Do you know how indigo becomes attached to cotton? Does it form hydrogen bonds? If so, is it known which locations it prefers. Is there information regarding the chemistry of this process anywhere?

Indigo is an interesting dye. It does not attach to the fiber by means of hydrogen bonds. Unlike most dyes, indigo has a neutral charge.

Indigo in its blue form is not soluble in water. In order to solubilize it for a dyebath, you must reduce the indigo. This is easily done by using sodium hydroxide and sodium dithionite, which are commonly purchased as lye and Rit Color Remover. The reduced leuco form of indigo is yellow in color.

When cotton or another natural fiber is dipped into a solution of reduced indigo, some of the dissolved indigo penetrates inside the fibers, between the cellulose or protein molecules (depending on the fiber). When the dyed fiber is then exposed to oxygen, the solubilized yellow leuco form of the indigo is oxidized once more to the insoluble blue form. While any dye on the outside of the fibers may be easily rubbed off, the dye that is trapped within the fibers is quite colorfast.

It's important when dyeing with indigo not to use an overly strong solution of indigo, because this results in too much dye being deposited on the outside of the fibers. Instead, multiple dips in a somewhat weaker dye solution work much better at building up a sufficiently deep color without resulting in a large amount of unfixed dye remaining on the outside of the fibers. Poorly applied indigo dye is not uncommon and results in complaints of dye rubbing off of indigo-dyed clothing onto other items, even when dry, a problem which is called crocking.

Since the indigo dye is held in place by its physical location rather than by chemistry, the physical structure of the fiber is important. Synthetic fibers such as nylon, which can be dyed very well with most dyes that work on wool, are too smooth to hold the indigo dye. Indigo works well for dyeing fibers as chemically different as cotton and wool, but it does not work well on smooth synthetic fibers such as nylon and polyester.

Indigo is a vat dye, Colour Index Vat Blue 1. It is available from both plant-derived and synthetic sources. For more information about vat dyes, start with these pages: "About Vat Dyes", "How to fix indigo dye", and "What kinds of chemical bonds attach dyes to fibers?".

John Shore's 1995 book, Cellulosics Dyeing, includes good information on both vat dyes and on the structure of cotton fibers as they relate to dyeiung. It is very expensive if you buy it from a used book supplier in the US or Canada, but it costs only £9, plus shipping, if you order a new copy directly from the Society of Dyers and Colourists, in the UK.

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