Microscale Vat Dyeing with Indigo | Microscale Synthesis of Indigo in Vat Dyeing

the Isatis Tinctoria plant in Europe. Due to the demand of this supply of blue dye, a trade route between India and Europe was established.

The first synthetic dye was generated in 1856, when William Henry Perkin, a student at the Royal College of Chemistry (England) accidentally discovered a synthetic mauve dye. This in turn, led to the first commercial development of an organic compound. In the 1880’s, Adolph von Baeyer undertook the study of indigo and determined its molecular structure and synthetic routes. For this work, von Baeyer received the Nobel Prize in chemistry in 1905. It was commercially available until after 1900 due to the problems faced when trying to mass produce the invention. However, by WWI, almost all the world’s manufactured dyes were produced in Germany and synthetic dyes had almost entirely replaced the extraction of dyes from natural sources.

Dyeing with Indigo:
To fully understand the process of indigo dyeing, we must first look at the chemistry behind it. Indigo dye (indigotin) is derived from indican, which is a water-soluble, colorless substance present in indigo-bearing plants. When indigo-bearing plants are crushed and placed in water the indican is released into the solution through bacterial action. When air is beaten into the solution, the indican combines with oxygen to produce indigotin or “indigo blue.” Indigo blue is insoluble and settles out of the solution to produce the blue indigo dyestuff.

Indigo is insoluble in water but is easily reduced by sodium dithionite (sodium hydrosulfite, Na2S2O4) in a strong alkaline solution to produce a water-soluble leucoindigo. This leuco base is strongly attracted to cellulose because they have large pores into which the dye molecules easily fit. Upon exposure to the oxygen in the air, the dye molecule rapidly oxidizes to its insoluble form, but by then it has already been trapped inside the cellulose fiber; due to its immense size, it cannot be removed at all easily. When a fiber is placed in the indigo white solution, the indigo white penetrates into the fibers and makes loose chemical combination which is oxidized back to the insoluble indigo blue. The colors from this dyeing process, the vat method, are very resistant to washing because the dye is insoluble in water. However, synthetic fibers are not good candidates for vat dyeing because they have tightly packed fibers not easily penetrable by dye molecules. For this lab we will be using cotton which is a natural organic polymer consisting of >95% pure cellulose, a linear polymer of glucose.