Langmuir 1996,Il)3483-3490
3483
Aspartic Acid Adsorption onto Ti02 Particles Surface. Experimental Data and Model Calculations Carla E. Giacomelli, Marcel0 J. Avena, and Carlos P. De Pauli* INFIQC, Departamento de Fisicoquimica, Facultad de Ciencias Quimicas, Universidad Nacional de Cbrdoba, C.C. 61, SUC.16, Cbrdoba, Argentina Received February 28, 1995. In Final Form: May 22, 1995@ The adsorption of L-aspartic acid (AA)at the Tio2-KNo3 aqueous solution interface was studied as a function of pH and electrolyteconcentrationusing direct adsorption, OH-desorption,electrophoresis, and XPS measurements. A multisites surface complexation model was used to describe the charging and adsorptive behavior of the particles. Adsorption and electrophoretic data show that the amino acid uptake is characteristic of adsorbing anions and almost ionic strength independent. Although XPS data indicates that AA interacts with T i 0 2 surface sites through the amino groups, neither XPS nor kinetic results give conclusive evidence for the adsorptionmechanism (ligandexchangeor hydrogenbonding). Model calculations together with OH- desorption data, however, allow us to concludethat ligand exchangereactions, by which AA forms inner sphere surface complexes, take place. The model also predicts that only terminal (TiOH) groups react with AA. The results exemplify the usefulness of modeling when a quite complete set of experimental data cannot elucidate the type of reaction taking place between the solute and the surface.
Introduction The study of amino acid adsorption at metal oxide surfaces has received considerable attention in the last years due to its relevance in several fields of chemistry, biology, and medicine. The case of amino acids and other biological molecules interactingwith T i 0 2 surfaces is now of great interest because they play an important role in determining the biocompatibility of Ti and Ti alloys, which are widely used as medical implants and prostheses materials. The investigations have usually been focused on identification of adsorbing species, their structure, and the type of interaction that takes place between the adsorbing molecule and the surface. Direct adsorption measurements, electrophoretic mobilities, acid-base potentiometric titrations, and several spectroscopic techniques have been usually employed in these s t ~ d i e s J - ~ Both electrostatic and nonelectrostatic (usually named "specific)))adsorption processes can take place when a metal oxide is immersed in an aqueous amino acid solution. A great variety of adsorption mechanisms have been proposed. Tentorio and Canova,l analyzing direct adsorption and electrokinetic data, concluded that serine does not adsorb a t the T i 0 2 surface because only electrostatic processes take place (serine and Ti02 have similar isoelectric points (IEP)). The case of lysine and glutamic acid adsorbed a t the same T i 0 2 surface is somewhat different: they adsorb even under conditions of strong electrostatic repulsion. These authors suggested a hydrogen-bonding mechanism between deprotonated amino and carboxylic groups and surface hydroxyl groups as a possible explanation for the observed ads0rption.l This mechanism correlates well with the very low AH value @
Abstract published inAdvance ACSAbstracts, August 1,1995.
(1)Tentorio, A.; Canova, L. Colloids Surf 1989,39, 311. (2)Holmberg, K.;Stark, M-J. Colloids Surf 1990,47,211. (3)Torres, R.; Blesa, M. A.; Matijevic, E. J. Colloid Interface Sci. 1990,134,475. (4)Ballion, D.; Jaffrezic-Renault, N. J. Radioanul. Nucl. Chem. 1986, 1, 133. ( 5 )Kaneko, S.;Mikawa, M.;Yamagiwa, S.-I.Colloids Surf 1990,46, 203. (6)Liedberg, B.; Carlsson, C.; Lundtrom, I. J. Colloid Interface Sci. 1987,120,64. (7) Micera, G.; Strina Erre, L. Colloids Surf 1987,28,147. (8) Uvdal, K.; Bodo, P.; Liedberg, B. J. Colloid Interface Sci. 1992, 149,162. (9) Schmidt, M.; Steinemann, S. G. Fresenius J. Anal. Chem. 1991, 341,412.
