pubs.acs.org/Langmuir © 2009 American Chemical Society
Control of r-Alumina Surface Charge with Carboxylic Acids Sergio Bertazzo and Kurosch Rezwan* Advanced Ceramics, University of Bremen, Am Biologischen Garten 2, 28359 Bremen, Germany Received August 23, 2009. Revised Manuscript Received November 4, 2009 In this work, we studied the surface charge of R-alumina treated with carboxylic acids with different carbon chain length. The results show the possibility of controlling surface charges of alumina by using different concentrations of carboxylic acids or changing the size of the carbon chain of the acids. We also report that part of the acid found on the surface is strongly bound, therefore making it possible to obtain pH-resistant samples of R-alumina with an isoelectric point (IEP) of 5.5. It is found, that IEP values obtained for modified samples have a linear correlation with the number of carbon atoms of dicarboxylic acids for up to five carbon atoms. From a practical perspective, the method presented in this work has many advantages. First, it maintains the same hydrophilicity of the alumina surface. Second, the modification of the surface is stable in a long-range of pH. Finally, the presented method is easy-to-use and cheap, as the modification consists of only two simple steps carried out at low temperatures with inexpensive and nontoxic reagents.
Introduction Alumina is widely used, e.g., as a chromatography column filler and a xerographic toner additive and also as a part of prostheses and implants. Three characteristics of this material are particularly relevant for its applications: chemical stability, electrical surface properties and mechanical properties. Out of such characteristics, surface charges are of special interest in practically all applications. As a consequence, several methods for the modification and control of alumina surface charges have been developed to date.1-4 Control over charges can be achieved by binding different functional groups to the surface of aluminas. Furthermore, effective control over charges and functional groups on alumina surfaces may significantly expand applications of this material in the medical field. So far alumina has been employed as a component of prostheses where good mechanical properties are required, and its use is not so spread, especially when alumina is in direct contact with the organism.5-8 Some works in the literature9-11 clearly relate the bioactivity of materials to their surface characteristics. Several studies suggest that bioactivity is mainly due to the presence of chargeable functional groups such as hydroxyl, carboxyl, amino, and nitro *Corresponding author. Telephone: þ49 421 218 4507. Fax: þ49 421 218 7404. E-mail:
[email protected]. (1) Veregin, R. P. N.; McDougall, M. N. V.; Tripp, C. P. J. Imaging Sci. Technol. 2001, 45, 174–178. (2) Popat, K. C.; Mor, G.; Grimes, C. A.; Desai, T. A. Langmuir 2004, 20, 8035– 8041. (3) Karaman, M. E.; Antelmi, D. A.; Pashley, R. M. Colloid. Surf. A 2001, 182, 285–298. (4) Hidber, P. C.; Graule, T. J.; Gauckler, L. J. J. Am. Ceram. Soc. 1996, 79, 1857–1867. (5) Boehler, M.; Knahr, K.; Plenk, H.; Walter, A.; Salzer, M.; Schreiber, V. J. Bone Joint. Surg. Br. 1994, 76B, 53–59. (6) Hench, L. L. J. Am. Ceram. Soc. 1991, 74, 1487–1510. (7) Griss, P.; Heimke, G.; Andrianwerburg, H. V.; Krempien, B.; Reipa, S.; Lauterbach, H. J.; Hartung, H. J. J. Biomed. Mater. Res. 1975, 9, 177–188. (8) Hayashi, K.; Matsuguchi, N.; Uenoyama, K.; Sugioka, Y. Biomaterials 1992, 13, 195–200. (9) Kurella, A.; Dahotre, N. B. J. Biomater. Appl. 2005, 20, 5–50. (10) Jones, F. H. Surf. Sci. Rep. 2001, 42, 79–205. (11) Thull, R. Materialwiss. Werkst. 2001, 32, 949–952. (12) Fischer, H.; Niedhart, C.; Kaltenborn, N.; Prange, A.; Marx, R.; Niethard, F. U.; Telle, R. Biomaterials 2005, 26, 6151–6157. (13) Aizenberg, J. Adv. Mater. 2004, 16, 1295–1302.
