Thin Liquid Films - Surface and Colloid Chemistry: Principles and Applications - page 169

large increase in η s . On the other hand, additives that are reported to inhibit gushing,
such as EDTA (ethylenediamine acetic acid—a chelating agent), have been reported
to decrease the η of beer. This relation between η s and gushing suggests that an effi-
cient gushing inhibitor should be very surface active (in order to be able to compete
with gushing promoters), but incapable of forming rigid surface layers (i.e., high η s ).
Unsaturated fatty acids, such as linoleic acid, are potent gushing inhibitors since they
destabilize the surface films.
The surface viscosity, η s gram/second, was investigated by the oscillating-disc
method. It was found that low η s (0.03-0.08 g/s) beer surfaces presented a nongush-
ing behavior. Beers with high η s (2.3-9.0 g/s) presented a gushing behavior.

8.3.3 a n T I f o a m I n g a g e n T S
In many cases, foaming is found to be undesirable (such as in dishwashing and
wastewater treatment), and in many industries, such as in wastewater treatment, all
foaming needs to be eliminated. The main criteria for antifoaming molecules have
the following characteristics:

They do not form mixed monolayers.
Surface viscosity is reduced (thus destabilizing the foam films).
The boiling point is low (such as with ethanol).

8.4 aPPlIcatIonS oF FoamS

8.4.1 W a T e r p u r I f I c aT I o n
The biggest challenge mankind is facing is the need for supplying pure drinking
water worldwide. The world population increase (from 1900 to 2000 by a factor 4)
is much faster than the availability of clean drinking water supplies. Further, the
increased need for water in industrial production also adds a further burden on water
supply. The purification of water for households has been developed during the past
decades, but the pollutants found in wastewater are of a different origin and concen-
tration. Solid particles are mostly removed by filtration, but colloidal particles are
not easily removed by this method. Solute compounds are rather difficult to remove,
especially toxic substances with very low concentration. Flotation has been used
with great advantage in some cases where sedimentation cannot remove all the sus-
pended particles. The following are examples where flotation is being used with
much success:

Paper fiber removal in the pulp and paper industries
Oils, greases, and other fats in food, oil refinery, and laundry wastes
Clarification of chemically treated waters in potable water production
Sewage sludge treatment

Many of the industrial wastewaters amenable to clarification by flotation are colloi-
dal in nature (e.g., oil emulsions, pulp and paper wastes, and food processing). For