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DIVISION S-2—"PARTICLE INTERACTIONS IN COLLOIDAL SYSTEMS"

Small-Angle X-Ray Scattering Study of the Quasi-Crystal Structure of Montmorillonite-CTAB in Suspension

^a Dept. of Chemistry and Biochemistry, South Dakota State Univ., Brookings, SD 57007-0896
^b Solid State Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831

* Corresponding author (james_rice{at}sdstate.edu)

	ABSTRACT

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The quasi-crystals of hexadecyltrimethylammonium (CTA⁺)-montmorillonite formed in suspension were investigated by small-angle x-ray scattering (SAXS). The d-spacing of the quasi-crystals increased from 17.1 to 21.7 Å, corresponding to an increasing surfactant loading from 20 to 250% of the clay's cation exchange capacity (CEC). One-dimensional grating theory was used to describe the scattering in the Porod region so that the number of clay layers and crystal size were obtained. The quasi-crystal reaches maximum size at a surfactant loading equal to the clay's CEC. The scattering peaks in the Porod region are due solely to quasi-crystals. Small-angle x-ray scattering avoids intermediate peaks resulting from the artificial interstratification of free and complexed layers in conventional x-ray diffraction analysis.

Abbreviations: CEC, cation exchange capacity • CMC, critical micelle concentration • CTA⁺, hexadecyltrimethylammonium • CTAB, hexadecyltrimethylammonium bromide • SAXS, small-angle x-ray scattering

	INTRODUCTION

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FOR MORE THAN 50 YR the interaction of organic cations with naturally occurring silicate clay minerals has drawn a great deal of research interest due to their industrial significance (Jordan, 1949). Early research findings in this field were summarized in an excellent volume by Theng (1974). Recently, renewed interest in clay–organic reactions, particularly cationic surfactant intercalation of swelling clays, has come from the extraordinary capability of the modified clays to retain organic pollutants (Sheng et al., 1996; Jaynes and Boyd, 1991; Li and Rosen, 2000), which reduces the transport potential of contaminants through natural porous media. In the past, characterization of cationic surfactant-clay complexes by x-ray diffraction has been performed on air-dried oriented samples, with or without humidity control (Theng, 1974; Xu and Boyd, 1995). The study of the organic-clay complexation reaction in suspension, a state which more closely replicates a natural environment, has been restricted to adsorption isotherms and colloidal behaviors (Kunyima et al., 1990; Dékány et al., 1994; Permien and Lagaly, 1995; Xu and Boyd, 1995).

Small-angle x-ray scattering is capable of revealing internal and surface structures of colloids at a length scale from 0.5 to 1000 nm (Schmidt, 1995; Schmidt et al., 1996). This technique has been used to determine the interparticle spacing and potential minimum of free smectite particles in suspension (Hight et al., 1962; Norrish and Rausell-Colom, 1963; Andrews et al., 1967; Taylor and Schmidt, 1969). Crystal complexes, like organic-mineral complexes, with regular structures and a short d-spacing contribute to the scattering in the relatively large-angle region, and the scattering data contain information regarding the size and d-spacing of the crystal and the number of platelets in a crystal domain (Hight et al., 1962; Ciccariello and Sobry, 1999). The scattering from structured lamellar microdomains with a relatively large d-spacing can be analyzed using the approach of Shibayama and Hashimoto (1986).

In this report, we use SAXS to investigate the internal structure of montmorillonite-cationic surfactant complexes in suspension, and the precipitated complexes, as a function of varying surfactant loading. The d-spacings of quasi-crystals formed under these experimental conditions were obtained directly from SAXS patterns, and the number of platelets per crystal (thus the crystal size) were estimated by curve fitting of the experimental data with a one-dimensional grating equation.

	THEORY

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If a system contains N_p identical crystal domains, each consisting of N planar lamellae of thickness t, surface A, and spacing d, the scattering intensity can be expressed in the form of elliptic functions (Miller and Schmidt, 1962; Ciccariello and Sobry, 1999). Because the elliptic expression is difficult to implement, Ciccariello and Sobry (1999) approximated the scattering intensity with its asymptotic leading term under the condition qt > 2 or q 2/(Nd), where q is the scattering vector, defined as q = (4 /) sin(/2), with being the x-ray wavelength and the scattering angle. The total asymptotic scattered intensity, I_t,as (q), is given by

[1]

where I⁰ = 8N_pA(_m - _p)²d⁴, with _m being the electron density of interlayer and/or medium (There is very little electron density contrast between water and hydrocarbon chains), _p the electron density of the lamella layer, Q = qd, and = t/d. This equation is most accurate in the q range where the condition qd > qt > 2 is met, and reasonably accurate in the q region where qd > 2 > qt (Ciccariello and Sobry, 1999). If the above conditions are fulfilled, the experimental intensity is

[2]

In practice, a Porod plot [q⁴I(q) vs. q] of the observed intensity can serve as a check for the applicability of Eq. [2]. This equation is actually a one-dimensional grating equation, from which d, N, and t can be obtained by curve fitting to experimental data.

