276 Emulsions and foams
Antifoaming agentsThe prevention of foaming and the destruction of existing foams is
often a matter of practical importance; for example, polyamides and
silicones find use as foam inhibitors in water boilers. Antifoaming
agents act against the various factors which promote foam stability
(described above) and, therefore, a number of mechanisms may be
operative.
Foam inhibitors are, in general, materials which tend to be
adsorbed in preference to the foaming agent, yet do not have the
requisites to form a stable foam. They may be effective by virtue of
rapid adsorption; for example, the addition of tributyl phosphate to
aqueous sodium oleate solutions significantly reduces the time
required to reach equilibrium surface tension^230 , thus lessening the
Marangoni surface elasticity effect and the foam stability. They may
also act, for example, by reducing electric double layer repulsion or
by facilitating drainage by reducing hydrogen bonding between the
surface films and the underlying solution.
Foams can often be broken by spraying with small quantities of
substances such as ether and n-octanol. As a result of their high
surface activity, these foam breakers raise the surface pressure over
small regions of the liquid films and spread from these regions,
displacing the foaming agent and carrying with them some of the
underlying liquid^229. Small regions of film are, therefore, thinned and
left without the properties to resist rupture.