Polarized ATR-FTIR Study of Smectite in Aqueous Suspension

Cliff T. Johnston* and Gnanasiri S. Premachandra. Soil and Environmental Sciences, 1150 Lilly Hall, Purdue University,. West Lafayette, Indiana 47907-...
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Polarized ATR-FTIR Study of Smectite in Aqueous Suspension Cliff T. Johnston* and Gnanasiri S. Premachandra Soil and Environmental Sciences, 1150 Lilly Hall, Purdue University, West Lafayette, Indiana 47907-1150 Received February 5, 2001. In Final Form: April 14, 2001 Na- and Ca-exchanged smectite in aqueous suspension have been studied using the polarized attenuated total reflectance (ATR)-FTIR technique. These spectra correspond to the first reported use of polarization methods to study smectite particles in aqueous suspension. Similar to the behavior of hydrous metal oxides, a thin, stable layer of smectite particles was found to coat the ZnSe internal reflection element (IRE) upon exposure to a dilute aqueous suspension (solids concentration of 10 g dm-3). The As spectrum was very similar to the transmission IR spectrum of the smectite indicating that the ATR-FTIR spectra are composed of a thin film of smectite in contact with a thick layer of water. As and Ap polarized in situ ATR-FTIR spectra of wet and dry Na- and Ca-SWy-1 indicated that the smectite particles were highly oriented with the (001) face parallel to the surface of the ZnSe IRE. The Ap polarized ATR-FTIR spectra clearly resolved the position of the perpendicular Si-O stretching vibration at 1075 cm-1 for Ca-SWy-1 and at 1084 cm-1 for Na-SWy-1. The thickness of the smectite deposit was estimated using a modified Beer’s law plot obtained using transmission IR methods. The absorption coefficient for the Si-O band was 3.6 × 104 cm-1, or 1.5 mAU per fundamental layer (0.96 nm) of smectite. On the basis of these values, the thickness of the smectite layer coating the ZnSe IRE was estimated to be in the range of 25-50 nm.

1. Introduction Smectites are an important class of nanosized materials that are used in many industrial processes related to catalysis, ion exchange, and sorption. Dioctahedral smectites (e.g., montmorillonite) have an approximate structural formula of Nax[SiaAl8-a]AlbMg4-bO20(OH)4, where x ) (12 - a - b) and are characterized by a large specific surface area (∼700 m2 g-1) and high surface charge (80120 cmolc kg-1). A projection of smectite along [010] is shown in Figure 1. Isomorphic substitution within the crystal structure produces a net negative surface charge that is compensated by exchangeable cations (e.g., Na+ and Ca2+). These cations have large hydration energies and impart a pronounced hydrophilic character to the clay. Since the first reported IR study of smectite-water interactions in 1937,1 IR and Raman methods have contributed significantly to our understanding of their structure, bonding, and reactivity. Vibrational methods have been used extensively to study guest molecules at the clay-water interface.2-4 In particular, considerable attention has focused on the vibrational properties of water itself on smectites.5-8 Much of the attention in prior IR and Raman studies has focused on the behavior of the sorbed species and not on the properties of the smectite itself. In this study, attenuated total reflectance (ATR)* Corresponding author. Phone: (765) 496 1716. E-mail: clays@ purdue.edu. (1) Buswell, A. M.; Krebs, K.; Rodebush, W. H. J. Am. Chem. Soc. 1937, 59, 2603. (2) Mortland, M. M. Adv. Agron. 1970, 22, 75. (3) Johnston, C. T.; Sposito, G.; Earl, W. L. Environmental Particles; Buffle, J., Van Leeuwen, H. P., Eds.; Environmental Analytical and Physical Chemistry Series, Vol. 2; Lewis Publ.: Boca Raton, FL, 1993; p 1. (4) Johnston, C. T. Organic pollutants in the environment; Sawhney, B., Ed.; Clay Minerals Society: Boulder, CO, 1996; p 1. (5) Russell, J. D.; Farmer, V. C. Clay Miner. Bull. 1964, 5, 443. (6) Poinsignon, C.; Cases, J. M.; Fripiat, J. J. J. Phys. Chem. 1978, 82, 1855. (7) Johnston, C. T.; Sposito, G.; Erickson, C. Clays Clay Miner. 1992, 40, 722. (8) Xu, W.; Johnston, C. T.; Parker, P.; Agnew, S. F. Clays Clay Miner. 2000, 48, 120.

