Langmuir 1999, 15, 6961-6968
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A Spectroscopic Study of Phthalate Adsorption on γ-Aluminum Oxide O. Klug* Department of Inorganic Chemistry, Luleå University of Technology, S-97187 Luleå, Sweden, Evox RIFA AB, Box 98, S-56332 Gra¨ nna, Sweden
W. Forsling Department of Inorganic Chemistry, Luleå University of Technology, S-97187 Luleå, Sweden Received February 2, 1999. In Final Form: June 1, 1999 Surface complexation of phthalic acid/phthalate has been investigated on synthetically produced, nonaged γ-aluminum oxide by infrared and Raman spectroscopy. Effects of time, pH, and ionic strength have been studied both on the total adsorbed amount of phthalate and on the surface complexes. The spectroscopic results indicated the formation of two different types of complexes: outer sphere and inner sphere. The relative concentrations of these complexes were shown to vary considerably with pH but very little with increasing ionic strength, which equally reduced the amount of both types of complexes. Considering the electrostatic interaction between the surface and adsorbate, a complexation model was proposed that is in accordance with the spectroscopic results. Adsorption on the synthetic γ-alumina was compared to the adsorption on γ-aluminum oxide films produced by anodic oxidation, where the adsorbed amount was greatly reduced; however, both inner- and outer-sphere types of complexation have been observed.
Introduction Anodically oxidized aluminum foil is commonly used by electrolytic capacitor manufacturers as it constitutes the anodic part of a capacitor winding. To improve the component, extensive research is being performed to characterize the oxide-electrolyte interface. As a step in an ongoing research project on capacitor chemistry, UV, infrared and Raman spectroscopic techniques were applied to investigate electrolyte ingredients adsorbed or complexed at the surface of capacitor foils. In the present work adsorption of o-phthalic acid/phthalate, a compound generally used in capacitor electrolytes, was studied by infrared and Raman spectroscopy. The anodic foil of a capacitor is an etched, anodically oxidized aluminum foil with γ-aluminum oxide on its surface. The rough surface and the thin oxide layer on the metallic aluminum phase make it difficult to apply some of the conventional spectroscopic techniques such as infrared and Raman. There are three principal ways to overcome the difficulties: 1. The application of surface-enhanced Raman spectroscopy (SERS), where the surface roughness is an advantage and the enhancement offers lower detection limits at the same time. However, SERS requires the deposition of a silver sol after the adsorption.1,2 Recent investigations showed that the spectra obtained under SER conditions originate from a silver phthalate complex rather than from the primarily adsorbed phthalate.3,4 2. The chemical dissolution of the metallic aluminum part of the oxidized aluminum foil before the adsorption takes place. In this process a bromine-containing methanol (1) Haigh, J. A.; Hendra, P. J.; Forsling, W. Spectrochim. Acta, Part A 1994, 50A, 2027. (2) Klug, O.; Sza´raz, I.; Forsling, W.; Ranheimer, M. Mikrochim. Acta 1997, 14, 649. (3) Klug, O.; Parlagh, Gy.; Forsling, W. AIP Conf. Proc. 430 (Fourier Transform Spectroscopy); American Institute of Physics: New York, 1998; pp 614-617. (4) Klug, O.; Forsling, W. Manuscript submitted to Mikrochim. Acta.
solution has been utilized to dissolve the metal.5 The residual oxide could be used as substrate. The disadvantages of this method were its long preparation time and production of a lot of waste material. 3. Using a model compound, like a nonaged, synthetically (not anodically) produced γ-alumina which is a convenient substrate for the adsorption experiments due to its higher surface area. The aim of the present work is to investigate the adsorption of phthalic acid/phthalate on synthetically produced, nonaged γ-aluminum oxide powder as a model substrate and to compare the adsorption to that on anodic γ-aluminum oxide, prepared as described under (2). It has to be emphasized that in order to avoid modifying the surface properties of either substrate, they were not brought into any hydrothermal equilibrium prior to the adsorption studies. Experimental Section Materials. The synthetic γ-aluminum oxide (>99.995%) used was produced by Sumitomo Chemical Company Ltd., Japan, and distributed by Mandoval Ltd., England. Its specific surface area, BET (N2, 5 points), was 152 m2 g-1. The oxidized aluminum foil was provided by Evox RIFA AB and was made by Becromal S.P.A. with a forming voltage of 690 V. The aluminum-free anodic γ-aluminum oxide films were prepared by treating the anodized foil with a 10% (v/v) brominecontaining methanol solution at the boiling point of the solvent.4 The dissolution took a few days, and the oxide layers on both sides of the foil were spontaneously detached. These were washed thoroughly and then dried at 60 °C. The BET (N2, 5 points) surface area of this sample was 22 m2 g-1. To maintain the same area/ volume ratio in the adsorption tests, seven times more anodic oxide was used for the same volume of solution as for synthetic alumina powder. The pure phthalic acid (>99%), potassium hydrogen phthalate (>99.9%), and potassium phthalate (>98%) were bought from Merck. Milli-Q, Millipore deionized water, was used for all solutions. (5) Klug, O.; Parlagh, Gy.; Forsling, W. J. Mol. Struct. 1997, 410411, 183.