(-2.257 kJ mol-l) measured by Okazaki et al. for lysine adsorption on TiOz.'O Several other mechanisms have also been postulated. For example, Kaneko et al.,5 analyzing direct adsorption data, suggested that the adsorption of L-glutamic and L-lysine on silica-titania, silica-magnesia, and silica-alumina gels takes place according to electrostatic, ion exchange, and complexation processes depending on the solution pH. A different outlook arises when spectroscopy is used to study amino acid adsorption. Liedberg et al. interpreted IR reflection-absorption studies of L-histidine and Lphenylalanine adsorbed on oxidized Cu surfaces in terms of ligand exchange reactions; -NH2 and -COO- groups replace surface hydroxyls or oxygen ions forming CuNH2 and Cu-COO- bonds, respectively.6 Schmidt and Steinemann, on the other hand, attributed the modifications in XPS data of several amino acids adsorbed at Ti02 surfaces to a ligand exchange reaction by which only -COO- replace surface hydroxyl^.^ EPR data of several ternary CUI1-amino acids-surface complexes were also interpreted in terms of ligand exchange processes.'J1J2 Surprisingly, neither electrostatic nor hydrogen-bonding reactions were suggested in these studies. The use of theoretical models is also relevant to understanding the adsorptive behavior of a surface. Models were widely applied to fit the adsorption of carboxylic acids (salicylate, citrate, succinate, oxalate, formate, phthalate, benzoate, etc.) on o x i d e ~ , ~ , ' ~but -~' there is a lack in the modeling of the amino (or imino) acid uptake. Moreover, only tentative conclusions about the adsorption mechanism could be obtained in these cases. Matijevic et al.,for example, described the adsorption of iminodiacetic acid (IDA) onto hematite particles using a surface complexation model which allowed the identifica(10)Okasaki, S.;Aoki, T.; Tani, K. Bull. Chem. SOC.Jpn. 1981,54, 1595. (11)Micera, G.; Strinna Erre, L.; Dallocchio,R. Colloids Surf: 1988, 32,237. (12)Micera, G.;Strinna Erre, L.; Dallocchio,R. Colloids Surf: 1988, 32,249. (13)Thomas, F.;Bottero, J. Y.; Cases, J. M. Colloids Surf: 1989,37, 281. (14)Liivgren, L. Geochim. Cosmochim. Acta 1991,55,3639. (15)Kallay, N.;Matijevic, E. Langmuir 1986,1, 195. (16)Zhang, Y.; Kallay, N.; Matijevic, E. Langmuir 1986,1, 201. (17)Regazzoni,A. E.; Blesa, M.A.; Maroto, A. J. G.J. Colloid Interface Sci. 1987,122,315.
0743-7463/95/2411-3483$09.00/00 1995 American Chemical Society
Giacomelli et al.
3484 Langmuir, Vol. 11, No. 9, 1995 tion of the singly charged IDA anion as the unique adsorbed species.ls However, such as formulated, the adsorption reaction can be ascribed to either ligand exchange or hydrogen-bonding reactions, so it was not possible to confirm the actual process. D. Ballion and Jaffrezic-Renault4used a simple ligand exchange mechanism to fit glutamic acid adsorption on AlzO3. However, they could reproduce their data only in a limited pH range. On the other hand, Helfferich presented several equations to describe the amino acids uptake by strong acid cation exchangers, but no experimental data to compare were presented.lg The differences in data interpretation when adsorption and spectroscopic studies are applied are evidence that, despite the great amount of experimental data, full understanding of the amino acid-oxide surface interaction has not been reached yet. On the other hand, the lack of modeling and the fact that only tentative conclusions have been obtained using different adsorption models reveal the need for further studies and model calculations. In this work we present a study of the aspartic acid-Ti02 system in aqueous solutions of varying pH and supporting electrolyte concentration. The MUSIC model described by Hiemstra et a1.20,21is used to fit surface charge, electrokinetic, and adsorption data. As far as we know, this is the first work that tries to prove the ability of the MUSIC model to describe the adsorption of amino acids at the metal oxide-aqueous solution interface. Complementary XPS studies are also included to provide evidence for the amino acid attachment mode.
In aqueous media, these surface groups can react with H+ ions according to the following processes:
~i
0h.u-2) n
+ H+
Ti OH(nJ-1) n
Ti OH("."1). n
9
+ H+* Ti,OH,'"."';
Kn,l
(1)
Kn,,
(2)
Hiemstra et al. have concluded that the logarithms of the proton afiinity constants for the two consecutive equilibria depicted above differ by 13.8units, the protonation of hydroxo groups being more difficult than that of surface oxo groups. This implies that only one protonbinding reaction per surface group can be observed within the normal pH range. According to these authors, adsorption at the triply coordinated TisOO groups does not occur in the normal pH range because the H+ affinity constant ofthis reactionis extremely low (log -7.5). Therefore, the actual charge at the TiOz-aqueous solution interface is due to the protonation ofthe singly and doubly coordinated groups. The H+ adsorption reactions at the TiOz-solution interface can thus be described by
+
TiOHU3- H+
TiOH?'
