6) Tejedor-Tejedor, M. I.; Yost, E. C.; Anderson, M. A. Langmuir 1990, 6, 979. (7) Tejedor-Tejedor, M. I.; Yost, E. C.; Anderson, M. A. Langmuir 1992, 8, 525.

phosphate interactions with iron oxide colloids. In addition to imaging the colloids using atomic force microscopy (AFM),8,9 we have also used scanning probe methods to directly measure the interactions of phosphate species with the colloids using chemical force titrations.10,11 In a chemical force titration experiment, the adhesive forces between the AFM tip and the sample surface are measured directly as a function of the pH of the solution in which both AFM tip and the sample surface are immersed.12 There are several advantages to this approach: First, it is possible to determine the surface pKa of the colloid under the relevant environmental conditions. More importantly, this type of experiment may be carried out with chemical specificity, by ensuring that the AFM tip is coated with a self-assembled monolayer (SAM) terminated with a particular functional group. In our previous work, the compound bis(11-thioundecyl)phosphate was used to examine phosphate interactions with the colloid surface. From the chemical force titration curves obtained, we showed that phosphate species interact preferentially with amphoteric Fe-OH sites on the colloid surface and further that the adsorption of simple analogues of humic substancesssuch as gallic acidswould reduce the forces associated with phosphate interaction at the colloid surface, as well as shifting the pH at which the maximum adhesive interaction occurred. We also found that the method of colloid preparation also had an important effect on the force titration profiles. In particular, an iron or aluminum oxide colloid that was precipitated in the presence of an adsorbate in solution, such as phosphoric, gallic, or tannic acid (known as a co-precipitated colloid), (8) Kreller, D. I.; Gibson, G.; VanLoon, G. W.; Horton, J. H. J. Colloid Interface Sci. 2002, 254, 205. (9) Kreller, D. I.; Gibson, G.; Novak, W.; VanLoon, G. W.; Horton, J. H. Colloids Surf., A 2003, 212, 249. (10) Omoike, A.; Horton, J. H. Langmuir 2000, 16, 1655. (11) Omoike, A.; Chen, G.; VanLoon, G. W.; Horton, J. H. Langmuir 1998, 14, 4731. (12) Noy, A.; Vezenov, D. V.; Lieber, C. M. Annu. Rev. Mater. Sci. 1997, 27, 381.

10.1021/la0518290 CCC: $30.25 © 2005 American Chemical Society Published on Web 09/23/2005

Chemical Force Titrations on Iron Oxide Colloids Scheme 1

had significantly different surface pKa and point of zero charge (pzc) than adsorbate that was added after colloid formation had already begun (known as a postprecipitated colloid). However, this work did not study the adsorption of humic species on the colloids directly, only how these species, when already adsorbed to the colloid, could change the behavior of the phosphate tip interaction with the colloid surface. In this paper we report on a series of chemical force titration experiments in which the functional groups associated with the humic components of this system are deposited on the AFM tip and chemical force titrations are carried out using these surface modified tips. Au-coated AFM tips were covered with thiol SAMs terminated with either phenol or benzoic acid, and their interactions with various hydrous iron oxide colloid surfaces were studied using chemical force microscopy (CFM). Phenol and benzoate are two of the main functional groups found within humic materials, which include such complex mixtures as tannins or humic acid. Initial experiments were aimed at determining the surface pKa of the 4-(12mercaptododecyloxy)benzoic acid and 4-(12-mercaptododecyloxy)phenol SAMs (see Scheme 1) used to modify the AFM tip. We then report on the interaction of AFM tips coated in these SAMs with a series of amorphous iron oxide colloids and compare the chemical force titration curves with those derived from ζ potential measurements on the same colloids. The phenol and benzoic acidterminated SAMs have also been characterized using standard surface analytical methods such as scanning tunneling microscopy (STM), reflection absorption infrared spectroscopy (RAIRS), and X-ray photoelectron spectroscopy (XPS). Results from the scanning probe experiments will be discussed in the terms of the selectivity of the hydrous iron oxide colloids toward the adsorption of phosphate- and humic-containing species from solution. Experimental Section Synthetic Methods. The synthesis of bis(11-thioundecyl)phosphate13 (1), 4-(12-mercaptododecyloxy)benzoic acid14,15 (2), and 4-(12-mercaptododecyloxy)phenol16 (3) were carried out using previously published methods. The physical and spectral properties matched those reported in the literature. The various hydrous iron oxide colloid substrates used in the force titration experiments were synthesized at room temperature (293 K) via hydrolysis of ferric chloride. The procedure for preparation of the unmodified (native) hydrous iron oxide colloid has been published in detail elsewhere,8 and these colloids have been previously characterized using ζ potential, titration and X-ray diffraction methods.17 The colloids were further modified using one of the following reagents: KH2PO4 (phosphatemodified), gallic acid, and tannic acid. The modification procedure itself was of two different types: postprecipitated colloids, in which the modifying reagent was added after the iron colloids had precipitated and aged for 20 min, or co-precipitated colloids, (13) Nakashima, N.; Taguchi, T. Colloid Surf., A 1995, 103, 159. (14) DeVries, V. G.;, Moran, D. B.; Allen, G. R.; Riggi, S. J. J. Med. Chem. 1976, 19, 946. (15) Zhou, X. C.; Ng, S. C.; Chan, H. S. O.; Li, S. F. Y. Sens. Actuators, B 1997, 42, 137. (16) Taylor, C. D.; Anderson, M. R. Langmuir 2002, 18, 120. (17) Russell, J. D.; Parfitt, R. L.; Fraser, A. R.; Farmer, V. C. Nature 1974, 248, 220.