3364 DOI: 10.1021/la903140k
groups attached to the surface of materials.12-17 These studies have laid a basis for further research aimed at altering bioactive properties of alumina surfaces by adding distinct functional groups to them through different methods.12,16,18-21 Bertazzo et al.19,22 have recently demonstrated that it is possible to considerably alter the bioactivity of γ-aluminas by making their surface react with ethanedioic acid. Nevertheless, the effect of the presence of carboxylic acids has not been fully determined and, as a consequence, the factors which account for the alteration of surface bioactivity are not yet known. One hypothesis is that the presence of carboxyl groups on the surface of aluminas alters surface charge, and this new charge affects surface bioactivity. Also, a point yet to be clarified is whether it is possible to use the dicarboxylic acids in order to alter the properties of R-alumina, the alumina phase actually used in prostheses, because of its excellent mechanical properties. This kind of reactions between alumina surface and carboxylic acids are of the same kind as the reactions that lead to the formation of compounds known as alumoxanes.23-27 This class (14) Li, P. J.; Ohtsuki, C.; Kokubo, T.; Nakanishi, K.; Soga, N.; Degroot, K. J. Biomed. Mater. Res. 1994, 28, 7–15. (15) Lee, K. Y.; Park, M.; Kim, H. M.; Lim, Y. J.; Chun, H. J.; Kim, H.; Moon, S. H. Biomed. Mater. 2006, 1, 31–37. (16) Kaltenborn, N.; Sax, M.; Muller, F. A.; Muller, L.; Dieker, H.; Kaiser, A.; Telle, R.; Fischer, H. J. Am. Ceram. Soc. 2007, 90, 1644–1646. (17) Toworfe, G. K.; Composto, R. J.; Shapiro, I. M.; Ducheyne, P. Biomaterials 2006, 27, 631–642. (18) Fernandez, L.; Arranz, G.; Palacio, L.; Soria, C.; Sanchez, M.; Perez, G.; Lozano, A. E.; Hernandez, A.; Pradanos, P. J. Nanopart. Res. 2009, 11, 341–354. (19) Bertran, C. A.; Bertazzo, S.; Almeida, L. L.; Souza, A. V. Key Eng. Mat. 2007, 330-332, 753–756. (20) Uchida, M.; Kim, H. M.; Kokubo, T.; Nawa, M.; Asano, T.; Tanaka, K.; Nakamura, T. Key Eng. Mat. 2000, 192-1, 733–736. (21) Zreiqat, H.; Evans, P.; Howlett, C. R. J. Biomed. Mater. Res. 1999, 44, 389– 396. (22) Bertazzo, S.; Zambuzzi, W. F.; da Silva, H. A.; Ferreira, C. V.; Bertran, C. A. Clin. Oral Implan. Res. 2009, 20, 288–293. (23) Andrianov, K. A.; Zhdanov, A. A. J. Polym. Sci. 1958, 30, 513–524. (24) Callender, R. L.; Harlan, C. J.; Shapiro, N. M.; Jones, C. D.; Callahan, D. L.; Wiesner, M. R.; MacQueen, D. B.; Cook, R.; Barron, A. R. Chem. Mater. 1997, 9, 2418–2433. (25) Mason, M. R.; Smith, J. M.; Bott, S. G.; Barron, A. R. J. Am. Chem. Soc. 1993, 115, 4971–4984. (26) Landry, C. C.; Pappe, N.; Mason, M. R.; Apblett, A. W.; Tyler, A. N.; Macinnes, A. N.; Barron, A. R. J. Mater. Chem. 1995, 5, 331–341. (27) Landry, C. C.; Pappe, N.; Mason, M. R.; Apblett, A. W.; Barron, A. R. Acs. Sym. Ser. 1994, 572, 149–164.
Published on Web 12/17/2009
Langmuir 2010, 26(5), 3364–3371
Bertazzo and Rezwan
of compounds is characterized by the bonding of carboxyls with Al atoms or alumina surfaces. These compounds have already been thoroughly studied and an extensive literature can be found on them.23-27 The effect of carboxyl upon alumina surfaces can be assessed with a study of surface charges on alumina treated with distinct carboxylic acids. A simple method to control the charge caused by these acids is to vary their acid dissociation constants (pKa). This constant can be controlled while keeping surface properties (such as hydrophobicity) practically unchanged, by using a series of carboxylic and dicarboxylic acids with an increasing number of carbon atoms in the chain. This method of altering surface charges using the same functional group (carboxyl) allows us to determine the actual contribution of the functional group to the distinct behavior of the alumina modified with each of the acids. In the end, the approach adopted in this work has many practical advantages. The first one is that the method presented here maintains the hydrophobicity of the modified surface, a feature that distinguishes it from other surface modification methods.17,28-32 This is an important matter, since a slight alteration in the hydrophobicity of the surface can cause a significant change in the behavior of the material toward the most diverse systems, for instance, living systems.33-35
Materials and Methods Alumina Functionalization. Tests were carried out in order to determine the most effective method for altering the IEP of alumina samples (R-alumina, CT300SG, Alcoa, with 7 m2/g surface area and particle size ∼1 μm). Such tests used different reaction times and distinct solvents and involved a pretreatment of alumina with a NaOH solution and/or a piranha solution. The effect of the solvent was assessed using ethanol, ethyl acetate, and THF as solvents for the reaction. In order to test reaction time, reactions with ethanedioic, butanedioic and hexanedioic acids were carried out for 24, 48, and 96 h with acid concentrations of 7 102 and 1.4 μmol/m2 for ethanedioic acid and 7 102 μmol/m2 for butanedioic and hexanedioic acids (the use of smaller concentrations of these acids cause a slight change to the IEP of samples, which negatively affects the evaluation of the efficiency of the modification). The effect of pretreatment was determined using a NaOH 1 M solution for 30 min while other samples were treated with a piranha solution (3:1 sulphuric acid (97-99%)/H2O2 35% v/v) at 80 °C for 4 h