Langmuir 1990,6, 979-987 tributes to the strategy needed to develop a theory of electrochemical adsorption on transition metals.lg
Conclusions Adsorption of pyridine on gold exhibits a reversible pattern of behavior with respect to both the electrode potential and the bulk concentration of adsorbate. This behavior appears common to weak electrochemicaladsorption, for instance, to adsorption of acetic acid on platinum. The free, actually instantaneous, exchange between the molecules on the surface with those in the bulk of solution creates a solid base for the application of interfacial thermodynamics for collecting such significant quantities as standard Gibbs energy of adsorption and lateral interaction energy. In this work, the Bennes isotherm is used for quantitative determination of these quantities at two potentials which guarantee a structural uniformity of the adsorbate. A comparison of pyridine adsorption on gold with adsorption on platinum, reported in the companion paper, reveals interesting differences between the two systems. A flat-vertical molecular reorientation takes place on platinum and is enforced by the increase in the bulk concentration of pyridine in solution. Pyridine surface concen-
979
tration is not potential-dependent in the broad potential range, and no evidence was found that changing electrode potential can change orientation of the adsorbate. On polycrystalline gold, pyridine adsorbs predominantly in the vertical form with the parallel orientation identifiable only at low bulk concentrations. In this low concentration range, we observed that the flat/vertical reorientation was activated by a positive-going electrode potential polarization. Adsorption of pyridine on platinum is irreversible. That is, pyridine molecules are immobilized on the electrode, and no exchange between the surface and the bulk takes place unless desorption (hydrogenation) of pyridine is activated in the hydrogen range of the electrode potentials. The difference in the bonding strength on platinum and gold is a natural consequence of the difference in the electronic configuration between the two metals studied.
Acknowledgment. This work was supported by the Air Force Office of Scientific Research (AFOSR-890368). The authors wish to thank Krzysztof Franaszczuk (MSc.) of the University of Warsaw for writing a computer program for the Simplex method. Registry No. Au, 7440-57-5; pyridine, 110-86-1.
Characterization of Benzoic and Phenolic Complexes at the Goethite/Aqueous Solution Interface Using Cylindrical Internal Reflection Fourier Transform Infrared Spectroscopy. Part 1. Methodology M. Isabel Tejedor-Tejedor,’ Eric C. Yost, and Marc A. Anderson Water Chemistry Program, 660 N . Park St., University of Wisconsin, Madison, Wisconsin 53706 Received June 20, 1989. I n Final Form: October 25, 1989 In this paper, we would like to show how CIR-FTIR “in situ” spectroscopy can be used routinely to obtain information on the structure of complexes formed between oxo anions and the surface of colloidal metal oxides in aqueous suspensions. We present a detailed account of this methodology, which was used in the general study of phenolic and benzoic complexes at the surface of a-FeOOH particles suspended in aqueous media. Here, we focus only on the benzoate complex with a-FeOOH as an illustrative example of this methodology. Because the spectral bands used for structural diagnosis in surface complexation studies of organic groups on metal oxides are the vibrational modes of the ligands, this methodology requires the use of additional complementary CIR-FTIR studies of uncomplexed oxo anions in both D20and H10 solutions as a function of pH. The methodology furthermore utilizes well-understood spectral subtractions and occasionally isotopes for confirmatory identifications. By using this “in situ” CIR-FTIR technique, we have shown that the benzoate anion forms a bridging bidentate complex with adjacent iron atoms on the goethite surface.
Introduction IR spectra of species at the solid-aqueous interface have been very difficult to measure in the transmission mode, due to the strong IR absorbance Of this It was not until the mid-19709 with the blossoming of ATR (atten-
* To whom correspondence should be addressed. 0743-7463/90/2406-0979$02.50/0
uated total reflection) as a sampling technique, coupled with the computerization of the IR systems, or, even better, used in conjunction with the high sensitivity of FTIR spectroscopy, that IR sin investigations of adsorbed species at the solid-aqueous interface produced acceptable quality spectra. In the past 10 years, an array of research has been conducted in the area of aqueous-solid interfaces using ATR 0 1990 American Chemical Society
980 Langmuir, Vol. 6, No. 5, 1990 techniques: in the characterization of electrode ~ u r f a c e s , ~ - ~ in studies of biomaterials in aqueous environments,4 in the understanding of blood-material intera~tions,~+j and in modeling the behavior of surface-active adsorbates for the flotation of In these studies using ATR, there are two principal experimental configurations: the first uses the internal reflection element (IRE) as the adsorbent material i t ~ e l f ,while ~ . ~ in the second type of configuration an adsorbent is applied as a thin film to the
IRE.^^^ The study of a monolayer or a submonolayer concentration of adsorbate on a small surface area substrate, as is the case of adsorption directly on the IRE or polished thin films, has severe problems of sensitivity. In order to increase the signal to noise ratio (S/N), Neff et a1.l used the ATR internal element coated with thin metal films (these films constitute the electrode) in conjunction with electrode potential modulation in order to enhance sensitivity in their systems. Furthermore, several authors have demonstrated the feasibility of measuring monolayer coverages on electrode surfaces by using external specular reflection in thin-layer cells in combination with modulation techniques. Modulation can be achieved from perturbing equilibrium either by producing electrochemical events in a modulation cycle which relaxes to equilibrium (EM1RS)'O or by using polarization modulation and FTIR spectroscopy (FT-IRRAS)."J2 A publication by Tejedor-Tejedor and Anderson13 widened the field of ATR-IR in situ interfacial solid-aqueous applications