Van der Waals (VDW) interactions: these non-covalent molecular
interactions, are the sum of dispersion and repulsive energies.
The thermodynamic strength of VDW interactions is smaller than
8 kJ mol
–1
.
14
2.
Electrostatic interactions between static molecular charge distributions:
in dyeing processes these include not only, for example, obvious attractions
such as those between protonated amino groups in polyamide fibres and
sulphonated anionic dyes, but also, according to classical definitions,
hydrogen bonding.
13
A recent review points out that this may be an
inadequate definition
15
but for the purposes of this analysis it is suitable.
Hydrogen bond strengths vary considerably, but for neutral molecules
they lie in the range 10–65 kJ mol
–1
and when one of the components is
ionic this range rises to 40-190 kJ mol
–1
.
3.
Solvophobic or hydrophobic interactions: these arise from the effect of
non-polar parts of water-soluble solutes on the structure of water. When
such molecules are brought into aqueous solution through a relevant
solubilising group (e.g. sulphonate) then the water structure must change
to accommodate the non-polar or hydrophobic residues. This change
represents a gain in entropy for the whole system. Since most dyeing
processes are restricted to aqueous systems, hydrophobic interactions
© 2009 Woodhead Publishing Limited
Advances in wool technology
188
are likely to play an important role in determining both dye uptake by
fibres and subsequent wet-fastness. Zollinger,
11
in his George Douglas
lecture, highlighted hydrophobic interactions as being responsible for
over-dyeing effects on nylon as well as the unusually high substantivity
of dyes containing bulky aryl or alkyl residues for wool.
Since dyes and fibres such as wool, silk and polyester both contain aromatic
residues it is worthwhile to consider current thinking regarding
π–π interactions.
Hunter
14
points out that these are commonly used to explain interaction
between two or more aromatic molecules, but current evidence shows that
they are negligible compared with electrostatics.
16
On first sight it might be expected that, since aromatic groups are planar,
maximum VDW interactions occur in a perfectly flat stacked arrangement.
In water, hydrophobic interactions will also favour stacking, since the flat
π-
electron surfaces of the dye molecules are non-polar.
17
Thus authors of papers,
e.g. Giles
18
on dye-aggregation in aqueous solution tend to draw the aggregates
as perfectly stacked molecules with maximum overlap of free aromatic rings.
Hunter and Sanders
19
have studied the geometry and energy contour plots of
two stacked porphyrins and noted that the aromatic residues were in an
offset or staggered arrangement. To explain these observations the above
authors concluded that electrostatics provide a large repulsive force which
pushes the
π-systems away from the usually accepted maximum overlap
position; the best model involved a positively charged
σ framework sandwiched
between two idealised ‘
π-atoms’. Continuing this analysis led Hunter
14
to
the conclusion that certain face-to-face arrangements of aromatic systems
lead to repulsion and other arrangements such as face-to-edge and offset lead
to attraction. He was thus able to construct a very useful diagram, which is
reproduced in Fig. 8.4.
Angle
(degrees)
180
90
0
8
Offset
(Å)
8.4
Electrostatic interaction between two benzene rings as a function
of orientation (from Hunter
14
).
© 2009 Woodhead Publishing Limited
The coloration of wool
189
This new way of thinking about
π–π interactions
14
allowed a ready
explanation of the characteristic herringbone packing of aromatic hydrocarbons
in the crystalline state
20
and also explained the phenyalanine–phenylalanine
geometries found in X-ray crystal structure analysis results from certain
proteins.
21
The maximum electrostatic interaction for two
π electron systems
was subsequently calculated as 6 kJ mol
–1
, which is relatively weak. However,
dyes are poly-aromatic systems and in fibres such as wool the aromatic side-
chain residues in phenylalanine, tyrosine and tryptophan often occur in the
same region; thus the overall small contributions, when summed, become
significant. The author
22
contends that the previous explanations of non-
polar interactions in dye–fibre systems have neglected the special case of
aromatic
π–π interactions. Attention should be drawn to the anomaly that in
aqueous systems hydrophobic interactions increasingly break down above
about 60
°C and many of the synthetic polyamide fibre overdyeing studies
were carried out in boiling aqueous solution. It is thus proposed that, rather
than hydrophobic interactions, aromatic or
π–π interactions are largely
responsible for the over-dyeing effects observed when dyeing synthetic
polyamides with mono-sulphonated dyes.
