Page 261 - Basic Principles of Textile Coloration

250 ACID, PRE-METALLISED AND MORDANT DYES

13.4.2 Ion exchange and hydrophobic bonding

Wool strongly absorbs milling and super-milling dyes even under weakly alkaline
conditions when the fibre has more anionic carboxylate groups than ammonium
ion groups. These dyes give high dyebath exhaustion under these conditions. The
ion exchange mechanism is clearly an oversimplification and other modes of dye–
fibre interaction must operate in these cases.

The substantivity of acid dyes for wool generally increases with increase in the
dye molecule surface area but decreases when the dye has additional sulphonate
groups in the molecule. For large hydrophobic dye molecules, other types of dye–
fibre interactions become important besides the coulombic attraction between
ammonium ion groups in the wool and dye anions. These additional dye–fibre
forces may involve dipole interactions, hydrogen bonding, and dispersion forces
operating between groups in the dye molecule and appropriate groups in the
protein. Wool, silk and nylon all have regions along the polymer chains that are
quite hydrophobic, being void of ionic and polar groups. Dye–dye and dye–fibre
hydrophobic interactions might be expected in such regions.

Hydrophobic bonding is simply the term used to describe the tendency of
hydrophobic groups to associate together to avoid interaction with water, as in the
formation of soap micelles and dye aggregates. The driving force in hydrophobic
bonding is the collapse of the hydrocarbon–water interface that occurs when the
hydrophobic groups in the dye molecule interact with hydrophobic groups in the
wool. Water molecules tend to become more highly hydrogen bonded together,
and therefore more highly structured, at an interface with a hydrophobic group.
When the hydrophobic group is surrounded by other hydrophobic groups rather
than water, the water molecules liberated give a significant increase in entropy.
This explanation of the substantivity of the more hydrophobic acid dyes has
considerable merit.

Evidence for the importance of hydrophobic interactions comes from dyeing
experiments on chemically modified wool. Wool in which all the free amino groups
have been benzoylated by treatment with benzoic anhydride, shows enhanced
absorption of typical milling dyes, but reduced absorption of low molecular weight
levelling acid dyes that have several sulphonate groups per dye molecule. In wool
dyeing, dye–fibre interactions therefore range from purely ionic interactions
between dye anions and ammonium ion sites in the wool to purely hydrophobic
interactions between non-polar groups in the fibre and in the dye molecule. The
consensus seems to be that the extent of absorption of acid dyes depends on the
hydrophobic interactions between the dye and the wool, which also determine the