Langmuir, Vol. 21, No. 23, 2005 10609 in which the modifying reagent was added simultaneously with ferric chloride to solution and thus was present as the colloids formed. Details of the synthesis of postprecipitated colloids have been published elsewhere.8 Details of the synthesis of the coprecipitated colloids are given here. (i) Hydrous Iron Oxide Colloids Co-precipitated with Phosphate. KH2PO4 (8.82 × 10-4 M, 1 mL) was added to 1 L of distilled water to produce a solution of pH 4.5. To this solution was added FeCl3 (50 mL, 41% (w/v)), while being stirred at 300 rpm. The pH decreased to 3.3. The solution pH was then adjusted to a value of 6 using a 0.50 M NaHCO3 solution. The solution was aged for 20 min and settled for a further 10 min. The precipitates were collected by filtration. (ii) Hydrous Iron Oxide Colloids Coprecipitated with Gallic Acid or Tannic Acid. Gallic acid or tannic acid (1.76 × 10-4 M, 1 mL) was added to 1 L of distilled water. The remaining portion of the procedure was identical to that described for the colloid co-precipitated with phosphate, above. Instrumentation. AFM force-distance curves were acquired using a PicoSPM (Molecula Imaging, Tempe, AZ), and a Nanoscope IIE controller (Digital Instruments, Santa Barbara, CA). STM imaging took place using a similar system. All experiments were carried out at 25 °C. For force titration experiments, unbuffered aqueous NaOH or HCl solutions of pH ranging from 2 to 11 were prepared. Unbuffered solutions were used in order to avoid the potential influence of buffer ions (such as phosphate) from solution on the probe-substrate interactions. Experiments were carried out under low ionic strength conditions (I e 0.02 mol L-1). The only ions in the solution were those introduced by pH adjustment using NaOH and NaCl. Solutions were changed frequently to minimize the possible pH change due to atmospheric CO2, and the pH value was checked before and after the experiment. From 200 to 300 retraction force curves were collected for each sample. The magnitude of the adhesive interaction from these curves was averaged, and the errors bars indicated in the results reflect the standard deviation of the data. Each force titration curve (plot of adhesive interaction as a function of pH) was also repeated at least twice to ensure the reproducibility of the data. Variation in tip radii and calibration factors (i.e. force constant of the AFM cantilever) may affect the magnitude of CFM force curves. Although there was variation of about 10% in the absolute magnitude of the tip-sample adhesive force between runs, the shapes of the titration curves as a function of pH in different experiments were reproducible. It is important to note that in order to allow the best comparison of the shapes of the force-distance curves in various pH series, a single AFM tip was used to obtain all the data in each pH series presented. The AFM cantilevers (MikroMasch, Portland, OR) that were used for the adhesion force measurement were terminated with oxide-sharpened silicon tips. The force constants of the cantilevers were calibrated using the method of Hutter and Bechhoefer18 and ranged from 0.1 to 0.3 nN/m, consistent with the manufacturer’s published range of values. The AFM tips were initially coated with 100 Å of Au by physical vapor deposition, using a modified Edwards vacuum deposition system. The Au-coated tips were functionalized with bis(11-thioundecyl)phosphate (“phosphate-terminated” tip), 4-(12-mercaptododecyloxy)phenol (“phenol-terminated” tip), or 4-(12-mercaptododecyloxy)benzoic acid (“benzoic acid-terminated” tip) by immersing them for 18 h in a 1.0 × 10-3 M solution of these compounds in ethanol. Self-assembled monolayers of the above three thiol compounds were similarly formed on Au-coated mica slides. Prior to immersion in ethanol solution, the slides were annealed with a hydrogen flame. Surface characterization of self-assembled monolayers of both 4-[(11-mercaptoundecyl)oxy]phenol and 4-[(11-mercaptoundecyl)oxy]benzoic acid formed on the Au-coated mica substrates were carried out using STM, XPS, and RAIRS. STM was carried out using the scanning probe equipment described above. XP spectra were acquired on the VGEscalab spectrometer at the Waterloo Advanced Technology Laboratory (WATLab) at the University of Waterloo. The XP spectra were analyzed (peak areas and background subtraction) using the CASA XPS software package. The RAIRS spectra were acquired on a Digilab FTS 7000 FTIR spectrometer. (18) Hutter, J. L.; Bechhoefer, J. Rev. Sci. Instrum. 1993, 64, 1868.