10.1021/la990105j CCC: $15.00 © 1999 American Chemical Society Published on Web 08/14/1999
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Table 1. pH of the Bulk Solution during the Adsorption of Phthalic Acida pH OPA
KHOPA
KOPA
time (h)
0 M KCl
0.1 M KCl
1 M KCl
0 M KCl
0.1 M KCl
1 M KCl
0 M KCl
0.1 M KCl
1 M KCl
0.0 21 116 190 reading stability
3.19 4.39 4.41 4.52 (0.01
3.28 4.56 4.58 4.66 (0.01
3.23 4.63 4.64 4.73 (0.01
4.06 5.44 5.56 5.66 (0.01
4.18 5.56 5.72 5.80 (0.01
4.05 5.42 5.51 5.57 (0.01
6.39 7.13 7.02 7.07 (0.01
6.56 7.44 7.31 7.26 (0.01
6.33 7.24 7.19 7.14 (0.01
a OPA, phthalic acid; KHOPA, potassium hydrogen phthalate; KOPA, potassium phthalate. γ-Aluminum oxide powder (2 g/L) was suspended in 1 mM solution and kept stirred at room temperature. The reading stability indicates the maximum deviation during a 5 min measuring period.
Adsorption Studies. The batch samples were prepared by suspending 2 g of aluminum oxide in 1 L of aqueous solutions of phthalic acid/phthalate. The phthalic compound concentration was always 1 mM. To observe the effect of the medium, the ionic strength was set to 0.1 and 1 M using potassium chloride (Merck). After different equilibration times the samples were centrifuged (30 min at 20 000 rpm at 10 °C) in order to separate the solid phase from the liquid. The solid part was dried at 60 °C for about 10-12 h. Before drying, the samples may have contained some dissolved phthalate in the residual solution part attached to the solid. However, by measurement of the weight of the samples before and after separating the liquid and solid parts the calculated maximum contamination of dissolved phthalate in the solid part was so small that it was expected to have a negligible influence on the infrared and Raman analyses. To verify this in the case of 1 mM solutions, γ-alumina powder was mixed with the calculated maximum amount of phthalate which could come from the residual solution part, but neither infrared nor Raman spectroscopy demonstrated any phthalate in the matrix. Another question was the thermal stability of the surface complex. As alternative methods the aluminum oxide samples were freeze- or oven-dried and the Raman spectra were compared with wet samples. They all showed similar spectral features revealing the stability of the surface complex. In addition, comparing the IR spectra obtained after various drying times between 4 and 24 h at 60 °C showed virtually no difference either. Instrumentation. The UV measurements were carried out on a Perkin-Elmer Lambda 2 UV/VIS spectrometer. The total concentration of phthalic acid/phthalate residual in the supernatant was determined spectrophotometrically in the acidified solutions. The sample absorbances were measured at 280 nm in dual beam mode using deionized water as reference. The infrared spectra were recorded by using a Perkin-Elmer 2000 Fourier transform infrared spectrometer. The γ-alumina powder samples were analyzed by a diffuse-reflectance technique using the Perkin-Elmer DRIFT accessory and a sample dilution of 2% in desiccated KBr. A 5-8% sample dilution was used in the case of anodic aluminum oxide. In all cases 200 scans with 4 cm-1 resolution were accumulated. In all DRIFT spectra, intensities were recalculated using the Kubelka-Munk function.6 Raman spectra were obtained with a Perkin-Elmer 1760X FTIR spectrometer equipped with a NIR-Raman bench, which utilizes a Nd:YAG laser operating at 1064 nm. A 180° backscattering geometry and an indium gallium arsenide detector were used. The laser power was kept constant at 800 mW, and at least 400 scans (normally 800) with a resolution of 4 cm-1 were accumulated for the final spectrum. The chloride ion exchange on the alumina surface was verified by turbidimetric measurements. Silver nitrate solution was added to the sample, and then the turbidity (from light scattering) was measured using a HACH 18900 turbidimeter calibrated with known chloride-containing standards.
Results and Discussion The γ-alumina is considered as a transition oxide in the transformation route of gibbsite to R-aluminum oxide (6) Kortu¨m, G. Reflectance Spectroscopy; Springer-Verlag: BerlinHeidelberg, 1969; pp 106-111.
