Molecular Hydraulic Properties of Montmorillonite: A Polarized Fourier Transform Infrared Spectroscopic Study

Abstract

Understanding the rates at which fluid flows into clay interlayers at the molecular level is fundamental to designing an effective clay barrier system. In this work, molecular interactions at the Na-montmorillonite (MMT)–water interface, emphasizing the flow properties of the clay interlayer, have been studied at the molecular and nanoscale level using polarized Fourier transform infrared (FT-IR) spectroscopic and X-ray diffraction (XRD) techniques. Clay-water slurries were smeared on inert gold-coated metal substrates for FT-IR experiments and slurries were smeared on quartz plates for XRD experiments. By analyzing the O–H stretching and H–O–H bending vibrations in clay slurries, it was concluded that the molecular behavior of interlayer water is significantly different from the molecular behavior of bulk water. With increasing clay-water interaction time, it was also seen that the Si–O stretching bands of clay are being significantly altered by the water molecules in the interlayer. Using these spectroscopic techniques we have estimated the time required for water to flow into the clay interlayer. Further, by analyzing the particle size of the clay using atomic force microscopy (AFM) imaging, we were able to estimate the flow velocity of the water in the clay interlayer. This velocity is found to be 3.23 × 10⁻⁹ cm/s. This flow velocity was found to be of the same order of magnitude as the hydraulic conductivity of smectite-type clay reported elsewhere. Also described in this work is the correct positioning of the Si–O out-of-plane vibration band of MMT at the two-layer saturation level in the interlayer. This band was only observed in p-polarized spectra at 1211 cm⁻¹. Thus, we attribute this band to the Si–O out-of-plane vibration band.