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Figure 2. STM image of a self-assembled monolayer of 4-(12mercaptododecyloxy)benzoic acid on a flame-annealed gold film. Tunneling current, 0.040 nA; tip bias, +0.05 V; z-range, 7.5 Å. Figure 1. Chemical force titration curve showing the tipsample adhesive force as a function of pH between a Au-coated AFM tip and Au-coated mica substrate which have both been terminated with an SAM of 4-(12-mercaptododecyloxy)benzoic acid. The error bars represent the standard deviation in the adhesion force as measured from the average of 300-500 forcedistance curves. The line segments joining the points are meant as a guide to the eye.

Results The chemical force titration profile for a Au-coated AFM tip terminated with 4-(12-mercaptododecyloxy)benzoic acid (2) against a Au-coated mica slide modified with the same compound is shown in Figure 1. A maximum adhesion force of 26 nN occurs at a pH of 4.0 ( 0.5, with the adhesive interactions dropping off rapidly at higher or lower pH values. Such force titration profiles have been previously observed for systems in which the interaction between the tip and sample is dominated by hydrogenbonding forces.19,20 The position of the peak has been shown to be indicative of the surface pK1/2, that is, the solution pH value at which half the surface sites have been deprotoned. At pH values above the surface pK1/2, both tip and sample are deprotonated, and electrostatic repulsion dominates between the negatively charged groups on the tip and sample, leading to a repulsive interaction. Below the surface pK1/2, regular hydrogen-bonding interactions between the protonated tip and sample lead to relatively small adhesive interactions. At pH values close to the surface pK1/2, the tip sample interaction is dominated by ionic hydrogen bonding, in this case between deprotonated Ar-COO- on the one surface and a protonated Ar-COOH group on the other. Such an ionic hydrogen bond has been shown to be up to 40 times stronger than a regular (neutral) hydrogen bond,21,22 under conditions similar to those used here. A similar experiment was also carried out using a tip and sample both terminated with 4-(12-mercaptododecyloxy)phenol (3) to form phenol-terminated surfaces. Although peaks were seen in the pH 8 region of the force titration curve, the results were difficult to repeat, and in several cases further peaks were seen under lower pH conditions. Since phenols are prone to oxidation, particularly in basic solution, it would seem that the terminal groups on tip and sample were oxidized during the force titration experiment, which takes place over a period of several hours. Due to these nonreproducible results, no (19) van der Vegte, E. W.; Hadziioannou, G. J. Phys. Chem. B 1997, 101, 9563. (20) van der Vegte, E. W.; Hadziioannou, G. Langmuir 1997, 13, 4357. (21) Smith, D. A.; Wallwork, M. L.; Zhang, J.; Kirkham, J.; Robinson, C.; Marsh, A.; Wong, M. J. Phys. Chem. B 2000, 104, 8862. (22) Wallwork, M. L.; Smith, D. A., Zhang, J.; Kirkham, J.; Robinson, C. Langmuir 2001, 17, 1126.

