On the effect of van der Waals attractions on the critical salt concentration for inhibiting bubble coalescence

The coalescence of gas bubbles in many salt solutions such as NaCl is significantly inhibited if the salt concentration exceeds a critical concentration which is unique for each salt. It has been predicted as a function of the Hamaker constants of van der Waals attractions between two bubbles. In this paper, the Lifshitz theory on the van der Waals interaction energy and the available spectrum for water dielectric permittivity are applied to determine the Hamaker constants for liquid films between bubbles in pure water and salt solutions. Using the new values for Hamaker constants for thin films of saline solutions, we show that the effect of van der Waals attractions on the critical salt concentration is significantly weaker than previously hypothesized. The predictions based on van der Waals attractions significantly under-estimate the experimental critical concentrations. The failure of the van der Waals attraction in predicting the critical salt concentration highlights the urgent need of revising the available theories on bubble coalescence in salt solutions and saline water.