Molecular dynamic study on structural and dynamic properties of water, counter-ions and polyethylene glycols in Na-montmorillonite interlayers

Abstract

Understanding the interfacial behavior of PEGs and water-PEG interaction in montmorillonite (MMT) interlayer is of great importance to the design of high-performance drilling fluids. In this work, we apply molecular dynamic (MD) simulations to study the structural properties of three low molecular weight PEGs (PEG2, PEG4 and PEG8) in MMT interlayers. We found that PEG2 incline to be parallel to the MMT surfaces, while PEG4 molecules tend to form a crown-like structure, hosting Na+ ions within. PEG8 molecules likely adopt helical or coil configurations caging Na+ inside. Thanks to the wrapping and caging effect, hydration ability of Na+ ion is significantly reduced. Moreover, at a higher concentration with PEG4 and PEG8, more Na+ ions are trapped in the middle of the pore due to the PEG caging. As PEG concentration increases, some water molecules are displaced from the pore surface. As a result, the hydrogen bonding number between water and pore surface decreases. Furthermore, the diffusion coefficients of water and Na+ ions are suppressed in the presence of PEGs. Our study provides important insights into the structural properties of water, counter-ions, and PEG molecules in MMT interlayers and the optimization of high-performance drilling fluids.