Page 175 - Basic Principles of Textile Coloration

164 AUXILIARY CHEMICALS FOR WET PROCESSING AND DYEING

A similar mechanism applies for eliminating insoluble hydrophilic polar dirt.
Firstly, surfactant molecules adsorb onto the surface of the hydrophilic dirt, but, in
this case, the anionic ‘heads’ orient into the surface of the polar dirt. To avoid
contact of the hydrophobic ‘tails’ with the water, a second layer of surfactant
molecules forms with their hydrocarbon tails interacting with those extending
outwards from the first layer. This second layer of molecules has its anionic groups
exposed towards the water. Such dirt therefore rolls up and lifts from the surface
and is held in suspension, as before (Figure 9.6).

Although many surface-active chemicals can act as both wetting agents and
detergents, this is not always true. Wetting depends mainly on reducing the
surface tension of the wetting liquid, whereas detergency depends on micelle
formation and the ability of the surfactant to keep dirt in suspension. The
surfactant alkyl chain must be at least six carbon atoms long for surface activity to
occur but at least twelve for even minimal detergent action.

Having established the principles of surface activity and micelle formation, and
their role in wetting, emulsification and detergency by means of simple soap
molecules, we will now examine the chemical nature of the many types of
synthetic surfactant.

9.3 SYNTHETIC SURFACTANTS
In textile wet processing, a variety of synthetic surfactants have replaced the
natural soaps because of the ease of precipitation of the latter in acid solution or in
hard water. Most synthetic products do not have these disadvantages and are
widely used as detergents, wetting, dispersing and emulsifying agents, as well as
dyeing assistants. They are manufactured from both natural starting materials,
such as stearic or oleic acid, and from fully synthetic petrochemicals. Surfactants
are usually classified according to their ionic character:
(1) anionic surfactants, usually organic sulphates and sulphonates;
(2) cationic surfactants, typically quaternary ammonium or pyridinium salts;
(3) non-ionic surfactants containing polar polyethylene oxide chains;
(4) amphoteric surfactants, whose molecules have both anionic and cationic

sites.

9.3.1 Anionic surfactants
This is the major group comprising almost 70% of commercial surfactants.
Anionic surfactants are invariably the sodium salts of either aliphatic or aromatic