prior the NaOH treatment. After these pretreatments, samples were modified with ethanedioic acid, and the ζ-potential was obtained as a function of pH. After determining the most effective method for the modification of the alumina surface, the alumina was functionalized in two steps. The maximum concentration utilized was 700 μmol/m2 for all acids. For the ethanedioic, butanedioic, and hexanedioic acids, different concentrations, other than the maximum concentration, were used. We chose these acids because they provide a good range of carbon chain sizes (two, four, and six carbons). Thus, by (28) Advincula, M.; Fan, X. W.; Lemons, J.; Advincula, R. Colloids Surf., B 2005, 42, 29–43. (29) Tanahashi, M.; Matsuda, T. J. Biomed. Mater. Res. 1997, 34, 305–315. (30) Lee, M. H.; Brass, D. A.; Morris, R.; Composto, R. J.; Ducheyne, P. Biomaterials 2005, 26, 1721–1730. (31) Liakos, I. L.; Newman, R. C.; McAlpine, E.; Alexander, M. R. Langmuir 2007, 23, 995–999. (32) Liu, D. P.; Majewski, P.; O’Neill, B. K.; Ngothai, Y.; Colby, C. B. J. Biomed. Mater. Res. A 2006, 77A, 763–772. (33) Horbett, T. A.; Waldburger, J. J.; Ratner, B. D.; Hoffman, A. S. J. Biomed. Mater. Res. 1988, 22, 383–404. (34) Gallardo-Moreno, A. M.; Gonzalez-Martı´ n, M. L.; Bruque, J. M.; Perez-Giraldo, C. Colloids Surf., A 2004, 249, 99–103. (35) Yong, Y. H.; Park, K. H.; Park, Y. Y.; Jeon, Y. J.; Kim, Y.; Park, I.; Hahm, K. S.; Shin, S. Y. FEMS Microbiol. Lett. 2009, 292, 134–140.
Langmuir 2010, 26(5), 3364–3371
Article using these acids we can obtain a systematic overview of the differences in behavior of alumina treated with acids that have different carbon chain sizes. The first step for the modification was the immersion of 40 g of alumina powder into 500 mL of NaOH solution (1 M) for 30 min with stirring. After this period, the sample was washed with water and then dried at room temperature. The second step consisted in immersing different samples of 7 g of alumina treated with NaOH into a 100 mL tetrahydrofuran (THF) solution with stirring, with several concentrations (Table 1) of different carboxylic acids (all acids used are from Sigma-Aldrich g99% purity) at 70 °C, for 24 h for acid ethanedioic and 48 h for the other acids. After reflux with the acid solution, alumina was washed with ethanol and dried at 40 °C for 24 h. Chemical Stability Tests of Modification. Different methods were used in order to assess the chemical stability of the distinct modifications. Samples modified with ethanedioic acid were evaluated by two methods. In the first one, 3 g of alumina powder modified by ethanedioic, butanedioic and hexanedioic acid (7 102 μmol/m2) was immersed into 500 mL HCl 1 M for 30 min under agitation, then washed with water and oven-dried. For the second method, modified alumina samples were immersed into 500 mL of water under agitation for 24 h, period in which the water was changed 4 times. After the treatments described, the ζ-potentials of the samples were obtained for all samples. Adsorption Quantification. The quantity of ethanedioic acid on the surface after reflux was obtained using the determination of acid quantities remaining in the solution, according to a modified version of the method proposed by Bergerman and Elliot.37 In brief, the procedure used for quantification of ethanedioic acid was: 1 mL of the solution was filtered through a nylon syringe filter (VWR) with pore size 0.2 μm and heated up to 70 °C until the solvent completely evaporated. Then 1 mL of a H2SO4 1 M (Sigma-Aldrich) solution plus 1 mL of freshly prepared indole 0.01 M (Sigma-Aldrich) solution were added to the remaining solid and heated at 70 °C for 1 h. After that, at room temperature the absorbance of the solution at 525 nm wavelength was measured with a spectrophotometer (DR LANGE-ION 500). A calibration curve was obtained using a solution of known concentration and following the same procedure. The quantities of butanedioic and hexanedioic acids on the surface, after reflux, were determined by the quantity of remaining acid in the solution with a Hewlett-Packard gas chromatographer 5890 Series II coupled with a thermal conductivity detector, using a fused silica column of 30 m x 0.32 mm coated with dimethylpolysiloxane phase (Varian CP-Sil 5 CB). For that determination samples were filtered through a 0.2 μm syringe filter, and an aliquot quantity of samples (1 μL) was injected in the chromatograph. Calibration curves for each acid were obtained through preparation of solutions with known concentrations, of which 1 μL aliquots were injected in the equipment. The parameters utilized in the chromatograph for measurements of butanedioic acid were: injector temperature of 280 °C, detector temperature of 280 °C, and oven initial temperature of 80 °C with a heating rate of 15 °C/min up to 160 °C, followed by a heating rate of 50 °C/min up to 290 °C. As for hexanedioic acid measurements, the chromatograph was used with the following parameters: injector temperature was 330 °C, detector temperature was 300 °C, and oven initial temperature was 150 °C with a heating rate of 20 °C/min up to 250 °C. All the quantification experiments were carried out at least in duplicate and measurements using the respective methods described for each acid were carried out in triplicate. (36) Lide, D. R. CRC Handbook of Chemistry and Physics, 86th ed.; Taylor and Francis: Boca Raton, FL, 2006. (37) Bergerman, J.; Elliot, J. S. Anal. Chem. 1955, 27, 1014–1015.
