How do dye molecules attach to textile fibres?

Dye molecules attach to textile fibres through a process of absorption, where they concentrate on the fibre surface and penetrate its microstructure. This retention, known as substantivity, relies on four primary types of forces:12

• Ionic forces: These occur when charged dye molecules (anionic or cationic) are attracted to sites of opposite polarity on the fibre. For example, in wool dyeing, sulfate anions are replaced by dye anions to form ionic linkages.21
• Hydrogen bonding: This involves attractive interactions between weakly acidic hydrogen atoms and electronegative atoms like oxygen or nitrogen. These bonds often form between the dye and the amido groups of protein fibres like silk and wool.21
• Van der Waals forces: These are weak attractive forces between closely spaced molecules. They are particularly significant for cotton and other cellulosic fibres.12
• Covalent chemical linkages: These result from a chemical reaction between a fibre-reactive dye and the fibre, creating a secure, permanent bond.31

When a dye has little natural affinity for a fibre, a mordant may be used to create more polarized sites, facilitating better binding.42