Foam destruction by mixed solid-liquid antifoams in solutions of alkyl glucoside: Electrostatic interactions and dynamic effects

Antifoam substances are used in various technologies and commercial product s 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 c onditions. The reasons for this high antifoam selectivity to the used surfa ctant 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-C-12/14(glucopiranoside)(1.2) (APG) and th e anionic sodium dioctyl-sulfosuccinate (AOT). Foam tests demonstrate signi ficant differences in the antifoam activity for these two surfactants, alth ough their entry, spreading, and bridging coefficients are very similar. On e interesting feature is that the antifoams destroy APG-stabilized foams on ly under dynamic conditions (during shaking); the foam that "survives" the first several seconds after ceasing the agitation remains stable for many h ours. In contrast, most of the studied antifoams destroy rapidly and comple tely the AOT-stabilized foams without external agitation. In general, the f oams produced from APG solutions are significantly more stable. Additional model experiments show that the observed differences can be explained by th ree simple effects: (1) the kinetics of surfactant adsorption on the air-wa ter and oil-water interfaces is much slower in APG solutions; (2) the barri er 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 ant ifoam globules. One surprising conclusion is that the electrostatic interac tion between the charged air-water and oil-water interfaces is extremely im portant in solutions of the nonionic surfactant APG.