Langmuir 1986, 2, 203-210 used to eliminate the angle 8. Singularities in G ( h ) occur a t 8 = 90° or h = f(3 cos2 K -1)/2. Case H: Fast Reorientations about the C3Axis and the Director and Fast Change in 0. This is identical with case D as described previously. The complete NMR
203
spectrum is a pattern of three sharp resonance lines with an intensity ratios of 1:2:1. The satellite shifts are given by eq A25, where cos2 8 is replaced by (cos2 8). For these four cases, E-H,the calculated spectra are shown in Figure 8 for K = 90°.
“In Situ” Attenuated Total Reflection Fourier Transform Infrared Studies of the Goethite (a-FeO0H)-Aqueous Solution Interface M. Isabel Tejedor-Tejedor and Marc A. Anderson* Water Chemistry Program, University of Wisconsin-Madison, Madison, Wisconsin 53706 Received J u l y 30, 1985. I n Final Form: November 13, 1985 This paper examines the use of attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) as an “in situ” technique with which to study the goethite (a-FeO0H)-aqueous solution interface. Using a cylindrical internal reflection (CIR) cell with a ZnSe crystal we show that ATR infrared goethite spectra are similar to spectra recorded via transmission for bands arising from the bulk of the solid. These results are interpreted by using a thin film model. Interfacial spectra as a function of pH or pD, ionic strength, and nature of the “inert” electrolyte anion were also obtained. By employing this method we demonstrate that the positive and negative goethite particles induce structure on adjacent water layers. ”Inert” electrolytes affect this highly structured water in the order NO; > Cl- > Clod-. The signal intensity in the ATR-FTIR technique is largely dependent upon the dispersivity (state of aggregation) of the suspension; therefore, it is presently semiquantitative since an increase in goethite concentration (g/L) does not necessarily produce a proportional increase in signal intensity (unless the samples have identical degrees of dispersivity). Consequently,one must use internal standards, such as bulk OH or FeO groups, to compare peak heights for quantification purposes. Phosphate,which is known to form inner-sphere complexes with iron oxides, has also been examined both in solution and at the goethite interface by using ATR-FTIR. This technique shows that phosphate greatly perturbs the structuring of the water at the interface (more so than the “inert” electrolytes NO3-, C1-, and C104-) and it substitutes for surface OH groups. As such, ATR-FTIR should prove useful for studying surface complexation reactions in aqueous suspensions.
Introduction Our research is addressed to understanding those heterogeneous surface reactions that control the distribution of solute species between aqueous solutions and solid surfaces. Previous research on these interfacial reactions has largely resulted in model postulates concerning the molecularity of surface reactions since until now only indirect methods (e.g., adsorption isotherms) were available for these studies. Our own effort, therefore, has been directed toward finding a technique that identifies the chemical structure of (1)the adsorbent surface, (2) the adsorbate, and (3)any new species which may be formed by chemisorption. This information can be obtained from the interpretation of vibrational spectra of these chemical groups, and infrared spectroscopy is the most common method of studying vibrational modes. While IR studies have been successfully used for interpreting catalytic reaction mechanisms,l the use of IR in this research has largely been confined to identify reactions occurring at a gas-solid interface. In our studies, we are interested in characterizing the solid-water interfacial region. However, water has always been an anathema to the transmission mode of IR analysis. Water is such a strong absorber in the mid-infrared that extremely short-path-length cells are required in order to transmit enough energy to make useful measurements. Although such cells can be constructed with (1) Sheppard, N. N A T O Adu. S t u d y Inst. Ser., Ser. C 1980, 67.