to the study of interfacial reactions on suspended colloidal particles. However, in the case of these suspended colloidal particles, sensitivity cannot be enhanced by using the polarization modulation techniques mentioned above.llJ2 The reason why is that these techniques are based on surface selection rules, and hence a single orientation of the surface species in relation to the IR beam is needed. Moreover, the orientation of a species at the interface of the colloidal particles will not be unique but rather random in relationship with the IR beam. Since we cannot take advantage of the flat ATR optics in order to either increase sensitivity or perform orientation studies, we have elected to abandon this configuration in favor of the cylindrical optics offered in the CIR-FTIR technique. This technique has been shown to be more sensitive in randomly oriented systems as is the case for species in aqueous s01ution.l~ While one of our previous articles13 introduced the general concept of CIR-FTIR as a technique which could be employed in colloidal suspensions, it is the purpose of this paper to present a more detailed account of the ~~
~
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~~
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(1)Neff, H.;Lange, P.; Roe, D. K.; Sass, J. K. J. Electroanal. Chem. 1983,150,513. ( 2 ) Neugelaur, H.; Nauer, G.; Brinda-Konopik, N.; Kellner, R. Fresenius Z.Anal. Chem. 1983,314,266. (3)Mattson, J. S.;Smith, C. A. Anal. Chem. 1975,47,1122. (4) Castillo, E. J.; Koenig, J. L.; Anderson, J. M.; Lo, J. Biomaterials 1985,6,338. ( 5 ) Kellner, R.; Gotzinger, G. Fresenius 2.Anal. Chem. 1984,319, 837. (6)Gendreau, R. M.;Jacobsen, R. J. Appl. Spectrosc. 1978,32,326. (7) Mielczarski, J.; Nowak, P.; Strojek, J. W. Polish J . Chem. 1980, 54,279. (8)McKeigue, K.; Gulari, E. Surfactants in Solution; Mittal, L., Lindman, B., Eds.; Plenum Publishing: New York, 1984;Vol. 11. (9)Kuys, K. J.; Roberta, N. K. Colloids Surf. 1987,24,1. (10)Pons, S.;Davidson, T.; Bewick, A. J . Electroanal. Chem. 1984, 160,63. (11)Bewick, A. J.Electroanal. Chem. 1983,150,481. (12)Roe, D. K.; Sass, J. K.; Bethune, D. S.;Luutz, A. C. J.Electroanal. Chem. 1987,216,293. (13)Tejedor-Tejedor, M. I.; Anderson, M. A. Langmuir 1986,2,203. (14)Wong, J. S.;Rein, A. J.; Wilks, D.; Wilks, D., Jr. Appl. Spectrosc. 1984,38,32.
Tejedor-Tejedor et al. methodology t o be used in studying surface complexes in colloidal metal oxide suspensions. The second paper in this two-part publication will apply this methodology to the analysis of the similarities and differences in binding of phenolic and benzoic acid compounds to goethite (a-FeOOH) in an aqueous suspension. The methodology we discuss in this paper essentially consists of two parts: (A) analysis of the CIR-FTIR spectra of the dissolved adsorbate species in both H20 and D20 solutions at different pH (pD) values; (B) examination of the FTIRCIR spectra of a-FeOOH with adsorbate in both H2O and DzO.
Experimental Section A. Goethite Properties. Goethite was prepared following
procedures of Atkinson et al.15 and Machesky.16 Needles averaged 20 nm in width and 60 nm in length, and Nz BET analysis indicated a surface area of 81 m2/g. B. Adsorbate Properties. Sodium benzoate (Kodak Chemicals),phenol (ACSBaker Chemical),and p-hydroxybenzoicacid (97% minimum, Kodak Chemicals) received no additional purification. Sufficient amounts of 1-5 M KOH were added to 0.100 M stock solutions of the acid forms to allow dissolution as an anion. The studies on dissolved organics utilized 0.1 M solutions and were adjusted to an ionic strength (I)of 1 M with KCl (ACS Columbus Chemical Industries). Stock solutions for use in equilibrium adsorption studies and CIR-FTIR experiments on goethite suspensions were 0.1 M with no additional ions used t o buffer the ionic strength. KCl used in equilibrium adsorption research was ultrapure (99.999% Aldrich Chemical Co.). All stock solutions were made with Milli-Q water (Millipore Corp. system with Organex-Qcartridge added and 0.2-bm final filter) and stored until use in the dark at 4 "C. C. CIR-FTIR Techniques and Methods. The attenuated total reflection apparatus was a cylindrical internal reflection (CIR) cell (SpectraTech CIRCLE system) with a ZnSe crystal rod. Spectra were recorded interferometrically with a Nicolet 60 SX Fourier transform infrared (FTIR) spectrometer and a Hg-Cd-Te (MCT) detector. Single-beam IR spectra were the result of 2000 or more co-added interferograms and ranged from 700 to 4000 cm-1. Further description is presented in TejedorTejedor and Anderson.13 All final spectra were the result of subtractingeither the spectra of the suspension supernatant or the ionic strength and pH-adjusted Milli-Q water solution (reference)from spectra of the suspension or organic in solution (sample),respectively. Many reference and sample spectra were ratioed against empty cell spectra; this assisted in accurate removal of the HzO ~ e a k s . 1 ~ Spectral accuracy was improved by leaving the cell in place for each corresponding reference and sample spectra collection so that transmittance and average angle of IR beam incidence remained constant for both reference and sample. This method minimized potential problems of resolving peaks in regions of very strong HzO absorption as discussed in Powell et al.17 Organic compounds dissolved in HzO or D20 were analyzed by first collecting spectra on 1 M KC1 at a specified pH (reference) and then gathering spectra on 0.1 M (unless otherwise indicated) solutions at the same pH and KCl (sample). These two spectra were ratioed against empty cell spectra, and then against each other (scale factor = 1). The goethite stock suspension concentration was typically 35 g/L. This stock suspension was prepared either from the original goethite stock16 or by adding lyophilized (freeze-dried) goethite to the solvent (latter was always true for D2O suspensions)and sonicatingintermittently for 24 h. Suspensionsderived from freeze-dried goethite were then allowed 2-3 days for equilibration. Aliquots for experimentation were withdrawn from (15)Atkinson, R.J.; Posner, A. M.; Quirk, J. P. J . Inorg. Nucl. Chem. 1968,I O , 2371. (16)Machesky, M. L. Ph.D. Thesis, University of WisconsinMadison, 1985. (17)Powell, J. R.; Wasacz, F. M.; Jakobsen, R. J . Appl. Spectrosc. 1986,40, 339.