	MATERIALS AND METHODS

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The details of the clay preparation were described previously (Shang et al., 2001). Wyoming bentonite (montmorillonite, SWy-2) was obtained from the Source Clay Minerals Repository, University of Missouri-Columbia, MO. The mineral has a CEC of 102 cmol_c kg^-1 and a specific surface area of 750 m² g^-1 (Schultz, 1969; van Olphen, 1977). The clay sample was first treated with pH 5 acetate buffer to remove carbonates, then hydrogen peroxide (30%, v/v) to oxidize organic matter, and finally dithionite in citrate-bicarbonate buffer to remove free iron oxides (Shang et al., 2001). The treated sample was then shaken with 1 M NaCl to prepare the mono-cationic clay. Excess salts were removed by first washing with distilled-deionized water followed by centrifugation, and then dialyzing until a silver nitrate test for chloride was negative. The <0.08-µm Na-montmorillonite fraction was obtained by centrifugation (CRU-5000, IEC, Needham Heights, MA) to a dilute suspension, which was concentrated on a rotary evaporator to produce a 3% (w/w) clay stock. The exact clay concentration was determined by oven-drying a known volume and weight of clay suspension at 110°C and weighing.

A preliminary study with the adsorption of several cationic surfactants with varying hydrocarbon chain lengths onto the montmorillonite showed that hexadecyltrimethylammonium bromide (CTAB) would best suit our purpose. With this surfactant, montmorillonite particles become redispersed at an adsorption capacity of 2.5 times of the clay's CEC due to strong hydrophobic interactions in the bilayer and charge reversal. The surfactant was obtained from Aldrich and used as purchased. The critical micelle concentration (CMC) of CTAB is about 1 mM (Rosen, 1989). To determine the appropriate solution parameters for preparing surfactant-montmorillonite complexes, an adsorption isotherm was obtained as follows: A portion of the clay stock containing 50 mg montmorillonite on an oven-dry basis was shaken overnight with 50 mL of CTAB solution whose concentration varied from 1.0 to 4.0 mM. At the end of shaking, the mixture was filtered through a 0.2-µm membrane and CTAB in the filtrates was determined by the mixed indicator titration method (Milwidsky and Gabriel, 1982). The difference in CTAB concentration before and after adsorption was plotted against the equilibrium concentration (not shown). The isotherm shows H-type adsorption; below the clay's CEC (90 cmol_c kg^-1 by CTA⁺ retention), it is a cation exchange reaction in which the CTA cations are preferentially sorbed; sorption reaches a plateau at an equilibrium concentration equal to or greater than CMC, and the amount of CTA⁺–CTAB retained is equivalent to 250% of clay CEC.

The montmorillonite-CTA⁺ complexes with CTA⁺ loadings of 5, 10, 20, 50, 100 and 250% of montmorillonite's CEC were prepared as outlined above. To avoid admicelle formation for CTA⁺ loadings equal to or less than CEC, an initial concentration of 0.5 mM was used and the solution:clay ratio was adjusted accordingly. The complexes retained on the filter membrane were redispersed in 5 mL of the filtrate by ultrasonification (for the 250% CEC loading, the complexes were dispersed in 1 mM CTAB). The final clay concentration was 1% (w/w). It was shown that at this clay concentration particle interaction is minimal (Shang et al., 2001), and the scattering at large q is strong compared with that of the solvent background.