during dehydration.7 It is not stable in the presence of water, and its hydroxylation has been extensively studied8-10 leading to the conclusion that there is a slow phase transformation at the surface resulting in formation of a bayerite layer. In the initial period of the phase transformation the extent of the hydroxylated surface is better to follow by pH measurements, because the usual spectroscopic techniques are insensitive on the monolayer level. Generally a fresh alumina-water suspension changes its pH slowly with time resulting in irreversible titration curves. The pH is also affected by adsorption, and therefore it was measured for every sample after separating the liquid part from the solid. Table 1 summarizes the pH values obtained after various adsorption times. The suspensions prepared from γ-alumina and the aqueous solution of phthalic acid or its monopotassium salt showed a monotonic pH increase during the whole adsorption period. The samples prepared from potassium phthalate reached the highest pH after 21 h. This latter discrepancy could be due to a unique combination of the simultaneous adsorption of protons and phthalates. It is important to emphasize that as the surface of the substrate in the present study gradually changes, none of the measurements can be assumed to reflect equilibrium conditions. The potassium chloride used for ionic strength adjustment can suppress the surface hydroxylation and in addition the chloride ions can be involved in ion exchange at the solid-liquid interface. To verify this phenomenon the residual chloride was determined by turbidimetry in the solution part of a blank alumina sample suspended in 0.01 M KCl solution. A lower concentration of KCl compared to the adsorption measurements (0.1 and 1 M) was essential to observe the change, because of the magnitude of measurement uncertainty (Figure 1). The number of surface sites reacting with chloride in our experiment seems to exceed the proton-active sites determined on similar, but aged (in water), γ-alumina: 1.1 sites nm-2.11 One reason may be the uncertainty of the measurement, but it could also be that certain types of surface hydroxyl groups do not react in acid-base titrations but they can participate in ion-exchange. The surface of aluminum oxide features hydroxyl groups of different chemical affinities whose distinct nature has been extensively studied, e.g., by Peri and Hannan.12 (7) Brown, J. F. J. Chem. Soc. 1953, 13, 84. (8) Dyer, C.; Hendra, P. J.; Forsling, W.; Ranheimer, M. Spectrochim. Acta, Part A 1993, 49A, 691. (9) Laiti, E.; O ¨ hman, L.-O.; Nordin, J.; Sjo¨berg, S. J. Colloid. Interface Sci. 1995, 175, 230. (10) Laiti, E.; Persson, P.; O ¨ hman, L.-O. Langmuir 1996, 12, 2969. (11) Person, P.; Nordin, J.; Rosenquist, J.; Lo¨vgren, L.; O ¨ hman, L.O.; Sjo¨berg, S. J. Colloid Interface Sci. 1998, 206, 252. (12) Peri, J. B.; Hannan, R. B. J. Phys. Chem. 1960, 64, 1526.
Phthalate Adsorption on γ-Aluminum Oxide
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Figure 1. Concentration of chloride ions in the solution phase of a 0.01 M KCl-alumina suspension (2.0 g/L solid).
Figure 2. UV spectrum: 1 mM potassium hydrogen phthalate solution (C); 1 mM potassium phthalate solution (B); supernatant of 1 mM potassium hydrogen phthalate-γ-alumina suspension after 190 h, I ) 0 (A).
The values of the surface charge of the alumina samples (or PZC) were impossible to quantify, because the system was neither in equilibrium nor reversible. However, the sign of the surface charge was predicted from electrokinetic potential measurements. Since all potentials were positive, a positive surface charge was assumed, which is often the case for other hydrous oxides too, in the investigated pH ranges. Potassium ions are not likely to adsorb on the alumina since the surface was positively charged under our experimental conditions. It was verified from the UV spectra of the supernatant from a suspension of potassium hydrogen phthalate solution (Figure 2). With adsorption of hydrogen phthalate the solution phase changed to phthalate, implying that the K+ ions became more abundant relative to the residual phthalate ions in solution state. Despite nonequilibrium conditions the amount of adsorbed phthalate approaches a nearly constant level, or at least the speed of adsorption significantly slowed (Figure 3). The amount of adsorbed phthalate appears to be significantly more in our case than the value obtained from measurements on aged γ-alumina: 1.1 phthalate/ nm2 (pH ) 4.18-5.80, I ) 0.1 M) compared to ≈0.7 phthalate/nm2 (pH ) 5, I ) 0.1 M).11 The difference illustrates the higher reactivity of the surface without the aqueous aging process. Assuming that phthalate molecules are orientated perpendicular to the surface with the carboxylic functions
Figure 3. Concentration of the residual phthalate in the bulk solutions from the adsorption tests.
toward the alumina, an approximate maximum area of 0.24 nm2 area can be calculated for each phthalate molecule. Considering this maximum value, all adsorption data presented in Figure 3 would correspond to a submonolayer coverage. The amounts of adsorbed phthalate are similar in the case of the pure acid and of the potassium hydrogen phthalate. On the contrary, the phthalate concentration in the bulk solution of the potassium salt did not change as much, indicating less adsorption at higher pH. The increasing ionic strength diminished the adsorption independently of pH. The ionic strength dependence of surface reactions is commonly utilized to provide an operative distinction: strong and weak dependences indicate outer- and innersphere complexation, respectively.11 In the following, infrared and Raman spectra of the adsorbed phthalate compound are analyzed in order to describe the surface complexes formed at different pHs and ionic strengths. Infrared and Raman spectra of monocarboxylates (RCO2-) can display two bands in the region between 1300 and 1700 cm-1, which are ascribed to the symmetric and asymmetric stretching vibrations of the carboxylate group.13 In the case of di- or polycarboxylates, the carboxylate group frequencies can be coupled depending (13) Colthup, N. B.; Daly, L. H.; Wiberley, S. E. In Introduction to Infrared and Raman Spectroscopy; Academic Press: San Diego, CA, 1990; pp 317-318.
