Chemistry Reference
In-Depth Information
Figure 8.5.
Mechanisms of foam breaking: (a) surfactant displacement (monomolecular
film); (b) lens formation.
surfactants, or organic or silicone materials that act by spreading on the interface
and displacing the stabilizing surfactant species.
A foam breaker that acts by spreading may do so by adsorption as a monolayer,
displacing the surfactant molecules that would normally assist in foam formation
and stabilization, or as a lens, accumulating in spots along the interface and leaving
''weak spots'' that can be easily ruptured by mechanical or gravitational forces
(Figure 8.5). In addition, such defects in the lamellar walls will not normally be
subject to the healing Gibbs-Marangoni effects. In either case, it is assumed that
the spreading foam breaker sweeps away the stabilizing layer, leading to rapid bub-
ble collapse. The rate of spreading of the defoamer will, of course, depend on the
nature of the adsorbed layer present initially. If the surfactant can be easily dis-
placed from the interface, the defoamer will spread rapidly, resulting in fast
foam collapse, or essentially no notable foam formation. If the surfactant is not dis-
placed rapidly, on the other hand, spreading will be retarded, or even halted. Foam
collapse will then be a much slower process relying on the thinning or weakening of
the lamellae by other drainage mechanisms.
Studies of the relationship between the action of defoamers and the concentra-
tion of the surfactant revealed that if the surfactant concentration was below the
cmc, the defoamer was most effective if it spread as a lens on the surface rather
than as a monolayer film. That is, the defoamer produced a defective lamellar struc-
ture that could not withstand the mechanical rigors of the foaming process for suf-
ficient time for the normal stabilizing mechanisms to take hold. In the presence of
micelles, the defoamer may be solubilized in the micelles, which can act as a reser-