further titration experiments were performed using the phenol-terminated tip. Self-assembled monolayers of 4-(12-mercaptododecyloxy)benzoic acid and 4-(12-mercaptododecyloxy)phenol were also characterized by STM imaging, XP, and infrared spectroscopy. STM images of both thiol compounds showed that there was no evidence of ordering on the surface. A typical image, shown in Figure 2, show steps and etch pits typical of alkanethiol adsorption on gold, but no evidence of atomically resolved order could be achieved with STM on these samples. High-resolution XP spectra were acquired from binding energy regions containing peaks arising from all elements that should be present on the samples: C 1s, O 1s, S 2p, and Au 4f. A low-resolution survey scan of the entire kinetic energy range did not indicate the presence of any other elements, except for small quantities of Si, which presumably arises from the mica support. XP spectra were compared to those acquired using a SAM of 12-mercaptododecanoic acid on Au. The O 1s peak areas were determined, and SAMs of the phenol compound 3 and 12-mercaptododecanoic acid, each of which contain two O atoms, gave XPS peak areas of 98 ( 2 and 103 ( 2, respectively. The benzoic acid compound [2], which contains three O atoms gave a peak area of 123 ( 2. While the benzoic-terminated SAM indeed has a larger O 1s signal, as expected, the peak areas are not in the correct ratio. Because the samples had to be transported to an external site and were exposed to air for some time before analysis, there is the probability of contamination by O or C. Signals from sulfur should not, however, have any signal arising from background contaminants. The S 2p peak areas were 4.7 ( 1.5, 5.7 ( 1.8, and 4.9 ( 1.6 (arbitrary units) for compounds 2, 3, and the 12-mercaptododecanoic acid control, respectively. This result demonstrates that the benzoic acid- and phenol-terminated SAMs have a density on the surface that are similar to that for the 12-mercaptododecanoic acid control case, but the small S 2p peak intensity does lead to large errors in the determination of peak area. The nearest neighbor distance for SAMs of 12-mercaptododecanoic acid have been previously determined to be about 0.5 nm from STM imaging experiments,23 and the density was about 9 × 10-10 mol‚cm-2.24,25 A previous determination of the density for a SAM of the phenol compound 3 was 3.7 × 10-10 mol‚cm-2.16 RAIRS spectra of self-assembled monolayers on Au of both 4-(12-mercaptododecyloxy)benzoic acid and 4-(12(23) Terrance, F.; Vela, M. E.; Salvarezza, R. C.; Arvia, A. J. Electrochim. Acta 1998, 44, 1053. (24) Imabayashi, S.; Iida, M.; Hobara, D.; Feng, Z. Q.; Niki, K.; Kakiuchi, T. J. Electroanal. Chem. 1997, 428, 33. (25) Walczak, M. M.; Popenoe, D. D.; Deinhammer, R. S.; Lamp, B. D.; Chung, C. K.; Porter, M. D. Langmuir 1991, 7, 2687.

Chemical Force Titrations on Iron Oxide Colloids

Figure 3. Reflection adsorption infrared spectra for selfassembled monolayers on Au(111) of the alkanethiols (a) 4-(12mercaptododecyloxy)benzoic acid (2) and (b) 4-(12-mercaptododecyloxy)phenol (3).

mercaptododecyloxy)phenol are shown in Figure 3. Both compounds show a strong peak in the 1250 cm-1 region. Taylor and Anderson16 also obtained an IR spectrum of the phenol on Au(111), which looks similar, in most aspects, to the one reported here. They attributed the 1250 cm-1 peak to an asymmetric O-C vibration of the aromatic ether group. Presumably the same transition is responsible for the peak also seen in the benzoic acid species. The peak observed at 1500 cm-1 for the phenol species has been attributed to a semicircular in-plane stretch of the benzene ring.26 The benzoic acid species has peaks at 1433 and 1612 cm-1 which may also be associated with a similar benzene ring stretch or, in the latter case, the CdO stretch of the acid group.26 The main significance of these results is that all these vibrational transitions involve changes in dipole along the plane of the benzene ring. Since the selection rule for RAIRS states that only vibrations perpendicular to the surface will be allowed,27 this result demonstrates that the benzene rings must lie at least partially upright with respect to the surface. In the higher wavenumber region, both compounds exhibit peaks at 2857 and 2927 cm-1 typical of methylene stretches of alkane linker group. Again, Anderson and Taylor found that, for the phenol species, these peaks appeared higher in intensity than those for the corresponding vibrational mode seen in dodecanthiol monolayers on Au(111), suggesting greater disorder in the phenol-terminated species and in agreement with the STM images obtained. The benzoic acid compound shows a similar peak intensity, demonstrating a similar degree of disorder. Interestingly, we also observed a very intense peak at 3625 cm-1 for the phenol compound that was not reported by Anderson and Taylor but which presumably arises from the O-H stretch of the phenol.26 In the benzoic acid, there is a much less intense peak observed at 3018 cm-1 that may arise from the acid O-H group.26 The presence of these peaks in both cases suggests that there is limited intermolecular H-bonding between the phenol or benzoate groups within the monolayer Figure 4 shows the force titration profile once again using the benzoic acid-terminated AFM tip, this time using the iron oxide colloid as a substrate. The inset to Figure 4 shows ζ potential data for the iron oxide colloid. This time, the titration shows a broad peak centered near a pH of 4-5, which maximizes at a force of about 30 nN. ζ potential measurements on the bare colloid were also (26) Lion-Vien, D.; Colthrop, N. B.; Fateley, W. G.; Grasselli, J. G. The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules; Academic Press: San Diego, CA, 1991. (27) Woodruff, D. P.; Delchar, T. A. Modern Techniques of Surface Science; Cambridge University Press: Cambridge, U.K., 1986.