Langmuir, Vol. 6, No. 5, 1990 981
Characterization of Benzoic and Phenolic Complexes this stock. In D2O suspensions,goethite was washed once with D20, centrifuged,supernatant discarded, and resuspended with fresh DzO to eliminate residual H2O (all D20 work was performed under a N2 atmosphere). Goethite suspensions (H20 or D20) prepared for CIR-FTIR analyses were adjusted to different pH values (pDs) at a fixed ionic strength of 0.01 M KC1. Experimental aliquots were brought to ionic strength (KCl) and pH (or pD) conditions by using microliter quantities of 0.01-1 M KOH or HCl (dissolved in D20 for appropriate experiments) and left for 24 h. Prior to CIR-FTIR analysis, pH (pD) was checked and adjusted if necessary. Immediately before spectra were collected, suspension samples were centrifuged. Supernatant was used as reference, and this volume (-3 mL) was kept separate from the concentrated ("pellet") suspension. The remaining suspension was resuspended by mixing and used as a sample. This procedure produced solid concentrations for CIR-FTIR analyses at levels of about 70 g/L (=2 times original).
Results and Discussion The interpretation of the IR spectra of surface complexes is normally based on differences between the spectra of the surface complex and that of chemical species in the bulk solution. For systems in which the adsorbate/ adsorbent are phenolic and benzoic ligands/metal oxides, the spectral differences upon specific adsorption will be the ones caused by the coordination of the organic ligands to the metal atoms of the surface. Although the metalligand vibrations would provide direct information about the coordinated bonds, in many cases they are not necessarily the best spectral bands to use for structural diagnosis. This situation arises when ligands are coordinated to the metal through an oxygen atom. In this case, the metal-ligand vibration would be in the same spectral range as that of the metal-oxygen vibrations of the metal oxide adsorbent. Also, these metal-ligand vibrations will adsorb in a region of the spectrum where water as well as the most commonly used optical materials for IR analysis also adsorbs IR radiation, thus complicating the interpretation of the spectra. Because of these problems, we deduced the formation of metal-ligand bonds from the behavior of the ligand vibrations. When the symmetry of an oxo anion ligand (e.g., benzoate) is changed by coordination, differences in the spectrum of the anion should be expected because of changes in the selection rules for the IR-active fundamental vibrations.18 Even if the oxo anion has the same symmetry after coordination, the position of the bands in the spectra will be shifted since the formation of a metal-oxygen bond (the oxygen of the oxo anion ligand) corresponds to a decrease in the bond order associated with the oxygen-central anion atom. A. Spectra of Species in Solution. Position and intensity of the group frequencies of these oxo anions are influenced by factors other than coordination to metals: the physical state of the sample, the nature of the solvent, solution concentration, etc. The magnitude of the changes depends on the nature of the vibrating group, and in solutions, on the electrical properties of the solvents. Unfortunately, there is not a simple relationship that allows one to predict the direction and magnitude of the frequency sifts when the sample passes from a phys(18) Nakamoto, K.Infrared and Raman Spectra of Inorganic and Coordination Compounds; Wiley: New York, 1986. (19) Mehrotra, R.C.; Bohra, R. Metal carboxylates; Academic Press: New York, 1983. (20) Wexler, A. S.Spectrochim. Acta 1977,23A,1319. (21) Pouched, C. J. The Aldrich Library- of. IR SDectra: Aldrich Chemical Company, 1981. (22) Mecke, R.;Langenbucher, F. Infrared Spectra of Selected Chemical Compounds; Heyden and Son: London 1965.
Table I. Influence of the Matrix on the Frequency Values of Carboxylic and Phenolic Vibrations vibrational frequency, mode
compd sodium acetate
cm-I
rhodium acetate potassium trifluoroacetate rhodium trifluoroacetate sodium benzoate
u,(COz-) u,(COz-) u,(COz-) u,(COz-) u,(COz-) u,(COz-)
iron benzoate phenol
v,(COz-) v(Ph-OH)
thallium phenoxide
v(Ph-OH)
0
1578 (solid)l8 1556 (HzO solution)o 1604 (so1id)'s 1678 (solid) 1664 1562 (solid)m 1542 (HzO solution)" 1525 (HzO solution)" 1225 (so1id)Zl 1255 (CSz solution)22 1240 (HzO solution) 1260 (so1id)Zl
Our data.
Table 11. Group Frequency Values (cm-') for Phenolic and Benzoic Compounds*
structural unit COzH
COzD
coz-
vibrational mode and approximate frequencies
u(C-OH)
u(C=O) 1690 1690
1280 1400 1350
Y-Ym
yam
1560
1400 1000
1400
Ph-OH
u(Ph-OH) 1230
Ph-OD Ph-R O
6(C-O-H) 1280
1230
G(Ph-O-H) 1360 1260 lo00
u(C=C) 46 bands: 1620-1300
In the spectral range 1700-900 cm-I.
icochemical state to another. In addition, since there is a general paucity of data with respect to IR studies of species in aqueous solution, we were faced with the necessity of measuring and interpreting the spectral features of dissolved species in order to have a true reference for the solution and surface complexation studies. Table I illustrates how for some carboxylic and phenolic vibrations shifts in their frequency values due to matrix effects can be of the same order of magnitude as the ones attributed to coordination with metals. Furthermore, due to the paucity of literature data, we needed to fill in some of the missing assignments of the absorption bands to the vibrational modes of the structural groups composing the molecule. It is only after the spectra are well understood that structural information can be extracted from the shift in the spectral bands upon complexation. In order to help the reader in the discussion, Table 11 shows the group frequencies for phenolic and benzoic compounds in the spectral range 1700-900 cm-l.23 1. Effect of pH. Variations in the solution pH of these oxy acid organic ligands can induce protonation or deprotonation with a resulting change in the ligand symmetry. This change in symmetry can affect the position and the number of bands in the IR spectrum. Hence, in IR studies of H2O solutions, pH becomes an important variable and can be used as an analytical tool for spectral interpretation. Parts a and b of Figure 1 show spectra of phenol in aqueous solution as a function of pH. The spectral changes are interpreted in terms of acid-base reaction equilibria: ~
~~
~
(23) Varsanyi, G. Vibrational spectra of benzene deriuatiue; Academic Press: New York, 1969.
982 Langmuir, Vol. 6, No. 5, 1990
Tejedor-Tejedor et al.
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WAVENUMBERS (cm-l) Figure 1. CIR-FTIR spectra of 0.1 M solutions of phenol at I = 1 M KCl: (a) in HzO, p.H 5.5; (b) in H20, pH 11.4; (c) in D20, pD 4.9. Scale: HI603 (height at 1603 cm-l) = 0.05 au.