Figure 1. [010] projection of the smectite structure showing the exchangeable cations and the water molecules in the interlayer region.

FTIR methods are used to examine the properties of the clay structure itself under hydrated conditions. Recent developments in ATR-FTIR methods have provided new insight into the structure and reactivity of hydrous metal oxides and clay minerals in aqueous suspension. Tejedor-Tejedor and co-workers9 were among the first to use ATR-FTIR methods to study the aqueous surface chemistry of goethite. When the ZnSe internal reflection element (IRE) of the cylindrical internal reflectance ATR cell was placed in contact with an aqueous suspension of goethite (R-FeOOH), a thin, stable layer of goethite particles was found to coat the IRE. The observed ATR-FTIR spectrum of the goethite-coated IRE was very similar to the transmission infrared spectrum of goethite and indicated that only a thin coating of goethite was present. In aqueous suspension, the ATR-FTIR spectrum (9) Tejedor-Tejedor, M. A.; Anderson, M. A. Langmuir 1986, 2, 203.

10.1021/la010184a CCC: $20.00 © 2001 American Chemical Society Published on Web 05/17/2001

ATR-FTIR Study of Smectite in Aqueous Suspension

was made up of a thin coating of goethite and a thick layer of water in contact with the goethite. Subsequently, ATRFTIR methods have been used to study the aqueous surface chemistry of goethite,10-17 hematite,18-21 rutile and anatase,22-24 various aluminum oxides,11,25-28 smectite,29-32 and kaolinite.32,33 By controlling the solution phase in contact with these clay- and oxide-modified IREs, these methods provide a very useful in situ method to study the interfacial surface chemistry of clay minerals and oxides in the presence of water. There has been some debate recently about the assignment of the ν(Si-O) bands of smectites. Four ν(Si-O) bands occur in the spectra of dioctahedral smectites at 1120, ∼1080, 1150, and 1110 cm-1.34,35 On the basis of pleochroic studies, the 1086 cm-1 band was assigned to the perpendicular, out-of-plane, ν(Si-O) mode (related to the a11 mode of talc).35 Recent infrared external reflectance studies of smectites support these assignments.36,37 Recently, Yan et al.31,38 showed that the positions and relative intensities of the 1046 and 1020 cm-1 bands are influenced by water content. They speculated that the Si-O stretching vibrations responsible for these bands are somehow coupled to the vibrations of interfacial water molecules, although no details about the proposed coupling mechanism were provided. In a related study, Shewring and co-workers39 reported that the perpendicular ν(Si-O) band at 1086 cm-1 of smectite decreased upon lowering the water content. They assigned the 1086 cm-1 band to the (10) Barja, B. C.; Tejedor-Tejedor, M. I.; Anderson, M. A. Langmuir 1999, 15, 2316. (11) Biber, M. V.; Stumm, W. Environ. Sci. Technol. 1994, 28, 763. (12) Ostergren, J. D.; Bargar, J. R.; Brown, G. E.; Parks, G. A. J. Synchrotron Radiat. 1999, 6, 645. (13) Ostergren, J. D.; Trainor, T. P.; Bargar, J. R.; Brown, G. E.; Parks, G. A. J. Colloid Interface Sci. 2000, 225, 466. (14) Ostergren, J. D.; Brown, G. E.; Parks, G. A.; Persson, P. J. Colloid Interface Sci. 2000, 225, 483. (15) Sun, X. H.; Doner, H. E. Soil Sci. 1996, 161, 865. (16) Tickanen, L. D.; TejedorTejedor, M. I.; Anderson, M. A. Langmuir 1991, 7, 451. (17) Tickanen, L. D.; TejedorTejedor, M. I.; Anderson, M. A. Langmuir 1997, 13, 4829. (18) Bargar, J. R.; Reitmeyer, R.; Davis, J. A. Environ. Sci. Technol. 1999, 33, 2481. (19) Bargar, J. R.; Reitmeyer, R.; Davis, J. A. Environ. Sci. Technol. 1999, 33, 2481. (20) Eggleston, C. M.; Hug, S.; Stumm, W.; Sulzberger, B.; Afonso, M. D. Geochim. Cosmochim. Acta 1998, 62, 585. (21) Hug, S. J. J. Colloid Interface Sci. 1997, 188, 415. (22) Hug, S. J.; Sulzberger, B. Langmuir 1994, 10, 3587. (23) Kesselman-Truttmann, J. M.; Hug, S. J. Environ. Sci. Technol. 