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Table 2. Band Assignments of Phthalate Vibrations (cm-1) in the Frequency Region of the Carboxylate Stretchinga ref 14
ref 16
ref 17
our work
IR solid
Raman solid
Raman solution
vibration
IR-ATR solution
vibration
IR-CIR solution
vibration
Raman solution
vibration
1382 vs 1410 s 1418 vs 1554 s 1565 s
1385 s 1411 s 1420 s 1554 w 1569 vw
1383 sh 1408 vs
14; ν(CC) νs(OCO)
1385 1403
νs(OCO) νs(OCO)
1380-85 1401-05
νs(OCO) 14; ν(CC)
1380-85 m 1405-06 s
νs(OCO) νs(OCO)
1557 w 1567 vw
νas(OCO)
1555 1564
νas(OCO)
1552-56
νas(OCO)
1550 sh 1561 sh
νas(OCO)
a
vw, very weak; w, weak; s, strong; vs, very strong; sh, shoulder.
Figure 4. Raman spectra: 50 mM phthalic acid solution, pH ) 2.22 (A); 20 M potassium phthalate solution, pH ) 6.90 (B); solid potassium phthalate (C) with reduced intensity.
on the structure and symmetry of the molecule. For phthalate ions the two carboxylate groups were suggested to be rotated 33° out of the plane of the ring classifying the ion into the C2 symmetry group.14 In contrast, Jessen and Ku¨ppers found that the carboxyl groups are rotated out from the ring plane with only 12° in the hydrogen phthalate ion and reported a CS symmetry.15 The C2 for the phthalate and CS for the hydrogen phthalate cannot be valid at the same time. Assuming that each carboxylic group rotates the same degree, C2 symmetry can be achieved if the direction of the rotation (over the C-C axes) is the same for both carboxylic groups and CS symmetry is obtained if the rotations are in opposite directions. In the spectroscopic investigation of the phthalate ion Arenas and Marcos reported two symmetric and asymmetric νC-O bands which were explained by the cooperative and opposing movements of the rotated carboxylic groups.14 Similar results were obtained from both semiempirical quantum mechanical calculations and experimental results.16 However, the band assignment of the carboxylate vibrations is not consistent in the different references and is summarized in Table 2. Owing to the significance of these bands in the present study of the surface complexes, we also carried out Raman measurements in both liquid and solid states (Figure 4). From the complete disappearance of both the 1384 cm-1 shoulder and the 1405 cm-1 band in acidic solutions, we suggest that these bands (14) Arenas, J. F.; Marcos, J. I. Spectrochim. Acta, Part A 1979, 35A, 355. (15) Jessen, S. M.; Ku¨ppers, H. J. Mol. Struct. 1991, 263, 247. (16) Nordin, J.; Persson, P.; Laiti, E.; Sjo¨berg, S. Langmuir 1997, 13, 4085.
could belongsin agreement with Nordin et al.16sto the symmetric carboxylate stretching. On the other hand, the ring vibration in the same region could be the small shoulder shown in the spectrum of the solid potassium phthalate at 1420 cm-1 (Figure 4 insert) as we have proposed previously.5 Complexation of phthalate at metal (hydro)oxide surfaces has been explained by formation of either outersphere or inner-sphere complexes. In the surface complex either or both carboxylic groups of phthalate can be involved. In addition, the binding can be mono- or bidentate depending on how the oxygen atoms are coordinated to the surface sites. Benoit at al.18 proposed that surface complexation of phthalate at the water-δ-aluminum oxide interface occurs through a ligand exchange reaction where a bidentate chelate at the edge of an aluminum octahedron was suggested to form. Nordin et al.16 provided evidence for the coexistence of more than one surface complex at the boehmite (γ-AlOOH)-water interface by studying the symmetric stretching region of the carboxylate ion. Supported by band intensity changes with pH and ionic strength they proposed an inner- and an outer-sphere complex. γ-Aluminum oxide contains fewer surface hydroxyl groups than boehmite, and therefore the preference for inner- and outer-sphere complexation might be different. To evaluate the type of complex dominating in our samples, DRIFT-IR and Raman spectra were recorded for the solid phase of the batch samples referred to in Figure 3 and Table 1. The spectra of the surface complexes resulting from 1 mM phthalic acid solution (pH ) 3.19-4.4) without added KCl are shown in Figures 5 and 6. These would be the most probable conditions for observing outer-sphere complexation since the adsorbed amount of phthalate is large and higher ionic strength has been shown to favor the inner-sphere complexation.16 Of the two carboxylate frequency regions, the one corresponding to the symmetric stretching is more favorable to analyze in the infrared spectroscopy because there are fewer bands and less interference from water vapor. However, with the sample compartment purged continuously with dry nitrogen the interference from the water vapor in the spectra was reduced, and thus the asymmetric region will also be discussed. The infrared spectra of the surface species exhibit strong absorbance in the area of both carboxylate stretching vibrations. At the same time the carboxylate character of the complex is also reflected in the 1400 cm-1 region of the Raman spectra. In both techniques the spectral features are composed of broad overlapping bands. To resolve the possible contributing bands appearing as shoulders in Figures 5 and 6, the inverse second derivative spectra were calculated by various degrees of smoothing. The (17) Tejedor-Tejedor, M. I.; Yost E. C.; Anderson, M. A. Langmuir 1992, 8, 525. (18) Benoit, P.; Hering, J. G.; Stumm, W. Appl. Geochem. 1993, 8, 127.
