In situ x-ray diffraction studies of alkyl quaternary ammonium montmorillonite in a CO2 environment

An in situ study was conducted using an x-ray diffractometer and a specially designed high pressure cell to examine the effects of carbon dioxide at different pressures and temperatures on three different modified montmorillonite species. These organoclays possessed organic pillars of quaternary ammonium surfactant with either one, two, or three long (C(16)-C(18)) alkyl chains attached to expand the galleries of the clay mineral. The three clay species were tested between 50 degrees C and 200 degrees C with carbon dioxide pressures between 0.1 and 8.4 MPa. Under these conditions, the three organoclays exhibited marked differences in their basal spacing depending on the surfactant used. The physical state of the intercalated surfactant was found to be of critical importance, with no changes in basal spacing noted until the organic component began to melt, in spite of CO(2) being present. A pressure effect was also noted which delayed melting of the surfactants as the pressure of the system increased. In all cases, further cation exchange with residual sodium cation present in the galleries was observed in the presence of CO(2) and above the melting state of the surfactant. The study included examination of the effect of rapid depressurization on the clay structure, which produced only a small change in basal spacing.