and
+
Ti,0U3- H+ Ti,0HY3' MUSIC Model of the Ti02-Aqueous Solution Interface The classical treatment of the metal (hydrloxidesurface is often physically unrealistic because it assumes the presence of only one type of reactive surface g r ~ ~ p . ~ ~ - ~ ~ To overcome this limitation, Hiemstra et al. proposed the where { } denotes surface concentration and ~ . J Ois the mean multisites complexation (MUSIC) model which predicts surface potential. the presence of several types of surface groups on the According to the MUSIC model, the surface densities basis of crystallographic considerations.20p21 The existence of singly and doubly coordinated groups (Nsl and Nsz) of these groups in Ti02was demonstrated from low-energy are, respectively, electron diffraction (LEED) measurement^.^^ In the case of Ti02 surfaces, for example, the model predicts the Ns, = {TiOHP'} {TiOHU3-} (5) presence of three kinds of groups (singly,doubly and triply coordinated)which can be represented by Ti30°, TizOV3-, and and nou3-or, in a general form, @
+
Ns, = {Ti,0H1/3+)+ {TiO,OU3-] where n.v - 2 is the formal charge of the groups, n being the number of Ti4+ ions coordinating surface oxide ions and v the formal bond valence which is defined as the cation charge divided by its coordination number in the solid bulk. (18)Torres, R.;Kallay, N.; Matijevic, E. Langmuir 1988,4, 706. (19)Helfferich, F. G.React. Polym. 1990, 12, 133. (20)Hiemstra, T.;Van Riemsdijk, W. H.; Bolt, G. H. J . Colloid Interface Sci. 1989,133, 91. (21)Hiemstra, T.;De Witt, J. C. M.; Van Riesdijk, W. H. J.Colloid Interface Sci. 1989,133, 105. (22)James, R. 0.In Adsorption of Inorganics at Solid-Liquid Interfaces; Anderson, M. A,, Rubin, A. J.,Eds.; Ann Arbor Science: Ann Arbor, MI, 1981;p 219. (23)Parks, G.A.InMineral-WaterInterface Geochemistry;Hochella, M. F., Jr., White, A. F., Eds.; Mineralogical Society of America: Washington D. C., 1991;p 133. (24)Davis, J. A.; Kent, D. B. In "Mineral-Water Interface Geochemistry"; Hochella Jr., M. F.. White. A. F.. Eds.:' Mineraloeical Societv of Amknca: Washington D.'C., 1991;p i77. (25)Hochella Jr., M. F. In "Mineral-Water Interface Geochemistry"; Hochella Jr., M. F., White, A. F., Eds.; Mineralogical Society of America: Washington DC, 1991;p 87. L
a0 is related to the concentration of different surface groups by
oo= el${Ti0Hf3+}
+ i{Ti,0HU3+} 3
-
i{TiOHU33-} 3 - 2{Ti202/3-}) 3 (7) where e and F are the electronic charge and the Faraday constant, respectively. Equations 3-7 represent the MUSIC model for Ti02 as proposed by Hiemstra et al. The model must be combined with an electrostatic one in order to describe charge distribution and potential drop across the solid-liquid interface. It has been shown that the MUSIC model in combination with the Stern model leads to a good description of charge and potential data.26 The set of equations conforming to this electrostatic model is summarized in Table 1. (26)Hiemstra, T.;Van Riemsdijk, W. H. Colloids Surf 1991,59,7.
Aspartic Acid Adsorption onto Ti02
Langmuir, Vol. 11, No. 9, 1995 3485
Table 1. Equations Conforming to the Stern Model of the Electrical Double Layep
1+ K,+[K+Iexp(-ey&T)
+ KNos-[N03-lexp(e~dkT) (8) (9)
VO - v d =
+
Ud = 0 = -0.1174[IcN0~1msinh(evd2kr)
00 f Ust
(10) (11) 'vd is the diffise layer potential and C is the capacity of the region located between the surface and the d planes. Ud
Some model equations need to be appropriatelymodified to consider aspartic acid adsorption. The modifications depend on the reaction mechanism (i.e., electrostatic interaction, ligand exchange, or hydrogen bonding) and will be considered later.