8.2.3
Pathways to dye diffusion
The most widely accepted view regarding the pathway to dye diffusion,
developed by CSIRO workers,
23–25
is that the dye molecules pass rapidly
through the heavily water swollen cell membrane complex (CMC) proteins,
initially between the cuticle scales, and thence into the cortex. In one approach
23
the above workers prepared anionic metal-complex dyes as heavy metal (Pt,
Pd and U) chelates which, owing to their nuclear denseness, have high
electron scattering power, and hence show up in transmission electron
microscopy (TEM); they were able to determine the location of dye according
to dyeing time and dyebath temperature. The results showed that these anionic
dye molecules enter the wool fibre between cuticle cells and then diffuse
into the non-keratinous endocuticle and cell membrane complex material,
into the inter-macrofibrillar material and into the nuclear remnants; in the
case of the dyes studied significant amounts were finally associated with the
more hydrophobic proteins of the A-layer in the exocuticle and with the high
sulphur matrix proteins in the cuticle. Brady
26,27
used fluorescence spectroscopy
to following the diffusion pathways of a fluorescent rhodamine dye (C I
Acid Red 52) and came to very similar conclusions – reinforcing the thesis
for dye diffusion via the inter-cuticular, non-keratinous route into the fibre.
It is clear that fibre reactive dyes may show a different diffusion profile since
they would form covalent bonds and become immobilised earlier than non-
reactive dyes. This latter proposal was proven by Lewis and Smith
28
using a
vinylsulphone derivative (VS), a sulphatoethylsulphone derivative (SES)
© 2009 Woodhead Publishing Limited
Advances in wool technology
190
and a non-reactive sulphanilic acid/hydroxytriazine derivative of a sulphonated
di-amino-stilbene as fluorescent brightener models for reactive and hydrolysed
dyes. The uptake pathways of such colourless models when dyed at the boil
for different time periods, could thus be followed using fluorescence microscopy
– UV excitation at circa 340 nm resulted in the emission of intense blue
visible light. It was thus shown that the highly reactive free VS form did not
penetrate the fibre but reacted with the endocuticule and the endocuticular
intercellular regions; the less reactive SES form partially penetrated the fibre
but was also mainly associated with the above regions; the non-reactive
model gave full penetration of the fibres after dyeing for 1 h at the boil.
Figure 8.5,
taken from the original thesis,
29
shows these fluorescence
microscope results after dyeing 15 min and 60 min at the boil. These are very
important findings and demonstrate that the distribution of fibre-reactive
dyes has to be very different than acid dyes without fibre-reactive residues
(acid, acid milling and metal-complex dyes). It is particularly significant
that reactive dyes tend to selectively covalently bond to and hence modify
proteins in the endocuticular regions.
The important question as to why the wool fibre cuticle surface does not
allow trans-cellular diffusion of dyes was actually addressed prior to the
above studies by such workers as Hall;
30
this author proposed that dyes gain
entry to the wool fibre interior via the junctions between the scales rather
than directly through the scale surface. The presence of a barrier to dyeing
SES
VS
Non-reactive
15 min
60 min
8.5
Fluorescence-microscope images of fibre cross-sections from
wool ‘dyed’ with reactive/non-reactive dye models showing diffusion
pathways.
© 2009 Woodhead Publishing Limited
The coloration of wool
191
at the scale surface is widely accepted [e.g. Rippon
25
]; the nature of this
barrier has become the subject of intensive research since it is expected that
a simple process to remove it would render wool more readily dyeable and
printable – and possibly even more importantly readily shrink-resisted. Current
evidence is that the surface barrier is a proteo-lipid with the lipid being
covalently bonded to the protein via a thioester linkage to cysteine.
31
A
simple process to remove the lipid would produce a modified fibre
32
which
would be more readily dyed and printed, and even shrink-resisted compared
with the untreated fibre; chlorination does partially remove this surface lipid
but its use may give environmental problems related to the production and
discharge of organo-halogen pollutants. Rippon
25
has developed an amphoteric
surfactant, Valsol LT or Sirolan LTD, for treating wool in a simple pre-scour
prior to dyeing – since this treatment brings about some lipid removal the
wool becomes more readily dyeable at 80–90
°C. Perachem Ltd have patented
a method for rapid removal of covalently bonded surface lipid which is
currently being trialled.
33
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