Microscopic Distribution of Quaternary Ammonium Salt Organic Modifiers in the Interlayer Space of Montmorillonite: Molecular Simulation Study

This study employs molecular dynamics simulations to construct designed unit cells of organic montmorillonite (OMMT) modified with four types of quaternary ammonium salts. The effects of modifier type and quantity on the basal spacing of montmorillonite (MMT) were analyzed. Molecular motion, morphology, interaction energy (<i>E</i>_int), and hydrogen bonding interactions were investigated to elucidate the molecular-level mechanisms between modifiers and MMT. The results indicate that the organic modification of MMT proceeds in three distinct stages: the filled stage, saturated stage, and supersaturated stage. During the filled stage, the basal spacing remains largely unchanged while <i>E</i>_int increases rapidly. In the saturated stage, the basal spacing expands as the growth rate of <i>E</i>_int slows. In the supersaturated stage, the basal spacing continues to increase while <i>E</i>_int stabilizes. The transition from the filled to saturated stage is governed by the van der Waals space occupied by the modifiers. Within the MMT interlayer, the modifiers adopt a bilayer morphology, with the nitrogen atom heads adhering to the MMT surfaces and the tails self-assembling. These findings provide theoretical insights into the basal spacing expansion and organic modification mechanisms of MMT, thereby facilitating improved material compatibility.