some difficulty, they are almost impossible to fill or empty and are subject to clogging when used to study suspensions. Internal reflection spectroscopy (IRS) offers a solution to this problem. Since beam penetration into the solution is extremely small with this technique, effective path lengths of the correct magnitude for most aqueous solution studies (15 pm) can easily be obtained. Although internal reflection cells made with flat plates have been used previously in the analysis of aqueous sol u t i o n ~this , ~ ~crystal ~ design is not efficient with respect to energy throughput in the circular beam of the FTIR spectrometers. To avoid beam vignetting and a consequent increase in the energy throughput of the system, Wilks, in 1982, proposed a new internal reflection element (IRE) design employing a polished cylindrical rod with coneshaped ends.4 This cylindrical internal reflectance (CIR) cell has been shown to perform very well for qualitative and quantitative analysis of aqueous solution^.^^^ On the basis of these results, we decided to utilize this design for IR studies of aqueous colloidal suspensions. This paper evaluates the CIR technique as an “in situ” tool with which to study the surface chemistry of aqueous (2) Yang, R. T.; Low,M. J. D. Anal. Chem. 1973,45,2014. (3) Mattson, J. S.; Jones, T. T. Anal. Chem. 1976,48, 2164. (4)Wilks, P., Jr. Industrial Research and Development 1982, Sept, 132. (5) Messerchmidt, R. G. Scan Time 1983,2,3. (6)Wong, J. S.; Rein, A. J.; Wilks, D.; Wilks, P., Jr. Appl. Spectrosc. 1984,38,32.
0143-7463/86/2402-0203$01.50/0 0 1986 American Chemical Society
204 Langmuir, Vol. 2, No. 2, 1986 suspensions. Furthermore, this paper provides information on the influence of pH, ionic strength (I),and nature of the electrolyte on the interfacial structure of water in the goethite (a-FeO0H)-aqueous solution system. Solvent structure effects have recently “resurfaced” and they have been raising many important questions in colloid and surface ~ c i e n c e . If ~ one could “see”, via spectroscopy, the structure of water near interface, this would be very helpful in describing a variety of phenomenon occurring in the colloidal and surface science field. For example, this technique could then be used in the development of a model for the interaction of two electrical double layers which includes water polarization at the surface.&1°
Experimental Section Apparatus. The ATR apparatus is a cylindrical internal reflection cell which was obtained from Barnes Analytical (CIRCLE). Its internal reflection element is a rod-shaped crystal of ZnSe, 80 mm long and 6 mm in diameter. The energy drop for the ZnSe-HzO system is approximately 700 cm-l. The crystal is designed to have one average angle of light incidence of 40’ and 16 reflections, five of which “see” the sample. The sample holder is stainless steel “open boat” with a cavity 25 mm long and 3 mL in volume. The spectra were recorded interferometrically with either a 60SX or 170SX Nicolet Fourier transform infrared spectrometer and a broad-range MCT detector. The single-beam spectra recorded by using the CIRCLE cell were the result of 2000 coadded interferograms, The spectra were recorded between 4000 and 600 cm-’ at a resolution of four wavenumbers, with a gain set as high as possible and full aperture. HappGenzel apodization was used for all spectra. In every case, the spectrum of the particles in suspension is the result of subtracting the spectrum of the supernatant (reference) from the spectrum of the slurry (sample),both previously ratioed against the spectrum of the empty cell. The scale factor of both sample and reference has always been unity. The cell remains in place throughout the running of every single-beam spectra of the empty cell, reference, and sample so that its transmittance and average angle of incidence are constant. Sample Preparation. Samples for ATR consist of goethite (a-FeOOH)suspensions in either H20or DzO,adjusted to different pH and ionic strength values. Additionally, some samples are treated with sodium phosphate. The concentration of the slurry is 50 g/L, unless indicated otherwise. The goethite is added to the DzOsolvent and this suspension is sonicated intermittently over a 24-h period. The samples are left to reach equilibrium for 2-3 days. Aliquots for analysis are withdrawn from this stock suspension. In the case of DzO, the solid in the stock is separated by centrifugation and treated again with another aliquot of DzO in order to eliminate residual HzO. Ionic strength and pH are brought to steady-state conditions and left for at least 24 h. Prior to running the spectra, pH is checked again and adjusted if necessary by using the acid or base corresponding to the predominant anion or cation in the ionic media. All manipulations are performed under nitrogen atmosphere when DzO is employed. A short time before running the spectra, the samples are centrifuged; half of the supernatant is used as reference, the solid resuspended in the other half and used as the sample. This procedure yields a solid concentration of 100 g/L in the CIR cell. Goethite was prepared by using the method of Atkinson et al.” The OH/Fe ratio was 2 and aging time was 50 h. Morphology of the samples was checked with electron microscopy showing that the single crystallites are needle-shaped, having the (100) plane as their predominant face. These needles average 80 nm in length
Tejedor- Tejedor and Anderson
’
I
L u r- ’
Ya + s
W
/\v-,
‘
1140
/ Id
LL 1
/\ 1660(
4000
P i
/ L 3000
‘
l----_La 2000
1000
4000
WAVENUMBERS
3000
2000
loo0
(cm -1)
Figure 1. Infrared spectra of dry goethite (a-FeOOH): (a) diffuse reflectance spectrum of neat sample; (b) diffuse reflectance spectrum of sample mixed with NaCl, (c) transmission spectrum of goethite in KBr pellet (0.5% by weight); (d) attenuated internal reflection spectrum,using the CIR cell, of a goethite film deposited on the ZnSe rod.