Phthalate Adsorption on γ-Aluminum Oxide
Figure 5. Infrared spectra of phthalate adsorbed on γ-aluminum oxide from 1 mM phthalic acid solution after 21 h (A), after 116 h (B), and the blank sample after 116 h (C). (The spectra have been offset for clarity.)
Figure 6. Raman spectra of phthalate adsorbed on γ-aluminum oxide from 1 mM phthalic acid solution after 21 h (A), 116 h (B), and the blank sample after 116 h (C). (The spectra have been offset for clarity.)
frequencies of those shoulders, which proved to be real, are shown in Figures 5 and 6 with their wavenumbers. In the symmetric carboxylate stretching region two additional overlapping bands were found on the highfrequency side of the 1400 cm-1 band in the infrared spectra (1451 and 1427-1431 cm-1) and two bands were resolved in the Raman spectra (1452 and 1380 cm-1). (There were a few other weak features appearing in the second derivative of the Raman spectra, but their confidence level was not sufficient to be regarded as real shoulders or bands.) By the help of a similar treatment applied to the asymmetric stretching region of the carboxylate vibration of the Raman spectra, the low-frequency shoulder of the 1600 cm-1 band was determined to be at 1578-1581 cm-1. In the case of the infrared spectra (Figure 5) there was no mathematical refinement performed in the asymmetric carboxylate region since the observed area was well resolved and the superimposed water vapor bands could have made the derivative spectra particularly uncertain.
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Figure 7. Infrared spectra of phthalate adsorbed on γ-aluminum oxide at different ionic strengths. The adsorption took place for 116 h from 1 mM solution of phthalic acid without KCl (A), with 0.1 M KCl (B), and with 1 M KCl (C). (The spectra have been offset for clarity.)
The presence of more than two bands in the symmetric carboxylate stretching region suggests the existence of at least two different carboxylates or two different surface complexes. This is further supported by the slight relative intensity change of the vibrations at 1400-1405 and 1427-1431 cm-1 as a function of ionic strength (Figure 7). The ratio of the two bands should remain constant if they originated from the same complex. In agreement with the proposals for adsorption on boehmite made by Nordin et al.,16 the 1400-1405 and 1427-1431 cm-1 vibrations could be assigned to an outer- and an inner-sphere complex, respectively. The stronger interaction with the surface in the inner-sphere complex causes a greater wavenumber shift compared to the analogous band for the phthalate ion at 1380-1385 cm-1 (Table 2). An alternative explanation for the wavenumber shift could be a conformational change of the carboxylate groups. In the hydrogen phthalate ion the rotation of the carboxylate groups is unlikely due to the shared proton located between the carboxylates. In such a case, the increasing size of the other cation has a minor influence on the position of the symmetric carboxylate vibration.15 In the phthalate anion, however, the rotation of the carboxylate groups cannot be excluded. To define the possible carboxylate vibrations, first the vibrations of the aromatic ring have to be determined in the regions of interest. On the basis of earlier works5,14,16,19 and on the spectra in Figure 4, Table 3 includes the assumed ring vibrations presented in Figures 5 and 6. The two vibrations at 1451-1452 and 1487-1491 cm-1 seem to be present at any pH and ionic strength and their relative intensities appeared to be constant, supporting their assignments to ring vibrations. The origin of the small shoulder band at 1380 cm-1 is not completely clear. It could be linked to the symmetric stretching of uncomplexed phthalate. In this case, however, this band would be detectable in the infrared spectrum as well,14 which was not possible to achieve even by mathematical refinement. Because this shoulder is more and more intense with time, we assume that it eventually could come from Teflon as it was stripped from the stirrer bar by the vigorous stirring. Other bands from Teflon have also been reported after longer times of stirring in alumina samples.8 (19) Arenas, J. F.; Marcos, J. I. Spectrochim. Acta, Part A 1980, 36A, 1075.
