THE PHYSICAL CHEMISTRY OF DYEING AND TANNING

GENERAL INTRODUCTION

BY SIR ERIC RIDEAL Chemistry Department, King’s College, London, W.C.2

Received 8th September, 1953

It was a happy thought of the committee responsible for initiating this Dis- cussion of the Faraday Society in their decision to hold the meeting in this city of Leeds. The contributions made in the past and being made by workers here are recognized all over the world as of outstanding importance If one were to cavil at all at the efforts of the committee, one might suggest that the range of the Discussion is truly enormous. The subject-matter of dyes and dyeing, even for one particular class of dyestuff and one textile mzterial, is already very great and presents many unsolved problems, whilst the addition of the equally complicated subject of tanning is indeed piling Pelion on Ossa. I suppose the fiindamental questions which are to be asked are : “ how, in any particular casc, is the dye combined with the fibre?”, “what are the conditions of equilibrium? ” and, finally, “ what governs the rate of dyeing ? ”

The intcraction of a dye with a fibre may, as we see in this Discussion, vary from salt formation with wool to a simple solution mechanism for non-ionic dyes in polyethylene teraphthalate. In the former case we note that the free energy change in the conversion of the protons of the acid dye combining to form a covalent linkage with the carboxyl groups of the wool side-chains is one of the major factors to be considered; the dye anions must likewise penetrate to prcserve electric neutrality. We may consider the equilibrium between thc wool phase and the aqueous medium to be defined by a Donnan distribution with fixed protons and mobile dye anions or that both cation and anion occupy local- ized sites and that the equilibrium is governed by a Langmuir equilibrium. If this latter is true, we must assume that thcre is only one type of site, that there is no interaction between neighbours and that the ions are not mobile. The diffi- culty associated with the method of treating the system as a Donnan equilibrium is that the thermodynamic relationships give us merely the activities of the two phases whilst we are interested in the quantities in each phase. Clearly some method is required for defining the volume of the wool phase before we can obtain all the information we desire. In addition, the assumption that the anions are virtually free within the wool phase, being present merely to justify electro-neutrality, is scarcely in accordance with the facts. Not only do various anions vary in inter- action energy with the wool sites but, as we note, the resolution of optically active anions by adsorption on wool is confirmatory proof that anion sitcs are more than sites for simple electrostatic bonding. In the treatment of localizcd sitcs, the prcmises on which an exact theory can be based are as yet somewhat un- certain. For example, we have reason to believe that certain of the keto amino groups in the polypeptide chain can, under suitable conditions, function as sites.

Wc are likewise confronted with problems in the other extreme case whcre the simple assumption of a Nernst partition law can be regarded as the basic con- trolling factor in the uptake of dye. Wc note, for example, how rapidly the

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10 G E N E R A L INTRODUCTION

activity of a dye may change in the presence of a salt, how many dyes arc in cffect not simple monomers in solution but, as the concentration rises or on the addi- tion of salts, aggregated forms build up; dimers or even more complex micelles make their appearance. Again the concept of solution in the fibrous material is not a clear one. We recognize in the majority of synthetic fibres ordered and less ordered domains. That there is a melting range rather than a melting point is a recognition that we cannot divide the domains into simple crystallites and amorphous regions. Since relatively large entropy changes are involved in the passage of the dye from the aqueous into the non-aqueous phase; differences of this kind may well affect the partition coefficient in different regions of the fibre. We must also note that in aqueous phase the mechanism of solution, i.e. dis- persion or peptization of the dye by water involves not only a reduction of the dipolar cohesive forces but frequently polar interaction and frequently hydrogen bonding at specific points on the dye molecule. Must we not invoke similar considerations in the fibrous phase ? Here the adlineated macromolecules may well give rise to adlineation of the dye molecule. Instances of dichroism in dyeing immediately come to mind, as will be seen from the papers being com- municated. Equally complex are the factors which govern the rate of dyeing.

FIG. 1.-Direct dyes on cotton, 60" C, as function of l /n , where n is number of passages of dye liquor through fibre in 10 min (recalc. from data of Fern).

