Foamability of surfactant solutions: Interplay between adsorption and hydrodynamic conditions

Abstract

The foamability of surfactant solutions depends strongly on the adsorption properties of the surfactants, their concentrations, and the foaming test used for foam generation. The aim of the current study is to analyse the subtle interplay between the hydrodynamic conditions and the surfactant adsorption layers formed during foaming. We compare the results from three foam tests, characterized with very different hydrodynamic conditions. The Bartsch test (fast-foaming method) is characterized by a very short time, of the order of milliseconds, allowed for surfactant adsorption before the newly generated bubbles collide with each other. The other two “slow-foaming” tests, the planetary mixer and the foam rise method, are very different from this viewpoint – the characteristic time for surfactant adsorption before the bubble collisions is much longer, of the order of seconds. Surfactants of two different types, nonionic and anionic, with different chain-lengths are compared. The obtained results reveal that the long-chain nonionic surfactants with 16 carbon atoms are unable to stabilize the foams in the fast-foaming test, even at very high surfactant concentrations (50 mM), due to their relatively slow adsorption. Interestingly, the same surfactants are excellent foam stabilizers in the slow-foaming tests. On the other hand, the medium-chain anionic surfactants with 12 carbon atoms are very suitable for stabilization of the gas bubbles in the fast-foaming test, even at relatively low concentrations, due to their rapid adsorption and inherent electrostatic repulsion. In slow-foaming tests, however, the anionic surfactants show lower foamability to the nonionic ones. All results from the foam tests are explained in a unified approach which accounts explicitly for the rate of surfactant adsorption and for the characteristic time-scales of the various foaming tests.