Comparative study of alkylammonium- and alkylphosphonium-based analogues of organo-montmorillonites

Abstract The organo-montmorillonites are highly attractive materials because of their wide application potential. In this work the basic structural, vibrational and thermal characteristics of the samples from two series of structurally analogous alkylammonium- and alkylphosphonium-montmorillonites are reported for the first time. The first series consisted of the organo-montmorillonites prepared from commercially available tetramethyl-(Me4-N/P), tetrabutyl-(Bu4-N/P) and tetraoctyl-(Oc4-N/P) analogues, while the second series was prepared from tributylalkyl cations with the fourth chain of the increasing length (Bu3-N/P-CX, X = 4, 8, 12, 16). The XRD analysis revealed that for both series, when comparing the equivalent pairs of the surfactants, slightly higher expansion of the interlayer space was observed for phosphonium-based than for ammonium-based montmorillonites. Difference in the d001 values was in accordance with a higher covalent radius of P than N. The impact of the different central atom (N/P) of the cation head-group was also studied by IR spectroscopy in the middle and near region. As a result of differences in the charge density of the nitrogen and phosphorus cationic centers the positions of the C-H bands (stretching and overtones) of phosphonium-Mt were shifted to lower wavenumbers compared to their ammonium counterparts. The effect of the central atom of the head-group was significant for smaller cations, but gradually decreased with increasing size of the ammonium and phosphonium surfactant. DTG profiles showed a significant step above 200°C attributed to the release of the organic phase. The temperature of maximal decomposition rate was higher for phosphonium cations compared to ammonium analogues. The mass profiles and temperature-dependent IR spectra showed the evolution of CO2 and H2O species as the main products. In addition, the C-H stretching and bending bands of the unspecified organic phase were detected in the spectra.