Recently Dr. Davies and I have been looking at a few of these and the results we have obtained in several cases are rather surprising.

THE RATE OF DYEING.-Many studies have been made of the rates of diffusion of dyes within sheets of Cellophane or within textile fibres. Apart, however, from the diffusion within the solid, there are two other processes, which in practice may well be rate-controlling. The first of these is the diffusion of the dye through the aqueous layer in contact with the solid surface. With plane surfaces there is a layer of liquid, effectively unstirred for slow flow rates, this layer being of the order 0.01-0*1 mm thick. Stirring of the solution more vigorously will, of course, reduce the thickness 6 of this unstirred layer, while in the unstirred solution a value (about 0-5 mm) determined by thermal convection is attained. Diffusion through layers of this order of thickness may be a slower process than any of the other diffusive steps in attaining dyeing equilibrium. As we stir more rapidly, the layers not only become thinner: it also may become possible for the eddies which accompany turbulent flow to approach more closely to the interface.1

Fig. 1 shows the data of Fern recalculated to show how the rate of circulation of dye-liquor influences the rate of dyeing. If we try to extrapolate the curves to infinite rde of stirring (zero l /n) we see that, although further measurements in the fast stirring region are clearly required, the major part of the barrier to diflusion arises from the aqueous phase adjacent to the fibres.

Fig. 2 shows the effect for the same dye on a Cellophane sheet and on a piece of cotton. The stirring rate is clearly much more important for the fibrous material

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SIR ERIC RIDEAL 11

than for the sheet of Cellophane, on account of the liquid being less easily moved between the fibres ; the unstirred layers therefore increase rapidly in thickness at low stirring rates.

These effects can be represented, in the simpler cases, in terms of the Reynolds number of the system. This quantity, defined by pvZ/y (where p = density of liquid ; v = velocity of liquid relative to solid ; 7 = viscosity ; I = character- istic length), is a measure of the turbulence of the system. At low values of the Reynolds number (denoted hereafter by Re) viscous forces predominate : at larger values the forces of inertia predominate, and the flow pattern becomes very different. If Z is very small, we may see that Re is likely to be small under all conditions, but if 1 is large, the flow will be turbulent except at the lowest velocities. As a first approximation, one may show that 6 = Z/(Re)*, or, if the rate of dyeing is limited by the transport across the unstirred layer of thickness 6 (and is therefore proportional to 1/8), the rate of transfer of dye is proportional to (Re)a/l, Z being the dimension of the fibre in the direction of flow. This applies

(Slo ads3

( fml adsorphobn

b " C

*-I--

Slow // (sh-iiy fa t e ) f a s t

slirring sttcrtnq

Ce//ophane

if Re is large. Thus the rate of dyeing of a Cellophane slab should vary as the square root of the velocity of flow past it, i.e. rate cc 770.5 for this effect. If there is also a barrier within the cellulose, we may write : l/rate = const. u-o*~+const. The linearity is satisfactory (fig. 3), though it clearly requires testing over a wider range of conditions. The plot is derived assuming the additivity of the resistances to diffusion in a Cellophane sheet, and the intercept on the ordinate axis gives the resistance to diffusion due to factors other than those associated with the un- stirred aqueous layer. The empirical findings of Brunner (1904) and of McBain (1922) that 8~0.67 is constant can also be used to recalculate the data for Cello- phane, but these experiments do not. allow us to make any exact evaluation of the exponent, as the range of velocities is not great enough.

Recent work on turbulent flow at liquid-liquid interfaces in investigations of mass transfer 2 has suggested the following equation,

K (densitylviscosity) = const. (Re)O*67 - 10, if the viscosity of the second phase is high. Here K is the mass transfer coefficient. We may note that the exponent is higher than the values of 0.5 and 0.67 quoted

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12 G E N E R A L I N T R O D U C T I O N

above. The meaning of the constant is not clear as yet, except that for appreci- able transfer to occur (across the " unstirred " layer) a certain critical stirring rate must apparently be reached.