1999, 33, 3171. (24) Kavan, L.; Krtil, P.; Gratzel, M. J. Electroanal. Chem. 1994, 373, 123. (25) Nordin, J.; Persson, P.; Laiti, E.; Sjoberg, S. Langmuir 1997, 13, 4085. (26) Persson, P.; Karlsson, M.; Ohman, L. O. Geochim. Cosmochim. Acta 1998, 62, 3657. (27) Su, C. M.; Suarez, D. L. Clays Clay Miner. 1997, 45, 814. (28) Wijnja, H.; Schulthess, C. P. Spectrochim. Acta, Part A 1999, 55, 861. (29) Weissmahr, K. W.; Haderlein, S. B.; Schwarzenbach, R. P. Environ. Sci. Technol. 1997, 31, 240. (30) Hunter, D. B.; Bertsch, P. M. Environ. Sci. Technol. 1994, 28, 686. (31) Yan, L. B.; Roth, C. B.; Low, P. F. Langmuir 1996, 12, 4421. (32) Kubicki, J. D.; Schroeter, L. M.; Itoh, M. J.; Nguyen, B. N.; Apitz, S. E. Geochim. Cosmochim. Acta 1999, 63, 2709. (33) Su, C.; Suarez, D. L. Environ. Sci. Technol. 1995, 29, 302. (34) Ishii, M.; Shimanouchi, T.; Nakahira, M. Inorg. Chim. Acta 1967, 1, 387. (35) Farmer, V. C. The infrared spectra of minerals; Farmer, V. C., Ed.; Mineral Society: London, 1974; p 331. (36) Karakassides, M. A.; Petridis, D.; Gournis, D. Clays Clay Miner. 1997, 45, 649. (37) Karakassides, M. A.; Gournis, D.; Petridis, D. Clay Miner. 1999, 34, 429. (38) Yan, L. B.; Stucki, J. W. Langmuir 1999, 15, 4648. (39) Shewring, N. I. E.; Jones, T. G. J.; Maitland, G.; Yarwood, J. J. Colloid Interface Sci. 1996, 176, 308.

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development of the electrical double layer that occurred when the clay is hydrated and noted that the intensity of this band is strongly diminished in the spectrum of dry deposits. Polarization of the ν(Si-O) bands was not considered in these earlier papers.31,39 The objective of this study is to examine the IR bands of smectite in aqueous suspension using polarized ATRFTIR methods with emphasis on the ν(Si-O) bands. Polarized ATR-FTIR methods have been used successfully to study the orientation of guest molecules at interfaces. However, this method has not been used previously to study the vibrational modes of smectites in aqueous suspension. Smectite is an ideal candidate for study using polarized ATR-FTIR methods because the aspect ratio of smectite particles is exceedingly high (>1000). In addition, the thickness of the smectite film on the ZnSe IRE is estimated based on a modified Beer’s law plot of the Si-O stretching bands. ATR-FTIR methods have provided new insight into the surface structure and reactivity of positively charged hydrous metal oxides. This study seeks to extend this approach to the study of negatively charged, expandable clay minerals. 2. Experimental Section SWy-1 smectite (Crook County, WY) was obtained from the Source Clays Repository of The Clay Minerals Society.40 SWy-1 is a low-charge smectite with substitution in both the octahedral and tetrahedral sheets and a cation exchange capacity of 80 cmolC kg-1. The structural formula is M+0.62[Si7.80Al0.20][Al3.28Fe(III)0.3 Fe(II)0.04Mg0.38]O20(OH)4.41 Prior to size fractionation, a homoionic montmorillonite suspension was prepared by placing 10 g of the clay in 1.0 L of 0.5 M NaCl. The resulting suspension was washed free of excess salts by repeated centrifugation with distilleddeionized water. The 1000); thus, the particles are preferentially aligned with the (001) surface of smectite particles oriented parallel to the window surface. The cross-sectional area of one unit cell of smectite across (001) is 0.45 nm2/unit-cell. The layer thickness of the deposit was determined by dividing the total lateral area of the smectite particles by the area of the deposit (4 cm2). Although some migration of the clay particles occurs upon drying, removing the deposit around the edges is assumed to remove the poorly aligned smectite particles.

