摘要:

DISCUSSIONS OF THE FARADAY SOCIETY No. 16, 31854 THE PHYSICAL CHEMISTRY OF DYEING AND TANNING THE FARADAY SOCIETY Agents for the Society’s Publications : The Aberdeen University Press Ltd. 6 Upper ICirkgate AberdeenThe Faraday Society reserves the Copyright of all Conimunications published in the “ Discussions ” PUBLISHED , . . 1954 PRINlED IN GREAT BRlTAlN AT THE UNIVERSIIY PRESS ABERDEENA GENERAL DISCUSSION ON THE PHYSICAL CHEMISTRY OF DYEING AND TANNING A GENERAL DISCUSSION on The Physical Chemistry of Dyeing and Tanning was held in the Department of Chemistry, Leeds University (by kind permission of the Vice-Chancellor) on the 8th, 9th and 10th September, 1953. The President, Prof. Sir Hugh Taylor, K.B.E., D.Sc., LL.D., F.R.S., was in the Chair and about 154 members and visitors were present.Among the distinguished overseas members and visitors welcomed by the President were the following :- Prof. A. Banderet (Mulhouse), Dr. R. Casty (Basle), Dr. G. Coumoulos (Athens), Dr. K. H. Gustavson (Stockholm), Dr. P. Larose (Canada), Dr. H. Lundgren (California), Mr. B. Olofsson (Gothenburg), Dr. A. Parisot (Paris), Dr. J. Pouradier (Vincennes), Prof. A. Schobcrl (Hanover), Dr. H. E. and Mrs. Schroeder (Delaware), Dr. G. J. Schuringa (Delft), Dr. G. J. M. Sprokel (Delft), P. J. van Vlimmeren (Waalwijk, Holland), Dr. H. Zollingcr (Basle). 3A GENERAL DISCUSSION ON THE PHYSICAL CHEMISTRY OF DYEING AND TANNING A GENERAL DISCUSSION on The Physical Chemistry of Dyeing and Tanning was held in the Department of Chemistry, Leeds University (by kind permission of the Vice-Chancellor) on the 8th, 9th and 10th September, 1953.The President, Prof. Sir Hugh Taylor, K.B.E., D.Sc., LL.D., F.R.S., was in the Chair and about 154 members and visitors were present. Among the distinguished overseas members and visitors welcomed by the President were the following :- Prof. A. Banderet (Mulhouse), Dr. R. Casty (Basle), Dr. G. Coumoulos (Athens), Dr. K. H. Gustavson (Stockholm), Dr. P. Larose (Canada), Dr. H. Lundgren (California), Mr. B. Olofsson (Gothenburg), Dr. A. Parisot (Paris), Dr. J. Pouradier (Vincennes), Prof. A. Schobcrl (Hanover), Dr. H. E. and Mrs. Schroeder (Delaware), Dr. G. J. Schuringa (Delft), Dr. G. J. M. Sprokel (Delft), P. J. van Vlimmeren (Waalwijk, Holland), Dr. H. Zollingcr (Basle). 3CONTENTS General Tntroduction.By Sir Eric Rideal . The Kinetics of Acid Absorption on Wool Fibres. By R. F. Hudson , The Combination of Acids and Colour Acids with Keratin. By L. . Peters and G. H. Lister A Contribution to the Theory of Diffusion of Sorbed Substances into and out of Fibres. By Bertil Olofsson. . Study of Diffusion Processes in Tanning. By D. M. G. Armstrong . The Self-diffusion of a Dye in a Polar Polymer Membrane, By M. L. Wright . The Absorption of Sodium Sulphate and Sulphuric Acid by Hair. By D. L. Underwood and H. J. White, Jr.. The Kinetics of Absorption of Water and Aqueous Solutes by Dry Viscose Cellulose. By H. B. Mann and T. H. Morton . Theoretical Aspects of the Dyeing of Cellulose Acetate Rayon. By C. L. Bird, F.Manchester and Miss P. Harris . Researches on Monohyers- Part 4. A Study of Dyeing Processes by the Use of the Unimolecular Film Balance. By Miss Margaret M. Allingham, C. H. Giles and E. L. Neustidter . GENERAL DIscussIoN.-Dr. P. Larose, Dr. B. Olofsson, Dr. R. F. Hudson, Prof. R. M. Barrcr, Dr. K. H. Gustavson, Dr. G. A. Gilbert, Mr. G. King, Mr. D. M. G. Armstrong, Dr. C. H. Giles, Dr. H. E. Schroeder, Dr. J. L. Horner, Dr. Andrk Parisot, DI-. L. Valentine, Dr. M. L. Wright, Dr. Howard J. White, Jr., Mr. Donald A. Underwood, Dr. T. Vickerstaff, Dr. T. H. Morton, Dr. H. Zollinger . 5 PAGI! 9 14 24 34 45 58 66 75 85 92 105G CONTENTS Atomic Models- Part 3. Some Stereochemical Problems in Dyeing. By Conmar Robinson . The Adsorption of Dyes by Crystals. By J.Whetstone . Calorimetric Studies of the Reaction of Naphthalene Orange G with Amino Acids. By A. M. Derbyshire and W. J. Marshall . The Solubility and Activity of Orange IT in Sodium Chloride and Sodium Sdphate Solutions. By A. B. Meggy . The Selective Absorption of Optical Antipodes by Wool. By W. Bradley, R. A. Brindley and G. C . Easty . Tanning of Fatty Acid, Amino Acid and Protein Monolayers by Metal Ions. By J. H. Schulman and M. 2. Dogan. . The Interaction of Tanning Materials with Collagen Monolayers. By S. C. Ellis and K. G. A. Pankhurst . Viscosimetric Study of the Hardening of Gelatin by Chrome Alum. By J. Pouradier . Some Aspects of the Reaction of Basic Chromium Salts with Hide Protein. By K. H. Gustavson . Mucoid Material in Hides and Skins and its Significance in Tanning and Dyeing, By D. Burton and R. Reed . Mechanism of Absorption on Non-Ionic Dyes by PolyethyIene Tere- phthalatc. By M. J. Schuler and W. R. Remington . The Dyeing of Polyacrylonitrile Fibres with Anionic Dyes, By R. H. Blaker, S . M. Katz, J. F. Laucius, W. R. Remington and H. E. Schrocder The Dyeing of Synthetic Polypeptides. By C. H. Bamford, J. Boulton, W. E. Hanby and J. S. Ward . PAGE 125 132 140 149 152 158 170 180 185 195 20 1 210 222 GENERAL DIscussIoN.-Dr. H. Zollinger, Dr. C. H. Bamford, Dr. A. S. Dunn, Dr. H. E. Nursten, Dr. K. €3. Gustavson, Mr. A. N. Derby- shire, Dr. C. H. Giles, Mr. W. J. Marshall, Dr. G. A. Gilbert, Dr.CONTENTS 7 PAGE A. B. Meggy, Prof. W. Bradley, Dr. G. D. Cournoulos, D. J. H. Schulrnan, Dr. K. G. A. Pankhurst, Dr. H. Phillips, Prof. D. Burton, Dr. R. Reed, Dr. H. H. Sumner, Dr. M. J. Schuler, Dr. W. R. Remington, Dr. D. Patterson, Dr. T. Vickerstaff, Dr. P. Meares, Dr. J. Simons . . 229 Author Index . 251 Revicws of Books . . 252

ISSN:0366-9033    

DOI:10.1039/DF9541600001

出版商:RSC    

年代:1954

数据来源: RSC

摘要:

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 910 GENERAL 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 materialSIR 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 f a 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 quoted12 GENERAL INTRODUCTION 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 KitchencrSIR ERIC RIDEAL 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.

ISSN:0366-9033    

DOI:10.1039/DF9541600009

出版商:RSC    

年代:1954

数据来源: RSC

摘要:

THE KINETICS OF ACID ABSORPTION ON WOOL FIBRES BY R. F. HUDSON Queen Mary College, Mile End Road, E.1 Received 6th July, 1953 The kinetics of the adsorption of acids on wool initially in the isoelectric condition have been studied, varying the concentration of acid in solution, temperature, degrec of agitation, and the nature of the anion. The variation in rate with time is interpretcd quantitatively by assuniing the diffusion of acid through the fibre, accompanied by adsorption on specific sites (carboxyl groups) to be rate determining. The rnagnitudc of the apparent diffusion coefficients, calculated on the assumption of a linear relation between the concentration of acid in solution and sorbed on the fibres, indicates that the diffusion is controlled by a small concentration of H t.ion in the aqueous phase within the fibres, which may be estimated from the Donnan membrane theory. The activation energy of this process is found to be similar to the value for diffusion in aqueous solution and for the diffusion of water through the swollen fibre. In ion exchange and dyeing, however, where the rate is controlled by the coupled diffusion of two ions in opposite directions, high activation energies, which have bccn attributed to the deformation of polymer chains are observed. In acid sorption, the mechanical restraint is removed by the simultaneous swelling, so that less energy is required for migration through the fibrc. In acid solution, the carboxyl groups of the wool macromolecule are neutralized leaving frec charged amino groups which attract negative gegen ions thus main- taining electrical neutrality within the fibre.These anions are probably mobile within a Gouy double layer surrounding the wool micelles as they rapidly exchange with other anions in an external solution until equilibrium is reached.1 This process is largely responsible for acid dyeing2 although the simple exchange is modified by the high affinity of the dye anion which may enable it to cnter the fibre as the potential-determining ion? For this reason, the rate of exchange of simple ions has been studied and shown to be controlled by a coupled diffusion of the ions within the fibre.4 The diffusion process involves the simultaneous migration of the ion of two kinds in opposite directions which is observed to require considerable energy of activation.This has been attributed to the energy required to deform the polypeptide chains ; this may in turn be causcd by electro- static repulsion between the migrating ions which becomes significant in a gel of low water content comparcd with more highly swolIen cationic resins. Exchange in the latter is usually accompanied by low activation energies similar to the values observed for diffusion in water. Acid absorption on the other hand involves the simultaneous diffusion of hydrogen ions and anions into the fibre and in this sense is unidirectional. The hydrogen ions form covalent bonds with carboxyl groups, a process which may be assumed to be instantaneous, and the anions accompany the protons merely to maintain electrical neutrality.This process continues until the chemical potential of sorbed ions is equal to the chemical potential of the ions in the external solution. This equilibrium may be expressed quantitatively in two alternative ways, either by considering both ions to be localized on specific absorption sites according to a Langmuir type of adsorption, or by considering the proton to 14R . F. HUDSON 15 combine chemically with carboxyl groups as determined by the dissociation con- stant, and applying the Donnan membrane concept to the resulting system of fixed amino-ions, and mobile anions.6 The former developed by Gilbert and Rideal5 is highly successful in providing a quantitative measure of specific anion affinity, and the latter provides a more realistic approach but is far more difficult to apply rigorously.In equilibria involving simple acids, the anions of which exert no specific affinity, the two treatments are equally satisfactory owing t o their common thermodynamic basis. Both theories are necessarily equally correct, and whereas the Gilbert and Rideal theory provides some information on the interaction between anion and absorption site, the Donnan theory can provide a quantitative estimation of the concentration of free protons within the fibre. Although the free acid within the fibre makes a negIigible contribution t o the chemical potential of acid in this phase, it provides the driving force for diffusion through the fibre, and therefore studies of kinetics of acid absorption should provide data for further examination of the Donnan theory.EXPERIMENTAL Fine botany wool in the form of loosely knitted fabric was employed in this work and all the samples were taken from the same bale. 2 g samples, purified by Soxhlet extraction in ether and alcohol, were wetted for 12-15 h in distilled water. This treatment was repeated until no change in pH of the distilled water was observed. In the early experiments? each sample was fitted to a cylindrical glass frame attached to an electric motor. This rotating frame stirrer was lowered into a known volume of acid in a beaker to initiate the sorption? and 5-ml samples withdrawn at known times and titrated with standard alkali. With sulphurous acid, the closed vessel used previously 7 was used, and the samples estimated iodometrically. When a relatively small volume of liquid was used (e.g 50 ml/g wool), not more than 3 samples were withdrawn in any particular experiment so that the volume of the solution was not affected seriously. In most of the experiments? mechanical shaking was used instead of frame stirring, as this mode of agitation was found to be the more efficient.Each sample of wool was attached to a piece of strong thread and suspended in the neck of a flask above the acid. The two phases could be mixed simply by a sharp movement of the flask, and a stopwatch started at the same time. At the end of a pre-determined time, the wool was rapidly removed by means of the thread attached to the stopper of the flask, and a given volume of the acid titrated. By this procedure the timing could be made more accurate? and a larger volume of acid titrated each time, but each measurement involved a different sample of wool and solution.This is an advantage in that any experimental inaccuracies will be minimized by making a considerable number of these determinations in each case, and in spite of the extreme simplicity, the results were found to be highly reproducible. The total sorption was obtained by allowing several samples to remain in contact with the appropriate volumes of acid for at least 2 days. RESULTS THE RATE-DETERMINING MECHANISM.-PreViOUS studies Of the kinetics Of ion exchange,4 dyeing 8 and chemical oxidation 9 have shown that the rate is controlled either by diffusion of reactant to the fibre surface (liquid diffusion), or by diffusion through the fibres (fibre diffusion) provided that the absorption on or reaction at specific sites is rapid compared with the diffusion rate.As the ionization and neutralization of acids are instantaneous processes this condition is satisfied in the present case and consequently it is necessary to differentiate between liquid and fibre diffusion. Either mechanism may become rate- determining depending on the conditions of the experiment given by the following variables (a) agitation of the liquid, (b) temperature? (c) particle size, (4 fibre and liquid diffusion coefficients, (e) the distribution coefficient of solute between the two phases, (f) solution concentration. In the present case all the factors may be varied with the exception of particle size and the results will now be discussed in an attempt to elucidate the rate- determining mechanism.(a) THE INFLUENCE OF AcrrA-rroN.-In general the rate will be sensitive to agitation if the process is controlled by liquid diffusion? and the results shown in fig. 1 where the calculated diffusion coefficient is plotted against rate of frame stirring, show that the rate16 KINETICS OF ACID ABSORPTION reaches a maximum value at ca. 350 rev/min, using 0.1 N HCl. This may be due to a change in mechanism as in previous cases where a sharp change in activation energy is observed.79 9 Alternativcly the rate may reach a maximum because the stirrer reaches a limiting hydrodynamic eficiency. In any case, increasing the rate of the stirrer reduces the width of the liquid diffusion layer at the surface asymptotically so that it is usually P Stirring rate 300 400 500 , FIG.1.-The influence of agitation on the rate of sorption represented by the diffusion coefficient Of. 0 frame stirring ; A hand shaking ; x mechanical shaking. difficult to differentiate bctween the effect of stirring in reducing the diffusion layer, and a change of mechanism. For this reason mechanical shaking was employed in some experiments as an alternative form of agitation. The results also given in fig. 1 show that this form of agitation is more efficient than frame stirring and that the rate of absorption is almost equal to the limiting rate using frame stirring. These observations, TABLE 1 .-DIFFUSION COEFFICIENTS CALCULATED FROM EQN. (3) AND (4) AND ARRHENIUS ACTIVATION ENERGIES FOR SEVERAL ACIDS acid DI in ' t z p .solution at '5 15" C sec K CC HCI 5 19.0 35.0 HNO3 19.0 C~HSSO~H 19.0 36.0 picric 20.5 naphthalene 19.0 H2S03 - 236 55 0.90 2.4 138 55 0.90 - 84 55 0.90 2.4 105 63 0.78 1.25 258 63 0-78 - - 63 0-78 0.69 335 75 0.67 - 390 1-00 - - - - - 1OSD E cni2/sec caljmole 0.508 - 0.825 5.7 1.375 - 0.843 - 0.350 - 0.640 6.3 0.2 10 I - - I - 2 g fibres in 100 ml solution ; 0.01 N initial concentration. although inconclusive, indicate that at high stirring rates, the sorption is controlled by diffusion through the fibre. were differentiated readily by the considerable difference in temperature coefficient. The liquid diffusion mechanisni is normally accompanied by an activation cnergy equal to that for diffusion in aqueous solution, whercas the fibre diffusion niechanisni has an activation energy of 10-12 kcal/molc.(b) THE EFFECT OF TEMPERATURE.-In the processes SO far studied the two mechanismsR. F. HUDSON 17 The data given for several acids in table 1 show that the sorption proceeds with a low activation energy only slightly greater than the corresponding values in aqucous solution. Krcssman and Kitchener 10 have shown, however, that high activation energies are not nccessary for diffusion in porous solids, and for rigid solids containing molecular pores, the activation energy is frequently about the same as the value for aqueous diffusion although the size of the molecular pores may only be slightly greater than the size of the solutc molecules. The rate processes so far studied with swollen wool fibres which are characterized by high activation energies involve coupled diffusion of anion or reactants and products, and consequently the energetic and steric requirements may be entirely different from those involved in the unidirectional diffusion which accompanies sorption.Thus King 11 has found that the activation energy for the sorption of water in keratin is approximately 7-5 kcallmole at low regains, but decreases to 4.5 kcaljmole as saturation is approached. Thus although the low values of the activation energy of acid absorption do not support the fibre diffusion mechanism, the interpretation is not unambiguous in this case and the rate process must be examined further before a decision may be made. (c) THE FORM OF THE KINmcs.-(i) Difiusion across a liquidfiZrn.-The kinetics may be investigated further by analysis of the form of the sorption curve which will differ considerably for the two mechanisms. The first possibility will be considered by supposing that the diffusion of acid through the fibre is the more rapid process.If Dr is the diffusion Coefficient of HC1 in water, A the surface area of the fibres in contact with Y ml of solution, and S the thickness of the hypothetical diffusion layer at the surface, the rate of transport, d(x V)/dt is given by d(xV)/dt = (DiA/S) (a - x - CI), where Cr is the concentration of acid in a small volume element of solution adjacent to the fibre surface, a the initial concentration in solution and a - x the concentration at time t . The acid in the volume element is supposed to be in equilibrium with the sorbed acid within the fibre, given by the equation of the Gilbert and Rideal theory.5 [H-k]i[CI-lr = K[H+]f[Cl-b.For the absorption of pure acid [H+] = [Cl-] in both phases, and the activity within the fibre is given by the activity of a Langmuir absorbate 12 oJ(l - oi), where Bi is the fraction of the total number of sites available occupied by ions of species i. Thus The rate of transport is then given by whcre Xis the maximum total acidcombining capacity of a quantity of wool with surface area A . On dividing through by a and putting x/a = F. whcre F, is the value of F corresponding to maximum acid combining capacity. When K*F, dF/dt = 0, F = F, SO that Fm=Fw 4- . F, is given alternatively by 088 W/aV, a(l-F,) where W is thc weight of wool of area A in volume Vml of solution, so that a check is available on the maximum value of the absorption in a given experiment.The expected rate of sorption can be calculated from this expression for any value of F, as the value of Di for HC1 in water is known, and a value of 6 may be obtained from independent experiments. Thus in previous work with dilute chlorine solutions, using the identical apparatus and a stirring rate of 450 revlmin, 6 is ca. 0.3 x 10-2 cm.7 The value of S is slightly dependent on the diffusion coefficient and for a given medium and stirring condition S tc Db. Values of b are not known with certainty and values between 0.5 and 0.2 have been reported as a result of experimental and theoretical studies.If the higher and more likely value is taken, a value of 0.75 x 10-2 is obtained for S for hydrochloric acid. Using this value, calculated rates may be obtained under any given conditions at this stirring speed. The results given in tables 2 and 3 show that the observed rates ate 10-100 times smaller than the calculated rates, so that the diffusion of acid to the surface is greater than the rate of diffusion through the fibres.18 KINETICS OF ACID ABSORPTION TABLE 2 TABLE 3 0.092 N HCl dF/dt (sec-1) 0.01 N HCl dF/dt (sec-1) F Oh. calc. F obs. calc. 008 0.0035 0.070 0-1 00070 0.0653 0.09 0.00225 0.069 0.2 0.0026 0.0590 0.10 0.001 66 0.0665 0.3 0.001 14 0.0457 0.1 3 04009 0.060 0.4 0.000855 0-0365 0.18 0.0 c 0.7 1 0.0 - Observed rates of sorption, dF/dr, compared with theoretical values ; A = 103 cmz/g ; V = 50 ml; W = 1 g; K+ = 1/160; D1 at 18" C = 3 X 10-5 cm2/sec.FIG. 2.-Theoretical test of the rate-deter- mining mechanism governed by film diffusion for 0.01 N HCl at 19" C. The results in table 2 and 3 also shows that the observed rate curve is entirely different in form from the theoretically predicted curve. This is shown more clearly in fig. 2 where observed values of dF/dt are plotted against [l - F - K+{F/(Fm - ~ ) } / a ] . The calculated values of the rate rapidly approach the observed values as the ex- tent of sorption decreases, so that the rate must be governed by liquid diffusion in the very early stages of the process. Owing to the entirely different kinetic form of the steady-state diffusion and non-steady-state diffusion, it may be concluded that fibre diffusion controls most of the sorption.