Inhibition mechanism of cationic polyacrylamide on montmorillonite surface hydration: A molecular dynamics simulation study

Cationic polyacrylamide (CPAM), a typical drilling fluid shale inhibitor can prevent the clay swelling and maintain the wellbore stable. Understanding the adsorption behavior of CPAM on montmorillonite (MMT) surfaces is of great significance in explaining the excellent performance of cationic inhibitors. In this work, molecular dynamic simulations (MD) are applied to investigate the adsorption mechanism of CPAM on the Na-MMT surface. The simulation results show that there are two main adsorption configurations of CPAM: 1) tri-methylamine group adsorption; 2) the co-adsorption of trimethylamine group and amide group, the latter has stronger interaction energy, also trimethylamine group accounted for a large proportion of the interaction en-ergy. The CPAM coating will decrease the H-bond number, neutralize the negative charge of Na-MMT surface and weaken the surface hydration ability of Na-MMT. Due to the existence of the CPAM coating, the diffusion and transport ability of water molecules on Na-MMT surface are greatly reduced. This study provides a theo-retical explanation for the coating-forming mechanism and adsorption characteristics of CPAM inhibitors on Na-MMT surfaces and may shed light on the design of high-performance water-based drilling fluid systems.