3. Results and Discussion Polarized ATR-FTIR Spectra of Ca-SWy-1 Smectite. Polarized ATR-FTIR spectra of Ca-SWy-1 smectite in aqueous suspension are shown in Figure 3 in the 1800800 cm-1 region. The most intense band in this region is the H-O-H bending band of water (ν2 mode) at 1635 cm-1. In addition, smectite bands are observed at 1117, 1075, 1046, 1020, 918, 885, and 843 cm-1. These spectra were obtained by filling the horizontal trough ATR cell with the dilute aqueous suspension of Ca-SWy-1 smectite. The smectite bands in the 1000-1120 cm-1 region were observed immediately upon filling the horizontal trough ATR cell. The intensities of the bands in the Si-O stretching region were not diminished by removal of the clay suspension from the ATR cell followed by gently washing the surface of the ZnSe IRE with water. This indicates that the IRE is coated with a thin, stable layer of smectite particles. This result is in agreement with similar ATR-FTIR studies of TiO242 and goethite32,43,44 coated IREs. (42) Stirling, W. C.; Goodrich, M. A.; Frost, R. L. IEEE Control Syst. Mag. 1996, 16, 66.

Johnston and Premachandra

The positions of these bands are in good agreement with values obtained from transmission35,45 and ATR studies of smectites.31,39 The bands at 1020, 1046, and 1117 cm-1 have been assigned to in-plane Si-O stretching bands.34,35 Splitting of the ν(Si-O) bands into four components is due, in part, to the lower symmetry of the siloxane layer of dioctahedral smectites.35 The ATR-FTIR spectra of Ca-SWy-1 smectite particles coating the IRE are highly sensitive to the polarization of the incident beam (Figure 3). The As spectrum was obtained with IR radiation that is polarized perpendicular to the plane of the incidence (represented by E⊥ in Figure 2). Two strong ν(Si-O) bands at 1046 and 1020 cm-1 are the dominant features along with the less intense band at 1117 cm-1. Spectrum Ap (Figure 3) was obtained with polarization parallel to the plane of incidence (represented by E|| in Figure 2). The most prominent difference between the As and Ap spectra is the significant increase in intensity of the 1075 cm-1 band in the Ap spectrum. The relative intensities of the 1046 and 1020 cm-1 bands are weakly influenced by polarization. The positions, widths, and dichroic ratios for the bands in the As and Ap spectra are listed in Table 1. The dichroic ratio, RATR, is defined as eq 1.

RATR )

As A⊥ ) Ap A|

(1)

