Functional Group-Dependent Antiswelling Mechanisms of Cationic Inhibitors on Montmorillonite

Montmorillonite swelling poses critical challenges in petroleum engineering, causing wellbore instability and reduced formation permeability. While cationic inhibitors mitigate swelling by replacing interlayer cations, the influence of functional group chemistry on their efficacy remains incompletely understood. This study systematically evaluates the antiswelling performance of four cationic inhibitors: potassium ions (K+), primary amine (C12–NH3+), quaternary ammonium (C12–N(CH3)3+), and polyethylenimine (−[C2–NH2+]n−). Centrifugal swelling tests quantified macroscopic inhibition efficacy, while molecular dynamics (MD) simulations characterized the adsorption configurations, hydration structures, and interlayer interactions at the atomic scale. Experimental antiswelling rates followed the order: C12–N(CH3)3+ (44%) < K+ (52%) < −[C2–NH2+]n– (56%) < C12–NH3+ (60%). MD simulations revealed that C12–NH3+ exhibited the closest surface proximity (0.06 nm H-adlayer thickness) and the strongest electrostatic coupling, while −[C2–NH2+]5– formed interlayer bridges. In contrast, C12–N(CH3)3+ showed weakened adsorption due to steric hindrance from its bulky headgroup. The results show that antiswelling efficacy is governed by a balance of adsorption strength and hydration behavior. Primary amines and polyethylenimines outperform quaternary ammonium compounds due to superior charge density and conformational flexibility, providing a molecular basis for designing high-efficiency clay stabilizers in petroleum drilling operations.