(ii) DIFFUSION THROUGH THE FIBRE. -The values given in table 2 show that only 18 % of the acid is removed from solution during the diffusion corre- sponding to a pH change of 0.09 units, so that the fibre surface remains almost saturated during the sorption. The quantity of acid Qt diffused from a con- stant surface concentration Qs into a semi-infinite solid in time t accompanied by adsorption assumed to be linear with an adsorption coefficient K is given by 13 Qt = 2QSA(Dt/Kn)*. (1) It is found that QJQ, is proportional to t* for approximately 60 % of the reaction (fig. 3) giving the diffusion coefficient recorded in table 5. When more dilute solutions of acid are employed, however, the change in solution concentration is considerable, and the diffusion within the fibre proceeds from a variable surface concentration.Mathematical solutions for this condition have only recently been realized, and a formal solution of the general case cannot be obtained.14 A solution is available, however, for a linear absorption isotherm 15 based on the general diffusion equation, For diffusion into a cylinder this equation is transformed into polar co-ordinates using the relation The cylindrical fibre of radius a and infinite length is imagined to be surrounded by a cylinder of liquid of cross-section A excluding the space occupied by the fibre. AssumingR. F. HUDSON 19 the absorption to be rapid compared with the diffusion, Wilson 15 obtained a solution in the following form.where qn is the nth positive root of the Bessel function equation : and the constants a and fl are given by wnJo(qn) + Wl(qn) = 0, 0: = A/.rra*(K -1- 1) ; /3 = D/a2(K -1- 1). This solution is only convenient for moderate values of cc and f ; for small values of t and moderate values of a, Crank has transformed the above equation into the form 16 /2 % J 8 4 / 4D FIG. 3.-Rclation between the quantity of acid sorbed and the square root of time for 0.1 N HCl at 16" C with the following rates of frame stirring in rev/min : @ 200 ; x 300 ; 0 450; 0 550. In the present case the absorption is of the Langmuir type, but may be taken to be approximately linear under some conditions for a limited concentration range. This procedure is rendered possible as the form of the theoretical curves given by (3) and (4), is not highly dependent on the value of 01.The variations in cc over the various absorption ranges is illustrated by the data in table 4. TABLE 4 The eITect of acid conccntration on the value of a. 0.01 N 0.02 N 0405N 8% 0.005 0.004 0.003 0.002 0.0 1 71 0.08 0.07 0.04 0-02 0.0 1 51 % 0.015 [acidlj 20 0.45 0 -39 0.37 0.285 0.55 0.50 0.48 0.39 0.67 0.62 0.55 a 0.555 0 5 1 0.40 0.35 0.90 0.80 0.73 0-51 1-47 1-25 0.920 KINETICS OF ACID ABSORPTION A theoretical rate curve for HCI calculated from eqn. (4) is comparcd with the experi- mental data in fig. 4 ; the agreement is observed to be very close. The form of the sorption curve is in agreement with the rate-determining fibre diffusion mechanism, but is completely different from the curve predicted for a liquid diffusion pro- cess.To illustrate the operation of this mechanism further, a sample of fabric was rc- moved in the course of a typical run and after passing rapidly through a small mangle to remove adhering liquid, was allowed to remain out of contact with the solution for several minutes to allow the diffusion gradient within the fibres to decrease. On rc- immersion, the rate of sorption is found to be considerably greater than in an identical experiment without interruption (fig. 4). This supports the conclusion that the rate- controlling process is determined by the diffusion gradient within the fibre. CALCULATED DIFFUSION coEFFIcImm-The rates of absorption of several acids of initial concentration 0.01 N are given in fig.4 where QJQ is plotted against t . The approxirnatc values of D obtained from eqn. (3) and (4) and the values of a corre- sponding to the initial concentration are given in table 1. The calculated diffusion coefficients are seen to decrease with size of the anion and are in the same order as the diffusion coefficients in solution, as far as can bc ascertained. FIG. 4.-% sorption-time curves at 19" C for the following 0.01 N acid solutions : 0 HNO3; x HCI; 0 C6HsS03H; El C&b(N02)3. OH; A CioH7SO3H; @ interrupted curve ; - - - - theoretical curve. The high rate of sorption of nitric acid compared with hydrochloric acid is due to the greater distribution coefficient K, and the calculated diffusion coefficients are observed to be almost equal as in aqueous solution.The proportionality between the calculated diffusion coefficients in the fibre and in solution also indicates that diffusion proceeds through the aqueous phase within the fibre, in agreemcnt with the low activation energies. The order of magnitude of the diffusion coefficients is extremely low and inconsistent with these deductions. As the diffusion coefficients are calculatcd from the relation pt = D/a2(K + l ) t , the low values may be due either to the use of a low value of a or of K. In the present work the first possibility might follow from the USC of loosely knitted fabric, and if the value of a is identified with the radius of the yarn (ca. 0.25 mm), values of D of the order of 10-5 cni2/sec are obtained.Thc calculated values of ca. 10-8 cm2/sec are, however, in agreement with the values 0.5-5 X 10-8 cm2/scc, obtained by Lind- berg17 depending on the pre-treatment. He used chopped fibres, a large volume of liquid, and a different mode of agitation. This suggests strongly, therefore, that the low values of the diffusion coefficient given in table 1 arc not due to the USC of an incorrect value of a. If the values of K previously used arc too low, it follows that the concentration of free acid within the fibre is less than the conccntration in the external solution as these two concentrations were originally assumed to be equal. An approximate allowanccI<. F . HIJDSON 21 for a decreased internal concentration may be madc, if the internal concentration is given by kc, where c is the external concentration.The diffusion equation may then be written : W c ) D 32(kc) -~ __ -___ - -- Zit K/k -I- 1 3x2 ’ D wC --._ 3 C 3t K/k -1 13x2’ - _ Each fibre is imagined to be surrounded by a volume element of solution of cross-section A and concentration c. The corresponding hypothetical cross-sectional area of liquid of concentration kc would therefore equal A/k, so that the parameters of eqn. (2) and (4) now become and It follows, thcrcforc, that thc form of the thcorctical curvc, determined by a, is unaffected by this modification. The true diffusion coefficients derived from p are greater by the factor Ilk than the previous values. INFLUENCE OF THE DONNAN MEMBRANE-AS the hydrogen ions which enter the fibre combine extensively with the carboxyl groups, and the anions largely retain their mobility, the protein may be regarded as a highly ionized salt.According to the general principles established by Donnan, the concentration of free acid in the aqueous phase within the fibre cannot be equal to the concentration in the external solution. Difficulty is encountered in applying this theory owing to the definition of the aqueous phase within a swelling material such as a fibre. There is no doubt that small regions of water exist within the fibre, but the dimensions are not known with certainty. Estimation of the volume of free water within the fibre is also difficult, and the usual assumption that activities may be equated to concentrations renders the treatment necessarily approximate.From this treatment it follows that k = [H+[[I/[H’]~ is given by the ratio of the concentration of acid in the liquid phase to thc concentration of sorbed acid in the fibre, which is of the order of 10-2 in the pH 1-3 range. Using the values of Peters and Speakman 6 obtained by a more detailed treatment, the modified values of the diffusion coefficient Dj given in table 5 are obtaincd. TABLE 5 initial cone* t j (scc) ~ O S D (cin+x) ~ C I I ~ N 0.092 17 2.76 04030 0-02 110 0.88 0*00020 0.0 1 135 0.825 0.00 10 0.005 160 0.61 0.0035 106Df (cm2,kcc) 0.85 0.88 0-825 0.87 Thc effect of concentration on the diffusion coefficient D calculated from eqn. (3) and (4), and values Df allowing for the Donnan membrane. It is observed that the values of Dfare almost independent of acid concentration, and the magnitude of ca.10-6 cm2/sec is in harmony with the low activation energy. Several additional observations indicate the influence of the Donnan membrane in reducing the concentration of H-i- ions within the fibres. Thus it was observed that sulphurous acid is absorbed more rapidly than the acids given in table 1. Complete agreemcnt between all the observed and theoretical rate curves was observed for values TARLE 6 of Q,/Qco LIP to 0.85. tcmp 2 (SCC) 1OSD (crnZ/scc) 2 180 2.08 15 105 ’ 2.57 25 72 5.20 35 51 7.3522 KINETICS OF ACID AnSORPTION The rate of sorption of 0.01 N sulphurous acid at various temperatures ; 1 g wool in 150 ml solution ; stirring rate = 500 rev/min. The high diffusion coefficients are probably due to the undissociated SO:! within thc fibres, which diffuses together with the ions and is uninfluenced by the membrane.Lindberg 17 has similarly observed that H3P04 and HCQQH) are sorbed more rapidly than HCI and HNO3. Further, the addition of salt is found to increase the rate as well as the extent of sorption (fig. 3, in agreement with the increased conccntration of free acid within the fibre predicted by the Donnan equations. FIG. 5.-% sorption-time curves at 19" C for 0.01 N HCI in the presence and absence of salt; 0 no salt; x 0.2 N KCl. DISCUSSION The detailed discussion of the results has shown that the rate of acid sorption is controlled by a non-steady state diffusion, provided that sufficient agitation is employed. The kinetics alone may equally well be explained by diffusion into the yarn treated as a composite cylinder of closely held fibres as by diffusion through individual fibres.In the first case, values of the calculated diffusion coefficient of about 5 x 10-6 cm2/sec are obtained for HCI at 18" C in agreement with the low activation energy of ca. 5.5 kcal/mole. This possibility is dismissed, however, as the rate of sorption, given by the internal diffusion coefficient, is foilnd to be very similar for individual fibres and fabric, i.e. ca. 10-8 cm2/sec at 18" C. This low value suggests that the concentration of free acid which con- trols the diffusion through the fibre is much lower than the external concentration of acid in agreement with the predictions of the Donnan membrane concept. Estimations of the concentration of free acid supposed to be located in an aqueous phase within the fibre lead to true diffusion coefficient about 20 times smaller than the value for HCl in water.This reduced value is due to the small pro- portion of free water within the fibres accommodated in channels of molecular dimensions between the micelles. Owing to the anisotropic swelling of the fibres (1.8 % longitudinal and ca. 16 % transverse) a resultant increase of approximately 15 % in the surface area is produced, which gives the fraction of the surface covered by water molecules. If all this water is localized in micropores and is not chemically bound to specific groups in the wool macromolecules, the diffusion coefficient would be approximately seven times smaller than the corresponding \ralue in aqueous solution.The agreement between predicted and calculatedR. F . HUDSON 23 diffusion coefficients (i.e. within a factor of 2 or 3) must be considered satis- factory in view of the approximations and assumptions which had to be made. Tt has hitherto been observed that diffusion controlled reactions with swollen wool fibres were characterized by activation energies of about 10-12 kcal/mole.89 9 These high values were attributed to the energy of deformation of the protein chains to allow the solute molecules to migrate through the micropores. In exchange rcsins, the activation energy is determined largely by the pore size, and in most cases the values are of the same order as in aqueous solution.10 Increase in size of the ion has little effect until a critical size is reached when increasc in encrgy is observed.The exchange of anions on wool fibres requires activation energies of about 10-12 kcal/mole, and if the ions are assumed to be completcly mobile within the fibres, diffusion coefficients of the order of 10-9 cm2/sec are obtained for simple ions.4 This value is of a similar order of mag- nitude to the value of the self-diffusion coefficient of Br- ions in horn leratin,21 which relates to the B r - B r - exchange. It is interesting to observe that the temperature-independent parameters A of the Arrhenius equation are very similar (- loglo A = 1.5 and 2.0 respectively) for the exchange and sorption processes in spite of the considerable difference in activation energy. The low activation energies observed for the sorption of HCl, H2SO3 and C~HSSO~H, and by King11 for water, show that the macromolecular network offers no mechanical restraint to the penetrating solute.The diffusion in this case is accompanied by swelling to accommodate the molecules or solvated ions, and provided that the hydrated radii are less than the pore diameter the activation energy will be similar to that in aqueous solution. In ion exchange and dyeing (and electrical conduction 18), hydrated ions migrate simultaneously in opposite directions in the micropores. If the combined hydrated radii are greater than the mean pore radius diffusion can proceed only by deformation of the polypeptide chains or side chains which would protrude into the micropores. The increased energy cannot be due to electrostatic repulsion as diffusion controlled chemical reaction require similar activation energies.It is probably significant that the activation energy is not highly dependent on the size of the anion. Thus the value for the HSO~---CGH~S~~- exchange is almost equal to that observed for dyeing with Acid Orange 11. This supports the idea that the micropores are relatively large, about 30-40 8, as suggested by Speakman 19 but the diffusion is restricted by side chains, the deformation of which requires a constant energy almost independent of the size of the anions. The unidirectional diffusion of molccules or ion pairs below a critical size is not hindered by these side chains, and the diffusion therefore proceeds as in aqueous solution. I wish to thank Dr. P. Alexander and J. A. Kitchener and Mr. D. Reichcnberg for several helpful discussions. 1 Elod, Trans. Faraday SOC., 1933, 29, 327. 2 Steinhardt, Fugitt and Harris, J. Res. Nut. Bur. Stand., 1942, 29, 417. 3 Alexander and Kitchener, Textile Rw. J., 1950, 20, 203. 4 Hudson and Schmeidler, J. Physic. Chem., 1951, 55, 1120. 5 Gilbert and Rideal, Proc. Roy. SOC. A , 1944, 182, 335. 6 Peters and Speakman, J. Soc. Dyers Col., 1949, 65, 63. 7 Alcxander, Gough and Hudson, Trans. Faraday SOC., 1949, 45, 1058. 8 Speakman and Smith, J. SOC. Dyers Col., 1936, 52, 121. Alexander and Hudson, 9 Alexander and Hudson, J. Physic. Chem., 1949, 53, 733. Alexander, Gough and 10 Kressman and Kitchener, Faraday SOC. Discussions, 1949, 7,90. 11 King, Trans. Faraday SOC., 1945, 41,483. 12 Fowler and Guggenheim, Statistical Thermodynamics (Cambridge, 1939). 1 3 Hill, Proc. Roy. Soc. B, 1929, 104, 39. Textile Res. J., 1950, 20, 481. Hudson, Trans. Faraday Soc., 1949, 45, 1 109.24 COMBINATION OF ACIDS 14 Crank, Phil. Mag., 1948, 39, 140. 15 Wilson, Phil. Mag., 1948, 39, 48. 16 Crank, Phil. Mag., 1948, 39, 362. 17 Lindbcrg, Textile Res. J., 1950, 20, 381. 18 King and Medley, J . Colloid Sci., 1949, 4, 9. 19 Speakman, Proc. Roy. Sor. A, 1931, 132, 167. 20 Steinhardt and Harris, J. Res. Nut. Bur. Stand., 1940, 24, 335. 21 Wright, Trans. Faraday Soc., 1953, 49, 95.

