Investigation on the co-modification process of montmorillonite by anionic and cationic surfactants

To study how anionic surfactant sodium dodecyl sulfonate (SDS) and cationic surfactant cetyltrimethylammonium chloride (CTAC) enter the interlayer spaces of montmorillonite, montmorillonite, CTAC and SDS were exchanged consecutively and simultaneously. Three parameters were investigated: reaction times, surfactants contents and addition sequences. The results of XRD, thermal analysis and sulfur analysis stated that SDS could not intercalate into montmorillonite interlayer space alone. However, it can enter the interlayer space of montmorillonite in the presence of CTAC. The addition sequences of cationic and anionic surfactants affect the d-value and thermal stability of anionic-cationic montmorillonite. The data obtained from elemental analysis clarified the steps of this multistep reaction sequence and a mechanistic scheme has been proposed: a portion of cationic surfactants intercalate into the interlayer spaces of montmorillonite by cation exchange firstly, and then the anionic surfactant enter the interlayer spaces of montmorillonite by the synergistic effect with cationic surfactants. Molecular simulation indicated the arrangement of cationic and anionic surfactants intercalated into montmorillonite interlayer space.