Rate of Bubble Coalescence following Quasi-Static Approach: Screening and Neutralization of the Electric Double Layer (original) (raw)

Air-bubble coalescence in aqueous electrolytic solutions, following quasi-static approach, was studied in order to understand its slow rate in purified water and high rate in electrolytic solutions. The former is found to be due to surface charges, originating from the speciation of dissolved CO 2 , which sustain the electric double layer repulsion. Rapid coalescence in electrolytic solutions is shown to occur via two different mechanisms: (1) neutralization of the carbonaceous, charged species by acids; or (2) screening of the repulsive charge effects by salts and bases. The results do not indicate any ion specificity. They can be explained within the DLVO theory for the van der Waals and electric double layer interactions between particles, in contrast to observations of coalescence following dynamic approach. The present conclusions should serve as a reference point to understanding the dynamic behavior. A ir-bubble coalescence is frequently encountered in nature as well as in industry 1-37. In some cases, rapid coalescence is preferred, e.g. to avoid clogging of a system. In others, a relative slow coalescence is required, in order, for example, to keep the total interfacial area of the bubbles as high as possible. An example of a natural process is the formation of bubbles during breaking of seawater waves. This foam stays stable for a short time, during which the bubbles enhance the dissolution of oxygen and other weakly soluble gases 7,8. Eventually, the coalescence of the bubbles with the sea-air interface leads to transfer of gas and ionic aerosols to the atmosphere 5-7. Industrial examples include mass transfer processes in bubble columns, distillation towers, gas-liquid contactors, bioreactors, electrochemical cells etc. 8,9. Relatively slow bubble coalescence (seconds or minutes) in aqueous solutions containing certain concentrations of electrolytes, was apparently first observed in 1929 10. This phenomenon has been widely discussed 10-37 , since it appears to be in qualitative contradiction to the DLVO (Derjaguin, Landau, Verwey, and Overbeek) theory 38 , which describes the stability of colloidal particles in a medium, by calculating the combined effects of van der Waals (vdW) attraction and electric double layer (EDL) repulsion between the particles. It predicts accelerated bubble coalescence in aqueous solutions of electrolytes compared with purified water. So far, only partial explanations of this apparent contradiction have been given, based on surface elasticity 2,15,17,39,40 , surface diffusion 41 , ion partitioning at the tested liquid-air interface 37,42,43 hydration and structural forces 26,44,45 , effective/ partial immobilization of interfaces, and deformation effects 46-49. Various electrolytes have been investigated over the years, using a variety of methods to study bubble coalescence behavior. Since bubble coalescence is actually a process of thin film rupture, it has been studied by methods that apply to either whole bubbles or isolated thin films: a bubble column 11-20 , pairs of bubbles 12,19,21-26 , bubbles rising to the interface 27-30 , a liquid thin-film between two gas-phases 31-33 , and two bubbles in Atomic Force Microscopy (AFM) 34-36. These studies have revealed the effect of electrolyte type and concentration on the rate of bubble coalescence, mostly when the approach of the coalescing bubbles is dynamic (approach velocity higher than 10 mm/sec). The transition between rapid coalescence (order of magnitude of milliseconds or seconds) and relatively slow coalescence occurs only with certain electrolytes and at specific concentrations (termed ''transition concentrations'' 14). Craig et al., using the bubble column method, empirically classified the ions of the electrolytes as type a or b. They found that aa and bb electrolytes in aqueous solutions slow bubble coalescence, while ab and ba electrolytes have no meaningful effect compared with purified water 14. This classification was supported also by the results