Structural characterization of organo-montmorillonites prepared from a series of primary alkylamines salts: Mid-IR and near-IR study

Primary alkylammonium cations with progressively increasing chain length from methylammonium (CH3-NH3+) to nonadecylammonium (CH3-(CH2)(18)-NH3+) were used to prepare unique series of organo-montmorillonites (O-Mts). Infrared spectroscopy in the mid-IR and near-IR regions and XRD were used to characterize samples and to investigate the effect of the chain length on the arrangement of cations confined in montmorillonite (Mt) interlayers. For the first time detailed assignment of the vibration bands in the near-IR region (8000-4000 cm(-1)) was provided not only for O-Mts but also for selected primary alkylamines and alkylammonium salts, used for their preparation. The IR spectra showed that both, type of head-group, i.e. NH2 vs NH3+, and nature of the anionic counterpart, i.e. chloride vs Mt layer, significantly affected the positions the N-H vibration modes as a result of different H-bonds in O-Mts. The alkyl chain length had significant effect on the arrangement of the intercalated cations. With increasing chain length the stretching (vCH(2)) and first overtone (2vCH(2)) bands were shifted to lower wavenumbers indicating decreasing amount of disordered gauche in favor of ordered all-trans conformers. Significantly higher shift of the 2v(as)CH(2) band than v(as)XH(2) benefits the NIR region for conformational studies more than MIR.