Interaction of supercritical CO2 with alkyl-ammonium organoclays: Changes in morphology

This paper investigates the influence of supercritical carbon dioxide on the morphology and surface chemistry of three organic modified montmorillonite species. Alkyl based quaternary ammonium surfactants with differing numbers of chains attached, were chosen to vary the degree of CO(2)-philicity exhibited by the organoclay. In a high pressure batch vessel, the different organoclays were suspended in the supercritical solvent at temperatures of 50 and 200 degrees C and pressures of 7.6 and 9.6 MPa and then removed after de-pressurization at 0.2 or 4.8 MPa/s. The structures of these treated clays were characterized by X-ray diffraction (XRD), differential scanning microscopy (DSC), and thermogravimetric analysis (TGA), and their chemical properties were analyzed by various methods including atomic absorption spectroscopy, and water uptake measurement. Solute-solvent interactions plasticized the organic medium while suspended in the supercritical fluid, which resulted in greater chain mobility and further cation exchange. The results indicate that intercalated surfactants exhibiting a paraffin complex arrangement were most likely to experience significant basal expansion, provided the tilt angle was not already close to being perpendicular to the silicate surface. At the lower processing temperature condition, the chemistry of the clay surface was notably altered by the CO(2) associations with the Lewis acid/base sites, which significantly reduced the moisture adsorption capacity of the material. For those organoclays demonstrating basal expansion, it was noted that the resulting particle size was increased due to enhanced porosity.