To the best of our knowledge, polarized ATR-FTIR spectra have not been reported previously for smectites. However, Margulies and co-workers46 used IR transmission methods to measure the linear dichroism values for highly oriented smectite deposits. Their values are included in Table 1 for comparison. The dichroic ratios of 0.2 and 0.4 for the 1075-1088 cm-1 band of Ca-SWy-1 and Na-SWy-1 in aqueous suspension indicate that the smectite particles are highly oriented on the surface of the ZnSe IRE with the 001 plane of the smectite particles oriented parallel to the surface of the ZnSe IRE. An expanded region of the As and Ap spectra (water subtracted) is shown in Figure 4. For comparison, a transmission IR spectrum of a highly oriented deposit of Ca-SWy-1 smectite is included in Figure 4. There is good agreement between the As polarized ATR-FTIR spectrum and that of the deposit; however, the 1020 cm-1 band has lower intensity in the transmission IR spectrum of the deposit. On the basis of prior ATR-FTIR studies, this similarity indicates that the spectra are composed of a thin film of smectite and a thick layer of aqueous solution.9 These data indicate that the smectite particles attached to the ATR crystal are highly oriented in contact with the aqueous solution with the (001) face of the particles oriented parallel to the surface of the ZnSe IRE. The aqueous Ca-SWy-1 suspension was removed from the horizontal trough ATR cell, and the smectite film coating the IRE was allowed to dry under ambient conditions. Polarized ATR-FTIR spectra of the wet and dry smectite deposits are shown in Figure 5, and the band positions and dichroic ratios are given in Table 1. The ATR-FTIR spectra of the dry deposit show very similar polarization (i.e., dichroic) behavior in the Si-O stretching region. For both the wet and dry deposits, the 1075 cm-1 (43) Peak, D.; Ford, R. G.; Sparks, D. L. J. Colloid Interface Sci. 1999, 218, 289. (44) Yost, E. C.; Tejedor, M. I.; Anderson, M. A. Environ. Sci. Technol. 1990, 24, 822. (45) Farmer, V. C.; Russell, J. D. Spectrochim. Acta 1964, 20, 1149. (46) Margulies, L.; Rozen, H.; Banin, A. Clays Clay Miner. 1988, 36, 476.

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Figure 3. Polarized ATR-FTIR spectra of Ca-SWy-1 smectite in aqueous suspension containing 10 gclay/LH2O in the 1800-800 cm-1 region. The As and Ap spectra of the smectite-coated IRE element were obtained using a wire-grid polarizer oriented perpendicular and parallel to the plane of incidence, respectively. In addition, the ATR spectrum of water is shown. Table 1. Band Positions, Widths, and Dichroic Ratios for Wet and Dry Ca- and Na-Exchanged SWy-1 Smectite Ca-SWy-1 Smectite band position

bandwidth

dichroic ratio

wet

dry

wet

dry

wet

dry

1117 1075 1046 1021

1118 1079 1046 1020

18 43 22 38

25 43 24 46

1.69 0.20 1.01 1.16

1.39 0.28 0.99 1.01

Na-SWy-1 Smectite band position

bandwidth

dichroic ratio

literature (ref 46)

wet

dry

wet

dry

wet

dry

position

DR

1116 1084 1047 1024

1115 1088 1045 1021

20 32 19 38

28 32 27 44

0.80 0.40 0.70 0.70

0.82 0.12 1.18 1.37

1088 1042

0.46 1.25

band is highly polarized. The largest difference between the wet and dry spectra is the significant decrease in intensity of the OH stretch and HOH bending bands of water. Surprisingly little variation, however, is observed in the ν(Si-O) bands between the spectra obtained in aqueous suspension versus those of the air-dried film on the IRE. The position of the perpendicular Si-O band increases in frequency by 4 cm-1 upon drying. The intensities of the structural OH bending modes of the smectite at 920, 875, and 840 cm-1 are slightly more intense and better resolved in the spectrum of the dried smectite film compared to that in aqueous suspension. It is interesting to note that the dichroic ratio of the 1075 cm-1 band (apical Si-O stretch) is lower in the wet deposit compared to the dry deposit. A lower ratio is indicative of a higher degree of orientation indicating that the smectite particles are more highly oriented under hydrated conditions. Polarized ATR-FTIR Spectra of Na-SWy-1 Smectite. Polarized ATR-FTIR spectra of Na-SWy-1 are shown in Figure 6, and the band positions and dichroic ratios are given in Table 1. The positions of the three bands in the As spectrum at 1117, 1046, and 1020 are similar to those in the As spectrum of Ca-SWy-1. However, the position of the apical ν(Si-O) band (observed in the Ap spectrum) is

