Silane modification of layered silicates and the mechanism of network formation from exfoliated layers

The edges of a synthetic layered silicate, Laponite XLG, was successfully modified with various amounts of ethoxytrimethylsilane, a monofunctional silane compound. The occurrence of the reaction was confirmed with various techniques. Thermogravimetric measurements and Fourier-transform infrared spectroscopy (FTIR) carried out on both the treated filler and the reaction medium yielded a saturation like correlation as a function of the amount of the silane used for treatment. The analysis of the bonded amount of silane by FTIR spectroscopy showed that a side reaction takes place during modification. Although the expected dimerization did not occur, the silane hydrolyzed, thus systematically changing the amount of bonded silane determined from the FTIR absorption of oxygen containing groups. Only vibrations related to the absorption of the -SiCH3 group can be used for quantitative analysis. X-ray diffraction (XRD) measurements proved that the silane is bonded onto the surface of the silicate and it does not enter the interlamellar space. Approximately SO mg silane is bonded to 1 g silicate corresponding to almost complete reaction of the surface -OH groups. The results of the measurements were supported by model calculations and extremely good agreement was found between the calculated and the measured values. Surface modification did not hinder the formation of a house-of-card structure showing that the generally accepted mechanism of network formation needs revision. An explanation was offered based on the release of hydroxyl ions from the -MgOH moiety of the silicate. The modification of the edges of layered silicates may facilitate exfoliation or help functionalization, but does not prevent network formation and cannot be used for the control of rheological properties.