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Figure 4. Chemical force titration curve showing the tipsample adhesive force as a function of pH between a Au-coated AFM tip terminated with an SAM of 4-(12-mercaptododecyloxy)benzoic acid and a bare iron oxide colloid. The error bars represent the standard deviation in the adhesion force as measured from the average of 300-500 force-distance curves. The line segments joining the points are meant as a guide to the eye. The inset shows the ζ potential of the bare iron oxide colloid as a function of pH.

Figure 5. Chemical force titration curve showing the tipsample adhesive force as a function of pH between a Au-coated AFM tip terminated with an SAM of 4-(12-mercaptododecyloxy)benzoic acid and (a) a Au-coated mica substrate which has been terminated with an SAM of bis(11-thioundecyl)phosphate; iron oxide colloids that were (b) postprecipitated and (c) coprecipitated with K2HPO4. The inset shows the ζ potential of the postprecipitated and co-precipitated colloids as a function of pH.

carried out as a function of pH. As can be observed, the point of zero charge of this colloid is at a pH of about 5.0. In Figure 5, a set of three force titration profiles are shown, all of which were acquired using the benzoic acidterminated AFM tip. The three curves demonstrate the effects of phosphate species on the force titration profile. In curve 5a, which may serve as a control, the substrate was a Au surface coated with the phosphate-terminated alkanethiol, 1. In this case, we see a peak centered at a pH of 4. Reversing the chemical termination on tip and sample, i.e., a phosphate-terminated tip on the benzoic acid-terminated surface, gave a similar titration profile. The remaining curves are for iron colloids that were postprecipitated and co-precipited with K2HPO4, to form colloids with phosphate groups on the surface. It is evident that the method of colloid preparation has a strong result on the force titration profiles and, by extension, on the surface properties of the colloids. It is particularly notable that the profile of the curve for the postprecipitated colloid, Figure 5b, follows closely that for the control case, Figure 5a. The curve for the coprecipitated colloid, Figure 5c,

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Figure 6. Chemical force titration curve showing the tipsample adhesive force as a function of pH between a Au-coated AFM tip terminated with an SAM of of 4-(12-mercaptododecyloxy)benzoic acid and an iron oxide colloid (a) postprecipitated and (b) co-precipitated with gallic acid. The inset shows the ζ potential of the postprecipitated colloid as a function of pH.

Figure 7. Chemical force titration curve showing the tipsample adhesive force as a function of pH between a Au-coated AFM tip terminated with an SAM of of 4-(12-mercaptododecyloxy)benzoic acid and an iron oxide colloid (a) postprecipitated and (b) co-precipitated with tannic acid. The inset shows the ζ potential of the postprecipitated colloid as a function of pH.

maximizes at a much higher pH, but is of a similar magnitude. ζ potential measurements were acquired for both postprecipitated and co-precipitated phosphatecoated colloids. The isoelectric point is in both cases lowered compared to the bare colloid, at about a pH of 3.5 for the postprecipitated colloid and 4.5 for the coprecipitated colloid. Figures 6 and 7 show two more series of force titration profiles, again all using the same benzoic acid-terminated tip. In Figure 6, force titration curves on colloids reacted with gallic acid, while in Figure 7 the colloids were reacted with tannic acid. Again, it is evident that the process by which the colloids are producedsco-precipitated or postprecipitated with the respective acidsleads to large changes in the profiles. ζ potentials were acquired for the postprecipitated colloids as a function of pH, and are shown on the insets to these figures. Discussion Because the benzoate and phenol groups are the two main functional groups present within humic substances, such as gallic or tannic acids, we chose to use an AFM tip terminated with these species in order to simulate the interaction of humic species with iron oxide colloids and, in particular, to examine the competition of these species

