Effect of aging time, airflow rate, and nonionic surfactants on the surface tension of bulk nanobubbles water

•Effects of aging time and airflow rate on the surface tension of bulk NB water are investigated.•Effects of airflow rate on the surface tension of bulk NB water is discussed.•The surface tension of bulk NB water is affected by the presence of NBs in the gas–liquid interface.•The surfactant adsorption affected by NBs in the gas–liquid interface determines the surface tension. Nanobubbles have attracted considerable and ever increasing scientific and commercial interest in the past two decades. The main interest focuses on the observation that they are colloidally stable showing various potential applications including flotation, wastewater treatment, cleaning. Here, we report a systematic study studying the surface tension of bulk nanobubbles (BNBs) in water with respect to the aging time and airflow rate. Further, the interfacial tensions of methyl isobutyl carbinol (MIBC) and 2-octanol solutions in the presence and absence of BNBs has been investigated. This study showed that the surface tension of freshly generated nanobubble water is significantly lower than the surface tension of pure water and gradually increase with time until it reaches 70.5 mN/m. Further, a reduction in surface tension and an increase in nanobubbles size were observed when airflow rate was increased. The surface tensions of mixed nanobubbles with methyl isobutyl carbinol (MIBC) was lower than those without nanobubbles, unlike nanobubbles mixed with 2-octanol solutions which led to an increase in the surface tension compared to those without nanobubbles. A possible mechanism for the change in the surface tension in nanobubbles mixed with nonionic surfactant is discussed.