Conventional surfactants can be replaced with alternatives, which ensure 80 - 90% biodegradability after 24 hours, generate a lower COD, have a high dispersing power and very low foaming power, insists B H Patel.

For over 2,000-years, humankind has been using surfactants or surface-active ingredients in various aspects of daily life for washing, laundry, cosmetics, and house-cleaning ^(1-3). In the United States alone, over 10 billion pounds of detergents are used annually. Anionic surfactants represent 70 - 75% of the detergent market. Natural soaps are the oldest anionic surfactants and are used mainly in personal care and in the detergent industries. However, the development of more economical processes for the manufacture of surfactants has contributed to an increased consumption of synthetic detergents.

Non-soaps or synthetic detergents account for 84% of the total detergent market. In 1996, over 5 billion pounds of non-soap surfactants were produced. In the Asia-Pacific region, the total surfactant consumption grows at an annual rate of 3.9% with a projection of 5.8 million tons in 2010. From a global perspective, the consumption and proportion of surfactants exhibit a different pattern for the North American and Western European regions compared with the Asia-Pacific region or Japan in particular. However, the major surfactants common (with respect to detergent) to all regions are linear alkylbenzene sulfonates (LASs), alcohol ether sulfates (AESs), aliphatic alcohols (AEs), alcohol sulfates (ASs), and soap.

In the past decades, new surfactants have proliferated mainly as nonionic or non-soap surfactants offering unique properties and features to both industrial and household markets. Non-soap surfactants are widely used in diverse applications such as detergents, paints, and dyestuffs; as speciality surfactants in home and personal care; and in the cosmetics and pharmaceutical industries.

Since the 1960s, biodegradability and a growing environmental awareness have been the driving forces for the introduction of new surfactants. These forces continue to grow and influence the surfactant market and production. A new class of surfactants, carbohydrate-based surfactants, has gained significant interest and increased market share. Consequently, sugar-based surfactants, such as alkyl polyglycoside (APG*), are used as a replacement for polyoxyethylene alkylphenols (APEs) where biodegradability is a concern. They represent a new concept in compatibility and care ^(4-5).

Nonetheless, over 35 different types of surfactants are produced and used commercially in the formulation of home care, personal care, and industrial products.

Contrary to many textbooks that elaborate on surfactant physical properties or formulation guidelines, this chapter approaches the surfactant topic from both synthesis and manufacturing perspectives. It offers a comprehensive overview of the most commonly used industrial surfactants with respect to their synthesis and manufacturing processes; their reactions and applications; and their physical, ecological, and toxicological properties.

A concise and thorough description of the most pertinent synthesis routes is presented for the major types of surfactants predominantly used in the home and personal care industry. These surfactants are primarily anionic, nonionic, cationic, and amphoteric. Also reviewed is the synthesis of surfactants derived from carboxylation, sulfation, and condensation of fatty acid and phosphoric acid derivatives.

The most commonly used anionic surfactants are LASs, ASs, and AESs. Nonionic surfactants are produced mainly by alkoxylation technology, although amine oxides under alkaline conditions are also classified as nonionic. Section 3 discusses the synthesis, production, and applications of the most commonly used ethoxylated surfactants such as alcohol ethoxylates, nonyl phenol ethoxylates and fatty acid ethoxylates, fatty amine oxides (FAOs), and fatty alkanolamides (FAAs). Section 4 is concerned with a class of biodegradable and highly compatible carbohydrate- or sugarbased surfactants such as sorbitan esters, sucrose esters, and glucose-derived esters. Their syntheses encompass a significant list of renewable raw materials, including sucrose from sugar beet or cane, glucose from starch, and sorbitol as the hydrogenated glucose derivative.

The most commonly used sugar-based surfactants, such as APG and fattyacid glucamides (FAGs), are reviewed in depth. Cationic surfactants contain exclusively a quaternary tetracoordinated nitrogen atom (quaternary ammonium compounds). They are widely used as textile softeners in laundry formulations and in flotation. Amphoteric surfactants (including betaines) exhibit a zwitterionic character, ie, they possess both anionic and cationic structures in one molecule. Recent progress in the surfactant field focuses on polymeric, splittable, gemini, multifunctional, and biosurfactants.

Biodegradability of surfactants

Biodegradability is defined as the susceptibility of a surfactant to the common process by which organic matter in waste water is decomposed by bacterial action. Biodegradation involves the use of the material as a suitable food by the universally available micro organisms in various environments. In the relatively short time of 30 - 40 years since the first synthetics surfactants were introduced, hundreds of different products, based on many types of chemical compounds, have appeared in the market.

For general purpose use, especially in household detergents as well as for textile wet processing, the anionic alkyl benzene sulphonates have led the field in terms of volume of production. It was soon observed that these surfactants are hard compounds in that they are resistant to break-down by bacteriological action in waste treatment plants. These agents reduce the rate at which oxygen from air is dissolved in water. Thus, they retard the natural process of self-purification of streams and rivers. If the concentration of these materials is sufficiently high, aquatic fauna and flora may be harmed. These products foam even at concentrations of less than 1 ppm in water.

It is interesting to note that in USA cases have been recorded where owing to presence of these surfactants, drinking water foamed as it flowed out from taps. This is considered undesirable. The problems caused by alkyl benzene sulphonates created a general awareness of possible difficulties from other surfactants and there is now a concern about biodegradability of surfactants. In USA and UK considerable amount of research has been carried out on this aspect and it has been observed that the low rate of biodegradablility of alkyl benzene sulphonates is associated with the presence of branched side chain. If a linear side chain is present, the rate of degradation of alkyl benzene sulphonate increases markedly.

