Comparative sorption isotherms for colorants present in Dyers’ madder (Rubia tinctorum L.) provide new insights into historical dyeing

Dyers madder (Rubia tinctorum L.) has been famously used throughout history as a source of red dye. The sorption onto mordanted wool of the major colorant components of R. tinctorum, alizarin and the glycosides ruberythric acid and lucidin primeveroside, is studied herein. Sorption of a purified 1:1 mixture of ruberythric acid:lucidin primeveroside most closely followed a Temkin isotherm (R2 0.925), whereas alizarin followed a Freundlich isotherm (R2 0.940). These results were compared to HPLC chromatograms of English, Turkish and Iranian varieties of R. tinctorum before and after dyeing, where it was observed that the glycosides are shown to have the highest uptake onto wool. The higher sorption energy of the purified 1:1 ruberythric acid:lucidin primeveroside mixture (–11.4 kJ mol-1) compared to alizarin (–5.8 kJ mol-1) is in agreement with the HPLC results, indicating that the ruberythric acid/lucidin primeveroside mixture has a substantially higher affinity for wool compared to alizarin. Not only do the glycosides show higher affinity for the wool, but greater interactions between adsorbed species suggest a more extensive aggregation of dye on the surface of the wool. These observations are in contrast to much literature and bring into question previous conclusions that alizarin was the main dyeing species throughout history.