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WAVENUMBERS (cm-1) Figure 2. CIR-FTIR spectra of sodium benzoate solutions at I = 1 M KC1: (a) in HzO, 0.025 M, pH 3.5, Hlam = 0.007 au; (b) in HzO, 0.1 M, pH 5.5, H 1 3 ~= 0.12 au; (c) in DzO, 0.023 M, pD 3.9, HI392 = 0.02 au; (d) in DzO, 0.1 M, pD 5.9, Hi390 = 0.12
au.
m
(1
C,H,OH = C,H,O-'
+ HC
(1)
which has a pK1 = 9.8.24 After proton dissociation at pH 11.4, the band at 1378 cm-l in the spectrum at pH 5.5 (Figure la) disappears, and the one at 1240 cm-l (Figure la) shifts to 1270 cm-1 (Figure lb). On the basis of these observations, the band at 1378 cm-I is assigned to the in-plane phenolic bending vibration and the one at 1240 cm-l to the phenolic stretching. Previous IR studies on other benzene derivatives have revealed that the 1378-cm-1 band is due to the coupling of the in-plane OH phenolic bending and vibration 14 of the benzene ring.23 The shift to higher energies of the phenolic stretching upon deprotonation is expected since the oxygenbenzene bond will be stronger in phenoxide than in phenol, and it is in agreement with literature d a h z 5 Since the pK1 of the acid-base equilibrium for benzoic acid is 4.0,24the CIR-FTIR spectrum at pH 5.5, Figure 2b, should correspond to the benzoate anion and the one at pH 3.5 to a mixture of this anion and benzoic acid. The decrease in the intensity of the band at 1542 and 1388 cm-' when the pH drops from 5.5 to 3.5 (Figure 2a and 2b) strongly suggests that these bands correspond to the asymmetric (u-) and symmetric (us) stretching vibrations of the carboxylate group. These values are in the range of the ones reported in the literature for those of benzoate in a nonaqueous medium (1562-1515 cm-l for vas and 1400 cm-l for us m ~ d e ) . The ~ ~ .spectrum ~~ at pH 3.5 (Figure 2a) shows the bands at 1705 and 1277 cm-' that did not exist in the spectrum a t pH 5.5 (Figure 2b); the former should be assigned to u(-C=O) of the carboxylic group, but the origin of the latter may be connected with the strongly coupled u(C-OH) and 6(-C-O-H) in-plane modes of that g r o ~ p . This ~ ~ , band ~ ~ assign(24)Martell, A. E.; Smith, R. M. Critical Stability Constants; Plenum Press: New York, 1977; Vol. 3. (25) Parfitt, R. L.; Farmer, V. C.; Russell, J. D. J. Soil Sci.1977, 28, 29.
(26) Rao, C. N. R. Chemical Applications of I R Spectroscopy; Academic Press: New York, 1963.
n o r.!n
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a I
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I
1637
I
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1383
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1129
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WAVENUMBERS (cm-1)
Figure 3. CIR-FTIR spectra of p-hydroxybenzoic acid dissolved in HzO at different pH values and Z = 1 M KCl: (a) 0.025 M, pH 3.5, HI170 = 0.005 au; (b) 0.1 M, pH 5.5, H1383 = 0.15 au; (c) 0.1 M, pH 10, H1378 = 0.07 au.
ment will be verified later in this paper by the information gained from the spectrum of benzoic acid in D20. Parts a-c of Figure 3 contain the CIR-FTIR spectrum of p-hydroxybenzoate dissolved in H2O at different pH values. The corresponding equilibrium can be written as follows: C,H,OHCOOH = C,H,OHCOO-' pK = 4.6 (ref 24)
+ H+ (2)
C,H,OHCOO- = C,H,0C002- + H+ pK = 9.5 (ref 24) (3) The bands produced by the vas and vs carboxylate should
Langmuir, Vol. 6, No. 5, 1990 983
Characterization of Benzoic and Phenolic Complexes appear in the spectrum at pH 5.5; meanwhile, in the spectrum recorded at pH 3.5, these bands would be expected to be either very small or nonexistent. Also, by lowering the pH to 3.5, we can produce new spectral bands coming from the stretching and bending of the carboxylic group. On the other hand, bands corresponding to the phenolic stretching and bending should appear at very close frequencies in the spectra at both pH values. Following this reasoning, the bands at 1540 and 1383 cm-l of Figure 3b (pH 5.5) correspond to the vas and us of carboxylate, respectively. A t pH 3.5, the band at 1691 cm-1 of Figure 3a is assigned to the v(C=O); the band at 1273 cm-l, as in the case of benzoic acid, should be due to both the stretching and bending of the carboxylic group. By analogy with the spectrum of phenol at pH 5.5 (Figure la), the band at 1248 cm-' of Figure 3b will correspond to the phenolic C-OH stretching. According to the literature data for para-substituted phenols,= the band at 1271cm-' and the shoulder at approximately 1370 cm-l are associated with the in-plane bending mode of phenol and the vibration 14 of the benzene. At pH 3.5 (Figure 3a), the former vibrational mode is represented by the bands a t 1250 cm-' and the latter may be represented by the bands at 1384 and 1273 cm-l. Both the phenolic and carboxylic groups of p-hydroxybenzoate are deprotonated a t pH 10. As a consequence, the major change in the spectrum obtained at pH 10 (Figure 3 4 , with respect to the spectrum at pH 5.5 (Figure 3b), should be a shift to higher frequency value of the phenolic stretching and the disappearance of the phenolic bending. The band at 1248 cm-1 of Figure 3b (pH 5.5) is replaced by the one a t 1289 cm-l of Figure 3c (pH lo), and the band corresponding to carboxylate symmetric stretching (1378 cm-l in Figure 3c) is more symmetric than the one at pH 5.5 (1383 cm-l in Figure 3b), in agreement with the disappearance of the above-mentioned coupling between phenolic bending and aromatic skeletal modes. The coupling of these two vibrational modes explains the fact that both the band a t 1271 cm-l and the shoulder at 1370 cm-' are connected with the existence of an H atom in the phenolic group (see Figures 3b,c and 4b). It is interesting to notice that protonation or deprotonation of the carboxylic group does not affect the frequency values of either the benzene skeleton vibration or the ones produced by other substituents on the ring (e.g., phenolic). Deprotonation of the phenolic group shifts to lower energies the skeleton vibration of the benzene ring (the 8a and 8b vibrations appear at 1608 and 1594 cm-l at pH 3.5, Figure 3a; at 1608 and 1598 cm-l a t pH 5.5, Figure 3b; and drop to 1587 and 1564 cm-l at pH 10, Figure 3c). Upon deprotonation of the phenolic group, the vas of the carboxylate shifts 41 cm-' to lower wavenumbers (from 1540 cm-' at pH 5 3 to 1499 cm-' at pH 10). We believe that these observations are important because they can be used as a diagnostic tool to study the participation of the phenolic group in surface complexation reactions. 