ISSN:0366-9033    

DOI:10.1039/DF9541600014

出版商:RSC    

年代:1954

数据来源: RSC

摘要:

24 COMBINATION OF ACIDS THE COMBINATION OF ACIDS AND COLOUR ACIDS WITH KERATIN BY L. PETERS AND G. H. LISTER* Textile Chemistry Laboratories, Dept. of Textile Industries, The University, Leeds 2 Received 10th July, 1953 A study has been made of the equilibrium with, and rate of diffusion into, wool keratin fibres of (i) hydrochloric acid and (ii) the free acid of Orange I1 @-naphthol-azo-p-benzene sulphonic acid). The effect of temperatures and concentration has been examined and the results analyzed from a thermodynamic viewpoint. The entropy term TAS was found to provide the major contribution both to the equilibrium free energy AG" of desorption and to the activation free energy AG* of diffusion at the temperatures (60"- 100" C ) which are usual for the acid dyeing of wool, These phenomena can be attributed to the effect of the solvent (water) whose negative affinity for the dye accounts for the fibre having a large positive affinity, and whose hydration of the polar groups adds to the barrier to diffusion.I. EQUILIBRIUM The combination of keratin with acid dyes is very similar to that with simpler acids, and data on a wide range of compounds at different temperatures have been accumulated, mainly by the careful work of Speakman et d , 1 by Steinhardt, Fugitt and Harris 2 and by Lemin and Vickerstaff.3 The nature of the reaction has been analyzed from several points of view. Steinhardt et al. studied the rc- action as an equilibrium between the ions in solution and four species of reacting groups : iso-ionic keratin, keratin-anion, keratin-proton and keratin-acid com- pounds.Gilbert and Rideal4 extended the Langmuir adsorption equation to cover separate anion and cation adsorption at fixed sites in the protein structure, and Peters and Speakman 5 treated the equilibrium of strong mineral acids with wool keratin as a Donnan membrane effect. Controversy over the applicability of each of these theories is still raging6,7 and this Discussion may provide an opportunity to clarify ideas and attempt a reconciliation of the different view- points; but this can be done only if there exists an agreed body of consistent data on which to base a satisfactory decision. Despite all the above-mentioned extensive research, the published experimental data are somewhat inconsistent even for hydrochloric acid and the simple dye Orange I1 (p-sulphonate of benzene-azo-/l-naphthol).For instance, many different values for the pH at which the keratin is 50 % saturated (i.e. pK values) are re- ported in the literature. There are, thus, no definitive values and the discrepancies, although apparently small, prevent adequate interpretation of the mechanism of combination. For * present address : Messrs. Sandoz Products, Ltd., Bradford.L . PETERS AND G . H . LISTER 25 this reason a more intensive experimental study of the reaction of these two acids with keratin has had to be undertaken. Variability in the properties of the material due to the biological origin of the fibres or to diffcrenccs in initial processes of purification may account for some of the inconsistencies.The major source of error in the past, however, must have been due to hydrolysis of amide groups at high temperatures and low pH values, especially in the presence of dyc anions or high salt concentrations. The particular way in which the reaction R . CONH;? + H20 -- RCOO- + NH4+ has been formulated emphasizes that there is an increase in the number of sites for hydrion adsorption as a result of the hydrolysis. The usual procedure of attempting to correct for this effect by subtracting the amount of ammonia found in solution from the apparent acid combined is, therefore, not strictly accurate. At high pH values only a few of the carboxyl groups thus produced will be in the form of -COOH but at low pH values they will all tend to be so and the apparent value for combined H+ is then the true value.The degree 8, of saturation by H+ must thcn be calculated by expressing the amount combined as a fraction of the increased number of sites. When some hydrolysis has occurred, &, the degree of saturation by anions, will not necessarily equal 8, since the fibre can then be electrically neutral without this condition. Close inspection of the isoelectric region of the HCI/KOH titration curves of keratin given by Steinhardt et al. reveals that some hydrolysis of amide groups probably occurred, in the wool thcy uscd, during the scouring processes prior to their experiments, for there seems to be an e x e s of -COOH groups over -NH2 groups not found in solvent- scoured virgin wool. The most important effect of hydrolysis in situ, however, is on the relative con- centration of H+ and NH4+ in solution.Evcn when the amount of ammonia in solution is negligible as a corrcction factor to the acid-combining capacity, i.e. at high pH values, ammonium ions may nevertheless constitute a high pro- portion of the cations present and render pH no longer equal to - log [X-], where X- is the anion concentration in solution. For HC1 this effect is always practically negligible but for Orange I1 the discrepancy leads to quite large errors. The NH4+ concentration, at times, is even greater than the H+ concentration, so that the reaction becomes that of the ammonium salt in the presence of a small amount of acid rather than that of the free acid alone. A further obstacle to precise calculation of affinities is brought out by the data of Meggy 8 on the activity coefficients of Orange IT.These show that the anion concentrations of Orange IT may have to be multiplied by factors up to 100 to convert them into activitics in aqueous solution and this can lead to crrors in estimating the free energy of adsorption of as much as 2.7 kcal/mole (i.e. 2.3 RTloglo 100). But until precise values of the activity coefficients at the appro- priate concentrations and temperatures are obtained, the use of concentrations instead of activities will have to continue, with the consolation that at high tem- peratures such corrections are likely to be smaller than the above limit. From all that has been said above it is clear that to estimate the affinity of dycs for keratin it is necessary to determine, at one and the same time, not only the pH of the solution, but also the ammonium and anion concentrations in solution and the separate amounts of hydrion and anion combined-data which are not provided in any previously published paper.EXPEKIMENTAL MATERTALS.-Wod.--ThC wool keratin was that of thc root ends of fibres taken from the shoulder and back of a New Zealand Romney fleece. It was purified by soxhlet extraction with alcohol and then ether for 24 h each. The wool was then thoroughly washed in distilled water to remove soluble salts and combed to shake out grit. After26 COMBINATION OF A C I D S soaking in N/1000 hydrochloric acid for 24 h, the fibres were immersed in successive baths of distilled water until an equilibrium pH of 4-8 was obtained.The wool was then centrifuged, pressed between filter paper, and allowed to condition at 22.2" C and 65 % relative humidity. The conditioned material was found to have a 15.20 % moisture and 0.06 "/, ash content. Orange I I free acid.--Commercial Naphthalene Orange was re-crystallized from dis- tilled water, precipitated as the barium salt, washed thoroughly to remove sodium salts, and the free acid prepared from the residue by the addition of the theoretical amount of A.R. sulphuric acid. Repeated precipitation with A.R. hydrochloric ensured the complete removal of any remaining sulphuric acid and evaporation to dryness removed all traces of hydrochloric acid. DETERMINATION OF TITRATION cuRvEs.-The titration curve of the wool with hydro- chloric acid was determined at 22.2" C and 60.0 f 02" C and that with the free acid of Orange I1 at 40.0 f 0.1" C, 50.0 0.1" C, 60.0 f 0.2" C and 80.0 :II 0.5" C.Samples of the conditioned wool (2.2000 g) were immersed in measured amounts of acid solutions (approximatcly 270 ml) in flasks having ground-glass stoppers. To prevent concentra- tion errors by evaporation or condensation on the cooler parts of the flasks they were always completely filled and the stoppers sealed with paraffin wax. Two days were allowed for the wool to reach equilibriuni with hydrochloric acid and the times allowed for the free acid of Orange I1 were determined by control experiments. FIG. l.-Titration curve of HCl + keratin at 22.2" C and 60.0" C and of Orange I1 free acid at 80" C ; (a), (6) H+ combined against pH; (c) X- combined against - loglo X. Each solution was rapidly decanted from the wool through a filter pad of glass fibres and, after cooling to 18.0" C, the pH values and the concentration of acid and ammonia in solution was detcrrnined.Determination of hydrogen ion concentration.-When the pH value of the originaI or final solution was less than 2.5, the hydrogen ion concentration was estimated by potentiometric titration. For solutions of higher pH values the concentration was obtained from the pH value by means of a calibration curve. Determination of ammonia in solution.-Aliquots of the solution were made alkaline with M/20 sodium borate and distilled from a Kjeldahl flask into ammonia-free distilled water. The ammonia was estimated in the distillate by colorimetric comparison with standard solutions of ammonium chloride after treatment with Nessler reagent.Determination of anion concentratioa-For the hydrochloric acid titration curve the chloride ion concentration was determined by titrating with silver nitrate, using potassium chromate as indicator. In the experiments with the free acid of Orange TI, the chromo- phore concentration in solution was determined colorimetrically on a Spekker absorptio- meter. The amounts of acid or anion combined were estimated from the changcs in concentra- tion, assuming that the whole of the moisture content in the conditioned wool was water capable of diluting the acid. The results for Orange TI are shown in table 1, and for HC1 in fig. 1.t t TABLE COMBINATION OF KERATIN WITH ORANGE rI FREE ACID 27 final PH 2.08 2.66 3.03 3.49 3.72 3.88 4.20 4.65 4.94 2-05 2.35 2.63 2.8 1 2.94 3.00 3.32 3.60 3.88 4.23 4-61 2.48 3-03 3.18 3-49 3.74 3.83 4.07 4.26 4.34 4.40 4-80 2.40 2.68 3-1 8 3.45 3.68 3.87 3.98 4-14 4.26 4.49 4-65 -log x 2.05 2.47 2.92 3-39 3.58 3.69 3-98 4.43 4.7 1 2.02 2.30 2-57 2.62 2.85 2.90 3.09 3.41 3.60 3.96 4.40 2.42 2.88 3.00 3.23 3.43 3-50 3.69 3.88 4.00 4.05 4-29 2.3 1 2.54 2-91 3.12 3.32 3.49 3.60 3.75 3.84 4.1 1 4.28 ammonia amount combined in solution hydrions anions ( C N ~ (%I) (UX) (mmolcj100 g dry wool) ___ .- ~ _ _ _ _- 9% 7-2 3-8 1.2 1.0 1.0 0.6 0.2 0.1 10.3 7.4 5.1 4.0 3.6 3.6 2-4 2.0 1-7 0.6 0.2 7.4 5.7 5-0 3.8 2-8 2.4 1.7 1.1 0.8 0.7 0.5 13.1 11.5 8.5 5.9 4.0 2 7 2.1 1.5 1.3 0.6 0.4 98.1 90.0 83.3 68.8 53.6 43.4 27.6 12.1 4.8 98.8 93.4 86.8 83.2 79.3 77.5 69-1 56.1 41.7 24.0 11.9 98.3 89.7 85-5 74.2 58.7 47.6 37.2 26.6 21.2 18.7 6.1 11 1.1 106.7 93.0 77.1 62.7 48.4 40.2 29-5 25.1 14.3 8.8 86.2 80- 1 78.2 66-2 51.4 41.4 26-5 11.8 4.9 87.1 84.0 80.2 78.5 74.9 721 65.0 53.2 39.2 23.1 11-5 88.1 81.7 79.0 68.7 54.7 43.8 34.9 25.2 20.1 17-7 6.0 94.1 92.6 83.1 70.1 58.2 46.1 38.4 27.5 23.7 13.8 8.3 DEFINITION OF AFnNIm.-There are several difficulties in attempting to estimate the affinity of an acid for keratin from data such as those given above.Tn the titration of a soluble amino-acid with hydrochloric acid, for instance, the pH of the solution in which the -COOH group is 50 % dissociated (the pK value) is a good measure of the free energy of ionization of that group because it is un- complicated by significant association between the chloride ion and the charged amino group -NH3+.When an insoluble protein is titrated, however, the adsorption of hydrions must be accompanied by adsorption of anions and the free energy of this process will, of course, affect the equilibrium value. Gilbert and Rideal overcome this difficulty by defining the standard state of the acid on the protein as that in which half the available sites are occupied (8 = 4). They then take as a measure of the affinity of the acid, the total free energy AG28 COMBINATION OF ACIDS of its ions, i.e. 2*3RTIog1o[Hl[X], where [HI and [XI are the concentrations (assuming unit activity coefficients) of the hydrions and anions under the above conditions.This implies that the zero free energy of desorption is taken as that of a thcorctical acid which can only half-saturate keratin when its concentration is 1 N. For hydrochloric acid, their viewpoint is, fortunately, easy to reconcile with thc Donnan treatment. According to this theory thc product of thc equilibrium activitics of the ions in the external solution must be equal to that in the internal solution, while at the midpoint the internal anion concentration can be assumed to be of the order of 1 3 N because of the low affinity of C1-. This is not far from unit activity, so that - loglo [H][Cl] is not appreciably different from the internal pN. The internal pH value at the midpoint is a measure of the affinity of the -COO- groups for H+ (AG == 2.3 RTpK) which is thus almost identical with the above Gilbert-Rideal definition of the affinity of kcratin for both ions, The effect of the internal anion concentration a&, where a, is the amount of uncombined internal anion and v is the volume of the internal phase, is too large to be neglectcd for collagen9 because of the large volume swelling; it can be neglectcd for keratin because its volume swelling is small but even then only when the amount of anion combiiied (in rnmolelg) is of the same order as the internal volume (in ml/g).At very low degrees of combination this correction factor becomes significant .7 The free energy AC of desorption when 6 differs from & is assumed by Gilbcrt and Rideal to be given by the equation Unfortunately, when AG is calculated from this equation the value found is not constant but depends on the degree of saturation.This could be explained as due to some sites having different affinities for the ions, or what axnounis to the same thing, each site may be influenced by the state of combination of its neighbours. It is also not cerlain that the number of sites for anions is fixed and definite in the same way as for hydrions. The attraction between anions and the charged -NH3-l- groups is a long-range coulombic effect and need not be one- to-one as in covalent bond formation. The non-anionic moiety of the dyc molecule, too, is often hydrophobic and this could cause the dye to accumulate in the solid- liquid interface as a mobile monolayer-a form of adsorption different from that implied by thc Gilbert-Rideal theory. Because the value of AG caIculated from eqn.(1) is not constant but depends on 8, it is not correct to take the mean of several values, because the result thus obtained is determined by the pH at which the experiments were performed. This is also true of the speciously elegant desorption method devised by Gilbert 10 and used by Lemin and Vickerstaff.3 A more rigorous approach would be to take the total area under the titration curve since this would include small pro- portions of groups of different affinities and thus give a truer estimate of the total free energy associated with complete saturation. The titration curves, howcver, are very symmetrical, and because of this, it has been found that the following simpler method gives almost the same results.The ammonia correction cNH3 and the amount aH of H+ combined were used to cstimatc the degree to which the carboxyl groups are saturated by assuming thc maximum at any time to be (90 + cNHJ mmole/l00 g dry wool. The amount ax of anion combined was assumed to have a constant Iiinit of 90 mmole/l00 g, i.e. pcptide hydrolysis was neglected. The data of table 1 were then uscd to find by graphical interpolation the value of pH - loglo X at which was zero. The resuIts obtained are plotted in fig. 2.L. PETERS AND G . H. LISTER 29 These interpolated values represent mean activities of the acid in solution which is in equilibrium with acid on the protein in a standard state. From them, thcreforc, an estimate of the free energy of desorption may be calculated.In what follows, the symbol AGO will refer only to this definition. It is a true measure of affinity in that large positive values refer to high affinities and it is to be preferred to the usual convention which involves the uOe of negative values. thc heat AH" and entropy AS" of ionization of weak acids (including amino-acids) are not indepcndent of temperature, a parabolic curve being found when pH is VARIATlON OF HEAT AND ENTROPY WITH TEMpERATURE.-It iS Well known that FIG. 2.-Temperature variation of pH at midpoint of HCl + keratin titration curve and of pH -log10 X for Orange I1 + keratin. 0 experimental - theory. plotted against T. Harned and Robinson11 have shown that the parabolic curve can also be closely fitted by an equation of the form (2) where A, B and C are constants such that pK == (A/T) + BT - C, and AH" == 2.3 R(A - BTZ), ASo = 2.3 R(C - BT).(3) (4) To apply this equation, at least three points are needed and the data for HCl/keratin in table 1 have been fitted to (2) with the help of the experimentally-determined value of AH& = 3650 cal/mole 12 and eqn. (3). The equation thus obtaincd is ( 5 ) For Orange II/keratin the equation fitting all four points is (6) The graphs of these equations are shown in fig. 2. From the numerical values of the constants, estimates of AGO, AH" and AS" have been calculated for different temperatures and the results are illustrated in fig. 3. No great reliance, of course, is to be placed on the exact numericaI values because of the paucity of data from which they were derived but the following conclusions are qualitatively indubitable.At the temperatures at which dyeing is usually carried out (60" to 100" C) the heats of reaction of both HCI and Orange I1 are equally small (AH" less than 0.8 kcal/mole). On the other hand there is a considerable free energy of dc- sorption, that of Orange I1 (AGO = 1 1 to 12J kcal/mole) being larger than that pKHcl = (1912*3/T) + 0.014956T - 6.3969. pKo,, = (5353*2/T) + 0.043957 - 23.409.30 COMBINATION OF ACIDS of HCL (AGO = 64 to 7 kcal/mole) in agreement with its greater affinity. But the small values of AH" imply that the major contribution to the free energy of desorption is the entropy change ; for HCl in this temperature range AS" is - 16 to - 22 caI/deg. mole and for Orange 11 AS" is - 27 to - 43 cal/deg.mole; i.e. the system loses degrees of freedom when desorption occurs. magnitude of the entropy change contradicts any idea of the anion being adsorbed on specific sites because desorption leads to a more ordered state. However, when it is remembered that these thermodynamic quantities refer to the system as a whole, including the aqueous solvent, the paradox can easily be resolved. Desorption of hydrogen atoms from uncharged carboxyl groups leads to oricnta- tion of water molecules around the ions thus created and it is to this entropy change that much of the above effects can be attributed. The entropy of formation INTERPRETATION OF THERMODYNAMIC VALUES.-At first sight the Sign and large FIG. 3.-Calculated values for the free energy AGO, heat AHo and entropy ASo of &sorption of Orange I1 from keratin at different temperatures.of ice (corresponding to complete orientation of the molecules) is about - 5 cal/mole deg. and it would only need about two water molecules to be oriented around each charge to give an entropy change of the right order for HCI. The higher value for Orange I1 shows that its larger molecule may be displacing a greater number of water molecules (another 4). Such an interpretation leaves very little scope for theorics based on adsorption at specific anion sites, but it is quite in harmony with the conception of surface adsorption at a vast 9 internal micellar surface (106 cm2/g) in a mobile monolayer which inhibits water adsorption-and this could be the source of the affinity.On this view, the larger the surface area of a dye molecule the greater would be its affinity- view put forward by Meggy13 who has attempted to calculate affinities on this basis. Further work is in progress on the adsorption by keratin of weak acids having small organic anions. 11. RATE OF DIFFUSION The histological complexity of wool fibres and the amide hydrolysis which occurs at high temperatures make exact interpretation of diffusion studies difficult, but the approximate treatment given below is probably sufficient for the present.L . PETERS A N D G . H . LISTER 31 EXPERIMENTAL The rates with which wool fibres take up acid from solutions maintaincd at constant concentration were studied by means of a specially built apparatus described elsewhere.14 Measurements were made of the absorption by keratin of Orange I1 free acid from solutions whose pH values ranged from 2.60 to 4-60.The rates were measured at 40", 45", 50", 60" and 80" C over periods of 180 n i n at the high temperatures and 300 min at the lower temper- atures. The general form of all the results ob- tained is illustrated by fig. 4, which gives as an example the data obtained with Orange I1 acid at 80" C . The slope of the graphs of the amount u taken up at different values of t ) increases in all cases during the first 4 min but remains constant afterwards until ultimately the curve flattens as equilibrium is approached. Thc slope (s=- da/pt)) of this linear region was measured in each case, and its reciprocal l/s plotted against that of the hydrion concentration (l/[H '1 = antilog pH).These are straight lines, except at low pH values, and the results of all the experinients at different temperatures with Orange I1 are given in this graphical form in fig. 5 . For comparison, the rate of absorption of hydrochloric acid by a sample of wool was measured at pH 3.73 and continued until equilibrium was almost complete. The wool was then removed and thoroughly pressedibctween sheets of filter paper. Sufficient hydrochloric acid was added to the residual solu- tion! to:bbring!gthe'[pH value to 3.43 and the damp wool re-immersed in the stronger solution. Measurement of the rate of the further uptakc FIG. 4.-Rate of absorption at 80" C of Orange I1 free acid at different pH values.was also prolonged until the characteristic approach to cquilibrium appcared. Both sets of data are presented in fig. 6. FIG. 5.-Variation of the reciprocal of the slope (11s) for Orange I1 free acid with reciprocal of concentration of ( l / [ H i ] ) at different temperatures.32 COMBINATION OF ACIDS DIFFUSION INTO A cuLINDm.-Crank 15 has shown that diffusion into a cylinder of radius Y from a solution having a constant conccntration, should obey the equation (7) a/A = 4(at)h - m t - 7~(Xt)%/3 f . . ., (Z - 0 0 0 \800c lI " 060°C 50 "C o o %P 0 0 0 - /2 0 0 0 0 " O * " 40 "C 3 P H 4 5 - ,I3 FIG. 6.-Rate of absorption of HCI at 40" C . (a) at pH 3.73, followed by a further adsorption ; (b) from pH 3.43.L. PETERS AND G . H. LISTER 33 calculated from (9) and the data in fig.5. The values thus obtained are shown in fig. 7. The smallness of the values for Orange I1 in keratin (10-11 to 10-12 cm2/sec) is in contrast to the magnitude of the values found for direct dyes in cellulose which are of the order of 10-8 to 10-11 cm2/sec (see Vickerstaff, ref. (6), p. 235). Tho value for HCl at 40" C from fig. 6 is also very small (D = 1.2 X 10-10 cm2/sec). Crank assumes that D = K(P + l), where K is the true diffusion coefficient and P is the partition ratio in which the diffusing substance distributes itself between the material of the cylinder and the exterior solution. In the derivation of (7), P is taken as constant, which seems unlikely to be true for keratin. From fig. 7, however, it appears that D is not very pH dependent (except at 80" C) SO there is some justification for assuming P to be constant.It is difficult to decide what value to assign to P. For HCl it would be of the order of 3000 if all the acid combined could be assumed to be dissolved in the wool fibre. This would give K the value of 3.6 x 10-7 cm2/sec. Similar calculations with the data on Orange 11 are possible but the signficance and validity of such "correction" is hard to assess. In the subsequent analysis, therefore, D will be preferred as a measure of the rate of diffusion. FIG. 8.Variation of activation heat AH*, entropy AS* and free energy AG* of diffusion of Orange I1 in wool keratin. Treating the diffusion of acid into keratin as an activated process, the diffusion coefficient D may be assumed to have the form 16 D == h2(kT/h) exp (- AG*/RT) = X2(kT/h) exp (AS*/R) exp (- AH*/RT>, (10) where h is the distance moved by each molecule of acid in overcoming the activ- ation energy barrier (AG*).The plot of log (D/T) against 1/T, however, is not linear so that the heat of activation (AH*, the Arrhenius activation energy E ) varies with temperature just as does the equilibrium heat of desorption (AH"). Estimates of AH* at the mean of the temperature intervals have been calculated and these are shown in fig. 8. The activation entropy AS* cannot be unambiguoudy determined since h is unknown. If it is assumed that the sum of the equilibrium free energy AGO and the activation free encrgy AG* is approximately constant and if h is constant, the values of D given in fig.7 and those of AGO in fig. 3 lead to the estimate that AG* - 13.5 kcal/mole and h = 2.8 A. On the basis of this assumption, estimates of AS* have been derived and the values are given in fig. 8 together with those of AG*. If the above assumption is not valid, however, the graph of AS* will only be displaced vertically. Consequently it seems incontrovertible that, at high temperatures, the major contribution to the activation free energy barrier is that of the entropy term. B34 DIFFUSION OF SORBED SUBSTANCES The authors are grateful to Prof. J. B. Speakman for encouragement and guidance, and to Messrs. Sandoz Products Etd. for a grant to one of them (G. H. E.)-which enabled this work to be carried out. ISpeakman and Hirst, Trans. Faraday SOC., 1933, 29, 148. Speakman and Stott, Trans. Furuduy Soc., 1934, 30, 359 ; 1935, 31, 1428. Speakman and Elliott, Symp. Fibrous Proteins, J . Soc. Dyers Col., 1946, p. 116. ZSteinhardt and Harris, J . Res. Nat. Bur. Starid., 1940, 24, 335. Steinhardt, Fugitt and Harris, J. Rcs, Nat. Bur. Stand., 1940, 25, 519 ; 1941, 26, 293 ; 1942, 28, 201. 3 Lemin and Vickerstaff, Symp. Fibrous Proteins, J. Sue. Dyers Col., 1946, p. 129 ; Symp. on Dyeing, J. Soc. Dyers Col., 1947, p. 41. 4 Gilbert and Rideal, Proc. Roy. SOC. A , 1944, 182, 335. 5 Peters and Speakman, J. Soc. Dyers Col., 1949, 65, 63. 6Kitchencr and Alexaiidcr, J . SOC. Dyers Cul., 1949, 65, 284; Text. Res. J., 1950, 20, 203. Olofsson, J. Sue. Dyers Col., 1951, 67, 57 ; i952, 48, 506. Vickerstaff, Physical Chemistry of I>yeii.rg (Oliver and Boyd, 1950), pp. 289-96. 7 Peters and Speakman, J . Soc. Dyers Col., 1949,65,287 ; J. Soc. Dyers Col. (in press). 8 Meggy, this Discussion. 9 Speakman, Proc. Roy. SOC. A, 1931, 132, 167. 1" Gilbert, Proc. Roy. SOC. A , 1944, 1183, 167. 11 Harned and Robinson, Trans. Faraday Soc., 1940, 36, 973. 12 Speakman and Stott, Trans. Furuduy Soc., 1938, 34, 1203. 13 Meggy, J. SOC. Dyers Col., 1950, 66, 510. 14 Chamberlain and Lister, .?. Soc. Dyers Col., 1951, 67, 176. 15 Crank, Phil. Mag., 1948, 39, 362. 16 Glasstone, Laidler and Eyring, TCteory of Rate Processes (McGraw-Hill, 1911), p. 524.

