Thermal characterization of surfactant-modified montmorillonites

The thermal stability of surfactant-modified clays plays a key role in the synthesis and processing of organoclay-based nanocomposites. Differential thermal analysis (DTA), thermogravimetric measurement and differential scanning calorimetry (DSC) were used in this study to characterize the thermal stability of hexadecyltrimethylammonium bromide-modified montmorillonites prepared at different surfactant concentrations. Analysis by DSC shows that the molecular environment of the surfactant within the montmorillonite galleries is different from that in the bulk state. The endothermic peak at 70-100 degrees C in the DTA curves of the modified montmorillonites is attributed to both the surfactant phase transformation and the loss of free and interlayer water. With an increase of surfactant-packing density, the amount of water residing in the modified montmorillonite decreases gradually, reflecting the improvement of the hydrophobic property for the organoclay. However, the increase in the surfactant packing density within the galleries leads to a decrease in the thermal stability of the organoclays. With an increase of initial surfactant concentration for the preparation of organoclays, the surfactant-packing density increases gradually to a 'saturated' state. It was found that the cationic surfactant was introduced into the montmorillonite interlayer not only by cation exchange but also by physical adsorption.