shifted ∼10 cm-1 to higher energy and is more resolved in the Na-SWy-1 spectrum. In aqueous suspension, the intensity of the ν(Si-O)apical band has appreciable intensity in the As polarization unlike its behavior in the Ca-SWy-1 system. In contrast, the dichroic ratio for the dried deposit of Na-SWy-1 is 0.12, indicating a very high degree of orientation. Linear dichroism values46 for the ν(Si-O) bands of Na-SWy-1 smectite reported were 0.46 for the 1088 cm-1 band and 1.25 for the 1042 cm-1 band (Table 1). The small RATR value obtained in this study indicates a very high degree of orientation of the Na-SWy-1 particles on the ZnSe crystal for the dried deposit. Ap polarized ATR-FTIR spectra of Na-SWy-1 and Ca-SWy-1 smectite are compared in Figure 7. The positions and relative band intensities of the in-plane ν(Si-O) bands are similar for the two smectites. However, the position of the out-ofplane ν(Si-O) band is clearly influenced by the type of exchangeable cation. In prior ATR-FTIR studies of hydrous oxides in aqueous suspension, the particles have been considered to be randomly oriented and isotropic.42 In other cases, the use of cylindrical-shaped IREs precluded the observation of polarized ATR-FTIR spectra.16,17,44,47 Polarized ATR-FTIR methods are commonly used to determine orientation of adsorbed species at solid-liquid and solid-air interfaces.48 However, to the best of our knowledge, polarized ATRFTIR methods have not been used to study the orientation of clay or oxide particles attached to an IRE. Numerous studies have shown that the dichroic ratio DR ) As/Ap is a useful parameter to determine the average orientation of adsorbed molecules.49,50 In this study, we use this parameter to determine the orientation of adsorbed particles at the IRE-solution interface. In addition, this (47) TejedorTejedor, M. I.; Yost, E. C.; Anderson, M. A. Langmuir 1992, 8, 525. (48) Fringeli, U. P. Internal Reflection Spectroscopy. Theory and Applications; Mirabella, F. M., Ed.; Marcel Dekker: New York, 1993; p 255. (49) Vallant, T.; Kattner, J.; Brunner, H.; Mayer, U.; Hoffmann, H. Langmuir 1999, 15, 5339. (50) Picard, F.; Buffeteau, T.; Desbat, B.; Auger, M.; Pezolet, M. Biophys. J. 1999, 76, 539.

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Figure 4. As and Ap polarized ATR-FTIR spectra of Ca-SWy-1 smectite in aqueous suspension containing 10 gclay/LH2O in the 1200-950 cm-1 region. The spectrum shown at the top corresponds to an IR transmission spectrum of a Ca-SWy-1 smectite deposit on a ZnSe window.

Figure 5. Influence of water content on the ATR-FTIR spectra of Ca-SWy-1 smectite. As and Ap polarized ATR-FTIR spectra of Ca-SWy-1 smectite in the 4000-700 cm-1 region. The two spectra shown in the upper portion correspond to the As and Ap spectra of Ca-SWy-1 smectite in aqueous suspension. The two spectra in the lower portion correspond to a dry deposit of Ca-SWy-1 on the ZnSe IRE. These spectra were obtained by removing the aqueous suspension, gently washing the IRE cell with distilled-deionized water, and allowing the remaining smectite film to dry. The As and Ap spectra are shown by solid and dashed lines, respectively.

experimental approach may hold considerable promise to study the orientation of adsorbed species at the claywater interface. The out-of-plane ν(Si-O) band is clearly resolved in the Ap polarized ATR-FTIR spectra of both Ca- and Na-SWy-1 smectite. The observed dichroic ratios listed in Table 1 confirm the original assignments proposed by Farmer and Russell35 (see Table 2) and provide new insight about the assignment of the 1084-1075 cm-1 band proposed by Shewring39 who suggested that this band was associated with the formation of the diffuse double layer. As is clearly shown in Figure 6, the intensity of the perpendicular Si-O band at 1084 cm-1 (Na-SWy-1) is strongly dependent on

water content. Under hydrated conditions, the smectite particles have a more random orientation at the IREsmectite-water interface. In this configuration, the 1084 cm-1 band has appreciable intensity. Upon allowing the smectite deposit to dry on the IRE, the smectite particles become highly oriented with the (001) face of the smectite particles parallel to the surface of the IRE. Under these conditions, the intensity of the 1084 cm-1 band has very little intensity in the As polarized ATR-FTIR spectrum. In contrast, this band has significant intensity in the Ap polarized spectrum. The observations by Shewring39 are reinterpreted here and can be explained on the basis of polarization effects.