The SAXS measurements were performed with the 10-m SAXS camera at Oak Ridge National Laboratory, Oak Ridge, TN. The instrument is described in detail by Wignall et al. (1990). The sample mounting and scanning parameters have been described previously (Shang et al., 2001). The distance between the sample and the detector was 1.119 m for the large-angle region measurements (0.18 to 4.5 nm^-1) and 5.119 m for the low-angle region (0.053 to 0.9 nm^-1). Copper K radiation with a wavelength of 0.154 nm was used. Samples were mounted in a metal cell fitted with a Kapton window and having a 1-cm internal diameter and 1-mm thickness. The 1-mM CTAB solution and water were used for background subtraction. The detector was calibrated with an Fe-55 standard, and the scattering intensity of samples was standardized against secondary standards (PES-3 and vitreous C), azimuthally averaged, and reported as absolute intensities after background subtraction was performed.

	RESULTS AND DISCUSSION

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The montmorillonite-CTAB suspension was stable after ultrasonic dispersion up to a 20% CTA⁺ CEC loading. No precipitate (i.e., flocculate) was found in the system even after the samples were aged for several days. Flocculation and precipitation occurred when the CTA⁺ loading was between 50 and 100% of CEC. The volume of montmorillonite-CTA⁺ complexes reached a maximum at 100% CEC adsorption, and the redispersed sample at this saturation was a slurry. With a CTA⁺–CTAB loading at 250% of the CEC, montmorillonite-CTA⁺ complexes first were present as aggregates, and then became dispersed after ultrasonic treatment. This dispersion was stable for days, and the subsequent sedimentation was slow, suggesting that the complexes are not very large along the c-dimension.

Figure 1 shows the scattering intensity of the samples studied as a function of the scattering vector. For CTA⁺ loadings of 20% or less, the scattered x-ray intensity, I(q), is proportional to q^-2, which is typical scattering behavior from a thin-layer particle system (Porod, 1982). The slope of the scattering curves becomes steeper with increasing surfactant loading. A Bragg diffraction peak was found in the large q-region, suggesting the formation of quasi-crystals (Quirk and Aylmore, 1971) when the surfactant loading is 20% or greater of the mineral's CEC. The peak position shifts towards the low q region with increasing loading (Fig. 1).

View larger version (26K): [in this window] [in a new window]   Fig. 1. Small-angle x-ray scattering data for montmorillonite-hexadecyltrimethylammonium bromide complexes in dilute aqueous suspensions and flocs. CEC = cation exchange capacity, I = intensity, q = the scattering vector.

View larger version (26K): [in this window] [in a new window]   Fig. 2. Guinier plots of small-angle x-ray scattering data for montmorillonite in dilute suspensions with varying amounts of hexadecyltrimethylammonium bromide adsorbed. The Guinier Approximation states that: ln[I(q)q2] = constant – R2gq2 for thin layer scatters, where CEC = cation exchange capacity, I = intensity, q = the scattering vector, and Rg is the radius of gyration for thickness.

The d-spacing of the clay-CTA⁺ complex increases from 17.1 Å at a CTAB loading equivalent to 20% of the CEC to 21.7 Å at 250% of the CEC, suggesting different molecular orientations and arrangements in the interlayers (Table 1). In the dehydrated state, CTA⁺ molecules usually achieve a monolayer arrangement occupying 4.0 to 4.5 Å of interlayer space (i.e., the size of the head group) (Berr et al., 1986). A d-spacing of 14 Å is usually observed for Na-montmorillonite-CTA⁺ (Xu and Boyd, 1995). The greater spacing at 20% CEC CTA⁺ loading is believed to be due to the hydration of CTA⁺ head groups, although an orientational adjustment of CTA⁺ is also possible. A previous study has shown a d-spacing decrease upon drying for montmorillonite-cationic surfactant complexes (Xu and Boyd, 1995), and the decrease was attributed to the loss of interlayer water (Dékány et al., 1994). A d-spacing of 17.1 Å is equivalent to a water monolayer in the clay-CTA⁺ interlayer space. At a 50% CEC loading, the experimental d-spacing from this study is 19.0 Å (Table 1). A bilayer structure with a horizontal orientation in interlayers has been suggested since at this level of CTA⁺ loading, drying did not decrease d-spacing of the complexes lower than 17.6 Å (Xu and Boyd, 1995). This d-spacing has also been reported for a bilayer interlayer structure by Lagaly and Weiss (1969) and Lagaly (1982). Theoretical calculations also indicate a bilayer structure using the flat-lying area (130 Ų) of a CTA⁺, a CEC of 102 cmol_c kg^-1, and a specific area of 750 m² g^-1 of montmorillonite. A 9-Å interlayer distance, compared with 7.6 Å by Xu and Boyd (1995), may be ascribed to the hydration of head groups under suspension conditions. The d-spacing remains the same for bilayer formation up to 100% CEC loading in agreement with the findings of Xu and Boyd (1995). It should be emphasized that the interlayer bilayer discussed here differs from that formed solely by hydrophobic interactions on an external surface (Sharma et al., 1996) because at the loading between 50 and 100% of the CEC, the orientation and spatial arrangement of hydrophobic chains are restricted by electrostatic interactions between CTA⁺ head groups and the surface. The observed d-spacing (21.7 Å) at the 250%-CEC CTA⁺–CTAB sorbed coincides with that of a pseudotrimolecular interlayer model at air-dry conditions (Lagaly, 1982; Jaynes and Boyd, 1991). At this loading, counter anions (Br^-) must be associated with the cations to achieve charge balance, and thus extensive hydration is expected. However, the hydration of head groups and counter anions has not been shown to affect the d-spacing (Xu and Boyd, 1995). In summary, the SAXS results indicate that the change in d-spacing with CTA⁺ retention is step-wise and accompanied by distinctive molecular arrangements in the interlayer space. The discrepancy in d-spacing between hydrated (in solution) and dehydrated (dried) states exists at low CTA⁺ loadings, suggesting that the interlayers are extensively hydrated although the arrangement of CTA⁺s could be different in the two states. The advantage of SAXS over conventional diffraction techniques in structure analysis is that while complex crystals formed with CTA⁺ are observed in SAXS, the interlayer structure(s) that are formed with Na⁺ in oriented samples by artificial interstratification are not present.

