By far the largest class of surfactants in general use today is the anionic surfactants (including soaps). For thousands of years in the history of the application of surface activity to the needs of mankind, soaps derived from natural animal and vegetable fats and oils dominated the field. The major historical and economic advantage of fatty acid soaps has always been the ready availability of raw materials from natural and renewable sources; however, the weakness of this class has always been the sensitivity of the carboxlate salts which mainly constitute them, in many processes requiring surfactant action which led to the development of synthetic surfactants. Synthesis of petroleum surfactants from feed stocks spurred the development of soap free synthetic surfactants. Today, sodium petroleum sulphonate – phosphate esters – methyl ester sulphonates are widely used.
Classically, anionic surfactants can be divided in to
- Carboxyl joined directly to hydrophobic group.
- Carboxyl joined through an intermediate linkage
- Sulphate joined directly to hydrophobic group
- Sulphate joined through intermediate linkage
Alkane sulphonic acids
- Sulphonic group directly linked to hydrophobic group
- Sulphonic group joined through intermediate linkage
Alkyl aromatic sulphonic acids
- Hydrophobic group joined directly to sulphonated aromatic nucleus
- Hydrophobic group joined to sulphonated aromatic nucleus
Miscellaneous anionic hydrophilic groups
- Phosphates and phosphoric acids
- Persulphates, thiosulphates
- Sulphamic acids etc
Alkylates for alkyl benzene production
Catalytic cracking and reforming processes were developed to produce high octane gasoline. They essentially consist of breaking an alkane chain to produce an alfa-olefin and to reform molecules in a different way. The reformation happens with the attachment at the second carbon atom of the alfa-olefin, thus resulting in branching, which is the structural characteristic that confers a high octane number. Because of the 3 carbon atoms difference between the n-mer and the n+1-mer, it is easy to separate by distillation the tetramer, with some amount of trimer and pentamer, to adjust the required chain length.
The alpha-olefin resulting from polymerization is used as an alkylate in a Friedel-Crafts reaction that ends in an alkyl-benzene. By sulphonation and neutralization, an alkyl-benzene sulfonate of the detergent type is produced at a low cost, much lower than a soap from natural oil and fat origin. However, the alkylate is branched (see Figure), and this is quite an inconvenient because it is much more difficult to biodegrade it than the linear counterpart. As a consequence, this kind of so-called hard alkylate have been banned by legislation in most countries, to be replaced by their linear equivalents
Linear alkylates are produced either by separation from a petroleum cut containing a mixture of linear and isomerized substances or by synthesis through ethylene oligomerization. Extraction of linear paraffins from refinery cuts can be carried out by two methods. The first one uses molecular sieves of the zeolite type.
Alpha olefin sulphonates
Alpha-olefin sulfonates display a better hard water tolerance than LAS, but they are not as good detergents; they are used as additives, particularly in low phosphate formulas: C12-14 in liquids, C14-18 in powders.
Lignosulfonates are produced by the reaction of wood lignin with bisulfite or sulfate ions during the wood digestion reaction to make the pulp. Lignin three-dimensional polymer contains numerous aromatic rings as well as hydroxyl methyl-ether functions. A sulfonating agent is able to add a sulphonate group on aromatic rings or to sulfate to a hydroxyl group. In both cases the resulting sulphonate or sulfate increases the hydrophilicity of the polymer and can turn it water soluble. This solubilization in the so-called black liquor at alkaline pH is the way to separate lignin compounds from insoluble cellulose fibers. A typical commercial lignin compound contents lignin chunks with MW ranging from 4000 Daltons (about 8 aromatic ring units) to 20.000 or more. Lignosulphonates are used as dispersing agents in textile processing.
Di Octyl Sodium Sulfosuccinate (DOSS)
DOSS is the dioctyl (actually di-bis-ethyl-hexyl) sulfosuccinate is prepared from secondary octanol. A similar compound is attained with hexanol.
The chemistry of phosphorus is particularly complex. It is just necessary to point out here that the double valency (3 and 5) leads to three acids:
Anionic surfactants are used for special applications in textile processing when other surfactants do not perform. One of the salient features of anionic surfactants is there ability to withstand strong alkali of mercerizing strength. Many anionic surfactants can also act as hydrotropes, which serve to raise the cloud point of nonionic surfactants. Anionic surfactants are also used during emulsion polymerization of acrylic /latex binders.
Nonionic surfactants are particularly efficient at removing oily soils from synthetic fabrics but they are not as efficient at removing particulate soils as anionic surfactants. The main drawback of anionic surfactants is its high foaming which is not desired during processing, however the problem can be obviated by the use of special solvents.
C. N. Sivaramakrishnan