ISSN:0366-9033    

DOI:10.1039/DF9541600024

出版商:RSC    

年代:1954

数据来源: RSC

摘要:

34 DIFFUSION OF SORBED SUBSTANCES A CONTRIBUTION TO THE THEORY OF DIFFUSION OF SQWBED SUBSTANCES INTO AND OUT OF FIBRES BY BERTIL OLOFSSON Swedish Institute for Textile Research, Gothenburg, Sweden Received 30th July, 1953 Thc relation between the equilibrium sorption isotherm s = f ( c ) and the sorption- coupled diffusion for fibres has been discussed. For finite bath exact solutions of the diffusion equations are obtained if (I) the actual part of the isotherm (corresponding to the range of bath concentration) is approximated by s = Rc (R = constant), or if (11) the same part of the isotherm is substituted by a line s = Ac + A0 and another part relatcd to the range of fibre concentration is substituted by 3s/3c = const. (not dependent upon A). The validity of these relations is tested on HBrlwool sorption curves : a satisfying constancy of the diffusion constant for a certain experiment is found and the difference between absorption and desorption values is cxplained by theory 11.Further applica- tions of these theories are summarized, viz. washing out acids from wool fabrics, diffusion of dyestuffs into fibres and moisture sorption on fibres. When discussing exchange of substances between a liquid (water) bath and fibres we must first consider: (i) the configuration of the fibre sample (yarn, fabric) that determines if the single fibres or only " macrosurfaces " of the sample are freely accessible to the liquid ; (ii) the circulation of liquid between the contact region of the sample and the rest of the bath. Some experimental work has been done to clarify the problems involved but these are very complicated. Furthcr clarification is obtained from this work too.In the theories here developed we make the assumptions: (i) the singleBERTIL OLOPSSON 35 fibres are in direct contact with the liquid (the equilibrium water-fibre phase established at zero time); (ii) the circulation of the liquid is such that a change in concentration just outside the fibre surface will at once be distributed throughout the bath. TmoRY.-lf the fibre is an absorbent for the diffusing material we could define a “ free ” concentration c as well as a “ sorbed ” concentration s. Generally c and s are functions of place, radial co-ordinate r, and time t. However, we further assume that there is an immediate equilibrium established between s and c at a point (r, t ) and this equilibrium follows the relation s =f(c).The diffusion is governed by a diffusion constant D which is generally a function of c, of the special phases (layers) of the fibres, of the direction of sorption, etc. Since the radial diffusion is generaIly the dominant process we further assume a homogeneous fibre and a D-value that is a constant for the solute and fibre considered. We focus our attention on the appropriate manner of applying a relation s ==f(c) for sorption problems. The fundamental mathematics are given by Wilson 1 on diffusion coupled with absorption into a homogeneous cylinder from a finite bath. The diffusion equation is written as We assume the fibres (radius = a, cross-section rra2) to be distributed in a bath of cross-section = A (cross-section of fibres excluded).Thus A/nd represents bath volume/fibre volume. Initially the diffusing substance has a concentration cg in the bath and CO everywhere in the fibre phase. Thus the total amount of substance at t = 0 will be rra2Co + Aco = MO -1 Aco. CASE I.-We make the assumption used by Wilson 1 that s = f(c) has every- where the form We further substitute s = Rc (R = constant). (2) y = 2n cr’dr’ (0 < r’ < a) s: in eqn. (1) together with relation (2) and obtain (3) There are two boundary conditions. The total amount diffusing substance is constant or 2.rr J: (c -I- s)r‘dr‘ + ~c =z M~ + A C ~ , or using (2) and (3) Further we have the initial condition for amount of substance in the fibre (6) phase ((2) and (3) are used), therefore We make the substitution (R + l)y(r, 0) = rrr2Co = r2Mo/a2 (0 < r < a, t = 0).36 IIIFFUSION OF SORBED SUBSTANCES in (4), (5) and (6) giving The solution of (8) has the form f == 2 BnrJl(qnr/a) exp (- P ~ O .(1 1) n Here and in following formulae Jn(x) denotes the Bessel function of order n for the variable x . By substitution in (8) we find Pn Z= Pqn2, (12) whcre /3 = D/(R 4- I)$. (13) Substitution of (11) in (9) gives aqnJo(9n) + Wl(Yn) = 0, a = A/[na2(R + l)], whcre and substitution in (10) gives where and Mt is defined as total amount of diffusing substance in the fibre at time 1. From (2) and (3) we find and further when (7), (15) and (18) are used, MI = (R -I- l)(a, t ) + Aco(1 + .)/(1 +a), (20) in (20), f(a, t ) is substituted using (ll), (14), (15), (16) and (17) to give From (21) we find For absorption wc divide (21) by (22) getting When MO = 0 wc get from (18), K = 0, and thusRERTIL OLOFSSON 37 For desorption we divide (21) by (18) giving When co == 0 we gct from (18), K cc, and thus On comparing the “ pure absorption ” (24) and “ pure desorption ” (26) wc obtain the relation CASE IT.-Assumption (2) is often a very rough approximation.For dye- stuffs with their high affinity, and also for acids on wool, we know, however, that $9 c, and we further put s -I- c = c, and Cr’dr’ (0 < r <a). (3’) Now we make the transformation suggested by Standing et al.2 and Vickcrstaff 3 for dyestuff diffusion using (2’) and (2”) : giving the new form of (l), If W is substituted from (3’) we get To get an exact solution, we further assume that there is a linear relation between s and c at the fibre surface, Then the boundary condition for constant total amount of diffusing substances will be (compare (5)) s == xc 4- A() (r = a). (2’”) =: (M” + Aco)(l -1- A) - AA”, (5’) (6’) (7’) (8’) r - a and the initial condition corresponding to (6) We now make the substitution H(r, 0) = .rn‘2Co = Mor2/a2 (0 < r < a, t = 0).(Aco + M”)(h -!- 1) + AA” H(r, t ) = in (4’), (5‘) and (6’) getting r2 + F(r, t ) (A -t 1 ) d -i- (A/T) (9’)38 DIFFUSION OF SORBED SUBSTANCES The solution of (8’) is F = ~ i r ~ l [qnr/al exp (-- p i t ) , N and substitution in (8’) gives Pn’ == p‘qn2, (13’) NqnJdqn) i- 2Jl(qn) 0, ( 14‘) where a = A/[nu2(A 4- l)].(1 5’) D 3c where p ’ = - - a2 as* Substitution in (9’) gives Substitution in (10’) gives and Mt is found from (2”) and (3’), KO = CO/(CO I- cox + ho). and further, if (7’), (15’) and (18‘) are used, 4 c o -1- cox -I- AO” -I- ( K O / 4 l (a -1- 1)(A -1- 1) _ _ _ _ _ ~ Mt = F(a, t ) -I- - * Now F(a, f) is substituted, using (ll‘), (14’), (15’), (16’), (17’), whence A(c0 + cox -t XO” + ( K 0 / 4 1 (a -I- -I- 1) For absorption, we divide (21’), by (22’1, Mw = When CO = 0 we get from (18’), KO = 0, and thus For desorption, we divide (21’) by (compare (18’) and (15‘)) Whcn (i) thcre is no substance in thc bath at t = 0 or co = 0, and (ii) ho = 0, we get from (18‘), /co = co, and therefore The relation between (24’) and (26‘) is given by (27).BERTIL OLOFSSON 39 For the relation between (25’) and (26’) we get 1 = (1 - ;) -’- KO’ CASE 111.-Experimental equilibrium sorption isotherms generally give thc relation C = c + s =f(c).If (2’) is valid, a good approximation for this expression is s = f(c). This relation is generally not linear and hence it is not possible to get an exact solution of the diffusion problem. However, we may use a method in which differentials are changed to finite diffcrences. For the absorption this method is given by Crank 4 and thc corresponding formulae for general sorption are easily deduced. The equations governing thc process are 0 3 3c 3s -- r 3r (r;) = -+ z, s =f(c), (29) and the boundary conditions are Ac.,,. -1- 2nl; (c -1- s)r’cir’ == Aco + n-azco ::- Aco -t- )do, (30) (3 1) Reduced variables are introduced, t is changed to T = Dt/n2 and r changed to p == r/a, the fibre cross-section is divided in circular shells of thickness dp and thc time is divided in intervals d7.The formulae now used are to be found in Crank’s paper.4 APPLICATION.-kI applying these theories to experimental results it is con- venient to use graphical representations. As we generally measure Mt as a function of t, the different equations for Mt/M0 or Mt/Mw should be represented. Now it is possible to find relations between equations such as (27) and (28’), and only one of these equations for casc I and case I1 must be plotted. We have chosen eqn. (26) or (26’). We take a and pt (or P’t) as parameters. For a given a-value we find values qn from (14), using tabulated values for the Besscl functions JO and J1.5 The a and the calculated qn values are substituted in (26), together with chosen Pt values in the exponents pnt = qn2Pt, and thc corresponding Mt/Mo values calculated.In the graph Mt/M0 is plotted against the transformed variable p - lOO/(l -t- a) and plots for a given ,&-value are constructed. The form of these diagrams is shown by fig. 1. C := CO ( t = 0, 0 < r < a). For Pt = co we find from (26), This shQws that the “ limit curve ” is a straight line from its origin to p = 100, Mt/Mo =: 1.000. For small values of t, expression (26) gives a very slow con- vergency of the series in exp (- pnt). Here we must use asymptotic expansions of the Bessel functions. The appropriate methods are given and discussed by Crank .6 SORPTION OF EIBr ON WOOL The validity of these theories has been tested by some careful measurements on the kinetics of HBr sorption on wool.The reason for using HBr was that this acid had been employed by Larsson and Lindberg (unpublished) in some earIier investigations of de- sorption using radioactive bromide ion. The experimental arrangement in the present case was designed by Lindberg.7 The experiments here reported have also been made by Lindberg, in connection with his work on thc reIations between acid diffusion constants and surface treatments of w00l.7~8 The fibres are ground in a Wiley mill and suspended in the bath, whcre they are kept in rapid movement by a stirrer. The change of concentration is measured by the change in electrical conductivity of the bath.A conductivity cell in the bath is part of a resistance40 DIFFUSION OF SORBED SUBSTANCES bridge with a comparison cell in a constant concentration bath and the bridge is coupled to a recorder, where the relative bath concentration is thus registered as a function of time. If now the initial amount of HBr in bath and fibres is known and also the constant volume of thc bath, we can easily calculate values of Mt (and Mt/Mo, or Mt/M,). The following experiments were madc. A 0.5 g sample of wool was ground, extracted with ether and alcohol, suspended in 5 ml distilled water and added to 250 ml 0015 M HBr in the conductivity vessel. The absorption curve a t equilibrium was recorded (A). Now the sample was filtered and the solution replaced by distilled water.The sample (together with a small amount of equilibrium solution from (A)) was added and the de- sorption curve a t equilibrium recordcd (B). Now the sample was carefully washed with distilled water and after this a new absorption curve was obtained (C), the HBr concentration being now 0.001 M, this figure corresponding to the final bath concentration in (B). All the experiments were made in duplicate. To calculate p values according to case I from fig. 1 we must first determine a or p using eqn. (32), e.g. withp = 100 M,/Mo for desorption. In “ pure absorption ” (Mo-0) FIG. 1.--Mt/M0 as a function of p = l00/(l -I- a) and @ x 10-3 (the figures given on the curves) from (12), (14) and (26). we calculate a from B,/M, (B, is the amount of acid in the bath at t = m ) using (22) : For such a p value and MJMo from experiments (or from cxpcrimental Mt/M, using (27)) fit and thus l3 values are determined from the graphs, For all the experiments A, B and C the /3 values show a satisfactory time constancy.For A and also B, f i decreases ; for C it increases somewhat with time as seen from fig. 2. We may calculate a simple number average a = ZP/n (n = number of values) or a time average = Zpt/Zr; this is probably more significant since every /3 value is calculated as a constant for the time 0 to t (table 1). A small deviation of %/a from 1 denotes a good time constancy of 8. However, is very different in the three experiments A, B and C. From (13) we might suppose that thc reason for this is the variation of X.According to (2), R should be some sort of average of the quotient SIC within the concentration region for the actual cxpcri- ment. Now we can determine R from thc cz value using (15). For this we must determine A/m2 = bath volume/fibre volumc. If the dry fibre weight is g with the density p, the dry volume will be g/p and th:: swollen volume (g:p)(l -1- s), where s is the fractional volume swelling. Substitution in (15) gives R. We find (table 1) important differences in cases A, B aiid C . Howcver, if these values are substituted in (13) we get values of D/a* = B(R -t- I ) . From table 1 we find that A As p N 1.3 and s ci 0-3 we get nu2 =< g.BERTIL OLOFSSON 41 10 20 t(rnrn) 30 4 0 FIG. 2--15 (case I) and -15’ (case 11) as a function of t for HBr/wool sorption curves A, B and C .TABLE AD ADSORPTION AND DESORPTION OF HBr ON WOOL 0.5 g dry wool -1- 255 ml bath (A/,& 510) absorption A desorption B absorption C - sample sample sample rcfercncc -~ ~ _ _ _ _ _ _ _ 1 2 range of c mmole/ml 0.0147 --> 0.0136 range of s mmole/g wool 0.0 -+ 0.616 ci 13.5 12.5 - 8n 0.0546 0.0492 81 0.0516 0.0480 R 36-7 39-9 (min-1) 2.02 2.01 (D/a2)r (min-1) 1.95 1.96 R (average) 38.3 h 14.9 A0 0.3 18 a 322 P’n 0.0592 0.0521 F t 0.0572 0.0505 @’n 0.88 0.78 @’1 0.85 0.75 c - 3s/3c 21.2 (D/a’)n (min-1) 1.26 1 * 1 1 (D/a2)r (min-1) 1.21 1-07 1 2 0.0 -> 0.00083 0,615 -+ 0.182 2.7 2-1 0.0230 0.0242 00199 0.0238 187.3 237.3 4.32 5.76 3.74 5.66 212.3 169.4 0.009 3.0 0.0229 0.0204 0.020 1 0.0 193 3.88 3.46 3.41 3.27 14.6 0.33 0.30 0.29 0.28 1 2 040098 -> 0.00075 0.0 -+ 0.135 3.3 3.1 0.0123 0.01 13 0.0128 00119 152.5 161.7 1.89 1.84 1 -97 1-94 157.1 89-8 0.058 5.6 0.0140 0.0131 0.0142 0.0135 1-26 1-18 1 -28 1-21 160.9 2-25 2.1 1 2.29 2-17 (32) and (32‘) fig.1 and 2 (1 5 ) (1 3) (1 3) “ least square ” (1 5’) fig. 1 and 2 ‘‘ least square ” (1 3’) (13’)42 DIFFUSION OF SORBED SUBSTANCES and C give the same L)/cr?- value but B a D/& value about 3 times as large. This resuIt is probably a result of the failure of the theory of case I. We can also compare our R values from (15) with values of s/c from acid/wool titration curves, giving s in units of mmole/g dry wool as a function of pH. Thus s is determined using the same assumptions as for R from (15), (1 g dry wool 21 1 cm3 swollen wool), e.g.all the fibre-water phase is assumed to be accessible for diffusing ions to the same extent. This is not in agreement with the “ Donnan theory concentration ”, where only the swelling water is taken as a solvent medium for intcrnal ions,g but it is a rather practical assumption. c = aH/fH is calculated from pH = - l o g l ~ a ~ and fH values tabulated. The s =: f ( c ) curve from Steinhardt-Harris HCl + wool titrations at 25” ClO is drawn in fig. 3. On the same graph we draw the s = Rc curves from R values (table 1). We now com- pare the position of these lines with the corresponding regions of variation for c and s on the experimental isotherm (fig. 3 and table 1). In case C, the R line corresponds rather well to the isotherm if the region of c as well as the region of s is considered.In case A, R corresponds well to the small c region and also gives an acceptable average for the s region, although this region is very large and thus the curvature large and the FIG. 3.-Relations between experimental s = f ( c ) curve and approximations used for HBr + wool sorption curves A, B and C . line a bad approximation. In case B, R corresponds well to the small c region, but is wholly outside the large s region. From these considerations and the good time con- stancy of /3 we might conclude that the appropriate R value in case I shouId be taken from the linear approximation of the isotherm between the experimental limits of bath concenfraf ion. We now apply the theory of case 11. Here the relation (2”’) is valid only for the fibre surface and hence this necessitates a linear substitution for the isotherm in the small bath concentration region.We have used the method of least squares for the corresponding parts of the isotherm (fig. 3) to get the A and A0 values. a is now obtained from (15’), where A/rra2 is calculated as before. For absorption, /3’ values are calculated as before from fig. 1, using (27), (12’) and (24’). For desorption, (M/&bf~),~ = o3 to be used in the graphs is calculated from (28’), where KO = Co/Ao according to (18’), (CO = 0). As seen from fig. 2 and table 1 the j3’ values are not very different from the /3 values for case I. Of course the method here given to calculate A does not fit the theoretical curvc at t = co . This is really reflected in an increasing t h e dependence of /3’ at large t values.Of course we might also fit the A-value to the experimental t = co conditions, e.g. put A = R for case I, or make a compromise between this condition and the “ least square ” condition. We find that although this region for the A values is rather large (R -> A in table l), theBERTII- OLOFSSON 43 corresponding region of p’ values is small @ -+ in table 1). However, the real diffusion constant in case I1 is found from (13’) : D/a2 = /3’ 3s/3c. A constant value of #I‘ means a constant value of 3c/3s, e.g. we also here make a linear substitution. But the important thing is that 3c/3s is not necessarily related to A. 3c/3s should be some approximation for the region of concentration within the fibre. If we put 3c/3s = 1/A we get the same kind of differences between D/a2 in cases A, B and C as if dc/bs = 1/R, or = 1/(R + l), case I.Now we make the assumption that 3c/3s is governed by the region of change for s in the fibre. We thus calculate 3c/3s from the isotherm (fig. 3) by the least square method for the s regions in the three cases; we have roughly (table 1): A region = B region + C region. Table 1 gives the D/& values obtained from (13’). We also here find large differences in the three cases, but the im- portant fact is that D/a2 for desorption (B) is much smaller than for absorption (A and C), contrary to the previous results. Thus it is probably corrcct to attribute differences in D/a2 to the choice of R or We obtain the following conclusion. Theory 1 is generally applicable for experiments where the change in bath conccntration is not very large.When comparing diffusion constants theory I1 should be used. For absorption, how- ever, thcory I is also now applicable as the parameters h and 3c/3s in I1 may both be substituted by the same value for R in I. For desorption theory, I1 must be used, i.e. h is calculated just as R for I, but 3c/3s is taken as another average for the fibre concentration range. For our case B, we assume D/a2 = 1.95 (= the absorption value), thus 3c/3s = /3’/1*95 = 0-0227/1-95 (from table 1) = 1185.8. The value 85.8 is smaller than the R and A value in B but larger than the 3s/3c value (table 1). However, for assessing thesc conclusions calculations according to case I11 must be made. The relation (29) to be used is (3 3) deduced from the Gilbert-Ridcal theory.9 Here R’ is taken from the equilibrium condition (t = m) for the experiment considered and s, is put equal to 0.86.9 The result of these calculations will be given later. WASHING PROCEDURES.-TheSe considerations have found an interesting and useful application for analysis of washing processes.11 We have made experi- mental investigations of the kinetics of winch-machine washing of sulphuric acid from wool fabrics, using baths of pure water as well as of neutralizing agents. For water-washing with changes of water the desorption theory of case I is used. If there is a continuous in- and outflow of water during the experiment we have used the idea of an “ expanding bath volume ” for the theoretical interpretation. When neutralizing agent is present in an amount equivalent to the total amount of acid, wc have considcred A / r G in (15) to be infinite, e.g.a = 00 or p = 0. Howevcr, in such cases thcre is generally a “ sorption level ” left in the material, its “ height ” varying with the character and amount of neutralizing agent. If v. and p is determined from this level, then p > 0 gives a very good timc constancy of /3 (from fig. 1). The explanation of this “ fast level ” might be that on the “ small sorption part ” of the sorption isotherm (fig. 3) R is increasing towards infinity at the same rate as is A/ra2; a thus is finitc. We may consider the points (i) and (ii) of the introduction. Probably condition (ii) is satisfied in this washing procedure but not condition (i).The failure of condition (ii) should be reflected in bad time constancy of p (Larsson, Tenfalt, unpublished) which is not observed here. The failure of condition (i) means, firstly, a decrease of the diffusion rate,l2 which is observed here, being much smaller than for free fibres. With neutralizing agents, however, the contact between liquid and single fibres seems very satisfying, being of the magnitude of for free fibres. Now from thc thcory we haw deduced relations between bath ratio, tempera- ture of bath, time of washing and number of washings. Also the cost of washing may be considered as a function of thesc factors and thus it is possible to determine a combination of thcse factors that gives minimum costs. values. s/(S, - S) = R‘c44 DIFFUSION O F SORBED SUBSTANCES DIFFUSION OF DYESTUFFS.-A micro method for studies of dye absorption has becn developed.13 Singlc fibres are dyed under fixed conditions in a spccial apparatus ; after fixed intervals of time, these are immediately frozcn with carbon dioxidc and dried in Ihe frozen statc.Cross-sections are cut with a microtomc, microphotographs taken and the distribution of dye recorded with a photonictcr. Thus we obtain values of the total concentration C(r, t), or rathcr C(r, t)/C(a, 0), where C(a, 0) is the concentration at thc fibre surface (not outside thc fibre) initially. We apply the theory of case 11 and from (3’) obtain 3H/3r is calculated from (7’) and (11’) and after simplifications we get If the fibre is initially free from dye, we get KO = 0, (18’), and if we assume an in- finite bath, we get a = co, (15‘), the resulting equation being Now experiments with viscose model fibres and direct dyestuffs give two character- istic results : (i) the absorption of dye in the external layer is very rapid but the further diffusion inwards rather slow ; (ii) the dye moves inwards with a very sharp boundary.(i) may be explained by the large “ free concentration ” at the fibre surface. (ii) is possibly interpreted as a decrease of Ddc/?s when passing the boundary inwards. This means a decrease of 3c/3s, e.g. there is a decreasing relative amount of dyestuff free to diffuse, and this wiIl sharpen the boundary, increasing the speed of the back and decreasing the speed of the front dye particles.However, the experimental method must be refined for further investigations. DIFFUSION OF MOISTURE.-A~ apparatus for determining the kinetics of vapour sorption from the change in capacity of a cell with loosely-packed fibres has been constructed and absorption and desorption curves for wool at different humidities for a given velocity of air current studied (unpublished work). The theories have been applied (a = co). It is found that /3 changes significantly during one experi- ment and is also a function of the humidity limits used and of the direction of sorption (absorption or desorption). It is evident that the theory must be de- veloped in several respects. Thus the validity of the linear approximation of s ~7 f(c) should bc investigated. It is also most important to considcr the swelling effects and their hysteresis. The influence of the temperature gradient from sorption heat must also be examined. An important critical study by Crank and Park 14 discusses other facts about such sorption proccsscs. I thank Dr. J. Lindberg very much for putting his expcrimental results at my disposal and furthermore for stimulating work and discussions on the kinetics of “ textile wet processing ”. I am very grateful to Prof. N. GralCn for his active and encouraging interest in these problems. Technical assistance and help with calculations has been given by several persons at the Institute to whom I express my gratitude. 1 Wilson, Phil. Mag., 1948, 39, 48. 2 Standing, Wanvicker and Willis, J. Text. Inst., 1947, 38, T335. 3 Vickerstaff, The Physical Chemistry qf Dyeing (Oliver and Boyd, London), p. 123. 4 Crank, Phil. Mag., 1948, 39, 140. 5 Watson, Theory of Bessel Functions (University Press, Cambridge), p. 666,BERT I L 0 L 0 F S SO N 45 6 Crank, Phil. Mag., 1948, 39, 362. 7 Lindberg, Text. Res. J. (to be published). 8 Lindberg, Text. Res. J., 1950, 20, 381. 9 Olofsson, J. SOC. Dyers Col., 1952, 68, 506. 10 Steinhardt and Harris, J. Res. Nat. Bur. Stand., 1940, 24, 352. 11 Olofsson, Medd. Sv. Textilforskirgsinst., 1953, nr 34. 12 Olerup and Lindberg, J. Sac. Dyers Col., 1950, 66, 148. 13 Olofsson, Mecld. Sv. Textilforskningsinst., 1953, nr. 29. 14 Crank and Park, Trans. Faraday SOL, 1951,47, 1072.