Experimental Section Materials. Titanium dioxide (P25, Degussa) was a commercial product prepared from titanium chloride. This sample was selected because it has been used in many interfacial studies, so its general characteristics do not need to be discussed in detai1.27-35The absence of pores and the monodispersion of the material make it appropriate for studies in which mathematical models are employed. Table 2 gives some characteristics of the solid material as reported by the manufacturer. Several properties measured in this work are also presented for comparison. The presence of acidicimpurities in P25 T i 0 2 has been previously reported.36 This is evident from the low pH value (lower than the IEP) of TiOz-water dispersions (Table 2). However, since these impurities do not affect the normal charging, electrokinetics it was used without and adsorptive behavior of the further purification. L-Aspartic acid (AA)was a commercial product from Merck and was used as received. All other chemicals (mo3,KOH, and HN03) were of analytical quality, and water was purified with the Millipore Milli-Q system. Methods. All measurements were performed at 25 "C. Scanning electron microscopy (SEM)was performed using a Joel JSM-35C apparatus. Oxide samples were highly dispersed in water and were sonicated and deposited on aluminium supports. After drying in air, the samples were gold sputtered. Surface area was measured by Nz adsorption with an area meter Stroehlein instrument, taking 16.2 x m2/molecule for the Nz cross-sectional area. Surface charge density (~00) was measured potentiometrically in KNo3 solutions in the 3.5-10 pH range as described el~ewhere.3~ The reversibility of uo-pH curves was checked by titrating with both KOH and HN03 solutions. Electrophoretic mobility measurements were carried out using a Rank Bros. Mark I1 electrophoresis apparatus equipped with a cylindrical cell of 2 mm internal diameter either in the absence (0.03 M KNo3)or in the presence of AA (0.03 M KNo3 0.001 M AA). A 500 mL dispersion was equilibrated at pH 10 for 30 min, and a measurement was performed on an aliquot. HN03
+
(27)Bohem, H. P.Discuss. Faraday Soc. 1971,52,264. (28)Chibowski, E.;Gopalakrishnan, S.; Busch, M. A.; Busch, K. W. J. Colloid Interface Sci. 1990,139,43. (29)Akratopulu, K. Ch.; Kordulis, Ch.; Lycourghiotis,A. J. Chem. Soc., Faraday Trans. 1990,86,3437. (30)Herrmann, J. M.; Mansot, J. L. J. Catal. 1990,121,340. (31)Kim, D. S.;Kurusu, Y.;Wachs,I. E.; Hardcastle, F. D.; Segawa, K.J. Catal. 1989,120,325. (32)Cristiani, C.; Forzatti, P.; Busca, G. J. Catal. 1989,116,586. (33)Girod, G.; Lamarche, J. M.; Foissy, A. J. Colloid Interface Sci. 1988,121,265. (34)Moser, J.;Punchihewa,S.; Infelta, P. P.; Grlitzel, M. Langmuir 1991,7,3012. (35)Avena, M. J.;Cbmara, 0. R.; De Pauli, C. P. Colloid Surt 1993, 69,217. (36)Zalac, S.;Kallay, N. J. Colloid Interface Sci. 1992,149,233. (37)Avena, M. J. Ph.D. Thesis, Universidad Nacional de Chdoba, Argentina, 1993.
solution was then added to vary the suspension pH, and a new measurement on a new aliquot was conducted. This procedure was repeated until the pH was around 3. In order to check reversibility, KOH solution was then added to increase the suspension pH and measurements were again performed. The measured mobilities were taken as the average of 20 pairs of readings carried out by alternating the polarity ofthe electrodes. The data standard deviation (s.d.1 was evaluated from five independent experiments carried out with different dispersions. Adsorption measurements were made in duplicate in the 3.59.0 pH range in 0.004 and 0.1 M KNo3 solutions. A known amount of solid (approximately lg) was dispersed in 40 mL of a KNo3 solution. The pH was adjusted to the desired value (initial pH) by either HN03 or KOH addition, and then 40 mL of 1.7 x 10-3 M AA solution with the same pH and electrolyte concentration was added. ApH increase was observed after this mixing. It was decreased to the initial value using a validated HN03 solution. After 20 min of equilibration, the suspension was centrifuged and the AA concentration was measured. The amount of adsorbed AA was calculated from the difference between the initial and final AA concentrations (depletion method). During adsorption measurements, the number of OH ions released per adsorbed AA molecule was evaluated from the 6 l u m e of validated HN03 solution needed to shift the pH to its initial value. The same procedure was used to perform adsorption isotherms a t pH 4.0 and 4.8 in order to know the maximum surface coverage. AA concentrations were measured using the spectrophotometric ninhydrin method. It has long been known that the amount of colored product formed in the ninhydrin reaction is dependent on the pH, temperature and other conditions at which the reaction is run.38Consequently,several (fourto five)standard AA solutions prepared a t the same pH and ionic strength were run together with the samples. These solutions were also used to evaluate the sed.values. X-ray photoelectron spectra W S ) were taken in a Surface Science Instruments M-Probe ESCA with a base pressure of 1.0 x torr. A monochromatic AlKa radiation source was used. The measurements were carried out using a spot size of 200 x 750pm2(passenergy, 25 ev) with a resolution of 0.74 eV. Samples were prepared by mixing calculated amounts of T i 0 2 and AA solutions a t pH 4. ARer equilibration and drying they were pressed onto an insulating biadhesive tape. The (Is) level of carbon (atmospheric hydrocarbons) was taken as the internal standard with peak position at 284.6 eV. Numerical elaborations ofC1, and N1,spectra were performed by the instrument software. Since oxygen is present in both T i 0 2 and AA, the analyses of 0 1s peaks did not provide useful information and were not considered.
