Al sub(13)-pillared montmorillonite modified by cationic and zwitterionic surfactants: A comparative study

A series of inorganic-organic montmorillonites (IOMts) were synthesized by modifying polyhydroxy-aluminum (Al sub(13))-pillared montmorillonites (AlPMts) with the cationic surfactant, hexadecyltrimethyl ammonium bromide (C16), and the zwitterionic surfactant, hexadecyldimethyl(3-sulphonatopropyl)ammonium (Z16). The products prepared using different surfactant concentrations (0.4-5.0 CEC) were characterized by X-ray diffraction (XRD), X-ray fluorescence (XRF), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TG), and C, H, and N elemental analyses. The results showed that the structure of the IOMt was strongly influenced by surfactant type. In the case of C16-modified AlPMt (C-AlPMt), the surfactant molecules entered and expanded the interlayer space, leading to the partial release of pre-intercalated Al sub(13) cations. At high surfactant concentrations (> 2.0 CEC), the basal spacing of C-AlPMt showed no further expansion although the gauche/trans conformer ratio of the intercalated surfactant decreased. In the case of Z16-modified AlPMt (Z-AlPMt), however, the amount of Z16 intercalated did not increase when the surfactant concentration exceeded 1.0 CEC, while the basal spacing was identical with that measured for the unmodified AlPMt. This observation could be attributed to a difference in the capacity of AlPMt for intercalating C16 and Z16. It was proposed that C16 was intercalated through both cation exchange (with Al sub(13)) and physisorption, while the intercalation of Z16 was primarily mediated by electrostatic attraction between the interlayer Al sub(13) cation in AlPMt and the negatively charged group of Z16.