Inhibition of bubble coalescence by osmolytes: sucrose, other sugars, and urea.

We report on bubble coalescence inhibition by non-surface-active, nonelectrolytes urea and sucrose, and other small sugars, in aqueous solution. Urea has no effect on bubble stability up to high concentrations>1 M, while sucrose inhibits coalescence in the range 0.01-0.3 M, similar to inhibiting electrolytes. Urea and sucrose both increase bubble coalescence inhibition in inhibiting and noninhibiting electrolytes in a cooperative manner, but urea decreases the efficacy of sucrose in mixed solutions. Several mono- and disaccharides also inhibit bubble coalescence at approximately 0.1 M, and the sugars vary in effectiveness. Disaccharides are more effective than the sum of their individual monosaccharide constituents, and sugars with very similar structures (for instance, diastereomers galactose and mannose) can show large differences in coalescence inhibition and hence thin film stability. We conclude that solute charge is not required for bubble coalescence inhibition, which indicates that the mechanism is not one of electrostatic surface repulsion and instead an effect on dynamic film thinning other than Gibbs-Marangoni elasticity is implicated. Solute structure is important in determining coalescence.