Facile modification of montmorillonite by intercalation and grafting: The study of the binding mechanisms of a quaternary alkylammonium surfactant

Abstract

Modified clays are among the most investigated organic-inorganic hybrid materials. Traditionally, they are prepared via reaction of the clay mineral with an organic substrate that can be bonded by cation exchange or by covalent grafting. However, the possibility of bonding one organic substrate to the clay mineral via these two mechanisms simultaneously has not been deeply investigated. In this study, a simple modification of a clay mineral was investigated via the reaction of a quaternary alkylammonium surfactant that possesses an alkoxysilane group [(3-trimethoxysilylpropyl)octadecyldimethylammonium chloride] (TPODAC) with raw bentonite that contains montmorillonite (Mt) as the main phase (~ 90%) and acid-activated bentonite (H5-Mt). The structures, morphologies, and compositions of natural and modified montmorillonites were investigated via various methods, which evidenced successful organic modification. The surface area of Mt. (50 m2g−1) was as expected for sodium bentonite. After acid activation, an increase of the BET area (120 m2g−1), a decrease of the cation-exchange capacity (CEC), and a loss of crystalline order were observed. The presence of TPODAC in the hybrid materials was evidenced by three IR absorption bands at approximately 2930 cm−1, 2860 cm−1, and 1475 cm−1. Negligible leaching of TPODAC from the hybrid materials in aqueous media at pH 3, 7, and 10, in hydrophobic media, and in high–ionic strength media were observed, thereby demonstrating the excellent chemical stability of these materials. A quantitative evaluation of the contribution of each binding mechanism of TPODAC in the hybrid materials was conducted by applying a surfactant-mineral interaction model in which grafting, cross-linking, and cation exchange mechanisms were proposed. The amounts of surfactant in the hybrid materials that were predicted by the model were in agreement with the results that were obtained via TGA.