2. Effect of Using Deuterium Oxide as the Solvent. Earlier, the need was mentioned for measuring the IR of surface complexes in H2O and D20 in order to increase the quality of the spectral information. Even though these two solvents are chemically similar, the substitution of one by the other may introduce substantial changes in the spectra of the dissolved adsorbates which have exchangeable H (D) atoms. This can occur by a variety of mechanisms such as a change in the value of the mass of the vibrating atoms, decoupling/coupling of vibrations, differences in the tendencies to form deute-
t 1
P
$ m a
a I
1764
I
1637
I
1510
I
1383
I
1256
I
1129
I
1002
WAVENUMBERS (cm-1)
Figure 4. CIR-FTIR spectra of p-hydroxybenzoic acid dissolved in DzO at different pD values and Z = 1M KCk (a) 0.025 M,pD 4.1, H ~ w= 0.03 au; (b) 0.1 M, pD 6.0, Hlaw = 0.14 au; (c) 0.1 M,pD 10.3,H 1 3 = ~ 0.07 au.
rium, and hydrogen bonds, At the same time, the isotopic exchange of hydrogen by deuterium serves equally well as pH to assist in band assignments. By comparing the spectra of phenol dissolved in D20 at pD 4.9 (Figure IC)and in H20 at pH 5.5 (Figure la), one can notice that the band at 1378 cm-l only exists in H2O solution spectrum and the one at approximately lo00 cm-' only in the spectrum in D2O. These observations confirm the previous band assignment of phenol at pH 5.5 (Figure la); that is, the bands a t 1378 and 1240 cm-l are associated with the bending and stretching of the phenolic group, respectively, and the rest of the bands in the spectrum with the group frequencies of the benzene ring. The main spectral difference caused by the change of HzO by D20 in benzoic acid is the band at 1277 cm-1, which moves to 1352 cm-l (Figure 2a and 2c). We previously assigned the band at 1277 cm-I to the v(C-OH) and S(C-0-H) carboxylic modes. Since S(C-0-D) sits at much lower energy ( ~ 9 8 cm-I 0 (ref 27)) than the v(COD), this type of coupling cannot exist upon deuteration of the carboxylic group. Thus, the shift to higher energies of the v(C-OD) vibration with respect to the v(COH) has to be interpreted as due to the removal of the coupling between the two carboxylic modes. The real position of the single-bond carboxylic stretching should be regarded as being closer to 1352 cm-l than to 1277 cm-'. This is important to keep in mind when we later discuss the interaction of a surface metal with this group. The spectrum of p-hydroxybenzoate in D2O at pD 6.0 (Figure 4b) shows some differences with the one at pH 5.5 (Figure 3b). These differences support our previous band assignment of the shoulder at 1370 cm-1 and the band at 1271cm-l that we associated with phenolic bending. These bands do not exist in D2O (Figure 4b), but the spectrum in D2O shows a new band at 1012 cm-l which is the expected position for deuterated phenolic bending. (27) Pinchas, S.; Laulight, I.; Infrared Spectra of Labelled Compounds; Academic Press: London, 1971. (28) Singh, S.;Rao, C. N. R. Can. J. Chem. 1966,44,2611.
984 Langmuir, Vol. 6, No. 5, 1990
Tejedor-Tejedor et al.
ID r
N
I
m m
It000
3683
31'83
2684
21'84
1685
11b5
6h6
WAVENUMBERS (Cm-l)
Figure 5. General features of CIR-FTIR spectra of goethite particles in aqueous suspension with adsorbed p-hydroxybenzoate (spectrum of suspension - spectrum of supernatant) and Z = 0.01 M KCl: (a) in H20, pH 3.6; (b) in DzO,pD 4.0, H8el = 0.26 au.
Contrary to prediction, the position of the phenolic stretching in p-hydroxybenzoate is as well influenced by isotopic exchange of H by D in the phenolic group, even though these atoms do not directly participate in this vibrational mode. The magnitude of the shift (18 cm-1, see Figures 3b and 4b) is very small in comparison with the one of the bending vibration (350 cm-l) in which the deuterium participates. This small shift to higher frequencies of the phenolic stretching mode upon deuteration is observed as well in phenol solutions. Unfortunately, in the case of phenol it is difficult to judge whether this shift is real or due to the existence of a negative band between 1255 and 1196 cm-l in the spectrum of phenol in D20 (Figure l a and IC).The shift of 16 cm-' to higher energies of this vibrational mode in p-hydroxybenzoate, when comparing the solutions at pD 10.3 and pH 10.0 (Figures 3c and 4c), should be due to the fact that hydrogen bonds are stronger than deuterium bonds.26~27 The largest differences in the spectral features of p-hydroxybenzoate in H20 and D2O are produced when the pH/pD is below the first dissociation constant of this acid (compare Figure 3a, pH 3.5; Figure 4a, pD 4.11, since at these pH/pD values the isotopic exchange takes place in both substituents of the benzene ring. B. Surface Complexes. 1. Isolation of the Spectra of Surface Complexes. Spectral subtraction of a supernatant (reference) from a suspension (sample) yields the spectrum of goethite plus the interfacial region. Since the spectral features of the bulk particles can be obtained from the CIR spectra of dry goethite, one should be able to determine the spectral bands produced by species in the interfacial region. Parts a and b of Figure 5 show the spectra of goethite with adsorbed p-hydroxybenzoate over the spectral range 4000-700 cm-'. Bands at 3120, 891, and 790 cm-l are assigned to subsurface lattice goethite groups.13~29Bands at 2989 cm-l, 1592 cm-l in Figure 5a, and 2216 and 1177 cm-l in Figure 5b are (29) Cambier, P. Clay Miner. 1986,21, 191.