DOI: 10.1021/la903140k
3365
Article Table 1. Structural Formula, Acid Dissociation Constant36 and Concentration (mol of Acid per Area of Alumina Surface) of All Acids Utilized for the Modification of Alumina Surfacesa
a The parameters utilized for the modification were: alumina pretreated with NaOH solution, reaction occurred in THF at 70° C, for 24 h for acid ethanedioic and 48 h for all the other acids.
Fourier Transform Infrared (FTIR) Spectroscopy. Alumina treated with ethanedioic, butanedioic and hexanedioic acids in THF was characterized by Fourier transform infrared (FTIR) spectroscopy using Perkin-Elmer System 2000 equipment. Infrared spectra were obtained from KBr pellets, of which ∼1 wt % was the sample. Spectra were obtained between 4000 and 400 cm-1, after 16 accumulations, at 4 cm-1 resolution. ζ-Potential. Modified alumina samples were titrated so that the ζ-potential and electrical conductivity were measured simultaneously as a function of pH. The range of pH was from 2 to 10 with approximately 22 points at this interval. After each addition 3366 DOI: 10.1021/la903140k
Bertazzo and Rezwan of the titrant (KOH 1 M or HCl 1 M solution) an equilibration time of at least 30s was allowed. The ζ-potential of the suspension was measured using the electroacoustic colloidal vibration current technique in an Acoustic & Electro acoustic Spectrometer DT-1200 from Dispersion Technology. All measurements were run at least in duplicate (twice toward the acidic pole and twice toward the basic pole of equilibrium pH), and the curves presented are the means of the curves obtained. Contact Angle. The contact angle measurements were carried out with a micropipet and an optical microscope. The contact angle of water on alumina surfaces was measured immediately after a drop of water (1.5 μL) was released onto the surface of alumina disks modified with the different acids. In order to obtain the disks, 0.15 g of alumina was pressed into a disk-shape of 0.8 cm at 15 KN for 30 s. After that, the disks were heated at 900 °C for 1 h and then gently polished with sandpaper sheets (4000 grit). After polished, disks were immersed into 25 mL NaOH 1 M for 20 min, then thoroughly washed with distilled water and finally oven-dried at 40 °C for 24 h. Disc surface roughness was assessed by a Profilometer Sensofar PLμ2300. The obtained surface roughness Ra of the prepared samples was approximately 1.5 μm. Reactions between disks and the different dicarboxylic acids were carried out following the same parameters used for the modification of the powder. Conductimetric and Potentiometric Titration. With a view to determining the quantity of hydroxyl groups present on the surface of unmodified alumina conductometric titrations were carried out in suspensions of this material in water. Samples were first titrated with KOH in an automatic titrator coupled with an Acoustic & Electroacoustic Spectrometer DT-1200 from Dispersion Technology. During the whole titration process temperature was monitored not to vary by more than 1 °C. A detailed description of this procedure, together with a thorough discussion, can be found in a work by Campos et al.38
Results FTIR. The formation of chemical bonds between acids and the surface of alumina was investigated by FTIR. Figure 1 shows the FTIR spectra of untreated alumina samples, alumina treated with NaOH solution and refluxed in THF and alumina treated with ethanedioic acid in THF. Most of the samples showed no significant alteration in IR spectra after modification by carboxylic acids or reflux with ethanol or ethyl acetate, but the samples modified with ethanedioic acid at a higher concentration (700 μmol/m2) presented subtle bands between 1670 and 1700 and 1390-1420 cm-1, as shown in Figure 1. Such bands can emerge due to chemical bonds between carboxyl and the surface of alumina (structures represented in the insertion in Figure 1), as pointed out in literature.26 Contact Angle. Measurements of contact angle were made for all samples treated with the concentrations listed in Table 1. For the dicarboxylic acids, the contact angles obtained were 0°, since less than a second after the water drop reached the surface it was completely spread on the surface. As for the samples treated with acids that have just one carboxyl group, a slight tendency to hold the drop together a bit longer (still less than 1 s) could be observed, but then the drop was once again completely spread on the surface. Conductimetric and Potentiometric Titration. Using conductometric titration it was possible to determine that the Ralumina used in this work has a concentration of 4.3 ( 4 μmol/m2 (38) Campos, A. F. C.; Tourinho, F. A.; da Silva, G. J.; Lara, M.; Depeyrot, J. Eur. Phys. J. E 2001, 6, 29–35.
