Thermal Stability and Structure Transitions of Organoclays (Hsiao):
Thermal transitions and morphological changes in Cloisite organoclays were investigated by thermal methods and in situ simultaneous small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD). The surfactant component in organoclays was found to undergo a melting-like order-disorder transition between 35 and 50 oC. Multiple scattering peaks were observed in SAXS profiles. In the SAXS data acquired below the surfactant melting point, Ttr, the second scattering peak was found to occur at an angle lower than twice of the first peak position (i.e. non-equidistant scattering maxima). Above Ttr, the second peak was found to shift towards the equidistant position (the most drastic shift was seen in the system with the highest surfactant content). Using a novel SAXS modeling technique, we suggest that the appearance of non-equidistant SAXS maxima could result from a bimodal layer thickness distribution of the organic layers in organoclays. The occurrence of the equidistant scattering profile above Ttr could be explained by the conversion of the bimodal distribution to the unimodal distribution, indicating a re-distribution of the surfactant that is non-bounded to the clay surface. At temperatures above 190 oC, the scattering maxima gradually broadened and became non-equidistant again but having the second peak shifted towards a scattering angle higher than twice the first peak position. The changes in SAXS patterns above 190 oC could be attributed to the collapse of organic layers due to desorption and/or degradation of surfactant component.