Mechanism of biodegradation of straight chain alkyl benzene sulphonate

According to Barnes and Dobson the biodegradiation of straight chain alkyl benzene sulphonates starts by oxidative degradation of the side chain. First the terminal methyl group is oxidised to give a carboxylic acid (W-oxidation) (A). This acid is then degraded by ß-oxidation, with a sequence of reactions starting with oxidation at the ß-carbon atom (B) - and finishing with fission of chain to yield a new acid (III) containing two carbon atoms less than the original one.

This end of molecule then undergoes another ß-oxidation step. From the intimate knowledge of ß-oxidation process, it follows that branching of the chain in particular at or near the ß-carbon atom, causes steric hindrance or depending upon the actual configuration, completely blocks ß-oxidative degradation. A close look at structures of straight chain and branched chain alkyl benzene sulphonates will make the above point clear.

Bument et al have measured the biodegradability of straight chain and branched chain alkyl benzene sulphonates by the shake flask and the semi-continuous methods. It can be seen from their results in Table 1 that straight chain alkyl benzene sulphonates are completely biodegraded whereas the branched chain type is only partially biodegraded.

Biodegradability of other anionic surfactant

Both primary and secondary alcohol sulphates are easily biodegradable. Barnes and Dobson have compared the biodegradability of different anionic surfactants. It can be seen from Figure 2 that primary alcohol sulphates are biodegraded in a very short time followed by secondary alcohol sulphate and linear alkyl benzene sulphonates. The branched chain alkyl benzene sulphonates are not completely biodegraded even after seven days. Sulphonated a-olefins are also easily biodegradable.

Biodegradibility of cationic surfactants

Cationic surfactants are used in small quantities for specialised purposes. These are neutralised or rendered non-surface-active by reaction with anionic substances, which are present in most effluents from textile works. As a result, the amount of cationic agents reaching the sewage purification plants becomes so small that their effect is insignificant.

Biodegradibility of non-ionic surfactants

Most of the commercially important non-ionic surfactants consists of ethylene oxide condensates of various hydrophobic compounds. The ethenoxy chain itself can be degraded by biological oxidation. Non-ionic agents thus differ from alkyl benzene sulphonates in that the hydrophilic part of the molecule can be the site of biological attack. However, this process is slow and where there is a long polyethylene glycol chain a retarding effect on degradation comparable to that of the benzene ring in anionic agents mentioned above can occur.

The primary alcohol ethoxylates are probably the quickest biodegradable surfactants among non-ionics. The linear secondary alcohol ethoxylates approach the primary alcohol ethoxylates in biodegradability. Branched chain alkyl phenol ethoxylates like nonyl and actyl phenol ethoxylates are hard, ie, non-biodegradable. Substitution of straight alkyl chain for branched alkyl chain in the molecule of these non-ionic agents has only a minor effect on their biodegradability.

In general, hard non-ionic agents certainly present a pollution problem, which is especially important because they promote and stabilise the foam produced by anionic materials.

Surfactant's role in textile processing

Almost all textile processes use water as a process medium. In order to conduct these processes, the textile substrate must be totally wetted out. Surfactants are necessary to lower the surface tension of process solutions for uniform application. How fast and uniformly aqueous solutions wet textile substrates often impacts processing performance.

It is a common practice to use surfactants in almost all textile processes to help increase wettability, often referred to as wetting agents. In fibre manufacture of synthetic/regenerated fibres and yarn spinning of cotton, wool, and their blends, surfactants are often sprayed on the fibre and yarn surface to reduce fibre-fibre and fibre-metal friction, referred to as yarn lubricants In desizing and scouring of natural textile fibres and theirs blends, surfactants are often used as detergents and emulsifiers to help remove impurities that are originated from natural fibres or added in the early processes such as spinning and weaving.

In textile dyeing, surfactants are broadly used as dispersants and leveling agents to help uniform dying and better dye penetration. Dye fixatives and dye carriers are also surface active although they are not as common as other dyeing assistants. In textile finishes, surfactants are often used as fabric softeners to improve fabric hand or feel and used as antistatic agents to control static electricity built up on the surface of textile fibres, particularly on synthetic fibre due to their low moisture contents. Surfactants are also useful to control foam formation during textile processes, referred as antifoaming agents, particularly in dyeing and other processes with high-speed padding.

Conclusion

In the textiles industry, surfactants are consumed by practically all processes from the preparation and bleaching stage to the finishing of fabrics. After the dyeing and printing processes, habitually the fabric is subjected to one or several washes in which surfactants are used as washing agents, which often cause problems of pollution in wastewater due to the presence of foam and deficient biodegradability. The objective now is in replacing conventional surfactants with others that give 80 - 90% biodegradability after 24 hours, that generate a lower COD, have a high dispersing power and very low foaming power

References

1. Tadros, Tharwat F: Applied Surfactant: Principle and Applications, Wiley VCH Verlag GmbH & Co KGaA, 2005.
2. Shenai V A: Chemistry of Organic Textile Chemicals, Sevak Publications, Bombay, 1990.
3. http://en.wikipedia.org/wiki/Surfactant.
4. Myers Drew: Surfactants Science and Technology, 3^rd Ed, Wiley-Interscience, 2006.
5. Texter John: Reaction and Synthesis in Surfactant Systems, Marcel Dekker, Inc, 2001.

B H Patel
Department of Textile Chemistry,
Faculty of Technology & Engineering,
The M S University of Baroda,
Vadodara, Gujarat.
Email: bharatextile@gmail.com.