Aqueous Nanobubble Dispersion of CO₂at Pressures Up To 208 bara

This paper presents an experimental program for generating an aqueous nanobubble dispersion of CO2 at elevated pressures up to 208 bara. It directly sets total volume, external pressure, and temperature, and the overall composition is determined by constant mass expansion of aqueous nanobubble dispersion to a low pressure (e.g., atmospheric pressure) with material balance. Results with deionized water show that the CO2 content in the aqueous nanobubble dispersion increased with the system pressure. The largest CO2 concentration, 2.3 mol/L, was obtained at 207.8 bara, which was 42.9% greater than the inherent solubility of CO2 in water at 207.8 bara. The maximum solubility enhancement, 52.8%, in comparison to the inherent solubility, was observed at 138.9 bara. Generation of an aqueous nanobubble dispersion of CO2 was also tested with a buffer solution based on sodium formate, which resulted in 1.52 mol/L of CO2 at 208 bara. This was 77% greater than the inherent solubility of CO2, 0.86 mol/L, in a sodium chloride solution with the same ionic strength. An important observation from the thermodynamic analysis of experimental data is that nanobubbles themselves may not be the main storage of CO2, but their existence can increase the level of supersaturation of the aqueous phase by CO2. This was consistent with the direct measurement of bubble properties using nano tracking analysis, where the CO2 content as bubbles was much smaller than the inherent solubility of CO2, even with a bubble number density of 108 mL–1 and a bubble radius greater than 100 nm.