3950
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WAVENUMBERS (cm-l)
Figure 6. Effect of specific adsorption on the hydration profiles of the CIR-FTIR spectra of aqueous suspensions of goethite particles, at Z = 0.01 M KCI and pH 4: (a) goethite only, He91 = 0.24 au; (b) goethite plus phthalate, Hag, = 0.10 au; (c) goethite plus p-hydroxybenzoate,H ~ e = l 0.14 au. assigned to OH/OD surface groups and interfacial H2O/ D20 (ref 13) (the band at 2989 cm-l is masking the band at 3120 cm-l produced by the subsurface lattice OH). The small sharp peaks observed in the region 1700-1000 cm-l correspond to interfacial p-hydroxybenzoate vibrations. By isolation of the spectral features of the surface organic complexes from the rest of the absorption bands in Figure 5a and 5b, a better understanding of the profiles of these surface complexes and, even more importantly, an exact identification of the position of overlapped or broad bands can be obtained. In order to obtain correct results in these spectral subtractions, both the goethite (reference) and the goethite with adsorbate (sample) spectra should have the same profile except for the spectral features of the surface complexes. However, we have previously observed13 that specific adsorption often results in changes in the profile of bands corresponding to interfacial H20/D20. This is due to a variation in the double-layer thickness caused by the alteration of surface charge produced by specific adsorption over a significant amount of surface area. In Figure 6a, we show the spectrum of goethite in aqueous suspensions at pH 4 and I = 0.01 M KC1. From comparison of the spectrum of goethite plus phthalate30 (Figure 6b) or goethite plus p-hydroxybenzoate (Figure 6c) at the same pH and I , it is apparent that there is a change in profile of the bands corresponding to the interfacial solvent. Changes in the band produced by the bending mode of the interfacial H2O/D20 and the wing of the band corresponding to the stretching of the interfacial D2O affect in a very important way the spectral profile in the IR region of interest for surface complexation studies. The correct way to perform spectral subtractions in these systems is to use as a reference a goethite suspension having such pH and I values that a hydration profile is produced matching that of the sample suspension containing the adsorbate. Thus we are really matching hydration profiles for reference and sample rather than specific experimental variables. This subtraction technique, however, is not without its problems. For example, in open air, co32-and HCOs (30)Yost, E. C.; Tejedor-Tejedor,M. I.;Anderson, M. A. Second paper of this study.
Langmuir, Vol. 6, No. 5, 1990 985
Characterization of Benzoic and Phenolic Complexes
may be adsorbed on the surface of goethite in suspensions above the first pK1 of HzC03 (pH = 6.3).31 Very often, goethite suspensions at pH values close to their PHIEP (PHIEPof goethite = 9.7)31 are needed to match the hydration profiles of the systems goethite-anionic adsorbate at pH values below 6.32 Under these circumstances, carbonate complexes can be present in the spectrum of the reference (with absorption bands at 1502 and 1340 cm-l) and not in the spectrum of the sample. Thus, while we may have matched the hydration profile for the system, we introduce after spectral substraction negative bands of carbonate that can result in shifts in the broad bands of adsorbed organics. In these cases, it would be better to use as a reference spectra a goethite-only suspension at a pH value near that of the sample but at higher I in order to decrease the double layer and obtain a hydration profile close to the spectrum of the sample. 2. Spectral Region of Interest. We have already discussed the need for using the vibrational modes of the ligands as a method of identifying surface complexes in metal oxide/oxo anion systems. Data in Table I1 indicate that most of the important spectral bands used for structural diagnosis in surface complexation studies of phenolic and benzoic groups are in the spectral range 1700-900 cm-l. Information on the 0-H/O-D stretching of the phenolic and carboxylic group would be very valuable, but these vibrational modes are obscured by the strong absorption bands of the solvent H20/D20 and as well by broad bands of interfacial OH/OD (40003000 cm-l/2700-2000 cm-l; see Figure 5a and 5b). 3. The Need for Using H2O and DzO.In the spectra of goethite-H20 suspensions, the bands due to the bending vibrations of the interfacial HzO are intense and broad enough to obscure the 1620-1750-cm-l region, while in the spectra of goethite-D2O suspensions, the bands produced by these bending modes cover the 1170-1300cm-l region. This means these two solvents can be used to open complementary spectral windows (see Figure 5a and 5b). 4. Correlation of Surface Complex Spectra with Their Structures. The simple observance of IR bands of organic molecules along with the interfacial HzO/ DzO and bulk solid band features in the spectra of goethite-organic adsorbate suspension systems does not necessarily imply anything beyond the fact that there is a higher concentration of organic molecules at the solidliquid interface of the particles compared with the bulk of the solution. Information on whether these organics are simply electrostatically accumulated at the surface or are coordinated with the Fe surface atoms can only be ascertained by an investigation of band disappearances, similarities, and shifts in the spectra of organics in the interfacial region when compared with the spectra of organics in the bulk solution at the same pH/pD and I values. In order to better interpret band shifts and similarities, we should follow the same guidelines as is done for the group frequencies of carboxylic and phenolic functional groups upon complexation with metals in solution or solid state. These guidelines are explained below. Guidelines. Single-crystal studies of metal carboxylates have established that when the value of Au (Au = Y,, - us of COO-) is smaller for the metal complexed form than for the uncomplexed one (ionic form), a bidentatemononuclear complex (both carboxyl oxygens form a ring structure with one Fe) is observed. However, when the (31) Zeltner, W. A.; Anderson, M. A. Langmuir 1988,4, 469. (32) Hansmann, D. D.; Anderson, M. A. Enuiron. Sci. Technol. 1985, 19, 544.
n
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WAVENUMBERS (cm-I)
Figure 7. CIR-FTIR spectra of benzoate within the interface region goethite/O.Ol M KC1 solution of H2O (or D2O) as a function of pH (pD): (a) pH 3.5, H1401 = 0.02 au; (b) pH 5.5, Hl39l = 0.009 au; (c) pD 3.9, Hl4m = 0.02 au; (d) pD 5.9, H1393 = 0.009 au.
