Colouration anionic direct dyes

Anionic direct dyes are the most widely used colourant within the tissue, printing and writing paper sectors. Having a typical molecular weight of 550-2,000, they produce a wide range of shades, although certain bright shades cannot be achieved... 

Colourants should be added at the maximum consistency that allows for thorough mixing of the furnish and colourant to take place. Preferential dyeing of fibres can take place if dye and a limited portion of the fibre are kept in contact for any period of time. By adding colourants, particularly anionic direct dyes and pigments, at a lower consistency, the Yoshida and van der Waals forces are less effective and dyeing... 

Churchley, J.H. Greaves, A.J. Hutchings, M.G. Phillips, D.A.S. Taylor, J.A. A chemometric approach to understanding the bioelimination of anionic, water-soluble dyes by a biomass - Part 3 Direct dyes. J. Soc. Dyers Colour. 2000, 116, 279-284. 

Results concerning filtration studies with 0.2 pm titanium dioxide membranes supported on stainless steel or ceramic porous tubes were recently reported by Porter et al. [47,48]. Solutions containing sodium nitrate alone and in the presence of anionic, direct and acid dyes were filtered with adjusted solution pH. Electrolyte rejections and colour rejections were measured at pH values from 4 to 10. They showed that the charged membrane was responsible for ion rejection at low ionic concentration while rejection decreased to near 0% as the salt concentration was raised to 5000 ppm. These results are consistent with long range forces associated to Debye-length which can reach several hundred Angstroms in the solution for very low ionic concentrations. 

Direct dyes like the acid ones belong to strong electrolytes and are completely dissociated in water baths into coloured anions and sodium cations ... 

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