Near-IR study of the impact of alkyl-ammonium and -phosphonium cations on the hydration of montmorillonite

Organo-montmorillonites (O-Mts) are attractive inorganic-organic hybrid materials used in numerous industrial applications. In this work the adsorption of water onto montmorillonite saturated with the alkylammonium and alkylphosphonium cations tetramethyl-(Me4-N/P), tetrabutyl-(Bu4-N/P) and tetraoctyl(Oc4-N/P) was studied. The effect of the organic cation size and particularly of the central atom of the organic cation head-group (N vs P) was followed at different relative humidities (RH). The samples were characterized using carbon analysis, N-2 adsorption, X-ray powder diffraction analysis (XRPD), and infrared spectroscopy (IR) in the middle-IR (MIR) and near-IR (NIR) regions. Gravimetric analysis and NIR spectroscopy were used to monitor the hydration of the O-Mts. A gradual shift of the combination band of water molecules, (nu+delta)H2O, to lower wavenumbers on increasing the amount of adsorbed water indicated an increase in the number of H2O molecules involved in stronger hydrogen bonds. A shift of the 2 nu CH3 band to higher wavenumber with increasing RH, observed for Me4-N/P-Mts and Bu4-N-Mt, provided evidence that water molecules interacted directly with the methyl groups of the Me4-N/P and Bu4-N cations. Gravimetric analysis confirmed that at each RH the content of adsorbed water decreased in the direction Me4-N/P-Mts (sic) Bu4-N/P-Mts (sic) Oc4-N/P-Mts due to the increasing hydrophobicity of the organic cations. The area of the (nu+delta)H2O band, reflecting the degree of O-Mt hydration, confirmed the results obtained by gravimetry, i.e. the content of adsorbed water decreased as the size of the organic cation increased. The results of the NIR spectroscopy as well as gravimetric analysis showed a higher hydrophobicity of O-Mts saturated with alkylphosphonium cations compared to their alkylammonium counterparts. The obtained results showed that NIR spectroscopy is a very useful method for the characterization of organo-montmorillonite hydration. (c) 2022 Elsevier B.V. All rights reserved.