Langmuir 2010, 26(5), 3364–3371
Bertazzo and Rezwan
Figure 1. Infrared of alumina treated with NaOH, treated with NaOH and immersed in THF for 24 h at 70 °C, and alumina modified by ethanedioic acid with concentration of 700 μmol/m2.
Figure 2. ζ-Potential variation as a function of pH for samples of unmodified alumina, alumina treated with NaOH solution, alumina treated with piranha solution then NaOH solution, and alumina heated for 24 h at 70 °C in THF.
hydroxyl groups on its surface. This value agrees well with the surface concentrations reported in the literature.18,39,40 ζ-Potential. ζ-potential values obtained as a function of pH for unmodified alumina (R-alumina), alumina treated with NaOH solution, alumina treated with piranha solution then NaOH solution, and alumina heated for 24 h at 70 °C in THF, are all shown in Figure 2. Results of the ζ-potential measurements for alumina not treated with any of the acids show that the isoelectric points at pH (IEP) of samples do not vary significantly ((0.5) through the different kinds of treatment. On the other hand, treatment with piranha solution significantly altered the ζ-potential of samples at pH values inferior to their IEP. The ζ-potential was obtained for all samples presented in Table 1. In Figure 3 we present examples of ζ-potential as a function of pH for alumina samples modified with ethanedioic, butanedioic and hexanedioic acids at different concentrations. Figure 3 shows that the isoelectric point of alumina varies most when it has been modified by ethanedioic acid. In this particular case the IEP falls from ∼9.5 (the initial value of IEP for untreated alumina) to ∼3, when alumina is treated with a concentration of acid higher than 7 μmol/m2. For the modification by butanedioic (39) Tamura, H.; Tanaka, A.; Mita, K.-y.; Furuichi, R. J. Colloid Interface Sci. 1999, 209, 225–231. (40) Takagi, R.; Nakagaki, M. Sep. Purif. Technol. 2001, 25, 369–377.
Langmuir 2010, 26(5), 3364–3371
Article
acid, the lowest IEP value obtained is 7.2 (treated with a concentration higher than 140 μmol/m2), whereas for the modification by hexanedioic acid it is 8.2 (also treated with a concentration higher than 140 μmol/m2). The results of the ζ-potential tests as a function of pH for alumina samples modified with the other acids present the same behavior as the samples presented in Figure 3, following the same changes concerning the carbon chain size observed. A graph with the ζ-potential as a function of pH for alumina treated with different acids at the same concentration is presented in Supporting Information. The ζ-potential results for samples subject to different pretreatments and tested with different solvents are presented in the Supporting Information. With regard to the pretreatment, results show that the samples treated with NaOH solution present the lowest IEP values, varying between ∼3 (for samples treated only with NaOH solution) and ∼3.5 (for samples treated with piranha and NaOH solution). The sample of alumina not submitted to any pretreatment before the reaction with ethanedioic acid presented an IEP value of ∼5.3. Tests with different solvents showed that the samples that had reacted with ethyl acetate and ethanol present higher values (∼5.4) of IEP than the samples that had reacted with THF. Finally, the results obtained from chemical stability tests showed that after the different washing methods, all samples treated with ethanedioic acid had the same ζ-potential behavior, presenting an IEP value of ∼5.5. For butanedioic and hexanedioic acids, IEP values after stability tests always went up to 8.9 and 9, respectively. The plots obtained for samples treated with dicarboxylic acids at different reaction times (presented in Supporting Information) are similar to the ones shown in Figure 3. Results indicate that an increase in reaction time only varies IEP to a slightly lower value for ethanedioic acid (approximately 0.5 units). For butanedioic and hexanedioic acids there is no variation. To verify how the IEP varies as a function of the acid concentration we chose three different acids, starting with the smaller dicarboxylic acid (ethanedioic), and providing a good range of carbon chain sizes (two, four and six carbons). Figure 4 illustrates how IEP values of samples modified by ethanedioic, butanedioic and hexanedioic acids vary as a function of the original concentration of acid utilized. Figure 4 shows that a significant variation in the IEP of samples only occurs for acid concentrations of lower than ∼25 μmol/m2 while approaching plateau values at higher concentrations. Carbon Chain and IEP. The maximum IEP at pH shift values for alumina samples treated with the distinct acids are correlated with the number of carbons in the chain of each acid in Figure 5. Figure 5 shows that there is a strong correlation between the number of carbon atoms in the chain and the IEP shift of dicarboxylic acid treated samples. IEP values increase linearly from ∼3 (samples treated with ethanedioic acid) to ∼8 (samples treated with butanedioic acid) and remain practically constant after this value. In contrast, all samples treated with carboxylic acids (metanoic, ethanoic, and propanoic) present IEPs with a nearly identical value of ∼10.8. Surface Concentration of Dicarboxylic Acid. Once again, in order to obtain an overview of the behavior of alumina treated with different acids, the quantification of ethanedioic, butanedioic, and hexanedioic acids on the surface was performed. Figure 6 shows the quantity of ethanedioic, butanedioic, and hexanedioic acids present on the surface after reflux as a function of the initial concentration of acids normalized to the surface area. DOI: 10.1021/la903140k
3367
Article
Bertazzo and Rezwan
Figure 3. ζ-potential variation as a function of pH for alumina modified by different concentrations of (a) ethanedioic acid, (b) butanedioic acid and (c) hexanedioic acid. The sample with 0 concentration is the alumina sample treated with NaOH solution and refluxed in THF. An asterisk denotes an alumina sample pretreated with piranha solution before reflux with the acid.