and 35 nm in width. BET anaysis performed on these samples indicated a surface area of 80 m2/g. Electrophoretic mobilities of goethite suspensions were measured as a function of pH by using a PenKem System 3000 Electrokinetic Analyzer. The isoelectric pH of these particles is 9.5. The KBr in transmission FTIR is spectral degree in purity. DzOis 99.5% in DZ. Salts providing the ionic strength buffer are either NaC104,NaN03,or NaCl, all of them AR degree in purity. Diffuse reflectance is performed either on neat samplesof goethite or on goethite samples mechanically mixed with NaC1.
Results and Discussion Spectral Features of Goethite from Transmission, Diffuse Reflectance and Internal Reflection FTIR. Figure 1 shows the spectra of dry goethite recorded via transmission (in KBr pellets) (Figure IC),diffuse reflectance both neat (Figure la) and mixed (but not pressed) with NaCl (Figure lb), and internal reflectance (Figure Id). The diffuse reflectance spectrum of the neat powder contains the same bands as the transmission spectra of dry goethite films previously described in the literat~re.’~-’~ The band with a maximum at 3660 cm-’ has been assigned to surface OH groups which are both triply and doubly coordinated to iron; the peak with a maximum at 3487 cm-’ is due to surface OH singly coordinated to iron. A very intense band with a maximum at 3140 cm-’ has been identified as corresponding to the stretching frequencies of OH within the bulk iron oxide. The diffuse reflectance spectrum of the goethite sample mixed with NaCl (Figure lb) is better resolved than that of the neat sample (Figure la). Both have the same bands in the 3800-2200-cm-’ region but the broad band below 1000 cm-l of the neat sample becomes resolved in three (only two illustrated) strong sharp bands coming from Fe-0-Fe oxy-bridging vibrations and/or Fe-0-H bending vibrations. Agreement does not exist in the literature with regard to a more precise assignment of these two a-FeOOH bands. The transmission spectrum of this sample in KBr (Figure IC) is very similar to the diffuse reflectance spectrum of goethite mixed with NaCl (Figure lb); however, the small but sharp peaks assigned to the surface OH are not any longer obvious. A very broad shoulder appears between 3410 and 3330 cm-’ which could be due to some residual water in the KBr disc and/or to surface OH forming hydrogen bonds with the Br- ions. Figure I d shows the internal reflection spectrum of a dry goethite film on the
(7) Israelachvili, J. N.; Adams, G. E. J . Chem. SOC.,Faraday Trans. 1 1978, 74, 975.
(8) Radic, N.; Marcelja, S. Chem. Phys. Lett. 1978, 55, 377. (9) Gruen, D. W. R.; Marcelja, S. J . Chem. Soc., Faraday Trans. 2, 1983, 79, 225. (10) Macdonald, J. R.; Kenkel, S. W. J. Chem. Phys. 1984,80, 2168. (11) Atkinson, R. J.; Posner, A. M.; Quirk, J. P. J. Inorg. Nucl. Chem.
