Colloidal Properties of Air, Oxygen, and Nitrogen Nanobubbles in Water: Effects of Ionic Strength, Natural Organic Matters, and Surfactants

Colloidal properties of nanobubbles (NBs) in liquid such as surface charge and surface tension influence stability (coalescence or size distribution), reactivity, and performance of applications (e.g., detergent-free cleaning, water treatment, and remediation) were studied. These colloidal properties are often effected by environmental factors such as pH, ionic strength, and the presence of natural organic matters (NOM). This work performed holistic investigations of colloidal properties of three types of NBs (pure air, oxygen, and nitrogen) in the presence of electrolytes, NOM, and surfactants, which are not reported elsewhere. Three different types of NBs exhibited different bubble size distribution (160-340 nm in water) and zeta potentials (approximately-27 to-45 mV at neutral pHs) presumably due to differences in their surface tension or charges. All tested NBs exhibited high stability against coalescence even under high ionic strength and surfactant concentrations. Soft particle extended Derjaguin-Landau-Verwey-Overbeek theory analysis indicated that the energy barriers between two interacting NBs were extraordinarily high (>5,000 k_BT) in pure water, which may explain the high colloidal stability and resistance to coalescence. These results provide new fundamental insight into the physical chemical properties of NBs in water and aim to lay the groundwork toward the green sustainable engineering applications.