Distinguishing interlayer cations in montmorillonite by thermal analysis of demethylformamide-saturated samples

Derivative thermograms (DTG) for the desorption of dimethylformamide (DMF) from Mn+-SWy-2/DMF complexes can be used to distinguish between different exchange cations simply by the temperature at which the most strongly held DMF molecules are desorbed. The maxima for the desorption of this strongly bound DMF in the DTG traces for Mg2+-, Ca2+- and Na+-SWy were at 420, 330, and 220 C, respectively. Variable-temperature X-ray diffraction confirms that the DMF is present as a single layer of molecules in the clay gallery at these temperatures. Variable-temperature diffuse reflectance Fourier transform spectroscopy (VT-DRIFTS) has shown that the strongly held DMF molecules are directly coordinated to the exchangeable cations via the carbonyl group. The method is simple and robust, requires no specialized preparation methods and the diagnostic capability is not adversely influenced by the presence of moisture. VT-DRIFTS has shown that coadsorbed water is desorbed at temperatures below 100C