Liang and Horton

with phosphates. The phenol tip was, unfortunately, not stable enough to acquire good force titration profiles. The benzoate-terminated AFM tip was stable enough to produce good titration curves, however, and determining the surface pKa of the benzoate groups is of considerable importance in interpreting the more complex tip-colloid interactions. It is important to note that the solution pH (measured here) is different from the pH at the surface of either tip or sample: the surface pH is shifted relative to the bulk value due to the presence of a surface potential.21 However, under low ionic strength conditions, the shift is generally small, on the order of 0.5 pH units. We measured a surface pK1/2 of 4.0 ( 0.5 for the SAM of 4-(12-mercaptododecyloxy)benzoic acid; that is, at a solution pH of 4.0, half the surface sites are deprotonated. The surface pH under these conditions would correspond to the surface pKa. Because the difference between solution and surface pH should be relatively small, less than 0.5 pH units, we will consider this pK1/2 value is a good estimate of the surface pKa. The solution pKa of 4-(12-mercaptododecyloxy)benzoic acid has been measured to be 4.48.,14,15 close to the surface pK1/2 measured here. It is often observed that surface pK1/2 values differ considerably from the solution pKa of the same species. The difference in solution vs surface pH measurement contributes partially to this, but a more important contribution is due to in-plane hydrogen bonding between molecules on the surface and the limited ability of the solvent to the shield charged species at the interface as compared to in solution. Both effects should increase the difficulty in ionizing surface-bound species.28 This should cause an upward shift in the surface pKa value relative to the solution phase for acidic groups. However, the surface pK1/2 shift for 4-(12- mercaptododecyloxy)benzoic acid observed here is, within the error of the measurement, essentially the same as that for the acid in solution. We know from the STM images that the 4-(12mercaptododecyloxy)benzoic acid SAM is disordered on the surface, and thus there is probably relatively weak in-plane H-bonding at the surface and little shift from the solution pKa value. Force titration curves on the variously modified FeO(OH) colloids produce a range of features, some of which must arise due to the fact that the colloids contain a number of different types of ionizable surface sites with which the benzoic acid-terminated tip can interact. Crystalline iron oxides have three distinct types of ionizable site on the surface. The pKa values that have been measured and/or calculated in the literature range somewhat depending on the exact species and crystal face under study, but they include an amphoteric Fe-OH site, known as the A site, whose upper pKa (deprotonation to Fe-O-) is generally quoted in the range of 4.5-7.0 pH units.29 There are also two other sites that are deprotonated at higher pH values. The µ2-oxy, or C site, in which an O atom bridges between two Fe atoms is about 8.1. The µ3-oxy, or B site, in which an O bridges between three Fe atoms is about 9.8.30 Consideration of the ζ potential data shown in the inset to Figure 4 suggests that the most prevalant site on the bare FeO(OH) colloid surface must be an A-type one. This is because the pzc for this colloid is at a pH of 5.5. Since the B and C sites should still be protonated, and hence positively charged under these pH (28) Giesbers, M.; Kleijn, J. M.; Cohen Stuart, M. A. J. Colloid Interface Sci. 2002, 252, 138. (29) Felmy, A. R.; Rustad, J. R. Geochim. Cosmochim. Acta 1998, 62, 25. (30) Gualteri, A.; Venturelli, P. Am. Mineral. 1999, 84, 895.