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Figure 6. Polarized ATR-FTIR spectra of Na-SWy-1 smectite in aqueous suspension containing 10 gclay/LH2O (upper portion) and as a dried deposit in the 1200-900 cm-1 region.

Figure 7. Comparison of the Ap polarized ATR-FTIR spectra of Na- and Ca-exchanged SWy-1 smectite in the 1200-950 cm-1 region.

In a related ATR-FTIR study of smectite in aqueous suspension, a nonspecific coupling mechanism between the in-plane ν(Si-O) bands at 1044 and 1020 cm-1 and the vibrations of interlayer water molecules was invoked to account for changes in the position and relative intensities of these bands as a function of water content.31 They normalized the intensities of the four ν(Si-O) bands to the total intensity in the ν(Si-O) region and observed that relative intensities of the 1044 and 1020 cm-1 bands increased at lower water content. Taking into account the polarization results obtained here, their data can be explained on the basis of smectite particles taking on a more preferred orientation at the lower water content

without invoking a coupling mechanism of unknown origin. The clay suspensions studied here were relatively dilute (1 wt % or 10 gsmectite dm-3H2O). The corresponding solids concentration in the Yan et al.31 study ranged from 23 to 1400 gsmectite dm-3. Preferred orientation at the high solids concentration is probable. In addition, they observed a shift to lower energy of the positions of the 1044 and 1020 cm-1 bands. As shown in Figure 4, the As polarized ATR-FTIR spectra of smectite obtained in this study from dilute aqueous suspensions are in close agreement to transmission IR spectra of thin deposits. The similarity between the ATR-FTIR and transmission IR spectra are expected under the thin-film limit and indicated that a

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Table 2. Band Assignments position

assignment

3630 3361 3253 2141 1635 1117 1075 1046 1020 917

ν(OH) structural OH groups (ref 35) ν(OH) sorbed water (ref 8) combination band of H-O-H bending vibration (ref 8) combination of H-O-H bend and libration (ref 8) H-O-H bending vibration of water (ν2 mode) (ref 8) in-plane Si-O stretch (refs 34 and 35) out-of-plane Si-O stretch (refs 34 and 35) in-plane Si-O stretch (refs 34 and 35) in-plane Si-O stretch (refs 34 and 35) OH deformation band of the Al2OH group (δ(OH)) (ref 35) OH deformation band of the Fe3+AlOH group (δ(OH)) (ref 35) OH deformation band of the MgAlOH group (δ(OH)) (ref 35)

884 843

Figure 8. Modified Beer’s law plot of the absorbance of the ν(Si-O) cm-1 band measured at 1046 cm-1 plotted as a function of the deposit thickness expressed in nm.

thin film of smectite was present in contact with a relatively thick layer of water. At the high solids concentration used by Yan et al.,31 however, the thin-film approximation would no longer apply and the position of the ν(Si-O) bands is influenced by the thickness of the clay deposit at the IRE. Optical Estimation of Film Thickness. To test the thin-film approximation, the thickness of the smectite deposit on the ZnSe IRE was estimated using the optical density in the ν(Si-O) region measured using transmission methods. Smectite deposits of known thickness were prepared on an IR transparent window. A modified Beer’s law plot of the 1045 cm-1 Si-O stretching band of NaSWy-1 smectite plotted as a function of the thickness of the clay deposit is shown in Figure 8. The thickness of the clay deposit is represented as the average number of clay layers (the thickness of one clay layer is 0.96 nm). A linear relationship was observed with an R2 value of 0.93 and a y-intercept near zero indicating that Beer’s law was operative under these conditions. The slope of the modified Beer’s law plot was 0.0015 AU/layer. Thus, the absorbance of one fundamental layer of smectite (0.96 nm) in the ν(Si-O) region is 1.5 mAU. The absorption coefficient of thin films of SiO2, R, is defined as

R)

