Coarse-Grained Molecular Dynamics Simulations of the Aggregation Mechanism of Hydrophobic Nanoparticles and Cationic Surfactants in Aqueous Phase

This study employs coarse-grained molecular dynamics (CGMD) simulations to investigate the dynamic interaction mechanisms between hydrophobic silica nanoparticles (H-SiO₂ NPs) and the cationic surfactant cetyltrimethylammonium chloride (CTAC) in an aqueous environment. A coarse-grained model based on the MARTINI 2.2 force field was constructed, and simulations were performed for 250 ns at 298 K in a 20 × 20 × 20 nm³ system containing one H-SiO₂ NP, 1200 CTAC molecules, 1200 Cl^- counterions, and 64,428 water beads. Key findings reveal that CTAC molecules assemble onto the nanoparticle surface via a stepwise mechanism of "monomer adsorption-micelle migration-cluster attachment," ultimately forming a core-shell structure: an H-SiO₂ NP core surrounded sequentially by CTAC hydrophobic tails and hydrophilic headgroups. Radial distribution function (RDF) analysis demonstrates spatial segregation between CTAC tails (peak at 0.5-2 nm) and headgroups (peak at 4 nm), confirming their oriented alignment. The adsorption quantity progressively increased, exhibiting a stepwise transition at 205 ns due to micellar cluster attachment, culminating in approximately 350 CTAC molecules covering a surface area of 110 nm² (20% coverage). A continuous decrease in system potential energy verifies the spontaneity of the adsorption process. The resulting H-SiO₂ NP/CTAC aggregate attained a hydrated diameter of 5.5 nm, with partial exposure of hydrophobic C2 beads indicating incomplete surface coverage. By elucidating the dynamic adsorption pathway and core-shell formation mechanism of CTAC on hydrophobic nanoparticles, this CGMD study provides a theoretical foundation for optimizing nanofluid stability in petroleum extraction, designing high-efficiency heat-transfer working fluids, and enabling pollutant adsorption remediation.