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Table 3. Frequencies and Band Assignments for the Surface Complex Formed during Phthalate Adsorption on γ-Aluminum Oxide
Klug and Forsling Table 4. Ratio of the Inner-Sphere and Outer-Sphere Complexes in Adsorbed Phthalate on γ-Aluminum Oxidea inner sphere/outer sphere
observed frequencies (cm-1) infrared
Raman 1039
1085 1148-50 1167 1263 1292-93 1400-07
1163-66 1292 1416-19
1427-31 1451 1490-91 1556-63
1452 1487
1583-84 1610-11
1581 1603-04
band assignment δ(C-H) δ(C-H) in-plane bending δ(C-H) ν(C-COO) ν(C-COO) δ(C-H) coupled to ν(C-O) νs(O-C-O) of the outer-sphere complex ν(C-C) ring νs(O-C-O) of the inner-sphere complex ν(C-C) ring ν(C-C) ring νas(O-C-O) of the outer-sphere complex ν(C-C) ring ν(C-C) ring
In the asymmetric carboxylate stretching region one aromatic ring vibration can be assigned to the 1603-1604 cm-1 Raman band in Figure 6 and to the 1610-1611 cm-1 infrared band in Figure 5. Another ring vibration could be assigned to the 1578-1583 cm-1 band in the infrared and Raman spectra. Both assignments are supported by the consistence of the bands in the Raman spectra at different pHs (Figure 4). The other spectral features observed in this region should originate from the carboxylate vibrations. On the basis of Table 2 it seems reasonable to consider that the IR bands at 1556 and 1563 cm-1 are originating from the asymmetric carboxylate motions of the outer-sphere complex. This complex interacts weakly with the Al sites, resulting in small band shifts compared to the phthalate ion in solution (1556 cm-1). It has to be noted that sometimes the doublet around 1560 cm-1 appeared as a single band at an intermediate wavenumber (Figure 5b). Since there was no tendency between the varied experimental conditions and the splitting of this band, and in addition, water vapor features a strong absorption at 1559 cm-1 we suspect that the single-double behavior is due to the interference of the water vapor in the optical path of the spectrometer. The asymmetric carboxylate vibrations of the innersphere complexes are expected to shift to higher frequencies from the similar vibrations of the outer-sphere complexes because of the stronger interaction with the surface sites. As there were no more strong bands detected in the appropriate region of the infrared spectra, assignment cannot be done for the asymmetric carboxylate vibration of the inner-sphere complex. The spectroscopic results indicated that there are several (at least two) alternative types of surface complexation resulting in different surface species on γ-aluminum oxide. The tentative band assignments are summarized in Table 3. Assuming a similar molar absorptivity for the innerand outer-sphere complexes, their ratio can be compared based on the heights of the 1427 and 1402 cm-1 absorbances in the infrared spectra acquired under the varied experimental conditions. The time of the sample treatment did not alter the ratio significantly. The maximum deviation was in the range of 5-15%, which can be attributed to the uncertainty in the diffuse reflectance measurements.
phthalic acid solution (pH ) 3.19-3.23) potassium hydr. phthalate (pH ) 4.05-4.19) potassium phthalate solution (6.33-6.57)
0 M KCl
0.1 M KCl
1 M KCl
0.80 ( 0.03
0.85 ( 0.04
0.92 ( 0.04
0.76 ( 0.04
0.79 ( 0.05
0.85 ( 0.05
0.58 ( 0.05
0.60 ( 0.04
0.64 ( 0.05
a The ratio was calculated from the height of the shoulder/peak at 1427 cm-1 and of the peak at 1402 cm-1 in the infrared spectra after 21, 116, and 190 h of adsorption.
The different pH of the adsorption influenced the ratio of the two complexes: higher pH (especially in case of the potassium phthalate solutions) favored outer-sphere complexation as it is shown in Table 4. The pH dependence of the ratio can be explained by the different probability of complexation of the dominating phthalate species in the solution. Increasing ionic strength slightly enhanced inner-sphere complexation, but the effect was minor compared to that of pH, especially if taking account the uncertainty of the measurement. The effect of the ionic medium could result from the chloride ion exchange as it decreases the number of available sites for outer-sphere complexation. In the discussion of the structures of the surface complexes the carbonyl stretching vibration can be utilized in order to decide whether either one or both carboxylic groups are involved during the adsorption. The carbonyl group of phthalic acid has a strong infrared absorption at 1682 cm-1 in the solid state and at 1705-1710 cm-1 in solution.17 In our spectra of the adsorbed phthalateseven at pH as low as 3.19sthere was no evidence of such a band indicating that there is no pure covalent bond between the phthalic compound and the surface and both of the carboxyl groups are involved. The ortho position allows the carboxyl groups to influence each other resulting (1) in a very short neighboring O‚‚‚O distance15 and (2) the absence of the characteristic carbonyl stretching frequency in the hydrogen phthalate ion with intramolecular hydrogen bonds.20 In addition, when adsorbed on the surface there are additional opportunities to form hydrogen bonds with surface hydroxyl groups which can also hinder the carbonyl stretching. The complexity of this question is reflected in the literature where different models were postulated on similar mineral surfaces. Phthalate adsorption has been interpreted as outer-sphere complexes involving phthalate and hydrogen phthalate.21,22 Others reported two innersphere complexes, one of them monodentate and the other one bidentate, but both involving only one carboxyl group.17 Another interpretation of the two inner-sphere complexes was also reported as bidentate chelates.18 In our investigation the more precise definition of the structure of the complexes requires consideration of the speciation of phthalate in the solution state and the surface charge. The γ-alumina surface is positively charged when suspended in water. Therefore the attraction of negatively charged ions is expected. The relative concentrations of the phthalate species at a given pH can be calculated from (20) Ku¨ppers, H.; Takusagawa, F.; Koetzle, T. F. J. Chem. Phys. 1985, 82, 5636. (21) Balistrieri, L. S.; Murray, J. W. Geochim. Cosmochim. Acta 1987, 51, 1151. (22) Nilsson, N.; Persson, P.; Lo¨vgren, L.; Sjo¨berg, S. Geochim. Cosmochim. Acta 1996, 60, 4385.
