Synchrotron x-ray study of hydration dynamics in the synthetic swelling clay Na-fluorohectorite

We present time-resolved synchrotron x-ray diffraction from the swelling clay Na fluorohectorite during controlled hydration and dehydration. A comparison of bulk and surface scattering reveals that the time dependence of basal Bragg peak positions and intensities has two components, which come into play at distinct temperatures. Intercalation of water into the crystal structure commences at a temperature of about 60degreesC, on a time scale on the order of 1 hour. By contrast, percolation of water into the porous medium has a characteristic time constant of 3-4 h. This is the rate limiting process for hydration of the interior of the clay for T>40degreesC. We suggest that the temperature-dependent percolation step may account for some of the hysteresis reported in earlier diffraction studies of the hydration of similar systems.