Effect of pH value and zeta potential on the stability of CO2 foam stabilized by SDS surfactant and SiO2, ZnO and Fe2O3 nanoparticles

Recently, the synergistic effect of nanoparticles and surfactants has been utilized to create more stable CO₂ foams. The surface charge of nanoparticles plays a crucial role in enhancing foam stability. In this study, we investigated the impact of zeta potential, which represents the surface charge of dispersed particles, on the stability of CO₂ foams stabilized by nanoparticles (SiO₂, ZnO, Fe₂O₃) and sodium dodecyl sulfate (SDS) surfactant. Through Brunauer-Emmett-Teller (BET) analysis, we observed an inverse relationship between particle size and specific surface area with foam stability. ZnO, having the smallest particle size and specific surface area, exhibited a 97% increase in foam stability (half-life) compared to the state without nanoparticles. pH-dependent investigations revealed that the optimal foam stability occurred in a basic pH range of 11-12 for all three nanoparticles. Furthermore, the assessment of zeta potential at varying pH levels indicated an inverse correlation with foam stability, regardless of the combined components' nature. Analyzing foam stability across different nanoparticle concentrations, we observed three distinct regions. At low concentrations (0.01 mass%), the surfactant primarily contributed to lamella formation, while at higher concentrations (0.07 mass% and beyond), the nanoparticles dominated this process. In the middle concentration range (0.02 to 0.06 mass%), both surfactants and nanoparticles exhibited a balanced influence on foam lamella formation. Notably, foam stability tended to decline in regions where either surfactants or nanoparticles exerted dominance. Based on the results, the optimal nanoparticle concentrations for foam stability in the SDS surfactant-containing solution were 0.06 mass% for SiO₂, 0.04 mass% for ZnO, and 0.02 mass% for Fe₂O₃.