A foam is formed when a liquid
contains suspended gas. Examples include shaving foam (where butane is the
gas) and the foam layer created on the surface of a warm bath by adding a
surfactant like 'bubble bath.' In the latter case, the gas is air,
predominantly nitrogen and oxygen.
When a gas is suspended in a
solid, it is also termed a foam. This colloid type is relatively uncommon
in nature unless we broaden our definition of 'solid' to include rock, making
pumice stone a colloidal foam. Cushions and pillows rely on synthetic foams,
and there is ongoing research on metal foams, known for their remarkably low
density.
Surfactants, or agents
active on the surface, are large organic molecules slightly soluble in
liquids like water, inducing foaming. They primarily consist of a strongly
hydrophobic group combined with a strongly hydrophilic group. The hydrophobic
group, usually a hydrocarbon radical (R) with 10 to 20 carbon atoms, comes in two
types: those ionizing in water and those not.
Anionic surfactants carry
a negative charge [e.g., (RSO3N)2Na1], while cationic surfactants bear a
positive charge [e.g., (RMe3N)1Cl2]. Nonionic surfactants commonly feature a
polyoxyethylene hydrophilic group (ROCH2CH2OCH2CH2...OCH2CH2OH, often
abbreviated REn, where n is the average number of -OCH2CH2- units in the
hydrophilic group). Hybrid variations exist. About two-thirds of total
surfactants are typically ionic, and one-third are nonionic.
Surfactants tend to accumulate
at the air-liquid interface, with the hydrophilic group in the liquid and
the hydrophobic group in the air. During liquid aeration, like in water, these
compounds amass on the surface of air bubbles, creating a highly stable foam.
Before 1965, synthetic
detergents contained a problematic surfactant, alkyl-benzenesulfonate (ABS),
resistant to biological breakdown. Legislation in 1965 replaced ABS with
linear-alkyl-sulfonate (LAS), a biodegradable alternative. The
prevalence of surfactants from synthetic detergents has significantly
diminished the foaming issue. It's noteworthy that so-called "hard"
synthetic detergents remain extensively used in many foreign countries.
Two tests now ascertain
surfactant presence in water. The MBAS (methylene blue active
substances) test detects anionic surfactants by measuring color change
in a standard methylene blue dye solution. Nonionic surfactants are assessed
through the CTAS (cobalt thiocyanate active substances) test. Nonionic
surfactants react with CTAS to yield a cobalt-containing product, extractable
into an organic liquid for measurement. It's important to note that the CTAS
method involves sublimation to eliminate non-surfactants and ion exchange to
remove cationic and anionic surfactants.
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