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Table 5. Speciation of Phthalate (c ) 1 mM and I ) 0) at Different pHsa pH
H2L
HL-
L2-
3.19 4.05 6.39
22.4 2 ≈0
76 79 3.2
1.6 19 96.8
a The values are given in percentage. Phthalic acid, H L; hydrogen 2 phthalate, HL-; phthalate, L2-. (pK1 ) 2.89, pK2 ) 5.51 for I ) 0.23)
the pKa values of phthalic acid (Table 5).23 It can be seen that at pH ) 3.19 and pH ) 4.05 the hydrogen phthalate ion is dominant in solution, while at pH ) 6.4 it is the phthalate ion. The similar adsorbed amount of the phthalic compound at the two lower pHs compared to the much smaller amount at pH ) 6.4 indicates that the hydrogen phthalate ion has a preferential affinity for the surface. The alumina surface is still slightly positive at pH ) 6.4 (verified by electrokinetic potential measurements), so the adsorption of the phthalate dianion should be favored from an electrostatic point of view. The low adsorption in this case, however, indicates that there is some other hindrance of this type of complexation. Considering the alumina surface, sites with positive character are assumed to interact. On the γ-alumina surface both tAlOH and tAlOH2+ surface sites fulfill this criterion. However, the amounts and the ratios of these two sites are dependent on pH: at high pH the former is more abundant and at low pH the protonated latter species is dominating. Thus, there is a possibility of the hydrogen phthalate ions to form outer-sphere complexes with both surface sites:
tAlOH + HL- S tAlOH2+L2tAlOH2+ + HL- S tAlOH2+HLAccounting for the electrostatic forces only, the surface sites are assumed to coordinate to that oxygen of the carboxylate group which is not involved in the internal hydrogen bond, since that part of the molecule bears the strongest partial negative charge. From the residual concentration of the phthalate in the supernatant of the batch samples, the pH of the solution phase can be calculated. In the case of the lowest pH solution the calculated pH is 3.46, in contrast to the measured pH ) 4.5 after 190 h. The large difference could be explained assuming a ligand exchange reaction where the surface hydroxyl groups are exchanged by hydrogen phthalate ions leading to the inner-sphere complex. The hypothesis can be verified by calculating the pH of the solutions in such a case. Due to the quantitatively unknown chloride competition for ion-exchange sites, only I ) 0 situations can be considered. From the pK values (Table 5), the residual total phthalate concentration (Figure 3), and the estimated inner-/outer-sphere ratio (Table 4) and assuming one hydroxyl release per one innersphere complex, the pH of the solutions can be calculated (Table 6). (In the calculation for the potassium phthalate solution the number of potassium atoms per molecule was reduced from 2 equiv to 1.9 in order to achieve a similar starting pH as was measured. This modification is still within the purity specification of the chemical (98%).) At higher pHs the calculated values are in reasonable agreement with the measurements supporting the proposed mechanism. For the initial condition of pH ) 3.19 (23) Handbook of Chemistry and Physics; Weast, R. C., Ed.; CRC Press: Cleveland, 1975; p D-130.
