Crystallization of a layered silicate clay as monitored by small-angle X-ray scattering and NMR

The 48-h hydrothermal crystallization of a magnesium silicate clay called h ectorite has been investigated in detail. Tetraethylammonium (TEA) ions are used to aid crystallization and become incorporated as the exchange cat;io ns within the interlayers. Data from small-angle X-ray scattering (SAXS) us ing aliquots ex situ are consistent with results obtained previously by X-r ay powder diffraction (XRD), thermal gravimetric analysis (TGA), atomic for ce microscopy (AFM), and IR. All these techniques see clay crystallites beg inning to form within the first few hours of reaction. Si-29 NMR displays a visible clay silicate peak after just 1 h. Solid-state C-13 NMR shows evid ence of TEA-clay formation in as little as 30 min and also that 80% of the final TEA loading is accomplished in the first 10-12 h. Up to 36 h more is needed to incorporate the remaining 20% of TEA, indicating that a slower ev ent is dominating at the later stages of crystallization. Data from 13C NMR and SAXS are compared to and are consistent with data from earlier AFM exp eriments. All present a scenario where initial nucleation and crystallizati on end after about 14 h, after which this occurs to a lesser extent and pri marily agglomeration of particles is taking place. The SAXS data show this in progressively increasing power law values, indicating more "open" struct ures that condense into more dense structures with time. In addition, the f irst in situ study of clay crystallization of any kind was performed by in situ SAXS. A possible clay crystallization mechanism is proposed.