(12) Parfitt, R. L.; Russell, J. D.; Farmer, V. C. J. Chem. SOC.,Faraday Trans. 1 1976, 72, 1082. (13) Rochester, C. H.; Topham, S. A. J. Chem. SOC.,Faraday Trans. 1 1979, 75, 591. (14)Van der Woude, J. H. A,; de Bruyn, P. L. Colloids Surf. 1983,8,
1968, 10, 2371.
55.
Langmuir, Vol. 2, No. 2, 1986 205
ATR-FTIR Studies of Goethite-Aqueous Solution
2435
3305
1200
895
3045
2604
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3400
3260
2620
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WAVENUMBERS (crri’)
Figure 2. ATR-CIR spectra of goethite (a-FeOOH) suspended in water: (a) spectrum of slurry; (b) spectrum of supernatant; (c) difference spectrum (a) - (b). This system, pH 6.5, represents
goethite suspended in distilled deionized water. Goethite concentration was 100 g/L. ZnSe crystal. This spectra does not show the sharp peaks a t 3660 and 3488 cm-l coming from surface hydroxide, although there are small shoulders a t 3410 and 3530 cm-’. (We believe that peak broadening is due to the presence of residual water still found on the surface of the solid.) Comparison of this internal reflection spectrum with the transmission spectrum shows two main differences between their profiles. (a) In the transmission spectrum, the peak a t 795 cm-l is less intense than the one a t 897 cm-l. Meanwhile, in internal reflection the situation is reversed. (b) The ratio between the heights of the peaks at 897 and 3140 cm-l is equal to 1.7 in internal reflection compared to 2.5 in transmission. The first one (a) can be explained by considering that the thickness of the goethite film surrounding the crystal is larger than the penetration depth of the beam (d,) at 897 cm-’ since, under these conditions, the effective thickness would increase with X.15 However, if this is the only factor causing the difference of profiles between ATR and transmission spectra the ratio of peak height at 897 cm-l to that at 3140 cm-’ should be larger than 2.5. Figure 2, parts a and b, shows the spectra of an aqueous goethite suspension and its supernatant, respectively. The main features of these spectra are a broad band centered near 3305 cm-’, which arises from several fundamental stretching vibrational modes of the liquid water solution, and a band with maximum a t 1633 cm-l, coming from the bending mode of different types of water molecules that probably are present in liquid water. A more precise assignment of these bands is not unique in the literature but will depend on the model chosen for the liquid water structure. The nature of the models range from regarding the water as a mixture of discrete species with different number of hydrogen bonds per molecule (mixture model), to viewing the water as a completely hydrogen-bonded (15) Harrick, N.J. ’Internal Reflection Spectroscopy”; Harrick Scientific Corporation: Ossining, NY,1979.
3400
3260
2620
1
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WAVENUMBERS (crrl) Figure 3. ATR-CIR spectra of goethite suspended in D20: (a) slurry; (b) supernatant; (c) difference (a) - (b), pD 7. Goethite concentration was 100 g/L.
system but with a broad distribution of bond length and angles (“continuum” model).l6ls In any case, the position and the shape of these bands will be a function of the water temperature as well as the nature and concentration of the electrolyte. Moreover, the reference in every spectra should be the supernatant of the slurry and should be run a t the same temperature as the sample (slurry). Under these conditions, any change in the position and/or shape of these bands can be attributed solely to the presence of the solid particles and the interactions of these particles with water. In the internal reflection spectrum of goethite suspended in water (Figure 2c), the relative intensities of the peaks at 895 and 800 cm-’ are more comparable to those recorded in the transmission spectra (Figure IC)than in the internal reflection spectra of dry goethite (Figure Id). However, the relative intensity of the band assigned to bulk OH becomes even larger than in the internal reflectance spectrum of dry goethite and its maximum is shifted from 3140 to 3045 cm-’. In place of the shoulder a t 3410 and 3530 cm-l, the spectrum shows a more resolved and more intense band at 3400 cm-‘. In order to gain insight into the interpretation of this spectrum, we can separate bulk OH from surface OH by substituting D20 for H20. Figure 3 illustrates that the position and relative intensities of the absorption bands due to the chemical groups in the bulk of the solid, 3145, 898, and 800 cm-’, are the same as those in transmission (Figure IC). Furthermore, this spectrum in D20 has additional bands at 2604,2256, and 1219 cm-’. Since these bands are a consequence of the sample treatment with DzO,all of the bands should be produced by groups a t the solid-liquid interface (bands coming from the bulk solution are eliminated by subtracting the reference, Le., the su(16) More O’Ferrall, R. A.; Koeppl, G. W.; Kresge, A. J. J. Am. Chem. Soc. 1971,93,1. (17) Scherer, J. R.; Go, M. K.; Kint, S. J . Phys. Chem., 1970,79,4360.