Figure 8. Possible surface reactions producing the inner- and outer-sphere complexes in the adsorption of phthalate on γ -aluminum oxide. Table 6. Calculated and Measured pH Assuming a Ligand Exchange Reaction during the Inner-Sphere Complexation of Phthalate (I ) 0) pH of the starting solution
pH after 116 h of adsorption
measured
calculated
measured
calculated
3.19 4.05 6.40
3.15 4.39 6.46
4.41 5.56 7.02
3.77 5.64 6.85
the calculation yielded a considerably lower value than was measured. At such a low pH there can be other chemical reactions which were not accounted for in the calculations, although their influence on the pH may have become important. Proton adsorption as well as dissolution of the aluminum may play a significant role and both could be responsible for part of the pH increase. Since the effects of these “side” reactions were not quantified, the proposed mechanism cannot be confirmed in this case. On the basis of the assumed surface sites, adsorbing species in the solution and surface reactions, the scheme of the phthalate complexation is shown in Figure 8. There is a possibility of the formation of two outer- and innersphere complexes, respectively. The two outer-sphere complexes (I and II) are probably indistinguishable by means of spectroscopy, but their relative amounts may depend on the pH of the adsorption. At low pH structure I is assumed to be dominating and at higher pH structure II is assumed to be dominating. Owing to the absence of the νCdO band in the infrared spectra, it seems that the special intramolecular hydrogen bond of the hydrogen phthalate ion is conserved in the inner-sphere complex too, favoring structure III rather than structure IV. In addition a symmetric structure like a bidentate chelating complex (IV) should have a sharp, very intense, major Raman band around 1400 cm-1 from the symmetric carboxylate stretching. Such a band ought to be at least as high in intensity as the aromatic vibrations around 1600 and 1040 cm-1. Since only a weak broad band appears in this region (Figure 6), the inner-sphere complex is likely to be coordinated as a monodentate species where the mutual influence of the carboxyl groups is still present via the intramolecular hydrogen bond (III). Both inner-sphere complexes could be formed by ionexchange reaction. The one resulting in structure IV would
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Langmuir, Vol. 15, No. 20, 1999
Figure 9. Infrared spectra of phthalate adsorbed on anodic γ-aluminum oxide. The adsorption took place for 168 h from 1 mM solution, I ) 0, 10× enlargement (A), for comparison a similar spectrum using synthetic γ-aluminum oxide (B).
be favored at higher pH because the appropriate surface sites and the dianions are both available. However, the reduced amount of inner-sphere complexes at higher pH and the missing intense Raman band around 1400 cm-1 both are suggesting that this reaction has a minor contribution to the inner-sphere complexation. It could be because the release of a hydroxyl ion into a more alkaline solution is less probable. On the other hand the ion-exchange reaction at low pH is reasonable and could cause the higher ratio of innersphere complex in such conditions. It is important to keep in mind that the representation of surface sites such as tAlOH and tAlOH2+ can be an oversimplification describing the actual charge and bonding arrangements of the sites. However, it is of value that they still reflect the observed chemistry. The adsorption of phthalate on anodic γ-aluminum oxide was compared to the adsorption on the synthetic model compound in the low pH region. The adsorbed amount of phthalate was much less, and consequently the pH increase of the solution phase was smaller too. The infrared spectrum of the adsorbed complex showed two broad overlapping features centered at 1565 and 1407 cm-1 (Figure 9). The bands were weaker than those in the case of synthetic γ-alumina, and therefore they were magnified by a factor of 10 in the figure. The weak features of the spectra were partly caused by the lower surface area (two to three times less compared to the synthetic alumina in the DRIFT measurement conditons) and partly by the lower phthalate concentration in the matrix. The lower adsorption of the phthalate can be due to the stabilization of the anodic foil which involves treatment in a phosphate
Klug and Forsling
solution during the manufacturing process. Phosphate reacts with the aluminum oxide surface forming strong inner-sphere complexes,10 which can occupy optional sites for phthalate complexation. However, no phosphate bands were detected in the untreated sample suggesting that the phosphate concentration on the surface was lower than the detection limit of the measurement. On compariosn of the spectra obtained on the anodic oxide to the model substance, they are quite similar in the carboxylate stretching region. Both inner-sphere and outer-sphere complexes seem to be present. A little variation between the spectra is observed around 1565 cm-1, but that probably results from the water vapor disturbance. Similar to the infrared spectra the Raman spectra of the surface complexes formed on anodic aluminum oxide and synthetic γ-aluminum oxide were found to conform to those reported previously.4 The good agreement between both infrared and Raman spectra of the surface complexes collected from the anodic γ-aluminum oxide and from the synthetic γ-aluminum oxide makes it possible to study phthalate adsorption and surface speciation using the synthetic oxide as a model material. The much higher active surface area of the model material is advantageous in any experimental method to be applied. Conclusions Adsorption of phthalate on nonaged, synthetically produced γ-aluminum oxide was found to lead to a quasiequilibrium state where the amount adsorbed does not change significantly. From infrared and Raman spectra of the adsorbed material two different surface complexes can be proposed: an outer-sphere and an inner-sphere. The coordination probably starts with the reaction of the hydrogen phthalate ion with the surface. The outer-sphere complex is believed to be coordinated either tAlOH2 + L2- or tAlOH2 + HL-. The inner-sphere complex can be formed from the outer-sphere complexes or directly by ligand exchange where surface hydroxyl groups and hydrogen phthalate ions are exchanged. From spectroscopic observations a monodentate coordination is suggested for the inner sphere complex where the intramolecular hydrogen bond of the hydrogen phthalate ion is conserved. The tentative band assignments of the assumed surface complexes are presented. The surface complexes formed by the phthalate adsorption on the model γ-aluminum oxide were found to be similar to those obtained using anodic γ-alumina. Therefore the model material can be used in the future for studying phthalate complexation on anodic aluminum oxide. Acknowledgment. The authors thank Dr Allan Holmgren for his advice and useful comments throughout this work. The financial support given by Evox RIFA AB is gratefully acknowledged. LA990105J
