A thermogravimetric investigation of alkylammonium intercalation into rectorite

This study focuses on elucidating the structural detail of intercalated hexadecyltrimethylammonium (HDTMA) and dodecyltrimethylammonium (DDTMA) in rectorite using thermogravimetric (TG), Fourier transform infrared (FTIR) spectroscopy, and powder X-ray diffraction (XRD) analyses. The results showed that the majority of the intercalated alkylammonium decomposed between 200 and 300 °C when the amount intercalated was much higher than the cation exchange capacity (CEC) of the mineral. However, about 0.5 CEC of the intercalated alkylammonium was thermally stable up to 400 °C. The presence of this amount of alkylammonium was verified by FTIR and XRD analyses after samples were heated to 300 °C. These results suggested that two types of interactions and/or two types of sorption sites were responsible for the two-step decomposition of intercalated alkylammonium. The alkylammonium held by hydrophobic interaction decomposed at the lower temperature, while that held by cation exchange decomposed at the higher temperature. The XRD and FTIR results revealed different interlayer configurations of the intercalated alkylammonium. At the intercalation maxima, HDTMA adopted a vertical arrangement with all-trans configuration as indicated by a d-spacing of 49.5 Å, while DDTMA took a horizontal pseudo-trilayer arrangement with extensive gauche configuration.