Interfacial adsorption characteristics of surfactant‐modified nanoparticles: Equilibrium and dynamic effects of salinity at the oil–water interface

Abstract This study systematically investigated the interfacial behaviour of silicon dioxide (SiO 2 ) and aluminium oxide (Al 2 O 3 ) nanoparticles (NPs) functionalized with cationic (CTAB), anionic (SDS), and nonionic (Triton X‐100) surfactants under varying MgCl 2 salinities. Surface charge and wettability analyses revealed that CTAB–SiO 2 exhibited the most pronounced hydrophobization (contact angle up to 119°) and highest adsorption affinity, whereas SDS‐modified NPs showed stronger aggregation tendencies at elevated salinity due to electrostatic attraction with Mg 2+ . Adsorption isotherms followed Langmuir characteristics, confirming monolayer coverage with additional heterogeneity for CTAB–SiO 2 and Triton X‐100 systems. Equilibrium interfacial tension (IFT) consistently reached a minimum at 25,000 ppm, reflecting optimized interfacial packing, while dynamic IFT profiles showed accelerated relaxation in positively charged systems and retarded kinetics in negatively charged ones. These findings provide direct mechanistic evidence of how surfactant type and NP surface chemistry jointly dictate salinity‐dependent interfacial activity, offering a novel framework for tailoring nanofluid interfaces.