(484g) Molecular Dynamics Study on the Intercalation Behaviors of Polyethylene Glycols into Montmorillonite Interlayer

Intercalation of polymers into clay interlayer has attracted profound interests among scientists and engineers for various applications, including geological storage of toxic and radioactive wastes, oil recovery enhancement, nanocomposite, and drilling fluid additives. Montmorillonite (MMT), a common clay mineral, consists of stacks of negatively charged layers, neutralized by Na⁺ or Ca²⁺ counter-ions. The counter-ions and negatively charged MMT surfaces can attract water molecules, resulting in clay swelling. During shale oil/gas exploration and production, serious problems such as borehole instability are frequently encountered due to clay swelling, incurring expensive drilling costs.

Low molecular weight polyethylene glycols (PEGs) as environmentally friendly and biodegradable polymers capable of intercalating into MMT lamellae, have been widely employed to modify the MMT surface. Thus, understanding the structural properties of PEGs and their interactions with MMT interlayer species is of great importance to the design of high-performance drilling fluid additives. While experimental measurements can provide important insights into PEG adsorption behavior on clay surface, they cannot reveal the structural and dynamic properties of PEG molecules and their interactions with interlayer species including water and counter-ions from molecular perspectives. In this endeavor, molecular dynamic (MD) simulation has been proven to be a promising and powerful option to reveal the structural and dynamic behaviors.

Therefore, in this work, we study the structural properties of three low molecular weight PEG molecules (PEG2, PEG4, and PEG8) with varying chain length and quantities in MMT interlayers at 300 K and 1 bar by using MD simulations. The effects of PEGs on interfacial and mobility properties of water and counter-ions are also evaluated. We also investigate the hydration behavior of Na⁺ ions in the presence of PEGs and water. It is found that PEGs with varying chain length exhibit distinct structural properties intercalated into the MMT interlayers. PEG2 and PEG8 molecules incline to be parallel to the MMT surface, while PEG4 molecules tend to form a crown-like structure, hosting Na⁺ ions within. The introduction of PEGs lowers diffusion coefficients of water as well as Na⁺ ions. Due to the formation of Na⁺-PEG4 complexes, both Na⁺ ion mobility and hydration are significantly reduced.

Our study provides important insights into the structural and dynamic properties of water, counter-ions, and PEG molecules in MMT interlayers and the optimization of high-performance drilling fluid additives.