ISSN:0366-9033    

DOI:10.1039/DF9541600034

出版商:RSC    

年代:1954

数据来源: RSC

摘要:

BERT I L 0 L 0 F S SO N 45 STUDY OF DIFFUSION PROCESSES IN TANNING BY D. M. G. ARMSTRONG* Thc British Leather Manufacturers’ Reseal ch Association, Egham, Surrey Received Gth July, 1953 Whenever two phases are in contact, it is possible, from the apparent mass of one phase immersed in the other, to estimate the amount of matter in that phase and to follow any transport of matter from one phase to the other. The application of a pyknometric method to skin in water or tan solution is discussed. Two concepts are introduced: the specific apparent mass of a protein (shown to be equal to its density increment) and the apparent specific volume of fixed tan. In the diffusion of tannin into skin, part is fixed by the protein and thus there is attrition of the diffusate, and very often sharp diffusing fronts are formed.The theory for this type of diffusion process has been given by Hill and Hermans and it is now applied to tanning; it is found that the apparent mass of skin in tan solution of constant con- centration after correction for small changes in density of the solution, varies with the square root of time. This is verified by experiments with tannin from mimosa (Acacia molfissima) bark and tannic acid. Thc tanning process is shown to be much simpler with Mimosa tannin. The amount of tannic acid which can enter bovine skin reaches a limiting value as the concentration of the tan solution is increased, and it is thus possible that, as concentration increases, the amount of tan fixed decreases. The method used at present in the tannery to estimate the rate of tanning is to cut sections through the leather, since there is generally a sharp boundary between the tanned and untanned part.Mezey 1 followed the movement of this boundary by taking such sections, staining them with dichromate and measuring the distance travelled using a microscope. Stather 2 used this method with several tanning materials and established that the depth of penetration was proportional to the square root of time and also to the square root of the concentration of the tannin. He further found 3 that the quantity of tan fixed by small strips of skin was approximately proportional to the square root of the time that these strips had been in tan solution. Pyknometric methods of analysis, i.e. methods depending on measurements of the density of the material analyzed, have been proposed in particular by Russell 4 for the estimation of sulphate.As early as 1905, Parker and Russell 5 proposed such methods for the estimation of the dry weight of skin and give values of the density of skin and conversion factors for obtaining the weight of dry skin from the apparent mass of the latter in water. Brown and Holmes6 followed the rate of tanning as an undctermined function of the “ surface dry mass ” of the skin being tanned. * present address : The Royal Veterinary College, London, N.W.l.46 DIFFUSION PROCESSES If a piece of skin with water filling the interfibrillary space is immersed in a solution of a tannin, its initial apparent mass in that solution depends on the dcnsity of the latter ; as tannin diffuses into the skin it displaces water and the apparent mass increases. The amount of tannin entering the skin should thus be a function of the apparent mass of the piece and of the density of the solution.The advantage of such a method is that continuous observations can be carried out with a minimum of interference with the system studied (cp. Clack 7 and Wall, Grieger and Childers 8). As tannin enters the skin it combines with the protein, and this must slow down the diffusion. The theory of this type of diffusion process, which may be called diffusion with attrition of the diflusate, has been developed by Hill9 and more recently by Hermans.10 It predicts the formation of a sharp diffusing front moving over a distance proportional to the square root of time and of concentration in agreement with the findings of Stathcr.29 3 In this papcr we invcstigate thc use of pyknometric methods in the study of tanning process and dcduce relations, some of which are similar in form to those given by Adair and Adair 11 in connection with the density increment of proteins.Data obtained by these methods arc then used to show the validity of the Hill- Hermans 99 10 theory for the tanning process. EXPERIMENTAL The tanning materials used were a commercial mimosa extract (from the bark of Acacia rnollisima) in powder form containing 65 mg equiv. of salt as dcterniined using the u '//1 I// //I I/- i. I f 1 (Q) . - / / 1 B ,,, ,,. ' / FIG. 1 .-Experimental details. acid form of the ion exchange resin Zeo Karb 226, and 17 mg equiv.of acid per 100 g of dry extract, and B.D.H. tannic acid containing 120 mg equiv. of acid and 1-5 mg equiv. of salt per 100 g of dry acid. Solutions in distilled water of the required concentration were used. The skin was obtained from an ox and had been treated with cal- cium hydroxide and sodium sulphide to unhair it. It was neutralized by 0.1 M acetate buffer of pH 5, di- alyzed in distilled water and finally dehydrated with acetone which was then evaporated. Rectangular picas of various size up to 5 x 10 cm were cut from the dorsal region, where the thickness was ap- proximately 0-7 cm. Fine holes were drilled at two adjacent corners of the pieces; through these were threaded loops of fine Nylon thread or bent handles of Nichrome wire as shown in fin.1. The pieces were degreased using petroleum ether. After evaporating the latter, they were placed in a beaker of distilled water in a vacuum desiccator which was evacu- ated at the water pump for 6 h, left under a partial vacuum overnight and a further evacuation carried out the next day. In this way all the air could be removed from the pieces as shown by measurement of their apparent mass. The mass of fully soaked pieces, both tanned and untanned, was detcrmined after lightly pressing between' filter paper, yielding " the surface dried mass ". The pieces were immersed in tan solution or water in 1 I. beakers placed in a thermostat at 20 k 0.02" C. In general half an hour was required to attain the temperaturc equilib- rium. Between measurements the pieces were allowed to rest sideways at the bottomD. M.G. ARMSTRONG 47 of the beakers ; in later experiments they were kept isolated from thc sides of the beakers as shown in fig. 1 (b). For measurement of apparent masses, an analytical balance was placed on a table spanning the thermostat tank. The left-hand pan arrestor was removed and a detachable suspension fitted to the bottom of the pan as shown in fig. I@). The loop of Nylon was placcd on a hook at the bottom of the suspension ; for pieces fitted with a Nichrome handle, the latter was kept immersed and passed over a glass hook (the volume of which had been determined) also immersed in the solution and connected to the suspension by fine platinum wire. The density of the solution was calculated from the apparent mass of a glass sinker (calibrated in distilled water at 20" C ) in place of the piece.Ap- parent masses were estimated to thc nearest 0-1 mg but in their measurement, the damping was such that the sensitivity of the balance was reduced from 5 to 1 or 2 divisions per mg ; thus it is unlikely that readings more accurate than 11; 0.15 mg could be obtained. When dealing with dense tan solutions C. > 1.1 g/ml) the pieces were loaded to prevent them from floating, using a small calibrated glass sinker filled with mercury (fig. l(b)). To find the dry mass of skin or leather pieces, these were placed in an air oven at 104 f 1" C for 24 h then weighed in situ by hooking the piece by its handle to a suspension passing through the top of the oven. There was only a further very small percentage loss in mass after 24 h.Before any piece was placed in the oven it was left to dry (on a sheet of polyethylene) at room temperature, protected from the dust. To determine the concentration of soluble tan in the solution a portion was centrifuged for 30 min at 3500 rev/min and the clear solution decanted; portions were transferred to flat weighing bottles, weighed and evaporated to dryness on a water bath. The residue (never > 9 g), after being hcated in the oven at 104" C, was weighed and the density of the remaining centrifugcd solution determined. To investigate the increase in tan fixation with increase in temperature (the "hot pitting " process) the beakers containing pieces of skin in tan solution were sealed with Cellophane painted with molten paraffin wax and then placed in an oven at 37" C .The procedure in a diffusion experiment was then as follows: a piece of skin was weighed in water (then, in some cases, swollen in acid solution) and its apparent mass therein and the density thereof being measured. Its surface dry mass was determined and the piece quickly transferred to the tan solution of known density and concentration. The apparent mass of the piece was then determined at intervals together with the density of the solution. At the end of the experiment, or on transfer to another solution, the piece was surface-dried, weighed and the concentration of the solution determined. The values of specific volume obtained were corrected for air buoyancy effect, using the approximate formula : where v is the corrected specific volume, v* the uncorrected value, pa the density of air and p2 the density of the solution in which the determination of apparent mass was made.The correction is within experimental error. v == V* -1- v*Pa/Pz - paw*', RESULTS AND DISCUSSION We define (i) fixed tan as any tannin material combined with the protein, i.e. no longer in solution; (ii) soluble tan all matter, non-volatile at 104" C, in solution irrespective of whether it includes substances which are not tannin ; (iii) 20" C as standard temperature. 1. THE APPARENT SPECIFIC VOLUME AND DENSITY INCREMENT OF SOLUBLE TAN.- The apparent specific volume urn (as defined by Kraemer 12) can be obtained from the density p2 of the solution and the concentration cST of the tan : where po is the density of water.The unit is ml/g if densities are expressed in g/ml and csr is in g of tan per ml of solution. The density increment kk is defined 11 by the equation p2 = PO + k&,, (1 62) . k . . ir 1 - pov;; (1.3)48 DIFFUSION PROCESSES if v ~ r does not vary with the concentration then k$ is constant and p2 varies I inearly with concentration. In general, vsr is nearly constant for high-molecular weight non-electrolytes. With freshly prepared solutions of tannic acid, VST is constant over a wide range of concentrations as shown in fig. 2, the value being 0.593 ml/g. With mimosa, we have not investigated the change of VST with concentration but have obtained with cST -- 0.118 the value 0.601 3: 0.009 ml/g (standard deviation for 5 determinations).I I c s 7- 0.1 0.2 0.3 0.4 0.5 0-6 FIG. 2.-Values of a , and am from varying concentrations of tannic acid solution. A values of vST for freshly prepared solutions ; 0 values of a, for aged solutions (40 to 60 days) and after use in tannage ; 0 values of am. 2. THE SPECIFIC APPARENT MASS OF SKIN PROTEIN.-when immersed in Water a mass mp of skin protein has an apparent mass m;,o and we define the apparent volume Vp of the protein by the equation, m;, 0 = mP - VP PO? :. m;, o/mp = 1 - vPpo = ki, since up, the apparent specific volume of skin protein, is the ratio of the apparent volume to the mass of the protein. Thus the ratio of the apparent mass of the protein to its mass is equal to its density increment. Since we are dealing with a fibrous protein in such a state that changes in concentration are limited, we call k; the specific apparent mass of the skin protein.From the value of k; and measurement of the apparent mass of a quantity of skin (or any fibrous protein) in water, we can calculate its mass. By weighing pieces of skin after drying at 104" C for 24 h, then in distilled water at 20" C, we obtained kg = 0.3125 f 0.0006 (standard deviation for 6 determinations). Some of these determinations were done by weighing the pieces in water first, then dehydrating, using acetone, drying at 104" C and weighing. In subsequent experiments, the mass of skin protein was obtained from its apparent mass. The value of up corresponding to this value of kg is 0.6890 -+ 04006 ml/g This value is lower than that (0.71 mI/g) calculated for collagen from the specific volume of the amino acid residues by the method of Cohn and Edsall13 using the composition of collagen given by Bowes and Kenten,l4 and is also lower than that of most soluble proteins which varies between 0.70 and 0.75 (Svedberg 15).This is not unexpected in view of the high percentage of amino acids residue with low specific volume present in skin collagen.D. M. G . ARMSTRONG 49 The variation of vp with temperature is not known. However, it is probable that the coefficient of volume expansion is about the same as that of muscle protein (7.4 x 10-4 per deg.) as determined by Wilkie 16 from the apparent mass of frog muscle in Ringer's solution in the temperature range 0-20" C. in contact with an aqueous tan solution, tan is fixed by the protein.We assume that in this fixation, the apparent specific volume of the skin protein remains constant. We do not assume that the apparent specific volume of the tan vpr when fixed and when in solution to be the same and vp varies, since in tanning the amount of tan fixed varies, whereas that of skin remains constant. To determine urn, the mass mrn of tan fixed on a mass mp of skin protein is obtained. We assume that the molality Fin of the tan in solution, in g tan/g water, is the same inside the piece as in the solution outside it ; in other words we con- sider the piece as an aggregate of tanned fibres immersed in a homogeneous tan solution. The surface dry mass of the piece, rnl, is then obtained, and the piece dried and weighed to obtain mDL, the mass of dry leather.3. THE APPARENT SPECIFIC VOLUME OF FIXED TAN.-when skin protein is placed The mass of tan in solution inside the piece is given by and The apparent mass of the piece m;,2 in the solution of density p2 can be con- sidered as equal to the sum of the apparent masses of the skin protein m;, 2 and that of the fixed tan rnA.2. but We now define k&, the specific apparent mass of fixed tan, as k; = 1 - povPT. (3.7) Values of vm for mimosa and tannic acid were obtained from measurements on pieces which had attained equilibrium (or quasiequilibrium) with the tan solution. From pieces tanned in mimosa solution in the concentration range 0.1 1 to 0.1 3 g/g solution of density 1-05 g/ml, of pH 4-1, and containing 50 mg equiv.salt/l. we obtained as value for urn, 0.630 f 0.003 ml/g (7) * ; tanning at pH 3-22 with the same salt concentration, gives a value 0.6270 whilst tanning at pH 3.2 with the salt concentration at 220 mg equiv.11. with KC1, @2 = 1.061), a value 0.6325 was obtained. From pieces in a solution of higher concentration 0.208 g/g solution and density 1.08324, with tannage carried out at pH 3-2 both in the presence and absence of large amounts of undissociated acetic acid (930 mg equiv/l. of solution) we obtained the value 0-6273 f 0.0004 (6) inl/g. With tannic acid, the values ob- tained varied with concentration as shown in fig. 2. Large bulks of concentrated tan solution are difficult to dry and hence pieces tanned in such solution will con- tain large amounts of tan in solution and it is very probable they cannot be dried in 24 h.Thus, to obtain a more accurate value of urn for tannic acid, pieces of skin 5 x 7.5 cm were tanned by the method of Gustavson and Nestvold 17 by im- mersion in concentrated (0.5-0.2 g/ml) acetone solution of tannic acid and each piece then placed in 11. of distilled water, which was changed after 2 days, and * the number in parenthesis refers to the number of determinations.50 D I F F U S I 0 N P R 0 C E S S E S then kept in that same water under toluene for one month, after which the same measurements were carried out to yield a value of vPY for tannic acid of 0-6170 5 0.0013 (8) ml/g. In this way uncertainties of drying were much reduced since there was a minimum amount of soluble tans in the piece.Since vST is smaller than VPY, there should therefore be expansion in the overall volume of the system with tanning. us suppose that we have a piece of skin of surface dried inass ml, and overall volume V1 (obtained from ml, the apparent mass r n ; , ~ , and p2) containing mp of skin protcin, mm of fixed tan, msT of soluble tan, mo of water. Since 4. VARIATION OF APPARENT MASS WITH ENTKY OF TANNING MATERIAL.-kt mo = WI - ~ P v P + mmVm + mST%T)IpO, mi, 2 = k&p f k b m + k h m - (p2 - PO) VI, (4.1) (4.2) SO that for a finite change at constant rnp k&Am, -1- k&Ani, = Am;,, -t 032 -- p0)AVl -I- V1Ap2; (4.4) this expression neglects a term in Ap2AV1, which is lcgitimate since our experi- ments are carried out at constant concentration and thus p2 is nearly constant and changes in V1 during tanning are small.We now define Arnf such that Am? = Am;, 2 + (p2 - PO)AVI -t- W p 2 . (4.5) Am? is thus the change in apparent mass of the piece corrected for changes in volume and density : it is solely a function of the change in tan content of skin. If we start the experiment with no tail in the piece thcn we can write k&nm f kkm, = Am?. (4.6) In most experiments it was found that V1 remained constant; to obtain the curves shown in fig. 4(a) and 5(a), V1 was 12.97 ml originally, 13.22 ml after 1 day, 13.28 ml at the end of the experiment, differenccs which are not significant since the value of V1 depends on how hard the piece is pressed between filter paper before weighing and therefore to obtain values of Am?, AV1 can generaIly be taken as zero.With pieces fitted with Nichrome handle and sinker, the value of V1 used is the combined volume of the piece and all the attachments including the volume of the hook at the end of the suspension. 5. LEATHER ANALYsIS.-The composition of pieces of leather, when quasi- equilibrium conditions have been attained in the tan solution, can be obtained from their apparent mass in that solution, the mass of skin protein having been deduced from the apparent mass of the skin in water. The mass of fixed tan is then to obtain the mass rnsr of soluble tan inside the piece, the weight concentration as, and the surface dried mass of wet leather must be obtained and nkT = Cm1 - h P + ~ m ~ l a s r . (5.2) Q~ is the number of g of soluble tan in 1 g solution and is related to cbT and (5.3) msr as follows : It is assumed in (5.1) and (5.2) that the interfibrillary solution has the same composition as the external solution.