Structural-chemical evolution within exfoliated clays.
The exfoliated (delaminated) structures of lamellar clays offer potential as precursors for the formation of various nanostructured materials. In this article, Lucentite and Laponite phyllosilicate clays, which both have empirical formulas of Na(0.33)[Mg(2.67)Li(0.33)Si4O10(OH)2] but differ in nanodimensions, have been exfoliated. Experiments were carried out for mixtures containing approximately 1 wt % phyllosilicate in a 5% aqueous solution of poly(acrylic acid) at different temperatures. X-ray diffraction and photoemission spectroscopy measurements for the solid products recovered after stirring the mixtures at 20 degrees C showed that the fully extended chains of poly(acrylic acid) were intercalated within the interlayer spaces between the silicate plates of the clays. At 85 degrees C, however, the clays were exfoliated and/or partially exfoliated. Photoemission spectroscopy also indicated that the exfoliated structures primarily consisted of silica nanoplates. 29Si nuclear magnetic resonance and oxygen K-edge near-edge X-ray absorption fine structure indicated that the surfaces of the plates were terminated by high concentrations of the silanol (-SiOH) groups, which created structural branches during intercalation. A model was developed in which intercalation and the removal of ions from the clays after the poly(acrylic acid) interactions reduced the electrostatic van der Waals forces between the plates. It was also shown that the formation of branches created a steric effect that inhibited the stacking of the plates. Together these resulted in exfoliation.