Page 308 - Basic Principles of Textile Coloration

THE EFFECTS OF VARIATIONS IN DYEING CONDITIONS 297

dye. The salt provides sodium ions to counteract the negative surface potential of
the wet cotton. All fibres immersed in water develop a negative surface potential.
This is a characteristic of any two different phases in contact – one will develop a
small, electric charge at its surface, opposite in sign to the charge of the other
phase. This occurs because one surface is a better electron acceptor than the
other. In the case of cellulose at pH 7, water is a better electron donor than the
cellulose. Negatively charged carboxylate groups, from oxidation of the primary
alcohol group on carbon-6 in some glucose units, also augment the surface charge.

The negative charge of the cellulose surface repels anionic dye molecules. A
high concentration of surrounding sodium ions counteracts this. Rapid solution
flow also helps to break down the surface layer in which this charge is effective.
Direct measurement of the potential difference between the inside of the fibre,
where the dye is adsorbed, and the external solution is impossible. Streaming or
zeta potentials, which develop when aqueous solutions flow across a fibre plug
held between electrodes, can be measured. The values are probably lower than the
total potential difference between the inside of the fibre and the external solution.
As expected, the negative zeta potential of cotton is reduced by salt solutions,
particularly those containing polyvalent metal ions, and increased by adsorption of
anionic direct dyes.

The influence of added salt is the most important factor in the dyeing of
cellulosic fibres with direct dyes. In dyeing, there are two objectives: good
exhaustion and good colour uniformity. Both depend on the salt-controllability of
the dyes, or how the gradual salt additions during dyeing influence the rate of
exhaustion. Unfortunately salt-controllability is very dependent on the dyeing
temperature. For practical purposes, it is useful to have some idea of the degree of
exhaustion at different salt concentrations for each dye, possibly at different
dyeing temperatures. For two dyes having about the same substantivity, salt should
have a greater influence on the exhaustion of the one with the larger number of
sulphonate groups, and therefore the larger negative molecular charge. The fibre’s
negative surface potential repels the more highly charged dye molecules to a
greater extent so this dye will respond more to salt additions. This conclusion,
however, is not necessarily of great practical value because commercial direct dyes
already contain much electrolyte.

The nature of the anion of the added electrolyte has little influence on the
amount of adsorbed dye. Therefore, NaCl and Na2SO4, at the same total sodium
ion concentration, have about the same influence. A higher positive charge on the
cation promotes increased adsorption because polyvalent metal ions counteract