View larger version (16K): [in this window] [in a new window]   Fig. 3. Curve fitting to experimental Porod plot using Eq. [2] to find the number of clay layers per quasi-crystal at the hexadecyltrimethylammonium bromide loading of 100% cation exchange capacity. I = intensity, N = number of planar lamellae, q = the scattering vector.

The quasi-crystal structural information obtained by using the classical Scherrer equation is also given as comparison. At low CTA⁺ loadings, the predictions are in close agreement with those obtained using Eq. [2], but at high CTAB loadings, it is overestimated. This discrepancy could arise from the nature of Scherrer constant, which is an approximation.

Hight et al. (1962) also derived a one-dimensional grating equation to describe the scattering of montmorillonite quasi-crystals resulting from the exchange of adsorbed Na⁺ with Al³⁺ or Ca²⁺. Their equation was written as

[3]

where F² is a form factor, representing the scattering from isolated particles, and is given by

[4]

Equation [3] is not practical because the scattering from dilute montmorillonite suspensions (representing F²) is usually severely influenced by background scattering at large q, where scattering peaks appear, and thus a complete form factor can not be constructed. Hight et al. (1962) used a theoretically constructed form factor to overcome this. If Eq. [4] is inserted into Eq. [3], rearrangement will result in an expression identical to Eq. [2]. In addition, the application of Eq. [2] is more straightforward.

Another point that needs to be addressed is that the actual quasi-crystal size in this system may vary, but only an average size is obtained with this approach. It is safe to assume that the scattering contribution from free particles in the q-range of interest is negligible compared with that from quasi-crystals (see Fig. 1).

	CONCLUSIONS

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Small-angle x-ray scattering provides a reliable means to characterize in situ clay-surfactant quasi-crystal complexes without the interstratification effect encountered in conventional x-ray diffraction analysis. The scattering data in the Porod region were analyzed for crystal d-spacing, the number of layers per crystal, and the crystal size. The d-spacing of quasi-crystals increases in a stepwise fashion with the CTA⁺–CTAB loading which corresponds to the number of surfactant layers in the interlayer. The number of silicate layers per crystal also increases with CTA⁺–CTAB loading, and reaches the maximum at 100% of the clay's CEC.

	ACKNOWLEDGMENTS

 
The work was supported by a grant from the USDA's National Research Initiative Competitive Grants program through award number 98-35107-6515. The research at ORNL was sponsored in part by the U.S. Department of Energy under Contract No. DE-AC05-00OR22725 with the Oak Ridge National Laboratory, managed by the UT-Battelle, LLC.

Received for publication June 20, 2001.

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This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via ISI Web of Science (4) Citing Articles via Google Scholar Google Scholar Articles by Shang, C. Articles by Lin, J. S. Search for Related Content PubMed Articles by Shang, C. Articles by Lin, J. S. GeoRef GeoRef Citation Agricola Articles by Shang, C. Articles by Lin, J. S. Related Collections Soil Chemistry