- - aST = cST/J)2 GST/(1 -k mST)-D . M. G. ARMSTRONG 51 Values of changes in mpr from one quasi-equilibrium state to another can be obtained by differentiation of (5.1) similarly for changes in nzsT we use which combined with (1.2) followed by differentiation at constant mp yields niST E= (V1 - vmmm - vPmP)cST, k&AmsT == (mm/csT)Ap2 - (p2 - ~0)vrnAmm (5 5 ) (5.6) 6. NON-SOLVENT WATER.II-lt is possible that in tannage in sohtions of high concentration some of the watcr inside the leather does not contain any dissolved tan (called bound water by Cheshire and Holmes 18).If mNs is the mass of this water associated with a given sample mm = ~ I D L - mp - (mi - mDL - mNs)Gsr (6.1) where vo is the specific volume of water. Using these two equations, the two unknowns mm and mNS can be estimated. The accuracy of this estimate requires accurate values of Zn and vm. To allow for variation of vm from sample to sample, the value for the piece used would have to be determined. The best procedure is probably to obtain rnl for the tanned piece then to transfer it to water and allow most of the soluble tan to diffuse out, the amount of the latter being estimated from the value of Am? for that process using (4.9, Amp, being zero and k& being estimated from ZsT and p2. Since tan appears to diffuse continuously from a piece immersed in water, there must at all times be a small amount of tan in solution inside the piece and an allowance should bc made for this, using a suitable application of the diffusion theory.We have not carried out such experiments but have used eqn. (6.2) to estimate the value of mNs for skin protein in solutions of potassium chIoride varying in concentration from 0.5 to 2-5 N. mm is then zero and the amount of non-solvent water varied between 0.33 and 0.39 g per g of dry skin protein which is of the same order as the values found by Eilers and Labout 19 from the change in con- centration in NaCl solutions in contact with skin. (The method of determining mNs from the apparent mass has been used by Neale and Williamson20 with cellulose.) applies to the diffusion into a semi-infinite skin phase bounded by a plane surface in contact with tan solution, with the following boundary conditions : (i) at x = 0, cST = cE at all times, x being the distance measured from the plane surface and normal to it, and cE being the constant external concentration of the tan solution in g/ml solution; (ii) at time t = 0, there are sites in the skin phase where tannin can become fixed to the extent of cm g/ml of the skin; (iii) at t = CO, cST = cE everywhere and all sites are occupied ; (iv) at t = t, x = f this being the plane where cST = 0, i.e.the boundary be- tween the region where all sites are occupied and that where none are. At this boundary the rate of forward diffusion of the tan equals the rate at which the sites are occupied; thus tan cannot diffuse until all sites are occupied D being the diffusion coefficient.7. APPLICATION OF THE DIFFUSION THEORY.-The Hill-Hermans 99 '0 theory -- D(dcs,/d,r)t -- c*df/dt, (7.1)52 D I F F US I 0 N PI< 0 C E S S E S If the boundary moves slowly, then a steady state is cstablished behind it and (7.2) (7.3) dcsT/df = O = DdZCsT/dXz ; CST = cE(1 - x/C$), so that (7.1) can be integrated yielding the amount of tan fixed per unit area of surface u p to time f is f = (2DcE/c,,)’t*; n?P.r/A = c n f , :. mpr = A(2Dc,c,)hi, A being the area of the plane. The amount of tan in solution inside the piece per unit area is (7.4) .’. mST Z= [cE/(2(‘FT)ImFT; (7.8) combining (7.6), (7.8) and (4.6) we finally get (7.9) i.e. Amy is proportional to the square root of time at constant tan concentration. FIG.3.-Diagrams of tanning as a dif- fusion process. These show cross- sections through the skin normal to its surface at different times. The axis OX represents distances measured normally from the surface, whilst OC is used as 0 s x o x o concentration axis, for cm above OX and for cm below. 5 is the distance of the tanned/untanned boundary from the grain c. 7 C, I CW layer and 5’ that from the flesh layer (in practice is generally greater than t’). AmT = A[k& f /~~cE/(2c,)](2Dc,cn)BtP, :bi :M :L (b) shows the end of stage I ; (c) shows the end of stage 11. la ( 6 I ( C ) With skin, thc phase is separated by two parallel planes, at the flesh and grain side respectively. Thus there will be two boundaries moving toward each other until they meet.We may call the period up to that time, which is the period when most of the tan is fixed, the first stage of tanning. At the end of this stage, the concentration of tan in solution in the piece falls linearly from cE on the outside to zero at the meeting plane of the boundaries. Thereafter there is a diffusion of tan until the concentration inside is uniform and equal to cE; this period may be called the second stage of tanning. Subsequently (and perhaps concurrently) there may be a further deposition of tan, which we call the third stage of tanning. Fig. 3 depicts the process. In tlic derivation of (7.9) we have neglected the fact that the whole volume of the protein phase is not available to the diffusion process since a large part of that volume is occupied by skin protein and fixed tan.The volume available to diffusion VD will be VD = Vl - (VP + V*), (7.10) Vl being the volume of the piece of skin, V, and VFT the volumes of protcin and fixed tan respectively. From the definition of en, Crr -:- (mFT),lVLb (7.11) the value of mm being that at the end of stage I : substituting in (7.8) we obtain the value of msr at thc end of stage I , (nrsT) I - - c,v,/2. (7.12)D . M. G. ARMSTRONG 53 To estimatc the magnitude of D we shall make the assumption that the area available to diffusion AD is given by AD 7-z ( vD/ v1)A. (7.13) In our experiments we have used pieces of skin in the form of a rectangular parallelepiped and there will be an effect due to diffusion from the smaller sides of the piece; thus if the latter has dimensions 2, h and 6, b being the thickness, then the initial area available to diffusion will be (7.14) (7.15) Fig.4 shows the curve obtained by plotting values of Am? per g of skin protein (a) an ordinary piece of skin 3.5 x 6.5 cm and protein concentration 0.377 g/ml; (b) for a piece originally of the same size as the other but heat shrunk by heat- ing in distilled water to 67" C then cooled to 20" C (this treatment resulted in a 7 % decrease in overall volume). The tan solution was mimosa of concentration 2(lh + hb + lb)JG/v~, whcreas the final area when the six boundaries meet is approximately 2(1 - b)(h - b) vD/ v1. against days in tan solution for 0.4 FIG. 4.Variation of the apparent mass of tan per gram of skin protein with time.(a) 0 for a normal piece of skin in a mimosa tannin solution of concentration cST = 0.12 g/ml and pH 3.8. (6) A for a heat-shrunk piece of skin in the same solution. 0.1 17 g/ml, pH 3-8, and density 1.046 g/ml, the density remaining fairly constant during the experiment. The curves show that with heat-denatured skin the rate of uptake of tan is much slower. When Amf/m, is plotted against the square root of time (fig. 5) a siiaight line is obtained over a long period of time in agreement with (7.9) for both pieces. The graph for piece (a) shows three points of inflection; one during the first day, the others near the tenth and twenty-fifth days (points A and B on the graph). The first may be due to a dccreasein the area available to diffusion owing to diffusion through the smaller sides of the piece, the next marking the end of the first stage of tanning and the last the end of the second stage.To test this, the apparent mass k&,msr, of tan in solution in the piece at point R was calculated using (5.4) and (5.6) assuming quasi-equilibrium between B and the end of the experiment at F; this is justified by the form of the graph. Since at the end of stage I the amount of tan in solution is half that at the end of54 DIFFUSION PROCESSES stage 11, subtracting half the value of kkm, at B gives the value of Am; at the end of stage I. In this way point A was obtained, marking the end of stage I at a position very near the inflection point (the value of AnzT/m, at the end of stage I is probably higher since we have assumed that all dissolved matter in solu- tion is fixable tannin which is unlikely). Thus it would appear that stage I took 9 days and stage I1 (A to B) 15 to 16 days.The graph for piece (6) shows that the end of stage I had not been reached by the time the experiment was terminated which was verified by cutting a scction normal to the grain of the piece, when it was clear that only about gth of the piece had been tanned; it is well known that the hcat shrinking of skin powder leads to higher tannin fixation and thus from (7.4), if cm increases, the value of for a given length of time t decreases. FIG. S(6) shows a slight inflection which may be due to a secondary proccss of tan deposition, which is located in 5(a) between B and F (corresponding to a third stage of tanning). B (0) F o.’-ooo- o - - o o o ~ -0 F I ~ .5.-Graph of Amf/mp against the square root of time for the same pieces as in fig. 3. The point A marks the estimated end of the 1st stage of tanning; point B the cnd of the 2nd stage. A further example of the uses of the pyknometric method is shown in fig. 6. For these experiments 6 pieces of skin of dimensions 10 x 5 cm were used, They were swollen over three days in a solution of acetic acid, 0.2N in NaCl and of pH 3.2 prior to tanning to eliminate possible complications due to swelling during tanning. The final average skin protein concentration was 0.33 g l d , that before swelling being 0.37 g/ml. Two mimosa solutions were made up to contain the same concentration of extract (ca. 240 g/l.) and the acid and salt concentrations adjusted by addition of HCl and NaCl for solution I and acetic acid and NaOH for solution 11, the follow- ing analysis being obtained after the end of the experiment.P2 -. salt .- . - acid __ solid content mg equiv./l. gll. PH solution I 3.2 150 60 220 1.083 solution I1 3.2 135 930 240 1 a094 SoIution I1 was far more stable than solution I, since far lcss deposit occurs in it. After an initial drop of approximately 0.003, the density of the solutions (and hence, probably the concentration) remained fairly constant. One piece, no. 1, was placed in solution I for 44 days, then transferred t oD . M. G . ARMSTRONG 55 solution I1 in which it remained for 54 days. The solution and piece were then heated for 7 days at 37" C, cooled to 20" C to allow measurements to be carried out, then heated again for seven days.After cooling again to 20" C it was left in solution I1 for a further 5 days, then transferred back to solution I. Piece no. 2 was kept in solution I throughout the whole experiment, being heated in the same way and at the same times as piece no. 1. Piece no. 3 was kept in solution I1 and heated in the same way and at the same times as pieces no. 1 and 2. . It was transferred to solution I at the same time as piece no. 1 was trans- ferred back from solution 11 to solution I. Each of these experiments was done in duplicate. -Tk< end- of stags -K-fm--pkm-lm. .1- was-ta'mi-tas-ccmxi higatpoint -33 @I days). From this the value of Arnf/rnp at the end of stage I was calculated (point A). f . ..5 8io 20 30 $070 /OO f30 Day5 FIG.6.-Plot of Ami/mp against square root of time (in h) : (1) 0 for piece of skin no. 1 in mimosa solution I up to point C and from E to F, in solution I1 from C to E. (2) A for skin no. 2 in solution I throughout. (3) V for skin no. 3 in solution I1 up to point E. Values of Am1 Imp for pieces 2 and 3 are the ordinate values minus 0.125 and 0.25 units respectively. C is the point where transfer to solution I1 was carried out. Using eqn. (5.1), (5.4) and (4.4) it is possible to calculate the value C', which Arnf/mp should have, assuming that there is a complete diffusion process occurring to make the inter- fibrillary solution the same in all respects as solution 11; as the figure shows this value is not attained although Am? increases then decreases again, which implies that some of the tan previously fixed is being dissolved.Between C and D the calculated loss in fixed tan is 2.8 g / 1 0 g of skin protein (2.6 % for thc duplicate). The change in An$ on hot pitting is shown by the curve between D and E, E being 5 days after removal of solution and piece from the oven; from the increase in n4,2 using (5.1) and (5.4), the increase in tan fixed could be estimated. Table 1 gives the calculated values of the amount in g/l@O g of skin protein of fixed tan at various points in the process for the six pieces; the primes refer to duplicates.56 DIFFUSION PROCESSES Table 1 and fig. 6 show that there was no significant difference in tan fixation or uptake brought about by addition of undissociated acetic acid to the solution, either in tanning at 20" C or on hot pitting; its addition to a solution in contact with a piece previously tanned in the absence of this acid led to the dissolution of some of the fixed tan, but this was compensated by an added fixation on hot pitting.Fig. 6 shows that stage I requires 6 or 7 days for pieces 1 and 2, and stage I1 15 to 16 days. The end of stage I for piece 3 has not been estimated since the amount of non-tannins in solution was too high. From the slope of the plot TABLE 1.- piece 1 1' 2 2' 3 3' -AMOUNT OF TAN FIXED PER 1OOg AT POINTS SHOWN ON FIG. 4 B C D D toE F 83-68 85.65 82-85 19.61 1 10.0 86.98 88.69 86.10 19-55 115.6 86.22 I 93.63 12.98 112.2 79-43 89.72 9.50 104.4 - I 88.07 11.72 115.8 - - 93-04 12.96 108.9 the calculated value of mpr at A (assumed equal to the value at B) and the average value of cm of the solution over stage I (obtained from p2), the value of the diffusion coefficient has been calculated using the average of the two values obtained by the use of (7-14) and (7.15) for the area available to diffusion (b being obtained by dividing V.1 by Zh). Thus D = 2.3 x 10-7 cm2/sec and from this a rough estimate of the average radius of the diffusing particles of 9 x 10-7 cm can be obtained by using the Einstein-Sutherland 21.22 equation.2 3 4 5 6 7 8 9 1 0 .0.5 HOUr.5 FIG. 7.4raph of Amf/mp against the square root of time for pieces of skin tanned in solutions of tannic acid of various concentrations : (a) p,, cm = 0.09, p 2 == 1.042, (d) 0, CST = 0.18, p2 = 1.071, pH = 3 ; (e) 0 csr = 0.26, p2 = 1.107, pH = 3 ; cf) 8, pH = 4; (b) A, csr ~ 0 .2 1 , ~2 = 1.082, pH = 4 ; (c) V, CST =0.09, p2 = 1.034, pH = 3 ; CST = 038, p2 = 1.155, pH = 3, (s) A, CST = 0.62, p2 = 1.248, pH = 3. Fig. 7 and 8 show the variation of Ami/mp with the square root of time at different values of concentration of tannic acid (the concentrations remained nearly constant during the experiments). It is seen from fig. 7 that again over ap- preciable parts of the process the same relationship is obeyed, especially at lower concentrations. At very high concentration, curve ( g ) where cST = 0.62 there is an infcction in the curvc after 6 h as if the entry of tan were impeded, possibly due to deposition of fixed tan which reduces the area available to diffusion.Fig. 8 shows that after 2 days or less there is a marked inflection, but that there is still appreciable entry of tan for a long period.D . M. G . ARMSTRONG 57 By calculating the mass of tan in solution msr in the pieces at the end of the expcriments using (5.1) and (5.2), we can obtain the value of half the apparent mass of tan in solution in the piece k&r71,/2 and by dividing by inp and subtracting the result from the final value of Arnf/rnp we get a value of Amf/rnp (that at the points marked H on the curves) which is much higher than the inflection region of the curves. Thus the second slow part of the process cannot be solely due to stagc IT of tanning but involved also a third stage of deposition of fixed tan. It is probable that stage I is very rapid.Thus tanning with tannic acid is a more complicated process than with mimosa tannin. Also by comparison of curves 8(a) and 5(b) where concentrations were nearly equal, it is seen that there is a far greater amount of tan fixed by skin from tannic acid. The nature of the curves obtained with tannic acid is such that since stage I1 and 111 of tanning occur at the samc time it is not possible to estimate the diffusion coefficient by the method used with mimosa. yo, , . . 100 , , , , 200 . , . . 300 ] FIG. 8.Plot of AmT/mp against the square root of time for the whole tanning process, for the same pieces as in fig. 7. For the sake of clarity only the final part of some of the curves are shown. An attcmpt to estimate the value of cm and hence of D by taking values of the dope of the straight portions at two concentrations and assuming cm and D to be constant, thereby giving pairs of simultaneous equations by the use of (7.9), was unsuccessful since cpT probably, and D possibly, vary with concentration.Fig. 8 shows that the value of the apparent mass of tannic acid per gram of skin protein has a limiting value of about 0.78 (at 40 days), whereas one might expect that this apparent mass, and hence the amount of tan in the piece, would increase continuously with increasing concentration, until the whole of the water in the skin had been replaced by tan (when an = 1). Two possible implications can be drawn from this. As the concentlation increases so the amount of tan fixed per gram of protein decreases. Thus for piece ( g ) where the concentration is 0.62 g/ml using (5.1) and (5.2), assuming zlm to be 0.617, the amount of tan fixed per 100 g of skin protein is 81 g ; the corresponding amount of tan in solution was 94 6: and for other pieces, the values were respectively : piece cf) 148 and 59 ; piece (e) 162 and 37 ; piece (b) 137 and 32.Alternatively, as the concentration increases so the amount of non-solvent water increases so that, effectively, the difference between the concentration of the interfibrillary solution and that of the external solution increases.58 SELF-DIFFUSION OF A DYE The author wishes to thank the Director and Council of the British Leather Manufacturers’ Research Association for permission to publish this paper, Dr. M. P. Balfe for valuable discussions and Mr. D. J. Tate for his help in the experi- mental work. 1 Mezey, Collegium Haltingen, 1925, 305. 2 Stathcr, Collegium Haltingen, 1933, 9 and 316. 3 Stather and Laufmann, Collegium Haltingen, 1935, 420. 4 Russell, Znd. Eng. Chem. (Anal.), 1937, 9, 592. 5 Parker and Russell, Manchester, Liverpool and District Tanners’ Federation Year Book 6 Brown and Holmes, private publication, 1937. 7 Clack, A Research on Diflusiion in Liquids (Clack, Aberdeen, 1922). 8 Wall, Griegcr and Childers, J. Amer. Chem. Soc., 1952, 74, 3562. 9 Hill, Proc. Roy. SOC. B, 1929, 1Q4, 39. 10 Hermans, J. Colloid Sci., 1947, 2, 387. 11 Adair and Adair, Proc. Roy. SOC. A, 1946, 190, 341. 12 Kraemer, in Svedberg and Pcdersen, The Ultracentrifuge (Oxford University Press, 1 3 Cohn and Edsall, Proteins, Amino Acids and Peptides (Reinhold, New York, 1943)) 14 Bowes and Kenten, Biochem. J., 1948,43, 358. 15 Svedberg, Proc. Roy. SOC. B, 1939,127, 1. 16 Wilkie, J. Physiol., 1953, 169, 369. 17 Gustavson and Nestvold, Leder, 1951, 121. 1 8 Cheshire and Holmes, J. Znt. SOC. Leather Chem., 1942, 26, 237. Holmes and 19 Eilers and Labout, Symp. Fibrous Proteins (SOC. of Dyers and Colourist, Bradford, 20 Neale and Williamson, Nature, 1953, 171, 844. 21 Einstein, A m . Physilc, 1905, 17, 549 ; 2. Elelctrochem., 1908, 14, 337. 22 Sutherland, Phil. Mag., 1905, 9, 781. (Bolton, 1905), p. 45. 1940), p. 59. p. 370. Lee, J. SOC. Leather Trades Chem., 1949, 33, 21, 122. 1946), p. 30.