value of Au is greater than in the ionic form, a monodentate-mononuclear complex is seen. In binuclear bridging complexes, Au will change little compared to an ionic form.ls Regarding the phenolic group, we have not found in the literature an exact description as to how complexation affects its group frequencies as compared to that of carboxylic functional groups. However, some logical guidelines can be followed in order to make these interpretations. Complexation, for example, will cause the disappearance of the Ph-0-H bending and a shift in the frequency value of the phenolic stretching due to a change in the strength of the Ph-0 bond. Further assistance in identifying these bands can come from comparing the protonation of the phenoxide group with the complexation of this group. Since the metal-oxygen bond is more ionic than the H-oxygen bond, we should expect a shift to higher frequencies of phenolic stretching upon complexation with the metal. This frequency should be of an intermediate value between that of the free anionic phenol and that of the protonated species. Example: Interfacial Benzoate. The spectra of benzoate within the interfacial region of goethite are seen in a-d of Figure 7. In H2O at pH 5.5 (Figure 7b), the band at 1391 cm-l, assigned to the us of the carboxylate group, was the only one clearly observable. Its frequency value is very close to the one for the same vibrational mode in ionic benzoate in solution (1388 cm-' of Figure 2b). Bands between 1593 and 1523 cm-' seem to be real and can be attributed to the u(C=C) and the u,(COO-) vibrations. For interfacial benzoate at pH 3.5 (Figure 7a), the us of carboxylate shifted to 1401 cm-l as compared to 1390 cm-l in solution (see Figure 2a). Also, the v ( C - 0 ) of the carboxylic was not present in the goethite-benzoate spectrum as it was in solution at this pH value (band at 1277 cm-l in Figure 2a, note that no significant peaks appear
986 Langmuir, Vol. 6, No. 5, 1990
Tejedor-Tejedor et al.
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Figure 8. CIR-FTIR spectra of Fe(II1)-benzoate complexes in solution at pH 1.8: (a) [Fe3+Iha = 0.03 M [benzlhM = 0.03 M, HI413 = 0.03 au; (b) [Fe3+Itoa= 0.4 M, [benzIhal = 0.02 M, H1413 = 0.01 au.
below 1300 cm-l in Figure 7a). Above 1500 cm-', the bands were somewhat indistinguishable from noise. The spectrum of interfacial benzoate in D2O goethite suspensions at pD 3.9, Figure 7c, provided additional information on the region between 1450 and 1700 cm-'. Along with the 1400-cm-' band of the u, mode of carboxylate, there are a broad group of bands which are in a similar position as might be expected for the asymmetric stretching mode (maximum a t 1526 cm-l). Furthermore, bands assigned to u(C=C) vibrations were evident a t 1446,1492, and 1593 cm-1. Absorption bands produced by the u(C-0) and u(C=O) modes of benzoic acid in DzO (1690 and 1352 cm-1 in Figure 2c) are not present in the spectrum of interfacial benzoate a t pD 3.9. In summary, spectral data of benzoate on goethite surfaces relative to benzoate in solution indicated that at pH 3.5 and pD 3.9, where the benzoate/benzoic acid mixture exists in solution, no bands of the protonated form are present in the spectra of adsorbed benzoate, and the u, mode of the carboxylate group shifted approximately 10 cm-1 to higher wavenumbers. Precise assessment of the u, mode was not certain because broad, poorly distinguishable bands characterized the 1520-1570-cm-' region. The spectra of benzoic acid dissolved in HzO, at pH 1.8 and in the presence of Fe(III), are represented by parts a (Fe/benzoic = 1:l) and b (Fe/benzoic = 20:l) of Figure 8. Although the presence of minor quantities of benzoic acid is indicated by the band a t 1274 cm-I (compare Figures 8 and 2a), the main benzoate species is the one responsible for the absorption bands a t 1413 and 1523 cm-l (Figure 8b). Invoking the Au value criterium for carboxylate complexation as well as the direction for the shift of the us, the position of these bands, that we assign to us and uas of the carboxylate group suggests bidentate coordination of benzoate with Fe(II1). The adsorption u,/absorption us ratio is considerably smaller for complexed benzoate (Figure 8) than for the ionic one (Figure 2b). The change in relative intensity of these modes of vibration upon complexation with iron is probably due to an increase in coplanarity between the benzene ring and the carboxylic group which in turn gives rise to coupling between the 8a vibrational mode of the benzene ring and the u,(COO-).~O It is well established that copla-
narity increases the stability of aromatic carboxylatemetal complexes.32 The benzene ring 8a vibration (1565 cm-I), which was not perceptible in the aqueous solution spectra of benzoic acid (see Figure 2b), can now be seen in the complex due to this coupling. The differences in the solution spectra of Figure 8a,b have been invoked by changing the ratio benzoate/Fe in solution. It is likely that in the case of less Fe, more organic ligands are complexed with a given iron atom. This multiple coordination of organic ligands about a central metal atom has been shown to produce multiple bands between 1400 and 1550 cm-l, with the multiplicity being caused by coupling between the organic groups bound to the same metal atom.33p34 At pH 5.5 and pD 5.9 in benzoate/goethite suspensions (Figure 7b and 7d), no detectable shifts of the symmetric stretching with respect to free benzoate in solution (i.e., no shift larger than the spectral resolution used for obtaining this spectrum) could be observed. However, the relative intensity of the two stretching modes of the carboxylate group resembles that of benzoateiron solution complexes and not the one for ionic benzoate in solution (compare Figures 7d, 8, and 2d). Since we interpreted this change in relative intensity as due to a change in coplanarity between the benzene ring and the carboxylate group upon complexation, the conclusion is that interfacial benzoate, under these pH conditions, is forming an inner-sphere complex with the geothite. This complex is similar to the one that forms on the surface of goethite at pH 3.5 and is similar to the complex which forms in solution but weaker. The frequency values of the symmetric and asymmetric stretching carboxylate in the benzoate-goethite systems vary with pH (Figure 7c and 7a). We interpret this result as a change in goethite surface charge (higher charge a t pD 3.9 than a t pD 5.9) that affects the strength of the surface complex. The symmetric mode is found a t 1413 cm-l when the residual charge on the Fe ion in solution is either +2 or +3. Since the residual charge of iron atoms at the goethite surface is smaller than iron in solution, we predict a shift to lower wavenumbers for a complex of this type on the surface. Even though the exact value of Au for the surface carboxylate complexes is difficult to assess from the spectra of Figure 7 due to broad bands in the region of the asymmetric stretching, it can be determined that Au is not smaller than 125 cm-l, which is too large for a bidentate bonding of the carboxylate. We believe that a bridging bidentate complex is the one being formed by the benzoate at the goethite surface and by analogy in solution. This complex can be pictured as follows:
I: Summary We have chosen three model organic molecules to show how CIR-FTIR spectroscopy can be used routinely to obtain the mid-IR spectra of chemical species adsorbed at the solid/aqueous solution interfaces. The sensitiv(33) Gould, E. S.Mechanism and Structure in Organic Chemistry; Holt, Reinhart and Winston: New York, 1959. (34) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry, 4th ed.; Wiley-Interscience: New York, 1980.