Results All the titrations and electrophoresis curves were reversible in the studied pH range. The oo-pH and C-pH data for Ti02 in KN03solutions are given in Figure 1.The symbols show the experimental points, whereas the lines represent the theoretical curves calculatedwith the model and are not of immediate concern. The general shape of the ao-pH curves is typical for ( h y d r ) ~ x i d e s . ~The ~ - ~point ~ of zero charge (PZC)of the sample is 6.0 f0.1 and coincides with the IEP. These results are in good agreement with those previously reported in the literature for P25 Ti02.29131133-37 The symmetry in the ao-pH and C-pH curves around the PZC and IEP and the coincidence between these two values suggest that K+and NO3- have similar chemical affinities for the surface. Figure 2 shows the electrophoretic data obtained for T i 0 2 particles immersed in a KNO3 + AA solution. Those obtained in the absence of AA are replotted here for comparison. AA causes a decrease in the electrophoretic mobilities at pH values lower than 9 and shifts the IEP down to pH 5.2. Similar behavior is usually found with anions that specifically adsorb at the i n t e r f a ~ e . ~ ' , ~ ~ , ~ ~ (38)Lamothe, P.J.; McCormick, P. G. Anal. Chem. 1972,44,821.
3486 Langmuir,
Vol.11, No. 9, 1995
Giacomelli et al. Table 2. Some TiOa Properties valud Property 50 f 15 (47) density, g cm-3 30 (30) chemical composition, %
property BET surface area, m2 g-l average primary particle size, nm pH value (in 4%suspension) crystalline form isoelectric point
3-4 (4.4)b
Si02 Fez03
6.6(6.0)
A1203 a
3.8
'99.5 < 0.2 < 0.01 < 0.3
Ti02
mainly anatase
value*
Values in parentheses are those measured in this work. 0.5% in 0.002M KNo3 solution.
f
' be
0.8 0.6 0.4 0.2 nn
3 2
3
4
5
6
7
8
9 1 0 1 1
PH
PH
Figure 1. pH dependence of the surface charge (udand zeta potential (t)of Ti02 particles immersed in KNO3 solutions: (squares)0.1M; (downtriangles) 0.02M; (uptriangles) 0.03 M; (circles) 0.002 M. Open and solid symbols correspond to titrations performed with KOH and HNo3 solutions, respectively. Lines are model predictions with parameters listed in the first row of Table 4. Results obtained with the other set of parameters are not shown here because it leads to almost the same data fitting (similar 6 values). The error asociated with a0 data corresponds to the size of the symbols. Error bars in electrophoretic data represent the f 2 s.d. range.
-.-
2
3
4
5
7
6
8
9
IO
I1
PH
Figure 2. C-pH curves for T i 0 2 particles immersed in and (circles)0.03 M KNO3 + 0.001 M (triangles)0.03 M AA. Open and solid symbols correspond to measurements performed from alkaline to acid media and from acid to alkaline
media, respectively. Lines are model predictions with parameters listed in the first row of Table 4 together with pKU,le = -3.84 or pKu,18 = 0.52 (Table 5). Error bars are as in Figure 1.
Time-dependent adsorption experiments showed that fast equilibration was achieved; no variation in the uptake was found by equilibrating AA solutions and T i 0 2 dispersion for 10 min or 1 day. Due to the method employed in AA determination, it was not possible to measure AA
Figure 3. AA adsorption data as a function of pH and KNo3 concentration: (opensymbols) 0.1 M and (solid symbols) 0.004 M. The initial AA concentration was 1.7 x M. Lines are model predictions with parameters listed in the first row of Table 4 together with (A) pKu,14= 4.98 and (B)pKW16 = -3.84 or pKfi,18 = 0.52. Error bars represent the f 2 s.d. range.
concentration before 10 min because a finite time (5-10 min) was required to ensure proper mixing and to separate solid and solution phases for analysis. Thus, it might be possible that equilibration was reached in shorter times. Adsorption data as a function of pH and ionic strength are shown in Figure 3. Lines also represent model predictions. The relative error associated with AA quantification was approximately 5% (f2 s.d.). Since AA adsorption (AAads) was measured using the depletion method, the absolute error becomes greater as decreases. For example, M a d , is 1.14 f 0.02 pmol/m2 at pH3.3 and 0.05 f0.10pmol/m2at pH 8.2. Figure 3 shows that the uptake is almost insensitive toward KN03 concentration but exhibits a strong pH dependence which is also characteristic of adsorbing a n i ~ n s .Reversibility ~~,~ of the adsorption process was confirmed by taking TiOzAA systems previously equilibrated at pH 4-5 and alkalinizing to pH 9; the adsorption data shified accordingly. Figure 4 shows the adsorption isotherms. Data are plotted in terms of the linearized form of the Langmuir equation. The maximum amount of adsorbed AA (AA,,), as estimated from the slopes, was 0.96 molecule nm-2 in both cases. Figure 5 shows the amount of OH- released as a function of a d s at two pH values. The linearity of the curves indicates that the amount of OH- released per adsorbed AA molecule (rod is constant at a fmed pH and KN03 concentration. Even though r0H does not depend on the AA uptake, its value appreciably changes when either the pH or the electrolyte concentration is modified (Figure 6); ?-OHhas a maximum at pH x 5 and decreases as the ionic strength increases. The core level binding energies of GI,and N1, for pure
Aspartic Acid Adsorption onto Ti02
Langmuir, Vol. 11, No.9, 1995 3487
1000 0.7
-
0.8
-
0.5
-
Lo 0.4
-
0.3
-
0.2
-
800
5.
