Conformation heterogeneity and mobility of surfactant molecules in intercalated clay minerals studied by solid-state NMR

The conformation heterogeneity and mobility of surfactant molecules in intercalated montmorillonite clay minerals have been investigated using variable-temperature solid-state C-13 NMR and two-dimensional proton wide-line separation (2D WISE) H-1-C-13 NMR spectroscopy. Previous FTIR studies by Vaia et al. for the first time revealed the existence of a gauche conformation of surfactant molecules in clay minerals and further illustrated the transition from an ordered conformation, to a liquid crystalline state, to completely liquidlike behavior. The NMR study reported in this paper clearly demonstrates the coexistence of order and disordered chain conformations. Two main resonance peaks are resolved and associated with the backbone alkyl chains: the resonance at 33 ppm corresponds to the ordered conformation (all-trans), and the resonance at 30 ppm corresponds to the disordered conformation (mixture of trans and gauche). The NMR technique allows detailed characterization and quantification of the conformational heterogeneity, which is difficult to determine by other techniques. Furthermore, variable-temperature NMR also directly provides quantitative information on the rigidity of the different conformations. Results from cross-polarization time constant T-CH measurements along with 2D WISE NMR suggest that molecules in the ordered all-trans conformation are as rigid as those in solid crystalline materials and that molecules in the disordered conformation are similar to those in liquid crystalline materials. Upon heating, the molecules in the disordered conformation remain more or less unchanged, while the molecules in the ordered conformation become disordered. However, the intercalated molecules in the clay minerals never attained the complete liquidlike behavior even after all the bound surfactant assumed a disordered conformation.