Molecular-Scale Insights into the Aqueous Dispersion and Water-Oil Interfacial Behavior of Surfactant Functionalized...

Studying nanoparticle dispersion in water and their behavior at fluid interfaces is crucial for optimizing their performance in chemical enhanced oil recovery. Stable aqueous dispersion ensures efficient nanoparticle transport, while strong water-oil interfacial affinity promotes effective interaction with reservoir fluids. However, current understanding is limited by insufficient molecular-level insight into how surfactant type, surfactant coverage, and reservoir conditions such as pH regulate nanoparticle stability and interfacial behavior. This study employs molecular dynamics simulations to investigate the dispersity and interfacial properties of silica nanoparticles functionalized with various surfactants─anionic, cationic, zwitterionic, and nonionic─across surfactant coverages ranging from 0 to 4.6 molecules nm^-2. The impact of water pH, modeled via silica surface charge densities representing alkaline (-0.85 e nm^-2) and acidic (0 e nm^-2) environments, is also examined. Nanoparticle dispersity is evaluated qualitatively through equilibrium distributions and quantitatively via potential of mean force profiles as a function of center-of-mass distance. Results reveal that sufficient surfactant coverage is critical for maintaining good dispersity. At high coverage, anionic and zwitterionic nanoparticles remain well dispersed regardless of pH, whereas cationic nanoparticles exhibit diminished dispersity under alkaline conditions. Nonionic nanoparticles show poor dispersity across all coverages and pH values. All nanoparticle types exert minimal influence on the water-oil interfacial tension, and possible reasons for this behavior are discussed. However, their interfacial affinities differ: anionic nanoparticles display moderately strong adsorption at the interface, while zwitterionic nanoparticles are nearly excluded from it. These findings highlight the importance of surfactant coverage and surfactant type in optimizing nanoparticle performance. Anionic nanoparticles, in particular, exhibit robust resistance to pH variations and moderate water-oil interfacial affinity, highlighting their broad applicability for enhanced oil recovery.