Direct dyes hydrogen bonds with cellulose

These are defined as anionic dyes with substantivity for cellulosic fibres applied from an aqueous dyebath containing an electrolyte. The forces that operate between a direct dye and cellulose include hydrogen bonding, dipolar forces and non-specific hydrophobic interaction, depending on the chemical structure and polarity of the dye. Apparently multiple attachments are important, since linearity and coplanarity of molecular structure seem to be desirable features (section 3.2.1). The sorption process is reversible and numerous attempts have been made to minimise desorption by suitable aftertreatments (section 10.9.5). The two most significant non-textile outlets for direct dyes are the batchwise dyeing of leather and the continuous coloration of paper. 

With cotton (cellulose), the direct dyes usually contain o-hydroxy- or aminoazo compounds, in which chelation assists in the production of hydrogen bonds between the dye and the cotton (1) ... 

Thirdly thereis the Freundlich equilibrium applicable to the adsorption of direct and vat dyes by cellulosic fibres. In this case the attachment is not at specific sites so that there is no stoichiometric limiting factor. If attachment is brought about by hydrogen bonds and physical forces the limitation is the available surface within the pores. In this case, therefore, adsorption is rapid at first because the sites are easily accessible but becomes slower as the dye molecules have to seek out the more remote points of attachment. The [0]//[7)]s curve, therefore, is not a straight line nor does it reach a point at which it becomes parallel with the horizontal axis, as shown in Fig. 12.16. 

Most dyes do not chemically react with the cellulose molecule to affix the color. The roles of interchain hydrogen bonding and van der Waals forces in the application of azoic, direct, sulfur, and vat dyes are the physical and chemical effects and are not classical chemical reactions. Tme chemical reaction between cellulose and the dye molecule occurs with reactive dyes, which comprise different chemical types (see Section 9.6.2.3). Such chemical reaction results in covalent bond formation between the dye molecules and the C6 hydroxyl groups of... 

It will be seen that all the direct dyes so far discussed are extended, linear molecules—a characteristic of the class. It has been shown that the distance between the two azo groups in a substantive dye is 1.08 nm and that of the polymer unit length in cellulose is 1.03 nm, so that the operation of van der Waals forces is facilitated, though there is clear proof that this is not the only factor present since substantivity disappears in CA and CT fibres. H-bonding is invoked as a source of attachment, the hydrogen atoms of—OH groups in the cellulose unit bonding with N of an azo link... 

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