Lamellar nanostructure in Somasif-based organoclays

Thermally induced lamellar structure changes due to phase transition and degradation in organoclays based on a synthetic 'Somasif' mineral and two organic surfactants, di-methyl dihydro-ditallow ammonia chloride (DMDTA) and tri-butyl-hexadecyl phosphonium bromide (HTBP) were investigated by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), Fourier transform infrared (FTIR) spectroscopy, in situ simultaneous small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) over the temperature range 30-280 degrees C. Results indicated that the surfactant layer in 'Somasif-based organoclays underwent thermally induced melting-like order-disorder transition followed by desorption of surfactant molecules, resulting in drastic changes in the character of the layer periodicity. The transition temperature (T-tr), determined from the endothermic transition in DSC, was found to depend strongly on the type and the content of surfactant incorporated. Temperature-resolved SAXS indicated complex intercalated layered structures, containing multiple lamellar stack populations of two different organic layer thicknesses. A weak scattering peak (s(0)), located at exactly the half angular position of the strong first scattering maximum s(1) (s(0) = 0.5s(1)), was found in all tested 'Somasif clays. The presence of this peak can be attributed to a slight breaking of the translational symmetry in the layered structure, causing the 1 D repeat period in real space to be doubled. In other words, some portions of layers are grouped into pairs and a single pair forms the new repeat unit. This arrangement is reminiscent of the Peierls-like distortion.