... ....
,
N -
-
E
e
600 I
E.
i2
400 200
0.1 I
0
0.2
(
0.4
0.6
0.8
1.0
3
1
1.2
I
I
I
1
4
5
8
7
PH
[MI ")
Figure 4. Adsorption isotherms for two different pH values: (open circles) pH = 4.0; (solid circles) pH = 4.8. Data are represented in terms of the linearized Langmuir equation. The error bars were calculated from the errors asociated with the AA determination (Figure 3). Errors in the [AA]/&ds axis correspond to the size of the symbols.
o'8
Figure 6. roH-pH dependence in (solidsymbols)0.004 M KNo3 and (open symbols) 0.1 M KNo3. Error bars were calculated from the errors asociated withthe AA determination (Figure 3).
t
F 0.6 -
-E
E '
i
v
3 0.4 ! . r
0 0.2
1
0.0
0.5
1.o
2
3
4
5
I 1.5
(*mol m-*)
Figure 5. Influence of AA adsorption on the release of OHat two different pHvalues: (opencircles)pH = 4.0; (solidcircles)
pH = 4.8. Error bars as in Figure 3.
AA and that adsorbed on Ti02 are summarized in Table 3. Pure AA shows three CI, peaks with different binding energies. The peak at 288.6eV corresponds to carboxylic carbons and is denoted as CI. The peak at 286.6eV is due to the carbon of the -CH2- group (CII) and that at 285.6 eV corresponds to carbon bonded to the amino group (CIII). The CISbinding energies are in good agreement with those found in the literature for other amino The nitrogen binding energy, on the other hand, is found to be 401.2 eV which is characteristic of the -NH3+ state of the amino Carbon and nitrogen core level binding energies shiRed when AA was adsorbed. The unique modification found in the XPS spectra of adsorbed AA was in the peak positions; neither shoulders nor additional peaks were detected. The shiRing in CIS binding energies was (39)Clark, D. T.; Peeling, J.;Colling, L. Biochim. Biophys. Acta 1976, 453, 533. (40)Jung, G.;Ottnad, M.; Bohnenkamp, W.; Bremser, W.; Weser, U. Biochim. Biophys. Acta 1973,295,77.
6
7
8
Q 10
PH
Figure 7. Distribution of AA species as a function of pH. Data
were calculated using the following pK,, values: pKal = 1.93, pKa2 = 3.70, and pK.3 = 9.63. Table 3. Carbon and Nitrogen Core Level Binding Energies of Pure and Adsorbed AA sample CI1s CII 1s CIII1s N 1s 285.6 401.2 288.6 286.6 pure AA &ds &ds &dB
= 1.5pmol m-2 = 1.2pmol m-2 = 0.4pmol m+
288.3 288.3 288.5
286.6 286.6 286.6
285.9 285.8 285.8
399.7 399.5 399.4
somewhat small (0.0-0.3 eV) whereas that of NI, was more significant (1.5-1.8 eV). According to Clark et al., values near 399.4eV can be attributed to the -NHz state of the amino Discussion Four AA sgecies can be present in aqueous solution depending on the pH. The relative contributions of these species to the total AA concentration are depicted in Figure 7. All of them are potentially adsorbable, however, since adsorption and electrokinetic data are consistent with anion adsorption; LH- andor L2- must be, in principle, the adsorbing ones. Data in Figure 3 clearly prove that the AA adsorption mechanism is different from that of supporting electrolyte
Giacomelli et al.
3488 Langmuir, Vol. 11, No. 9, 1995 ions because variations in KN03 concentration did not modify the A A ~ p t a k e . ~Thus, ~ , ~ other ~ , ~ processes ~ , ~ ~ such or hydrogen b o n d i n p must be as ligand invoked to account for experimental data. Even though ligand exchange and hydrogen bonding reactions are representative of two very different states of the adsorbed molecule at the surface, it is very difficult to discriminate between them. Fast adsorption-desorption kinetics obtained in this work do not provide additional information because both processes must take place rapidly on TiOz surface^.^-^^ Similar conclusions can be obtained by analyzing XPS data (Table 3): a shifiing of around 1.8eV in the N1, binding energy upon adsorption can be attributed to either amino groups directly bonded to surface TiN 8,40,48 (ligand exchange) or to a certain grade of hydrogen bonding between hydroxylated surface groups and AA molecules.39 Although XPS data cannot ensure the type of adsorption reaction, the values of N1, and C1, binding energies before and after adsorption offer useful information about the AA attachment mode. The relatively high variations in the N1, binding energies indicate that -NH3+ groups deprotonate upon adsorption to interact with the surConversely, the insignificant variations in C1, binding energies suggest that -COO- groups remain unprotonated upon adsorption39and weak (if any) interaction among these groups and the Ti02 surface takes place; the fact that carboxylate groups shift their core level binding energy by 0.6-1.7 eV when directly coordinated to metal centers40 is an indication that, in our case, no ligand exchange takes place among -COO- and surface groups.
