Arrangement, conformation, and mobility of surfactant molecules intercalated in montmorillonite prepared at different pillaring reagent concentrations as studied by solid-state NMR spectroscopy

Abstract

The arrangement, conformation, and mobility of dodecyltrimethylammonium cations (DDTMA +) intercalated in montmorillonite prepared with different pillaring reagent concentrations have been studied by 13C relaxation time measurement, cross-polarization dynamics, and two-dimensional proton wide-line separation (2D WISE) spectroscopy, as well as X-ray powder diffraction (XRD). We demonstrate that the arrangement of DDTMA + and the mobility of various groups are different, depending on the pillaring concentration, but the conformations of alkyl chains are similar. XRD experiments illustrate that at three different pillaring concentrations (DDmt0.2, DDmt0.5, and DDmt1.0), the organic cations adopt a lateral-monolayer, lateral-monolayer, and pseudotrilayer arrangement, respectively. 13C MAS NMR reveals that the alkyl chains at the three concentrations uniformly display a large amount of mixed trans and gauch conformation (disordered) and a small amount of trans conformation (ordered). 13C spin-lattice relaxation time and 1H– 13C cross-polarization dynamics measurement, along with 2D WISE NMR experiments, indicate that the mobility is much different for various groups at a given concentration and for a given group at different concentrations. At each concentration N-methyl unusually possesses the highest mobility, even exceeding that of the terminal methyl; at different concentrations the N-methyl and terminal methyl in DDmt1.0 exhibit the highest mobility compared with the other two samples.