Atomic force microscopy (AFM) investigation of nanoscale bubble-microplastic interactions under varying ionic strength: Insights from extended DLVO theory.

This study investigates the interaction and deformation of air bubbles in contact with microplastic particles - specifically polyethylene (PE) and polystyrene (PS) - to advance understanding of flotation-based removal of microplastics from various aquatic environments. A range of NaCl concentrations was examined using atomic force microscopy (AFM) and a numerical model based on the augmented DLVO Young-Laplace equation, incorporating extended DLVO (eDLVO) forces. Key findings show that increasing NaCl concentrations (from 1 mM to 1000 mM) reduce the magnitude of the repulsive van der Waals interaction due to decreasing Hamaker constants. Similarly, the zeta potentials of both PE and PS particles decrease with salinity, indicating weakened electrostatic repulsion within the electrical double layer (EDL). Disjoining pressure profiles reveal a peak in repulsive pressure at a separation distance of ∼20 nm, which declines as salt concentration increases. The AFM experiments using PE colloidal probes partially agree with the numerical predictions. At low NaCl concentrations (<50 mM), no attachment was observed experimentally - contrary to model results - while at 500 mM, both approaches indicated bubble-particle attachment. These discrepancies are attributed to hydration forces, which act repulsively at low ionic strengths but can become attractive at higher concentrations. Overall, this study provides deeper insight into bubble-microplastic interactions under saline conditions, with implications for environmental remediation and materials science applications.