Page 218 - Basic Principles of Textile Coloration

DYEING KINETICS 207

In some cases, the entropy of dyeing is positive. This indicates an increase in
molecular freedom as a consequence of the dyeing process. This probably involves
an increase in freedom of water molecules that were initially in a highly structured,
hydrogen-bonded cage surrounding the hydrophobic dye molecules in the solution.
This cage collapses when the dye molecules transfer into the fibre, releasing water
molecules. This is the same effect that causes the entropy of micelle formation to
be positive (Section 9.2.2). Thus, the freedom gained by the liberated water
molecules more than offsets that lost by the adsorbed dye.

The preceding analysis assumes that the standard affinity, as well as the
enthalpy and entropy of dyeing, are independent of the temperature. Over a small
temperature range this is a reasonable assumption but temperature variations must
be considered in more precise studies.

The activity of the dye in the fibre cannot be determined and there is no choice
but substitution of the molar concentration. For different types of dyes, different
standard states are usually involved so that affinities of a dye for different types of
fibres often cannot be compared directly. Although this thermodynamic approach
is academically satisfying, it is based on some questionable assumptions. These,
and the fact that it often bears little relation to practical dyeing, have limited
studies in this field.

11.2 DYEING KINETICS

The rate of dyeing is of greater practical importance than dyeing equilibrium
because few commercial dyeing processes reach equilibrium. The time required is
simply too long. Unfortunately, dyeing rates are quite sensitive to changes in a
number of variables. These include the degree of agitation of the bath, the liquor
ratio, the type and construction of the material, the dyebath temperature and pH,
the concentration of dyeing assistants, as well as the dye substantivity. This is one
of the reasons why the fundamental study of dyeing kinetics has developed to an
even lesser extent than has the study of dyeing equilibria.

Practical dyeing rates depend upon the way dyebath exhaustion changes as a
function of time under the particular dyeing conditions. This may be determined
at a constant temperature although, for most practical purposes, the temperature
may vary during the test. The result characterises the dyeing properties of the dye
and is useful for selecting compatible dyes that exhaust at about the same rate
under the same conditions. Typical exhaustion curves show the initial rate of
dyeing, the gradual decrease in dyeing rate as the bath becomes exhausted, and