Page 172 - Basic Principles of Textile Coloration

SURFACE ACTIVITY OF DETERGENTS 161

In the interior of liquid water, each molecule is surrounded uniformly and
equally attracted by neighbouring molecules in all directions. The surface tension
originates from the unbalanced attraction of water molecules at the air–liquid
interface towards neighbouring molecules in the bulk of the water pulling them
inwards away from the air. These intermolecular attractions are mainly hydrogen
bonds. If a liquid such as water has little attraction for molecules in the interface
with which it is in contact, such as air, the surface tension will act to reduce the
liquid surface area in contact with that interface. This is why water droplets in air,
or on a wax surface, are approximately spherical. It is exactly the same
phenomenon that causes water molecules to become more structured around the
hydrophobic alkyl chain of a soap molecule. The air–water surface tension of a
surfactant solution decreases rapidly as the concentration increases because
surfactant molecules replace water molecules at the interface, with their alkyl
chains and carboxylate groups oriented away from and towards the water,
respectively. The surface tension increases somewhat, however, once the CMC
has been exceeded (Figure 9.4).

Surface activity and micelle formation are dynamic effects and surfactant
molecules are undergoing constant interchange between the interfacial
monolayers, micelles and solution. Micelles may have different shapes and sizes
depending on the surfactant, its concentration and the temperature. The
effectiveness of a surfactant in emulsifying oils and fats depends very much on the
micellar composition of the solution and therefore on the detergent concentration
and temperature. Once the CMC has been reached, the detergent action increases
only slowly and the use of a large excess of surfactant is wasteful (Figure 9.4).

It is not uncommon for a combination of two different surfactants to produce
an effect that is greater than the combined individual effects of the two
components. This is called a synergistic effect. It probably comes about from the
formation of mixed micelles, containing molecules of both surfactants, that are
more effective than micelles of the individual surfactants. These are complex
chemical systems. Many products have special formulations for a particular
purpose and the best combinations and concentrations for a particular process are
often determined by trial and error.

9.2.3 Emulsification, detergency and wetting

How then does the surface activity of a surfactant influence removal of dirt from a
fabric and its emulsification in solution? The emulsification of oil droplets in water