Model Calculations Ti02-KNOS System. Before considering AA adsorption, several model calculations were performed to evaluate the parameters describing the acid-base and electrical behavior of the Ti02-KN03 solution interface. Although seven parameters (Nsl, Ns2, K1,2, K2,1, KK+, KNO~., and C)are necessary to solve eqs 3-11, most of them can be independently estimated and do not need to be optimized. In fact, log K,,,,, KK+, KNO~., Nsl, and Ns2 values were estimated as follows. (i) Values log K1,2 and log K2,1 were assumed to be 6.5 and 5.5,respectively, in order to satisfy the relation.37
2PZC = log Kl,2+ log
(12)
The selected values are the same, within the expected error (0.5 units), as those theoretically evaluated by Hiemstra for TiO2. (ii) The equality KK+= KNO~. was assumed because of the lack of indication of any asymmetric titration behavior (Figure 1). (iii)Since some doubt exists about the actual Nsl = Nsz value ofthe T i 0 2 surface (Hiemstra et al. indicated a 4.44sites nm-2 value which was deduced from crystallographic considerations-rutile surface-whereas Bohem et al., using different experimental methods, found a value of ca. 2.40sites nm-2 for (41) Davis, J. A.; Leckie, J. 0.J. Colloid Interface Sci. 1980, 74,32. (42) Haves, K. F.: Papelis, C.; Leckie, J. 0.J. Colloid Interface Sci. 1988, 125,- 717. (43) Sposito, G. In Mineral- Water Interface Geochemistry; Hochella, M. F.. Jr.. White, A. F.. Eds.: Mineraloeical - Society of America: Washington DC, 1991; p 261. (44) Morrison, W. H., Jr. J. Colloid Interface Sci. 1984, 100, 121. (45) Thompson, G. A. K.; Taylor, R. S.; Sykes, A. G. Inorg. Chem. 1977,16,2880. (46) Comba, P.; Merbach, A. Inorg. Chem. 1987,26, 1315. (47) Hachiya, K.; Sasaki, M.; Karasuda, M.; Yasunaga, T. J. Phys. Chem. 1980,84, 2292. (48) Srinivasan,V.;Stiefel, E. I.;E l s b e q , A,;Walton, R. A. J.Chem. SOC.,Dalton Trans. 1973, 2, 200. '
Table 4. Parameters Describing the Ti02-KNOs Aqueous Solution Interface Nsl = Ns2 ~ K K=+ (sites/nm2) log K1,2 log K2,l CQ(F/m2) ~ K N o ~5 (C2/m4) -~ 4.44 6.5 5.5 0.75 0.39 0.00093 2.40 6.5 5.5 0.86 0.16 0.00095 Optimized parameters.
both types of surface groups in a Ti02 P25 sample27),both values (4.44 and 2.40 sites nm-2) were used in the calculations to avoid any misinterpretation due to an erroneous selection of the surface sites density. Numerical optimization to fit experimental 00 and 5 data (Figure 1) with the model was performed. The 6 function, defined as
(13) where X,, and Xcalcdenote experimental and calculated magnitudes, respectively, and m is the number of data points, was minimized by adjusting only C and ~ K K=+ p K ~ 0values ~using the Simplex method;49log KI,Z= 6.5, log 5.5,and Nsl = Nsz = 4.44(or 2.40)sites nm-2 were not varied during optimization. Since two different Nsl = Ns2 values were used in independent calculations, two sets of optimized parameters describing the Ti02-KN03 aqueous solution interface were obtained and are listed in Table 4. Figure 1,on the other hand, compares the calculated uo-pH and C-pH curves with experimental results. Both sets of optimized parameters lead to a good data fitting (see Figure 1 and 6 values in Table 4h50 Ti02-AA System. As was already stated, the AA adsorption mechanism must be different from that of the supporting electrolyte ions; in fact, model calculations assuming the adsorption of LH- and/or L2- at the Stern plane confirmed this assumption, predicting a remarkable ionic strength dependence of the AA uptake. No conclusive experimental evidence was obtained, however, for either ligand exchange or hydrogen-bonding reactions. Thus, both processes need to be considered in the calculations. The considered equilibra are summarized in Table 5. Equations 14 and 15 are ligand exchange reactions. According to them, LH- does not deprotonate upon adsorption; therefore, one of the carboxylate groups of the AA molecule should be coordinated to titanium centers because they are the unique electron donor groups in LH-. Although the suggested structures are not compatible with XPS data which reveal that -NH2 is the main interacting group, they will be considered for comparison. Hydrogen-bonding processes are represented by eqs 16 and 17. TiOH* *LHU3-and Ti2O- * *LHW3surface groups can be thought as TiOH1I3- and Ti2OW3-sites hydrogen bonded to the AA molecule through the -NH3 group. The existence of TiOHzW3+and TizOHU3+indicates that the corresponding deprotonated species (TiOHm- and TizOm-) are Bronsted bases capable of reacting with Hf and supports the postulated structure of the surface species. Equations 18 and 19 are also ligand exchange reactions. In these cases, L2- is the reacting species. The surface group formed can be considered as a L2- ion ligated to titanium centers through the -NH2 group which is now (49) Deming, S. N.; Morgan, S. L. Anal. Chem. 1973,45, 278A. (50) Note that Figure 1 only shows theoretical curves obtained with the set of parameters listed in the first row of Table 4. Those obtained with the other set are not shown because they lead to almost the same fitting results (similar6 values). This similarityin data fitting was also observed in all the other calculations.
