Interlayer conformations of intercalated dodecyltrimethylammonium in rectorite as determined by ftir, xrd, and tg analyses

Intercalation of a wide variety of alkylammonium cations into the interlayer spaces of swelling clay minerals leads to many different applications, ranging from surface-charge measurements to rendering the clay compatible for the preparation of clay nanocomposites, but knowledge of the exact conformation of the intercalated organic species is still incomplete, thus preventing a full understanding of this process. The purpose of this study was to investigate the interlayer conformation of dodecyltrimethylammonium (DDTMA) bromide in rectorite as affected by the amounts of DDTMA intercalated, using a battery of physical and spectroscopic methods. The capacity of rectorite to intercalate DDTMA was equivalent to 1.67 times the cation exchange capacity (1.67 CEC) of the mineral even though the initial input was as much as 5.00 CEC. When the DDTMA intercalated was <0.50 CEC of the mineral, minimal counterion sorption was associated with DDTMA intercalation. Derivative thermogravimetric (DTG) analyses revealed a single-peak decomposition temperature (T_peak) at 430°C. X-ray diffraction (XRD) analyses indicated a flat-lying monolayer of the intercalated molecules, while shifts in Fourier Transform infrared (FTIR) bands confirmed gauche conformation. These results suggested that cation exchange was the dominant mechanism. At the maximum intercalation, the DDTMA adopted a horizontal trilayer arrangement with mainly gauche conformation as determined by FTIR and XRD. Meanwhile a second T_peak appeared at 255°C, similar to the T_peak of solid DDTMA. Counterion bromide sorption accompanying DDTMA intercalation reached a capacity of 310 mmol/kg. The results indicated that van der Waals interaction was responsible for the DDTMA uptake at the amount beyond 0.50 CEC. When the amount of DDTMA intercalated was between 0.5 and 1.67 CEC, the XRD patterns showed non-integrality, i.e. the 002 reflection was split into two non-integral peaks with 2 × d₀₀₂ < d₀₀₁ and 3 × d₀₀₃ > d₀₀₁. They became integral at 1.67 CEC. The results suggest that the mineral might be composed of mixed layers of a monolayer intercalated rectorite and a trilayer intercalated rectorite, without a bilayer as intermediate, when the amount of DDTMA intercalated was between 0.5 and 1.67 CEC.