D (2.303 d )

max

Si-O band(@1046cm-1) is 3.60 × 104 cm-1. For comparison, values ranging from 1.35 to 2.54 × 104 cm-1 were reported by Trchova.51 The slightly larger value obtained here is attributed to the preferential orientation of the siloxane surface parallel to the depositional surface, thereby maximizing the effective cross section of the Si-O bonds to the incident IR radiation. One of the intriguing aspects of the measured absorption coefficient of the ν(Si-O) band of smectite is its magnitude. The sensitivity of modern FTIR spectrometers is in the range of 10-5 AU.52 On the basis of this sensitivity and the absorption coefficient determined from above, individual layers of smectite can be detected. Under these conditions, the peak ν(Si-O) absorbance for the Na-SWy-1 smectite is approximately 0.2 AU. The number of reflections used in this ATR-FTIR cell was nine corresponding to 0.02 AU/ reflection. On the basis of the measured absorption coefficient, the smectite film is estimated to be ∼15 layers thick. The d spacing of the smectite on the ATR crystal in contact with the aqueous solution is not known. For the purpose of this calculation, two layers of water molecules corresponding to a d spacing of 1.5 nm is reasonable. On the basis of this value, the thickness of the smectite deposit is ∼22 nm. The effective path length of the ATR-FTIR cell was measured to be 11.3 µm at 1635 cm-1 based on the molar absorptivity of the 1635 H-O-H bending band. The corresponding penetration depth based on the index of refraction of water is 181 nm. Thus, the thickness of the smectite deposit is significantly less than the effective penetration depth and the thin-film approximation does apply. This calculation provides a rough estimate for the thickness of the smectite deposit on the surface of the ZnSe IRE. Quantitative ATR-FTIR measurements are complicated by the wavelength effect,53 and a detailed quantitative analysis is beyond the scope of this paper. This calculation is included here to show that the spectra are composed of a very thin layer of smectite and a relatively thick layer of water. These results are in qualitative agreement with the ATR-FTIR study of goethite reported by Tejedor-Tejedor and co-workers9 who reported similar findings. In summary, we have shown that ATR-FTIR spectra of a smectite coating in contact with an aqueous solution can be obtained. These spectra are highly polarized resulting from the orientation of the smectite particles on the ZnSe IRE. In addition, the nature of the exchangeable cation (Na+ versus Ca2+) influenced the position of the Si-O band assigned to the apical oxygens. The coupling of the ν(Si-O) band to water vibrations proposed in an earlier ATR-FTIR study of smectite is re-evaluated in this paper and assigned to orientation effects. By use of a modified Beer’s law equation obtained from transmission IR spectra of smectite, the thickness of the smectite deposit coating the ATR element is 22 nm. In addition, we have shown that single-layer detection of smectite particles using ATR or transmission methods that may have significance for recent surfactant-modified smectite applications using Langmuir-Blodgett films54-57 is possible. LA010184A

(2)

where d is the film thickness and Dmax is the peak absorbance.51 The thickness of each deposit (data point shown in Figure 8) was determined, and the absorbance of the ν(Si-O) bands was measured. From eq 2, the experimentally determined absorption coefficient for the (51) Trchova, M.; Zemek, J.; Jurek, K. J. Appl. Phys. 1997, 82, 3519.

(52) Griffiths, P. R.; deHaseth, J. A. Fourier transform infrared spectrometry; John Wiley & Sons: New York, 1986. (53) Internal Reflection Spectroscopy. Theory and Applications; Mirabella, F. M., Jr., Ed.; Marcel Dekker: New York, 1993. (54) Hotta, Y.; Taniguchi, M.; Inukai, K.; Yamagishi, A. Langmuir 1996, 12, 5195. (55) Hotta, Y.; Inukai, K.; Taniguchi, M.; Yamagishi, A. J. Electroanal. Chem. 1997, 429, 107. (56) Song, C. J.; Villemure, G. J. Electroanal. Chem. 1999, 462, 143. (57) van Duffel, B.; Schoonheydt, R. A.; Grim, C. P. M.; De Schryver, F. C. Langmuir 1999, 15, 7520.