Aspartic Acid Adsorption onto Ti02
Langmuir, Vol. 11,No. 9, 1995 3489
Table 6. AA Adsorption Reactions Considered in Calculations eq no. 14 15 16 17 18 19 20 21 22
reaction
TiOHy3-
d(~m012/m4)
PKAA,N
+ LH- = TiLHU3-+ OH+ LH- = TizLH*+ + OH+ LH- = TiOH..-LHmTi202/9- + LH- = Ti20 - .LHV3TiOH*- + L2- = TiL*- + OHTizOH*+ + L2- = TizLvs- + OH2 TiOHU3-+ LH- = Ti2Lm- + OH- + H20 TiOHU3-+ TizOH*+ + LH- + H+ = TI& + 2H20
TiOH*Ti20H*+ TiOH*-
4.9aa (4.62Ib 5.15 (4.93) -3.84 (-4.46) -1.99 (-2.98) 0.52 (-0.094) 1.98 (1.66) 4.91 (4.49) -14.1 (-14.9) -9.18 (10.02)
+ Ti20Hm+ + LH- = Ti3(0H)2. * *L-
1.24a (1.06)* 1.61 (1.75) 0.47 (0.44) 1.95 (0.87) 0.47 (0.44) 2.17 (2.10) 1.15 (0.91) 1.45 (1.13) 0.71 (0.66)
a Values obtained using parameters listed in the first row of Table 4. Values in parentheses were obtained using parameters listed in the second row of Table 4.
an electron donor group. Although L2- is not the predominant species in the studied pH range (Figure 71, it does not represent a problem in modeling and data interpretation because reaction 18 is equivalent to, for example, the following reaction:
where LH- deprotonates to give the TiLm- surface species. Besides eqs 14-19, several other equilibra involving two or more surface groups can be postulated. Some representative equilibriainvolvingdouble ligand exchange or hydrogen-bonding processes are also listed in Table 5. Other reactions were not specifically considered because most of them are equivalent to some of those presented in Table 5. Equations 5-7 need to be modified to take into account AA adsorption when calculations are performed. A significant problem in surface complexation models is the definition of the number of surface sites covered by the adsorbate. For example, depending on the surface sites density value selected for calculations,the ratio Nsl/AA,, can be 4.61 or 2.50; this implies that each AA molecule might respectively occupy 4.61 or 2.50 surface sites when adsorbed. These noninteger values suggest steric problems rather than multiple sites reactivity4l and therefore, the approaches used by Davis and Leckie41were used to solve this computational difficulty. This implies that, for example, eqs 5 and 7 must be rewritten as follows
Ns, = {TiOHP'}
+ {TiOHU3-}+ G Nsl {TiLHU3-} zx
(5')
+
oo = e F ( ~ { T i O H ~ + }1{Ti,0H1/3' 3 }
-
1{TiOHU3-} - 2{Ti,0y33-} - 3(TiLHv3-}) 1 (7') 3 3 in order to appropriately consider LH- adsorption onto Ti02 according to eq 14. Optimization of KAA values was performed to fit Figure 3 data. Each equilibrium reaction (14-22) was separately considered to find the one that best describes the adsorptive behavior of Ti02 surface. The parameters of the TiOz-aqueous solution interface (both sets of parameters in Table 4) were not varied in these calculations. The respective optimized pKM and 6 values are given in Table 5. In Figure 3, on the other hand, predictions of eqs 14, 16, and 18 are plotted to show the relation between different 6 values and data fitting. The results shown in Table 5 clearly demonstrate that only eqs 16 and 18 produce relatively low 6 values and, consequently, the best fitting of adsorption data. Moael predictions using these equilibria also agree with C-pH data (Figure 21,
and in addition, the proposed interaction between the amino group and surface is supported by XPS results. The concordance between 00-pH,