The fitting in Figure 6 was obtained using eq 1. y ¼
abx0:5 1 þ bx0:5
ð1Þ
The equation represents an adsorption isotherm, indicating the mechanism by which the acids adhere to the alumina surface. According to Figure 6, the maximum quantity of acid on the surface is considerably larger for ethanedioic acid than for the other acids. On the other hand, surface concentrations of butanedioic and hexanedioic acids are similar. IEP and Surface Concentration. Figure 7 presents the correlation between the quantity of ethanedioic, butanedioic and hexanedioic acids measured on the surface of alumina and IEP values obtained for the corresponding samples. Figure 7 shows that ethanedioic acid alters most significantly the IEP of alumina surfaces, followed by butanedioic acid and then hexanedioic acid. 3368 DOI: 10.1021/la903140k
Discussion Previous studies of alumina surface modification with long chain alkanoic acids41 showed that the quantities of such acids on the surface are very similar to the amount found in this study for butanedioic and hexanedioic acid. Other works showed that, on planar surfaces of alumina, alkanoic42 and phosphonic acids43 form a self-assembled monolayer (SAM) with good stability regarding temperature, but not completely stable in solution. These results are again comparable to the results obtained here. These comparisons indicate that the modification presented in this work, using small carboxylic acids, has probably the same (41) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 52–66. (42) Lim, M. S.; Feng, K.; Chen, X. Q.; Wu, N. Q.; Raman, A.; Nightingale, J.; Gawalt, E. S.; Korakakis, D.; Hornak, L. A.; Timperman, A. T. Langmuir 2007, 23, 2444–2452. (43) Tsud, N.; Yoshitake, M. Surf. Sci. 2007, 601, 3060–3066.
Langmuir 2010, 26(5), 3364–3371
Bertazzo and Rezwan
Figure 4. IEP at pH values obtained for alumina samples as a function of concentrations of acid utilized. Experimental data represented by points and fitted curves in continuous lines.
Article
Figure 6. Quantity of acid adsorbed on the alumina surface as a function of initial concentration of acid. Experimental data represented by points with respective standard deviation and fitted curves in continuous lines.
Figure 7. IEP at pH as a function of the quantity of acid on the surface, obtained from the fitted curves presented in Figures 4 and 6.
Figure 5. Maximum IEP at pH shift of alumina samples treated with different carboxyl acids, correlated with the size of the carbon chains.
mechanism and behavior as modification of alumina with other acids. FTIR. FTIR results (Figure 1) show that samples treated with ethanedioic acid do not present any interaction with THF but unequivocally presented the bands referring to the covalent bond between the surface of alumina and the carboxyl. In spite of that, it is not possible to affirm that similar bonds do not occur for other acids. The formation of this chemical bond is well-documented by the literature.3,4,26,44-50 The difficulty in observing bands related to the carboxyl/ alumina bond is more likely due to the low surface area of the alumina and the low concentration of adsorbed acids. (44) Kasprzyk-Hordern, B. Adv. Colloid Interfac. 2004, 110, 19–48. (45) Gurian, P. L.; Cheatham, L. K.; Ziller, J. W.; Barron, A. R. J. Chem. Soc. Dalton 1991, 6, 1449–1456. (46) Jiang, L. Q.; Gao, L.; Liu, Y. Q. Colloids Surf., A 2002, 211, 165–172. (47) Evans, H. E.; Weinberg, W. H. J. Chem. Phys. 1979, 71, 1537–1542. (48) Benoit, P.; Hering, J. G.; Stumm, W. Appl. Geochem. 1993, 8, 127–139. (49) Wang, Z.; Ainsworth, C. C.; Friedrich, D. M.; Gassman, P. L.; Joly, A. G. Geochim. Cosmochim. Ac. 2000, 64, 1159–1172. (50) Kummert, R.; Stumm, W. J. Colloid Interface Sci. 1980, 75, 373–385.