(18) McCabe, W.C.; Subramanian, S.; Fisher, H. F. J. Phys. Chem. 1970, 79,4360.
206 Langmuir, Vol. 2, No. 2, 1986
Tejedor- Tejedor and Anderson
pernatant). The frequencies of these bands are in good agreement with the ones reported in the literature for the hydrogen-bonded FeO-D stretching vibration and stretching and being vibrations of hydrogen bonded DzO, respe~tively.’~J~ The stretching vibration band of bulk OH’Sand the band coming from the stretching vibrations of solvent molecules adjacent to the surface appear very well separated in the spectrum of goethite suspended in D,O (3145 and 2256 cm-’), Figure 3, but will overlap when the solvent is HzO and this overlapping explains the high relative intensity of the band at 3045 cm-’ of Figure 2 as well as the shift in its position. The stretching and bending frequencies of D20 in the spectrum of the bulk solution (Figure 3b) occur a t 2435 and 1200 cm-l, respectively. These two values anticorrelate with 2256 and 1219 cm-’ that we assigned previously to D,O at the surface and/or interface. This indicates that the mean D 2 0 molecule is more affected by hydrogen bonding in the interfacial region than in the bulk.20 In other words, the surface of goethite behaves as a promoter of structure in this solvent, which is in agreement with the findings of Dumont and Watillon in their studies of coagulation kinetics of ferric hydrosols.?’ Physical Arrangement of the Suspended Particles around the Crystal in the Cell. As we mentioned before, there is a remarkable resemblance between the part of the internal reflection and transmission spectra representing the chemical groups in the bulk of the goethite (Figure 3c and Figure IC). This feature is surprising if we assume that the signal is coming from particles distributed in a homogeneous suspension. We know15 that the intensity of the spectrum is related to the effective thickness, de: RN = (1- ad,)”‘ (1) where R is the reflectivity, N is the number of reflections, and a the absorption coefficient. For bulk materials, in which the thickness of sample (as in the case of these suspensions) is much greater than the penetration depth of the evanescent field (dp),the effective thickness is related to d, in the following way:
In this equation, nZ1refers to ratio of the refractive index of the water (or suspension) ( n z )to the crystal (nJ, the angle of incidence is 0, and E , refers to the electric field amplitude. On another hand d, is given by d, =
A1
2r(sin2 ’6
-
n21z)1/z
(3)
where A, = A/n,
This wavelength dependence in thick samples causes absorption bands at longer wavelengths to be relatively stronger in intensity when compared with their corresponding transmission spectra. It is only in the case of a thin sample film that the effective thickness is independent of wavelength
de =
nz1E02d 2 cos 8
~
(4)
(19) Nakamoto, K. “Infrared and Raman Spectra of Inorganic and Coordination Compounds”; Wiley: New York, 1978. (20) Cotton, F. A.; Wilkinson, G. ‘Advanced Inorganic Chemistry”, 4th ed.; Wiley-Interscience: New York, 1980. (21) Dumont, F.; Watillon, A. Discuss. Faraday SOC.1971, 52, 352.
2642
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0 19
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WAVENUMBERS (crrl)
Figure 4. Variations in the intensity of signal in ATR-CIR spectra of goethite in D20as a function of pD and I: (a) pD 11, I= M NaC1; (b) pD 8, M NaCl; (c) pD 4, M NaCl. Goethite concentrations used in all three samples are 100 g/L. when d
