Although Cotton is inherently anionic, cationic dyes are not used for reactive dyeing.Rather, common practice is cationisation of cotton followed by anionic dyeing.
The anionic charges of cotton are too weak to maintain a strong interaction with the cationic dyes.
I add my comments between the sentences of your question.
Although Cotton..
Cotton is too vague ! please define the kind of cotton.
is inherently anionic, cationic dyes are not used for reactive dyeing..
Dyes for reactive dyeing needs reactive groups (see attachment 1-3 ) and cationic dyes have not reactive groups (but you can introduce reactive group (s) in the molecule).
.Rather, common practice is cationisation of cotton followed by anionic dyeing.
According to the Marius answer, you must treat the cotton with chemicals (see references and Wikipedia) in order to have the possibility of a covalent binding (reactive dyes) or a strong interaction without reactive groups (see Astrazon dyes staining).
JRG
Thank You very much @ Marius Pelmus and @ Jean Rene Grezes for your answers. I would like to add more on this, which I found in below paper.
Salt-free dyeing of cotton cellulose with a model cationic reactive dye
Kawee Srikulkit et.al Coloration Technology, 116, 12, 2006
[Cationic dye ranges are not practically used for the dyeing of cellulosic fibre, even though they offer the possibility of a salt-free dyeing process with high tinctorial strength at relatively low levels of dye application. The
reason that they are not usually used for this type of dyeing procedure is due to their poor fastness to light. It is well documented that localised or delocalised positive charges on cationic dye molecules adversely affect their fastness to light] .
Need Comments on ..
How localised positive charges affect light fastness of cationic dyes?
Cationic dyes with localized positive charges (pendant dyes) have a limited effect on the light fastness of the chromofore, as they don't interact directly with the conjugated system.
On the other hand the cationic dyes with delocalized positive charges have high extinction coefficients, very bright colors and have poor to very poor light fastness character.
Check this book:
The Chemistry and Application of Dyes by David R. Waring, Geoffrey Hallas
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