Langmuir 2010, 26(5), 3364–3371
ζ-Potential. The results presented in Figure 3 and Supporting Information show how effectively the treatment with dicarboxylic acids alters the ζ-potential of alumina. In terms of effectiveness, we have first ethanedioic acid, which caused the major ζ-potential variation, followed by the other dicarboxylic acids as the number of carbon atoms in the chain increases. Interestingly, by using ethanedioic acid to modify samples it is possible to obtain IEP values ranging between ∼3 and ∼9.5, a variation obtained as we alter the concentration of the acid in the reaction medium. Using the same approach it is possible to obtain IEP values ranging between ∼4.5 and ∼9.5 for alumina treated with propanedioic acid, from ∼7 to 9.5 for butanedioic acid. Therefore, it is possible to use two different acids in order to obtain alumina samples that have the same surface charge. Besides, the ζ-potential for the samples treated with the different dicarboxylic acids shows that it is possible to control surface charge by variation in the size of the carbon chain, allowing the production of similar surface charges by using carboxylic groups with different pKa values. The IEP values obtained for the samples that had been through chemical stability tests (Supporting Information) show that part of the molecules of the acids present on the surface of alumina left the surface, an indication that some molecules of the acid are just physisorbed on the surface. The fact that we always obtain the same modified surface, demonstrated by the similar behavior of samples as to ζ-potential, shows that part of the modification is considerably stable regardless of pH and of the 24 h washing with water. DOI: 10.1021/la903140k
3369
Article
Figure 8. (a) Reaction of dicarboxylic acid with alumina surface. (b) Possible structures formed by dicarboxylic acids on alumina surfaces.
With respect to the different systems used in order to evaluate the most effective procedure for surface modification, results (Supporting Information) show that the most effective procedure is the one using the NaOH solution for pretreatment and THF as solvent. Such effectiveness was expected for the reaction in this system, given the well-known reactions3,4,26,44-50 that occur between the surface of alumina and carboxyl groups. The pretreatment with the NaOH solution already causes solely slight changes in the ζ-potential, as expected, due to the fact that the treatment with this solution cleans the surface and increases the amount of O- groups on the surface. Concerning the solvent use, we have just a small variation on the ζ-potential of the sample refluxed only with THF, because the interaction between THF and the alumina surface is practically nonexistent. Chemical Binding between Carboxylic Acids and Alumina Surfaces. According to the indications provided by FTIR (Figure 1) and chemical stability test results and the literature3,4,26,44-50 we can expect part of the carboxylic acids to react with the alumina surface, following the reaction path shown in Figure 8a. For the reaction paths between the alumina surface and the carboxylic acids possible structure formations are presented in Figure 8b, depending on the acid utilized. A thorough explanation of how the reaction between carboxyl groups and alumina surfaces takes place, as well as the discussion about the formation of the structures present in Figure 8b lies outside the scope of this work, but can be found in several and well documented works in literature.3,4,26,44-50 We could expect that the formation of the structures III and IV should increase the contact angle on the alumina surfaces, but the results showed no variation in the contact angle for any dicarboxylic acid utilized. Even so, the absence of change in the contact angle does not rule out the formation of structures III and IV, since the formation of the structures I and II and the presence of physisorbed acid molecules could maintain the hydrophobic surface. Some preliminary tests with NMR were carried out, but the small amount of acid present on the surface was not sufficient to produce a reliable signal that could be analyzed. The results of ζ-potential measurements with samples submitted to different pretreatments (Figure 3 and Supporting Information) show that the most effective modification of the alumina surface is obtained by using NaOH solutions. These results also indicate that the greatest advantage of using NaOH is that it allows a larger quantity of O- species to form on the surface of alumina and thus making the reaction presented in Figure 8a likely. A second indication that the chemical reaction between carboxylic acids and alumina occurs as shown in Figure 8a is that the 3370 DOI: 10.1021/la903140k
Bertazzo and Rezwan
IEP values obtained from alumina treated with acids in solvents have been higher than the values for alumina treated with THF. This is because the solvents used may also react with hydroxyls present on the alumina surface as it is the case for ethanol and ethyl acetate. The possibility of competition between the carboxyl group from acids and a functional group from the solvent on the surface of alumina seems to be the main factor to be considered in using solvents. This factor is even more important than the polarity of the solvent, as dielectric constants of ethyl acetate (6.08) and THF (8.54) are quite similar and ethanol (25.3) has a considerably higher dielectric constant.36 Finally, the results obtained from chemical tests for stability of the alumina surface modification are also in good agreement with the reactions proposed in Figure 8a. The fact that IEP values obtained after the stability tests are the same for all samples and are not the original value of alumina indicates that part of the molecules of the acids present on the surface is indeed strongly bonded to the surface. On the other hand, the molecules that are just physisorbed on the surface are washed off the surface with water and the HCl solution. Thus, by examining Figure 7 it is possible to say that, for the acids evaluated, the amount of molecules on the surface over ∼5 μmol/m2 is physisorbed, since after washing the samples always present IEP values that correspond to the concentration of ∼5 μmol/m2 molecules on the surface. IEP and Surface Concentration. Using the results of IEP as a function of the initial concentration of acid utilized and the quantification of acid adsorbed on the surface, it