Chemical Force Titrations on Iron Oxide Colloids

conditions, the negative charge on the colloid must be contributed by deprotonated A sites on the surface. The force titration data for the benzoic tip on the bare colloid may be understood in terms of the ζ potential data. The pzc of the colloid surface happens to conincide with the surface pKa of the benzoic acid-terminated tip. Thus, we should see a maximum interaction somewhere between about pH 4 and 5 since under these conditions we would have the maximum number of ionic hydrogen bonds available to form between tip and sample. The interaction forces drop off gradually to higher pH, as both the tip deprotonates and the ζ potential on the colloid grows progressively more negative. The very broad nature of the force titration curve is similar to the results we obtained previously using a 16-mercaptohexadecanoic acid-terminated tip on the same bare colloid. This result suggests that, like standard carboxylic acid groups, benzoic acid interacts nonspecifically with the bare colloid, through electrostatic interactions or hydrogen bonding to the colloid surface. Such a result is quite different from that observed using a phosphate-terminated tip. In our previous work, we found that with a phosphate-terminated AFM tip that there was an extremely strong and sharp peak in the adhesive force at pH values close to those for the surface pKa of the A-type site. The implication then was that phosphate groups adsorbed specifically at A sites on the colloid surface. Evidently, if phosphate species are competing with humic-based materials containing benzoate groups, it is the availability of the A-type sites that are important. In the next series of experiments, therefore, we examined the interactions of the benzoic acidterminated tip with colloids that had been precipitated in the presence of phosphate, in the form of K2HPO4. In the control experiment shown in Figure 5a we see a strong interaction between the benzoate-terminated tip and phosphate-terminated surface at a pH of 4.0. The SAM used to coat the surface, bis(11-thioundecyl)phosphate, is a monoprotic phosphate, with a previously measured surface pK1/2 of 4.0,8 very similar to that of the benzoic acid-terminated tip. Thus, the force titration profile in Figure 5a is easy to explain: the peak again appears at the pH where the maximum number of ionic hydrogen bonds between tip and sample may be formed. The profile for the benzoic acid-terminated tip on the colloid postprecipitated in the presence of phosphate, Figure 5b, looks very similar to that for the phosphate SAM surface. The postprecipitated colloid was formed by allowing the FeO(OH) colloid to form, followed by exposure to K2HPO4. Thus, one might expect that phosphate groups will be found on the colloid surface. The force titration profile does indeed suggest that this is the case and furthermore suggests that the phosphate has adsorbed in a bidentate fashion on the surface, leaving only one protic site available for interaction with the tip, much like the phosphate SAM surface. The ζ potential curve for the postprecipitated colloid is consistent with this interpretation: the pzc has shifted slightly downward in pH as compared to the unmodified colloid, which would be expected as the surface of a phosphate-coated colloid should be somewhat more acidic than that of the bare colloid. Furthermore, the ζ potential curve begins to plateau at pH values above 6, which would be consistent with the main ionizable groups on the colloid being monoprotic phosphates, which should have almost entirely deprotonated under these pH conditions. It is significant that the titration profiles obtained can be interpreted in terms of interaction with phosphate species on the surface, not bare colloid sites. Thus, it seems

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that the benzoic acid groups on the tip are unable to displace phosphate at the surface of the postprecipitated colloid. The force titration profile for the colloid coprecipitated with phosphate, in Figure 5c, is shifted to a higher pH value, and also appears somewhat broader, than that for the postprecipitated colloid. Because the co-precipitated colloid had K2HPO4 available during precipitation of the FeO(OH) from solution, we might expect phosphate species to be incorporated into the bulk, as well as on surface sites. The ζ potential curve for this colloid consistently shows a ζ potential that is more negative than that for the postprecipitated colloid, suggesting a higher concentration of ionizable sites on the colloid. The pzc is also shifted to lower pH, suggesting that those sites which are ionized on the surface do so at a lower pH. The force titration profile is certainly not consistent with the presence of phosphate adsorbed in a bidentate fashion, leaving a monoprotic phosphate-coated surface. If the phosphate concentration on this colloid surface is higher, then it is possible that it has adsorbed in a monodentate fashion, leaving the surface coated with diprotic phosphate sites. Solution pKa data of diprotic phosphate species, such as dimethyl phosphate, have values in the region of pKa,1 ) 1.5 and pKa,2 ) 6.3.31 Previous experience with our monoprotic phosphate has suggested that the pK values should shift upward by about 2 pH units when at the surface.8 This would thus explain the observation of the lower pzc, as the first surface pKa will be exceeded, and the colloid begin to become negatively charged, at a pH of about 3. The presence of the second pKa should presumably lead to a strong tip-sample interaction with maximum ionic hydrogen bonding at a pH intermediate to the surface pKa of the tip and surface pKa,2. We observe a peak at a pH of about 6, which would be consistent with our known pK1/2 value of the tip of 4.0 and a surface pKa,2 of the phosphate sites on the colloid of about 8. Alternatively, there may also be some interaction between tip and any bare B and C sites on the colloid surface. Our previous work examined the interactions of a phosphate-terminated tip with colloids that had been postprecipitated in the presence of gallic or tannic acid. Tannic acid, consisting of a series of five digalloyl units attached to a glucose ring, via ester linkages, is used as a model species to simulate the adsorption of humic substances with iron oxide colloids. Because gallic acid is one of the components of tannic acid, we also studied this much simpler system on the colloid surface. Previous infrared studies7 of gallic acid-coated colloids has shown the loss of the -OH stretching peak of the benzoic acid group at 3489 cm-1, suggesting that gallic acid adsorbed on the surface mainly through the benzoate group, via formation of a ester linkage to Fe-OH sites on the surface. The ζ potential measurements on the colloid postprecipitated with gallic acid show that the pzc has shifted to a pH of 2.5, considerably lower than that of the bare colloid, suggesting the presence of many acidic sites on the surface. Gallic acid contains two types of ionizable sites, the carboxylate group with a solution pKa of 4.2 and the first phenol group of 8.8.32 If all the gallic acid on the surface was indeed adsorbed through formation of a linkage through the benzoic group, we should expect to have the pzc increase, not decrease as seen here, since the only sites available to ionize would be the rather high pKa phenol groups. The force titration curves obtained in Figure 6 seem to be more consistent with a mixture of (31) Kumler, W. D.; Eiler, J. J. J. Am. Chem. Soc. 1943, 65, 2355. (32) Boyd, I.; Beveridge, E. G. Microbios 1979, 24, 173.