Langmuir 1990,6, 987-991 ity of the method can be optimized by subtracting from the spectrum of goethite plus adsorbate a spectrum of aqueous goethite only with similar hydration profile. The spectra of species at the solid/aqueous interface must be compared with the spectra of the species in aqueous solution, at the same pH, if meaningful information on the structure of the interfacial species is to be obtained. The use of both solvents, DzO and HzO, is needed in order to gain information in the spectral regions 17001500 and 1300-1100 cm-l, respectively. Moreover, the spectra of chemical species with exchangeable H/D atoms can be very different in each of the solvents. Consequently, the spectra of dissolved species in both solvents are needed as standards.
987
In some cases, information on the structure of the surface complexes can be gained by knowing the IR spectra of that metal-ligand complex in aqueous solution. As we have shown in this paper, we could not decide on whether or not benzoate formed a complex with the surface iron, at pH 5, without the IR studies of Fe-benzoate complexes in solution.
Acknowledgment. This work was funded by a contract from the Ecological Research Division, Office of Health and Environmental Research, US. Department of Energy (DE-FG02-87ER60508). We gratefully acknowledge all support received.
Effects of Ion Binding and Oily Conditions on the Adsorbed Monolayer of Crown Ether Compound at the Oil/Water Interface Tsutomu Watanabe,* Hideo Matsumura, Seiichi Inokuma, Tsunehiko Kuwamura, and Kunio Furusawa National Chemical Laboratory for Industry, Higashi, Tsukuba-shi, Ibaraki 305, Japan, Electrotechnical Laboratory, Umezono, Tsukuba-shi, Ibaraki 305, Japan, Faculty of Technology, Gunma University, Kiryu, Gunma 376, Japan, and Department of Chemistry, The University o f Tsukuba, Tennoudai, Tsukuba-shi, Ibaraki 305, Japan Received June 14, 1989. I n Final Form: December 4, 1989 To evaluate the adsorption-desorption behavior of ((octadecyloxy)methy1)-l8-crown-6, with and without complexing with metal ion at the oil/water interface, the II-A curve of the compound was measured under various salt conditions. The curve was analyzed in terms of the concept of adsorption equilibrium at the oil/water interface. It was found that a stable monolayer of the compound was formed at the interface under metal-complexing conditions, especially in the presence of Ba2+ ion in the water phase. These adsorbed molecules of the carrier cannot diffuse easily from the interface to nonpolartype oil phase when they complex with metal ions. The desorption rate of the carrier with metal ions from the interface to the oil phase showed a clear dependency on the oil solubility of the ion pair composed of the complex and counteranion.
Introduction A number of natural substances which increase the ion permeability of biological and artificial membranes have been identified.1-6 These mediators of ion transportation are classified into two groups, i.e., ion carriers and channel formers, which are called “ionophores” in general. The crown ether compounds are the most popular synthetic ionophores and can complex specific metal ions with high selectivity.2-6 Since these complexes are lipophilic, as a result of the hydrophobic nature of the
* Author to whom correspondence should be addressed National Chemical Laboratory for Industry, Higashi, Tsukuba-shi, Ibaraki 305, Japan. (1)Stark, G.Membrane Transport in Biology; Springer-Verlag:New York, Berlin, 1987;p 447. (2)Lamb, J. D.;Christensen, J. J.; Oscarson, J. L.; Nielsen, B. L.; Asay, B. W.; Izatt, R. M. J. Am. Chem. SOC.1980,102,6820. (3)Pedersen, C. J. Am. Chem. SOC.1967,89,7017. (4)Frensdorff, H. K. J. Am. Chem. SOC.1971,93,600. ( 5 ) Hirooka, M. Crown compounds; Kodansya: Tokyo, 1978; p 112.
crown ether’s rings, they can dissolve in nonaqueous media. When the transportation behavior across biological membranes is considered, the carrier mechanism can be modeled.‘+1° Such a mechanism involves a selective complex formation of the carrier with a specific metal ion at the membrane-solution interface and a diffusion process across the membrane. If the carrier has no affinity for the interface, ion transportation is determined principally by the diffusion process of the metal ion in the water phase and of metal-bound carrier and metal-free carrier in the oil phase. Actually, the adsorption of the carrier at the interface may occur to some degree, and (6)Izatt, R.M.;Bruening, R. L.; Bruening, M. L.; Lindh, G. C.; Christensen, J. J. Anal. Chem. 1989,61,1140. (7)Izatt, R. M.;Breuening, R. L.; Clark, G. A.; Lamb, J. D.; Christensen, J. J. J. Membr. Sci. 1986,28, 77. (8)Izatt, R. M.; Mcbride, D. W., Jr.; Brown, P. R.; Lamb, J. D.; Christensen, J. J. J. Membr. Sci. 1986,28,69. (9)Eisenman, G.; Sergio, S. M.; Szabo, G. Fed. R o c . 1968,27,1289. (10) McLaughlin, S. G. A.; Szabo, G.; Ciani, S.; Eisenman, G. J.Membr. Biol. 1972,9,3.
0743-7463/90/2406-0987$02.50/00 1990 American Chemical Society