was possible to correlate IEP values with the concentration of acid on the surface, as shown in Figure 7. From Figure 7 we find that the concentration of acids on the surface decreases as the carbon chain increases. This relation might be explained by the fact that as the carbon chain increases in size, pKa values of carboxyl groups will increase. This increase in pKa will reduce the charge of the acids while in solution. Thus, acids would display lower adsorption on the surface due to electrostatic interaction between the surface and the acid. Another point that could be raised about the relationship between IEP and acid concentrations on alumina surfaces (Figure 7) is the possibility to correlate the IEP obtained for alumina samples subject to stability tests and the concentration of acids on the surface (∼5.5 for the samples treated with ethanedioic acid and ∼9 for the samples treated with buthanedioic and hexanedioic acids). These IEP values correspond to acid concentrations of ∼3.6 μmol.m-2 for ethanedioic acid and ∼2.8 μmol/m2 for butanedioic and hexanedioic acids on the alumina surface as shown in Figure 7. These values of acid concentration on the surface of alumina are considerably inferior to the values obtained for the concentration of OH groups originally present on the surface of alumina (∼4.3 μmol/m2). IEP and Carbon Chain. Analyzing the graph presented in Figure 5 we can notice, first: by bonding carboxylic acids with hydroxyls on the surface the acids end up eliminating hydroxyl sites responsible for the original IEP of R-alumina. The fact that all samples treated with these carboxylic acids have nearly identical IEP values, regardless of the number of carbon atoms in their chain, corroborates this observation. The second information that can be extracted from Figure 5 is that IEP values have a clear correlation with the size of the carbon chain for samples treated with dicarboxylic acids. The IEP behavior of alumina samples treated with the different acids can be correlated with pKa values of the carboxyl in each of the acids. Figure 9 compares acid dissociation constants of the acids with IEP values obtained from samples and the quantity of carbon atoms in the acids. Langmuir 2010, 26(5), 3364–3371
Bertazzo and Rezwan
Article
The IEP behavior observed for samples treated with dicarboxylic acids that have between two and four carbon atoms in their carbon chain could thus be affected practically only by their pKa. For acids with four or five carbon atoms, the IEP value obtained is probably affected by the formation of the bridge structures (Figure 8b structures III and IV). Finally, for samples treated with acids that have more than five carbon atoms, the probability of formation of the distinct structures is similar since the acids in question have relatively flexible carbon chains.
Figure 9. IEP at pH from alumina modified with different acids, pKa values as a function of the number of carbon atoms in acids.
Figure 9 shows that the pKa of monocarboxylic acids has a variation between methanoic and the other acids. On the other hand, the same behavior does not occur for the IEP of samples treated with the same acids. This is in good agreement with the hypothesis that IEP values obtained for alumina samples treated with carboxylic acids are mainly (if not exclusively) due to the bonding of these acids with the surface of samples. As for dicarboxylic acids, it is also found that the behavior of their pKa (concerning both the first and the second dissociations) does not strictly follow the behavior of IEP determined for samples treated with them. While pKa1 shows a significant variation only between ethanedioic acid (two carbon atoms) and propanedioic acid (three carbon atoms), pKa2 shows a variation as far as butanedioic acid (four carbon atoms) is considered. In contrast, the IEP obtained for modified samples shows a linear variance up to samples treated with pentanedioic acid (five carbon atoms). The presence of acid molecules bound and physisorbed on the samples surface may not be the only explanation for the IEP behavior of samples treated with carboxylic acids. Similar IEP values were expected for samples treated with acids that have similar values of pKa, as seen for butanedioic and hexanedioic acids, for example. The possible explanation for the IEP behavior of samples can be the formation of distinct structures, as presented in Figure 8b, and the possibly preferential formation of one of them. Therefore, we can expect that the larger the number of carbon atoms in a chain, the more structures III and IV of Figure 8b will be formed.
Langmuir 2010, 26(5), 3364–3371
Conclusions The results presented in this work show the possibility of controlling surface charges of R-alumina using carboxylic acids. This control can be carried out by variation of two parameters. The first one is the selection of the quantity of acid remaining on the surface of the material. The second parameter for controlling surface charges consists in choosing the size of the carbon chain of the acid. The most efficient modification in surface charge of the samples is obtained using ethanedioic acid at concentrations higher than 7 μmol/m2 of the R-alumina surface. The results showed that part of the acid present on the surface of samples is strongly bound to it, therefore making it possible to obtain pH resistant samples of R-alumina with IEP of 5.5. The stability of the modification is one the most important characteristics when we think in the possible applications of the modified alumina, such as implants and chromatography column filler. Another advantage of the method proposed here is that it is simple and cheap, since the modification consists in two simple steps carried out at lower temperatures with inexpensive and non toxic reagents. Finally, the results also showed that IEP values obtained for modified samples have a linear correlation with the number of carbon atoms of the dicarboxylic acids, up to acids with five carbon atoms. Acknowledgment. We thank Prof. Celso A. Bertran and Daniela DP Campos for running the NMR tests. Supporting Information Available: Figures showing ζpotential graphics for alumina samples subject to different pretreatments, different solvents, and different reaction times and ζ-potential graphics for samples that had been through chemical stability tests. This material is available free of charge via the Internet at http://pubs.acs.org.
DOI: 10.1021/la903140k
3371
