Mechanism of different Auxiliaries used in Textile Wet Processing

Surfactants:
Surfactants are compounds that lower the surface tension of a liquid, the interfacial tension between two liquids, or that between a liquid and a solid. Surfactants may act as detergents, wetting agents, emulsifiers, foaming agents, and dispersants.

Surfactants are widely used in textile wet processes for the purpose of wetting, dispersing, emulsifying and cleaning. The molecular structures of surfactants have a distinctive hydrophilic moiety and a distinctive hydrophobic moiety. When they are used at a sufficient concentration, the surface/interface tension of the solutions lowered and micelles (an aggregate of surfactant molecules dispersed in a liquid colloid) are formed, which give the solution extra properties. According to their ionic properties in aqueous solution, traditional surfactants can be divided into four categories:

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


The lipophilic tails of the surfactant molecules remain on the inside of the micelle due to unfavourable interactions. The polar “heads” of the micelle, due to favourable interactions w ith water, form a hydrophilic outer layer that in effect protects the hydrophobic core of the micelle. The compounds that make up a micelle are typically amphiphilic in nature, meaning that micelles are soluble not only in protic solvents such as water but also in aprotic solvents as a reverse micelle.

Anionic surfactant:
Anionic surfactant are those whose bear a negative charge and migrate towards the anode or positive charge in solution. This class of salt like compounds include the soaps and many of the popular synthesis. When fats are hydrolyzed in presence of alkali fatty acids is formed which on neutralization with caustic soda form a sodium salt of fatty acid, known as soap.

The instability of of soap in acid solution is the main drawback. Synthetic detergent are surfactants developed to overcome the drawback of soaps. They are salt of strong acids and when pure they are practically neutral. In contrast to soap they are less liable to hard water precipitation & are readily rinsable.

Typical example of anionic detergent: Sodium stearate (soap), Alkyl benzene sulphonates, Alkyl sulphates, Alkyl sulphonates, phosphates ester.

Cationic surfactants:
Chemically they are just opposite to anionic surfactants and hence they are unsuitable for use as detergent or wetting agent. The hydrophobic part of the molecule of the cationic surfactant is the organic ammonium or pyridinium compound containing one or more hydrophobic residues. Example- catyl pyridinium chloride.

Cationic surfactant are rarely used in preparation unless applied as a lubricant or antistat at the end of the scouring cycle on goods that are to be dyed but not bleached.

A small quantity of cationic surfactant is also recommended as an aid in weight reduction in caustic treatment of polyester. Cationic surfactant are mainly used as softener, leveling agent, retardants in dyeing, water repellents, bacterial growth inhibitors and emulsifiers.

Nonionic surfactant:
Non ionic surfactant do not contain an ionisable group and have no electric charge. The most important non ionic detergent are those obtained by condensation of ethylene oxide or propylene oxide. Ethylene oxide can be reacted with fatty acid, alcohols and alkyl phenol, fatty amide etc to give polyoxyethylene compound of ether and amide known as ethylene oxide condensation product.

Non anionic surfactant are free from precipitation and redeposition onto the fabric and can be safely used which also permit the caustic to act as a lime-soap detergent. Its advantages are excellent compatibility with all classes of surfactants, good wetters and rewetters, good emulsifiers, excellent oil solubility and good component of oil emulsifier.

Example: Ethoxylated primary alchol, ethoxylated thio -ethers , Ethoxylated fatty acid, Ethoxylated fatty amide.

Amphoteric surfactant:
Amphoteric surfactant may be cationic, anionic and nonionic depending upon the PH of the aqueous solution. Amphoteric surfactant offer an excellent degree of lubrication, corrosion inhibitor and wetting action and provide a protective colloid for silk and wool processing. Then major uses of amphoterics are in the scouring and dyeing of protein fibres to prevent chafling, crack marks and crow’s feet. Amphoterics are comparatively expensive and some of them are not heat stable and hence can’t be used at elevated temperature.