Preparation and Structural Characterization of Novel Nanohybrids by Cationic 3D Silica Nanoparticles Sandwiched between 2D Anionic Montmorillonite Clay through Electrostatic Attraction

In our study, cationic colloidal silica was prepared using a silylation process to modify silica by chemically grafting it with (gamma-aminopropyl)trimethoxysilane (APS) under acidic conditions. The silylation of silica by APS grafting was characterized by FTIR and solid (29)Si NMR with its zeta potential measured for the confirmation of cationic nature for APS-modified silica (APS-silica). The strong electrostatic attraction of cationic APS-silica toward negatively charged Na(+)-montmorillonite (MMT) clay resulted in the formation of APS-silica/MMT nanohybrids during the mixing process. The coagulated nanohybrids acquired different charges depending on the weight ratios (WRs) of APS-silica to MMT in the nanohybrids. The surface potential of nanohybrids also varies with the amount of APS-silica imparted when measured using a zeta potential analyzer. Images obtained by transmission electron microscopy and field-emission scanning electron microscopy reveal the supracolloidal structure of the nanohybrids. APS-silica nanoparticles, with a diameter of 22 nm, were embedded in two-dimensional MMT arrays that are approximately 300 nm x 300 nm in size. The ordered structure of the nanohybrid was demonstrated in a small-angle X-ray scattering study. Also, the wide-angle X-ray diffraction of nanohybrids was employed to analyze the interlayer distance of MMT in nanohybrids. The precipitates of coagulated nanohybrids were separated from solution and weighed to yield the "coagulation efficiency" on the basis of the feeding weight of APS-silica and MMT. Coagulation efficiencies over 97.6% are obtained for WRs between 2 and 7. The calculation using coagulation-efficiency data gives a saturated WR of 8.1 that agrees with that of the theoretical packing of APS-silica nanoparticles covering the MMT surface. In addition to determining the ordered supracolloidal structure, this research also presents a novel method to exfoliate the 2D MMT nanoclay with 3D nano-APS-silica colloids by a self-assembly process through electrostatic attraction. A comparison of the Brunauer-Emmett-Teller surface area of MMT in nanohybrids with the theoretical packing of silica on MMT concludes the biased layout of silica colloids on MMT with partial overlapping and void development in the interstices of nanohybrids