Foam Destruction by Mixed Solid−Liquid Antifoams in Solutions of Alkyl Glucoside: Electrostatic Interactions and Dynamic Effects

Abstract

Antifoam substances are used in various technologies and commercial products to prevent the formation of undesirable foam. A typical problem in their application is that an antifoam that is rather active in a given surfactant solution might be very inefficient for other foaming media at comparable conditions. The reasons for this high antifoam selectivity to the used surfactant are still poorly understood. To gain a new insight into this problem, we compare the mechanisms of foam destruction by several antifoams for two surfactants: the nonionic alkyl-C12/14(glucopiranoside)1.2 (APG) and the anionic sodium dioctyl-sulfosuccinate (AOT). Foam tests demonstrate significant differences in the antifoam activity for these two surfactants, although their entry, spreading, and bridging coefficients are very similar. One interesting feature is that the antifoams destroy APG-stabilized foams only under dynamic conditions (during shaking); the foam that “survives” the first several seconds after ceasing the agitation remains stable for many hours. In contrast, most of the studied antifoams destroy rapidly and completely the AOT-stabilized foams without external agitation. In general, the foams produced from APG solutions are significantly more stable. Additional model experiments show that the observed differences can be explained by three simple effects: (1) the kinetics of surfactant adsorption on the air−water and oil−water interfaces is much slower in APG solutions; (2) the barrier to entry of the antifoam globules is much higher for APG; (3) the films stabilized by APG are much thicker and more resistant to rupture by the antifoam globules. One surprising conclusion is that the electrostatic interaction between the charged air−water and oil−water interfaces is extremely important in solutions of the nonionic surfactant APG.