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benzoic acid or phenol sites exposed on the colloid surface. In the case of the colloid postprecipitated with gallic acid, we see a fairly broad peak, centered at a pH of 6.0. This lies at too high a pH to represent the interaction of the tip solely with benzoic groups on the surface, as we have already seen from the benzoate SAM sample that in that case we should see a peak closer to a pH of 4. There is also a significant shoulder in the force titration curve at the high-pH end, which perhaps indicates H-bonding interactions persist with nonionized phenol groups still present on the surface under these conditions. The force titration curve for the co-precipitated colloid is even broader with a sharp peak at a pH of 8. Again, this would be consistent with a mixture of both benzoic and phenol groups on the surface, although, in the case of the co-precipitated colloid, it appears that the phenol groups are present in greater concentrations. The force titration data on the much more complex systems in which tannic acid was adsorbed on the colloid surfaces are more difficult to interpret. AFM images and nanoindentation measurements that we have previously obtained and reported on for similar colloids indicate a surface that is covered in a layer of organic molecules about 1 nm in thickness.11 The overall lower magnitude of the tip-sample interaction on both colloids is indeed consistent with this, as fewer hydrogen bonds appear to be formed between tip and sample on these colloids. This is also consistent with the ζ potential data which show that the pzc has once again shifted back to a higher pH value. The ζ potential data also indicate that there are a relatively higher fraction of phenol groups exposed on these colloids, as we observed a second rapid decrease in the ζ potential above a pH of 9, which is close the pKa of the phenol groups. The force titration data on the coprecipitated colloid also has a large peak in the region around pH 6-7, also consistent with the presence of phenol groups at the surface. Conclusions The adhesive interactions between an Au-coated AFM probe terminated with self-assembled monolayers of 4-(12-

Liang and Horton

mercaptododecyloxy)benzoic acid and a series of hydrous iron oxide colloids, modified with phosphate, gallic acid, and tannic acid, have been examined as a function of pH. The surface pKa of 4-(12- mercaptododecyloxy)benzoic acid has been determined to be 4.0 ( 0.5. The chemical force titraton profile for an AFM probe coated with this SAM and a bare iron oxide colloid demonstrates that the benzoic acid function group interacts with all three types of iron oxide sites present on the colloid surface over a wide pH range. Similar experiments on colloids precipitated in the presence of phosphate species shows that the benzoate group does not appear to displace the phosphate from the surface. Force titration profiles acquired on colloids precipiated in the presence of gallic or tannic acid show that binding of these species to the surface occurs mainly through the benzoate fucntional group, leaving higher concentrations of phenol sites at the surface. These results are applicable to understanding the processes taking place during water treatment processes, particularly the removal of phosphate species in the presence of humic or other organic compounds. In contrast to the benzoate group, phosphate has been found to adsorb mainly at A-type, Fe-OH sites on the colloid surface, with the adhesive interactions between a phosphate-coated tip and colloid maximized over a fairly narrow pH range. Thus, careful control of pH during adsorption of the phosphate species should maximize the selectivity of the colloids for removal of this species from solution. Furthermore, as we found that benzoate does not exchange effectively for phosphate on the surface, it would seem best to use postprecipitated colloids for extracting the phosphate from water sysems, that is, to precipitate the iron oxde colloids from solution before exposure to phosphate or humic species. Acknowledgment. We acknowledge the Natural Sciences and Engineering Research Council of Canada for financial support. LA0518290