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CHARACTERIZATION OF MONTMORILLONITE SURFACES AFTER MODIFICATION BY ORGANOSILANE

Kang Song and Giselle Sandí

Chemistry Division, Argonne National Laboratory, 9700 S. Cass Ave., Argonne, Illinois 60439, USA

E-mail of corresponding author: gsandi{at}anl.gov

X-ray powder diffraction (XRD), thermal gravimetric analysis (TGA), surface area measurements, and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy were used to examine the surface properties of organosilane-modified smectite-type aluminosilicate clays. Organic modified clays derived from the reactions of montmorillonite (containing 93–95% montmorillonite from a bentonite, <1% quartz, and 4–6% opal CT) with octadecyltrichlorosilane (C₁₈H₃₇SiCl₃) and octadecyltrimethoxysilane [C₁₈H₃₇Si(OMe)₃] are highly hydrophobic. Surface loadings of the modified clays depend on the organosilane and the solvent, and they range from 10 to 25 wt. %. The organic species are probably adsorbed to the outer surfaces and bound to the edges of the clay via condensation with edge–OH groups. Encapsulation of montmorillonite with C₁₈H₃₇SiCl₃ and C₁₈H₃₇Si(OMe)₃ resulted in a hydrophobic coating that acts like a "cage" around the clay particles to limit diffusion. Basal spacings of the organic modified clays remain at ~15 Å upon heating to 400°C in N₂, whereas those of unmodified clays collapse to ~10 Å . A considerable reduction in surface area (by 75–90%) for organic modified clays is observed, which is consistent with the existence of a surface coating. The solvent used can affect the amount of organic silane coated on the clay particles, whereas the difference between the products prepared using C₁₈H₃₇SiCl₃ and C₁₈H₃₇Si(OMe)₃ in the same solvent is relatively small. The carbon and oxygen K-edge NEXAFS spectroscopy of the modified montmorillonite surfaces showed that surface coatings on the outside of the clay particles exist. The encapsulating system may allow for economical remediation and storage of hazardous materials.

Key Words: Clay-Organic Interactions • Montmorillonite • Organic Coating • Silane • Surface Modification