The Effects of Ionic Strength and pH on Nanobubble Electrostatic Stability in Saline Solutions

Nanobubbles (NBs) have recently emerged as a medium uniquely suitable for a wide range of science and engineering applications, yet their behavior under different solution conditions remains poorly understood. In this study, we conducted a systematic investigation of the characteristics and stability of nitrogen NBs in solutions of varying salinities and pH. NBs were examined by scanning electron microscopy, nanoparticle tracking analysis, and zeta potential measurements. Optimal NB generation parameters (gas flow rate: 0.5 ft³/h; duration: 60 min) yielded NBs with high concentrations of (200 ± 20) × 10⁶ bubbles/mL, uniform sizes (100 ± 20 nm), and zeta potential of -34.0 ± 0.3 mV. Effects of salt (NaCl) concentration and pH were probed in two distinct scenarios: (1) direct generation of NBs in solutions with varying salinity or pH and (2) generating NBs in deionized water with subsequent addition of NaCl or pH adjustment. In the first scenario, NB generation at high salinity and high pH produced more concentrated NB solutions, rationalized by the salting-out effect and more negative NB surface charge at alkaline pH, respectively. However, despite the high initial concentrations, NBs formed at high salt exhibited very poor long-term stability, likely due to electric double layer (EDL) compression. Likewise, high pH proved less favorable for the NB stability over time than neutral, although generally more than the acidic pH. By contrast, salt addition postgeneration and pH adjustment in either direction led to a drop in the NB concentrations, more pronounced as the salinity increased or pH decreased. Similar to the first scenario, the long-term stability was also reduced by the added salt as well as acidic and, to a lesser extent, even basic pH. These findings provide important insights into optimizing the NB solutionsfor diverse applications, particularly for enhanced oil recovery, where stability under harsh salinity and pH conditions is essential.