In the dyeing process the mass of small fibres in close proximity complicates severely the exact treatment of diffusion rates through the aqueous phase. The data for the different dyes (fig. 1 and 2) are not spread over a sufficient range of stirring speeds for any exact mathematical treatment to be possible, but at high rates of stirring (or of circulation of the dye liquor) an exponent n of about - 0.5 is certainly required, approaching - 1.0 as the stirring rate becomes slower and Re is less, i.e. as the flow becomes more streamlined. This means that in the latter circumstances the uptake of dye depends simply on the amount of liquor- carried dye passing the fibre per second. A similar result was found by Alexander et ai.3

At high dye concentrations, Alexander and Hudson 3 found that the dyeing rate was controlled by the aqueous layer at low stirring speeds, but that at high stirring rates the rate depended on diffusion through the wool itself, the transport of dye to the surface of the fibres then exceeding the rate at which dye was removed from the surface by diffusion into the interior of the fibre. At low concentra- tions of Orange 11, diffusion through the aqueous layers was found to be always

V

FIG. 3.-Data of Neale et af. recalculated to show depcndence of reciprocal of dye adsorbed after 5 min OR cellulose sheet on inverse square rate of velocity of dye liquor

past sheet. (Chlorazol Sky Blue FF at 90" C.)

rate-determining. The energies of activation for diffusion were 5 kcal for dyeings in which control was by the aqueous layers, but 13 kcal for diffusion within the fibre. This suggests that an alteration of temperature can alter the rate-deter- mining step in the dyeing process, the hydrodynamics of the system being more important at higher temperatures.

Besides the aqueous layers adjacent to the fibres, therc are two other possible barriers to diffusion. If the dye is charged, e.g. carrying sulphonate groups, it may encounter a force of electrical repulsion within a few tens of angstroms from the surrace, due to the presence of the molecules of dye of similar electrical charge already adsorbed there. In recent years, considerable attention has been devoted to elucidating quantitatively the mechanism of the slow lowering of surface tension, and in particular the action of this potential barrier, with limited success. For exact interpretation, it is preferable to use the simplest possible systcms, and long-chain sulphates or quaternary ammonium ions have proved uscful. Work is at present in hand in this laboratory to evaluate exactly ihe barriers due to diffusion and to electrical repulsion, the latter effect clearly being of importance since a few years ago Sutherland and Rideal (unpublished) noted that salts present even in very low concentrations greatly accelerated the rate of lowering of surface tension, presumably by reducing the repulsive barrier. Alexander and Kitchencr

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S I R E R I C R I D E A L 13

(1950) have looked into some of the implications of potential barriers of this sort in the dyeing process.

To pass from the adsorbed state to the interior of the fibre may also require considerable energy. The ionized groups have to lose some of their co-ordinated watcr, and energy must be supplied to strip this away against the attraction of the water dipoles for the ion : that is to say, the process requires an energy of activation. The magnitude of this interfacial barrier has never been accuractely measured, owing to severe experimental difficulties. A preferred molecular orientation may also be necessary, adding an entropy factor to the work term. With regard to dyes the importance of this type of barrier to diffusion is still far from clear.

We may notc in concluding that unless further research can be devised to scparate and cstimate the magnitude of these different processes, the physical chemistry of dyeing can never be free from empirical factors. We may con- fidently hope that tracer techniques and further fundamental studies of the hydrodynamics and of the adsorptive and electrical factors operating at surfaces will prove fruitful in strengthening the foundations of the physical chemistry of dyeing.

1 Danckwerts, I d Eng. Clzem., 1951, 43, 1460.

2 Lewis (A.E.R.E., Marwell), unpublished data. 3 Alexander et al., Trans. Faraday SOC., 1949, 45, 1058, 1109; Text. Res. J. , 1950.

4 cf. e.g. Bircuinshaw and Riddiford, Quart. Rev., 1952, 6, 157.

Fern, unpublished data, quoted by Vickerstaff, The Physical Clzemistry of Dyeing (Oliver and Boyd, 1950).

20, 203, 481.

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