The role of fluid polarity in the swelling of sodium-montmorillonite clay: A molecular dynamics and Fourier transform infrared spectroscopy study

Abstract

Swelling clays are found extensively in various parts of the world, and sodium-montmorillonite (Na-MMT) is the main constituent of an expansive clay mineral. In this work, the swelling behavior of Na-MMT clay with a wide range of organic fluids, high polar through low polar fluids, is studied using a combination of Fourier transform infrared (FTIR) technique and molecular dynamics (MD) simulations. The construction of the representative clay–fluid models is carried out, and the nature of nonbonded interactions between clay and fluids is studied using MD. Our FTIR and MD simulations results suggest the significant nonbonded interactions between Na-MMT clay and polar fluids, such as formamide and water. The nonbonded interactions of Na-MMT with methanol and acetone are significantly less than those in Na-MMT with polar fluids. The interactions of the fluids with various entities of the clay such as SiO, FeOH, MgOH, and AlOH captured via the spectroscopy experiments and modeling provide a finer understanding of the interactions and their contributions to swelling. The MD simulations are able to capture the band shifts observed in the spectra obtained in the spectroscopy experiments. This work also captures the conformations of interlayer sodium ions with formamide, water, methanol, and acetone during swelling. These nonbonded interactions provide insight into the molecular mechanism that the polarity of fluids plays an important role in the initiation of interlayer swelling, alteration in the orientations, and evolution of microstructure of swelling clays at the molecular scale. Keywords: Swelling clays, Montmorillonite, Fourier transform infrared (FTIR) technique, Molecular dynamics (MD), Organic fluids, Polarity