Large-scale inhomogeneities in solutions of low molar mass compounds and mixtures of liquids: supramolecular structures or nanobubbles?

In textbooks, undersaturated solutions of low molar mass compounds and mixtures of freely miscible liquids are considered as homogeneous at larger length scales exceeding appreciably dimensions of individual molecules. However, growing experimental evidence reveals that it is not the case. Large-scale structures with sizes on the order of 100 nm are present in solutions and mixtures used in everyday life and research practice, especially in aqueous systems. These mesoscale inhomogeneities are long-lived, and (relatively slow) kinetics of their formation can be monitored upon mixing the components. Nevertheless, the nature of these structures and mechanisms behind their formation are not clear yet. Since it was previously suggested that these can be nanobubbles stabilized by adsorbed solute at the gas/solvent interface, we devote the current study to addressing this question. Static and dynamic light scattering was used to investigate solutions and mixtures prepared at ordinary conditions (equilibrated with air at 1 atm), prepared with degassed solvent, and solutions and mixtures degassed after formation of large structures. The behavior of large structures in strong gravitational centrifugal fields was also investigated. Systems from various categories were chosen for this study: aqueous solutions of an inorganic ionic compound (MgSO4), organic ionic compound (citric acid), uncharged organic compound (urea), and a mixture of water with organic solvent freely miscible with water (tert-butyl alcohol). Obtained results show that these structures are not nanobubbles in all cases. Visualization of large-scale structures via nanoparticle tracking analysis is presented. NTA results confirm conclusions from our previous light scattering work.