Azo and Anthraquinone Dyes

The low molecular weight dye molecules contain a chromophoric system, usually of the azo or anthroquinone, hydrophilic groups for water solubility and reactive groups, such as a heterocyclic halogen or an alphatic chain with reactive sites. The reactive groups can form covalent bonds with hydroxy and amino groups. 

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There is a wide variety of dyes unique to the field of hair coloring. Successive N-alkylation of the nitrophenylenediamines has an additive bathochromic effect on the visible absorption to the extent that violet-blue dyes can be formed. Since the simple A/-alkyl derivatives do not have good dyeing properties, patent activity has concentrated on the superior A/-hydroxyalkyl derivatives of nitrophenylenediamines (29,30), some of which have commercial use (31). Other substituents have been used (32). A series of patents also have been issued on substituted water-soluble azo and anthraquinone dyes bearing quaternary ammonium groups (33). 

Azine, oxazine, and thiazine dyes were historically more important than they are at present. However, at least one example of each, introduced more than 100 years ago, is still offered commercially today (1,2). Azo and anthraquinone dyes have largely displaced them in commercial appHcation. Azo dyes (qv) offer better fastness and broader shade ranges at more economical prices. 

Jarosz-Wilkoazka A, Kochmaska-Rdest J, Malarcyk E et al (2002) Fungi and their ability to decolourize azo and anthraquinonic dyes. Enzyme Microb Technol 30 566-572... 

From the selectivity point of view, LC-NMR coupling is especially suited to the analysis of compound classes such as nitroaromatics, phenols, aromatic amines, aromatic carboxylic acids, polyaromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and azo- and anthraquinone dyes. Another advantage of LC-NMR coupling for the investigation of aromatic compounds in environmental samples is that the position of substituents on the aromatic ring, e.g. in unknown metabolites or degradation products, can best be determined by NMR spectroscopy. 

Research to date has focused primarily on azo and anthraquinone dyes, due to their commercial importance. Environmental processes including biodegradation, photolysis, sorption to soils and sediment, and abiotic transformation in sediment /water systems have been studied. The quantity of dyes apparently entering and potentially passing through wastewater treatment systems unaltered has prompted research on the behavior of these chemicals in biological and other types of wastewater treatment systems. 

Fig. 13.28. Azo and anthraquinone dyes suitable for liquid crystalline media.

Disperse dyes are relatively small molecules, with very low water solubility, which possess a high affinity for hydrophobic fibres such as cellulose acetate, polyester or blends thereof. The dyes are applied by transfer printing or high temperature steam fixation. Azo and anthraquinone dyes constitute the major portion of disperse dyes. 

L.S. Sorokina, G.E. Krichevskiy Effect of physicalstmcture of polyamide and polyester fibres on photodestmction of dispersed azo and anthraquinone dyes. Proceedings of higher schools. Technology of textile industry (1978), No 4, 82-86 (in Russian). 

Disperse colouring materials are used principally for colouring cellulose acetate, polyamide and polyester fibres. Their chemical structure is similar to that of the fat-soluble dyes. They belong to the compound classes of nitrodiphenylamine derivatives and azo- and anthraquinone dyes without a sulphonate group. They are more or less easily soluble in organic solvents but insoluble or difficultly soluble in water. Mixtures of disperse dyes can be separated on alumina columns, using solvents such as ether, methylene dichloride, ethyl acetate and tetrahydrofuran [71]. PC-separations on cellulose paper are incomplete [14, 30, 31, 73, 85]. Separations are better on acetyl-paper [29, 30, 43] or pre-treated paper [19, 20, 25, 44, 73]. 

With regard to the dye-related parameter measurements, the dye concentration is usually kept at about 0.5-2%, depending on its absorption coefficient. For azo dyes the dye concentration is about 0.25-1% for dye doped TN, about 2-4% for Heilmeier displays, and about 2-5% for phase change displays. The cell thickness is usually about 5 pm for dye-doped TN at the first Gooch Tarry minimum [ 15,16,88], about 7-10 pm for dye-doped TN at the second Gooch Tarry minimum [15, 16, 88], about 5-15 pm for Heilmeier, and about 10-20 pm for phase change dichroic displays. Usually, the dye concentration is increased by a factor of about 1.5-2 when anthraquinone dyes are used instead of azo dyes, as anthraquinone dyes have less absorbance than azo dyes. Sometimes combinations of azo and anthraquinone dyes are used [16, 54]. 

Novotny, C. Dias, N. Kapanen, A. Malachova, K. Vandrovcova, M. Itaevaara, M. Lima, N. Comparative use of bacterial, algal and protozoan tests to study toxicity of azo and anthraquinone dyes. Chemosphere 2006, 63, 1436-1442. 

Smoke dyes are azo and anthraquinone dyes. These dyes provide the color in smokes used for signaling, marking, and spotting. 

Apart from the paper pubhshed by Scarpi et al. (1998) which deals with semi-permanent triphenylmethane, azo and anthraquinone dyes, all the others deal mainly with oxidative hair dyes. This is because oxidative hair dyes seem to be the most interesting topic at the moment. Also, as many triphenylmethane, azo and anthraquinone dyes are used as cosmetic colorants, the papers published on this matter and reviewed in Section 4.2 will be useful to determine these substances in hair-dye products. 

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