ISSN:0366-9033    

DOI:10.1039/DF9541600045

出版商:RSC    

年代:1954

数据来源: RSC

摘要:

58 SELF-DIFFUSION OF A DYE THE SELF-DILFFUSION OF A DYE IN A FOLAR POLYMER MEMBRANE BY M. L. WRIGHT Wool Industries Research Association, Torridon, Headingley, Leeds 6 Received 1st July, 1953 ’ Measurements are reported of the self-diffusion coefficient of the anion of the dye Orange 11 in a membrane of horn keratin. Estimates of the mobility of the hydrogen ion have been made, using this data in conjunction with membrane conductivity measure- ments ; the results have been qualitatively confirmed by means of membrane potential experiments. It has been found that the dye anion mobility is much smaller than that for the hydrogen ions and that it varies strongly with concentration. This work is concerned with the processes involved in the dyeing of wool and in particular with the kinetic aspects.Existing data on the diffusion of ions in wool fibres immersed in aqueous electrolyte solution are not numerous. There are several reasons for this, including those difficulties arising from the hetero- geneous nature of the fibres in a wool sample both along one fibre and from fibre to fibre, but in the main they are connected with the behaviour of the outermost layer or epicuticle of the wool fibre.l* 2, 3 I t is well established from visualM. L . WRIGHT 59 evidence that the dyc penetrates more readily through imperfections in the outer skin and that the diffusion rate inside the fibre is much greater than in this skin. In general the dye is constrained to pass through the narrow breaks in the outer surface which then act as line sources as the dye spreads out into the fibre interior.It is also known that this outer layer becomes damaged throughout the life of the sheep and in all stages of subsequent handling so that it is impossible to obtain a " standard " sample of wool with which to perform rate experiments. (Equili- brium sorption is little affected as the epicuticle forms only a small proportion of the total fibre weight.) Added to these are the complications arising from the concentration-dependence of the diffusion coefficient often found in such systems and also from the electrical diffusion potential ; these, however, may be eliminated by the measurement of self-diffusion using radioactive isotopes rather than " chemical " diffusion down a concentration gradient. Self-diffusion is the more fundamental property since a coeficient is obtained for each ion-species rather than a mean value depending on the properties of both anion and cation.The first two difficulties associated with the wool fibre still remain and therefore to simplify the problem a study has been made of membranes of fibrous protein and polymers which h v e properties related to those of the keratin found in the interior of wool fibres. The use of membranes confers two further advantages; firstly, their simple geometry enables an easier solution of the diffusion equation to be used than for fibres and secondly it is simpler when using a membrane rather than a bundle of fibres to ensure that diffusion in the polymer, and not some surface mechanism, is the rate controlling process.It must be emphasized that although the problems related to the surface properties have been temporarily side-stepped, this may well be the most important aspect of wool dyeing. Since there are no data yet available for the self-diffusion of large ions in an electrically charged network of polymer chains, it is felt that these data must first be obtained before proceeding to an investigation of the properties of the cuticle layers. The membrane data and methods described here will clearly be of direct interest in research dealing with synthetic fibres. This paper presents preliminary resuI ts for the self-diffusion in a membrane of cow-horn keratin, of the anion of the free acid of Orange I1 which was " labelled " with 35S, viz., OH Previous work has dealt with Ihe diffusion of a mineral acid 4 and a simple salt 5 in such a system; the original paper should be consulted for much of the detail.EXPERIMENTAL APPARATUS.-The diffusion apparatus is shown in fig. 1 ; it is designed to have a large cross-sectional area for diffusion and to operate using a small volume of solution. The membrane is held directly between the ground edges of the two glass cells which are pressed together by means of a metal clamp fitted with rubber inserts. The whole apparatus is rotated by an electric motor at 60 rev/min about a horizontal axis ; stirring in the solu- tion takes place as an air bubblc is left in each sidc when filling the cells with solution and a series of glass projections (only one of which is shown) also tend to break up the liquid flow.Either side may be filled as follows : after thc rotation has been stopped, a clutch allows the shaft to bc rotated by hand until the hinge axis is horizontal and then the apparatus itself may be swung into the horizontal position. After replacing the rubbcr stopper in the cell a piece of glass rod is used to close the narrow hole through the stopper ; this ensures that the cell contents are only slightly compressed. The apparatus together with the stock solutions and pippettes is kept inside an air thermostat which is controlled60 SELF-DIFFUSION OF A DYE to 0.1" C. The air in thc thermostat is kept saturatcd with water vapour to reduce cvapor- ation from the cdge of the membrane. MEMBRANE.-TO reduce equilibrium times, very thin membranes have been used ; some of these were about 25 p in thickness which is comparable to the diameter of an average wool fibre.The absence of small holes in thc membrane may bc tested by placing a dye solution at one side of the membrane and water at the other, both at room tcm- pcrature ; the dye has a negligible permeability through an intact membrane at low tempera- tures. Small areas of the membrane which are thinner than the rest may be detected by the difference in coloration after dyeing the membrane to a light shade. The thickness of the membrane was obtained by a micrometer and this was checked using density data after weighing the membrane at a known relative humidity. Before use, the mem- brane is eIectrodialyzed to remove any ionic impurities, using a platinum electrode placed at either side of the membrane in distilled water which is constantly renewed.To reduce degradation effects a new membrane is used for each concentration. Although it is possible to relate the permeability of one membrane to another by thickness measurements it is preferable to use conductivity measurements. THE DYE soLunoN.-The purified non-radioactive dye solution required for the experi- ment was prepared from /3-naphthol and sulphanilic acid using A.R. reagents. After recrystallizing once from aqueous solution the dye was redissolved and salted-out with 0.25 N NaCl. The salting-out procedure was repeated three times and thcn the dye was finally recrystallized from aqueous solution to remove the majority of the salt. (It is \ 1 rnerntmmz FIG.1 .-Diffusion cell. found that the precipitates obtained by salting-out with NaCl rather than with HCl are easier to filter.) The sodium dye, plus any salt remaining, were converted to the free acids by passing through a Zeo-Karb 225 ion exchange column in the hydrogen form. By using radioactive sodium ions in a trial experiment under the same conditions, it was found that the concentration of sodium ions was reduced lo00 times on each passage through the column. Following the method of Kressman 6 the solution which now contained the dye acid plus a small amount of HCl, was run down a column of Deacidite E which absorbed the mineral acid and allowed most of the dye solution to pass through. A sample of the dye solution was then dried in VCICUO to constant weight at 110" C and used to make a standard solution.The method of Durrurn,7 in which thc dye solution is flowed down a vertical paper sheet across which there is an electrical potential, was used to test for isomers or any other coloured matter having a diffusion coefficient different from that of the majority of the solution. In fact many of the methods developed for protein analysis can be used to test for dye purity ; the ionophoretic method of Tiselius 8 using a horizontal paper between two glass sheets is particularly usefuI. PREPARATION OF DYE CONTAINING 35S.-Apart from the normal necessity of thoroughly purifying colloidal substances before attempting to use them for physico-chemical measure- ments, it is also essential in this case to remove all traces of impurities containing radio- active sulphur remaining after the synthesis, such as Hzs*04 or NH2-*03H, since these smaller ions would diffuse much faster in the keratin than the dye anion.The dye containing radioactive sulphur was synthesized from sulphuric acid received fromM. L. WRIGHT 61 Harwell which contained 35s (half-life, 87 days). The sulphuric acid was fist converted to sulphanilic acid, which was purified by recrystallizing three times from hot water to remove all traces of HzS*04, and the filtrate tested using BaC12, since the barium salt of sulphanilic acid is soluble. The diazotized sulphanilic acid was then coupled to 15-naphthol. Any radioactive sulphanilic acid remaining was removed by salting-out twice with NaCl from a slightly alkaline solution in which the dye itself is very soluble.In a trial experi- ment using radioactive sulphanilic acid and non-radioactive dye, it was found that 97 % of the sulphanilic acid was removed on one salting-out, The solution was then acidified and the purification carried out as before. Paper electrophoresis was used to check that all the radioactivity was contained in the coloured spot after it had moved along the paper. RADIOACTIVITY MEAsurnMEN-rs-Since 35s is a weak beta emitter it is necessary to evaporate an aliquot of the solution to dryness before radioactive assay, taking precautions to obtain a layer of uniform thickness so that allowance can be made for the self-absorption occurring in the material itself. A gas-flow proportional counter in which the samples are placed inside the counter has been used.Although it is possible to count 355 using a thin end-window Geiger counter, the gas-flow counter method is more sensitive. PROCEDURE.-The membrane is brought to equilibrium at a particular temperature with non-radioactive dye solution. As the temperature coefficient of equilibrium sorption is not large it is possible to speed-up this operation by first increasing the temperature, renewing the solution at intervals and then returning the temperature to its correct value in the later stages. When chemical equilibrium is reached, the dye solution is drained from one side of the diffusion cell and this is refilled with dye of the same concentration but with some dye anions containing radioactive sulphur atoms.At intervals the contents of the other ccll are sampled, to measure the rate at which the radioactive ions are coming through the membrane. This is continued until the steady state has been established; the steady-state refers to the concentration gradient of the radioactive atoms. The extra- polation backwards of the steady-state slope gives an intercept on the time-axis known as the time-lag T from which the self-diffusion coefficient D* can be calculated using the Barrer 9 equation D = (Ax)2/6T, where Ax is the membrane thickness. The flow at the steady state gives a measure of the self-permeability coefficient P* and the solubility or partition coefficient S is given by the ratio P*/D*. The permeability coefficient is defined in terms of the gradient across the membrane as measured in the solution whereas that for the diffusion coefficient refers to the gradient as measured just inside the membrane.RESULTS Measurements have been made at 60" C for three concentrations of the dye acid, viz. at pH 3.5, 2.5 and 1.5, and the results for the self-diffusion coefficients arc shown in table 1. TABLE 1 pH D* (cmz secl) PH D* (cm2 SCC-I) PI* D* ( c d sec-1) 3.5 0.9 X 10-11 2.5 1-8 x 10-11 1.5 6.5 >: 10-11 DISCUSSION The diffusion of the ions of a simple salt in membranes of keratin,5 Cellophane 10 and Nylon 11 has been investigated in earlier work and it has been found that in general the diffusion coefficients of both ions are similar in magnitude and that there is only a slight variation with concentration. From these results and other published work it appears that the magnitude of the diffusion coefficient for simple salts follows a course parallel to the magnitude of the water absorption of the polymer a t saturation water vapour pressure, although it is to be emphasized that no suggestion is made that diffusion takes place in liquid water in the polymer except in the case of the most swollen ones.Approximate values of the diffusion coefficients for simple salts diffusing in some polar polymers are shown in table 2 ; the values are in terms of the diffusion coefficient of the electrolyte in aqueous solution.62 SELF-DIPFUSION OF A D Y E TABLE 2 CX, water %olymer/ rcf. sorption Ijwater gelatin 800 1 Lodge 12 sulphona ted pol y s tyrenc resin Celiophanc 100 10-1 Wright 10 horn keratin 33 10-3 Wright 4 66-Nylon 10 10-4 Wright 11 Teiy lene 1 veiy small Sunmer 14 (Dowex 50) (100) 2 X 10-1 Bauman and Eichhorn 13 The list ranges from the highly swollen gels of gelatin as instanced by the work of Sir Oliver Lodge in the classical experiments on ionic mobilities, to Terylene which is only slightly polar.For the highly swollen polymers the diffusing electro- lyte is little influenced by the polymer as there is only a small probability of the ion coming near an absorption site and so there is little difference in magnitude between diffusion in water and in the polymer for either a salt or an acid. (In several cases these results are somewhat arbitrary as they depend on how the polymer film has been prepared and in which direction the diffusion has been measured.) It is not suggested that the degree of water sorption is the only factor determining the value of the diffusion coefficient but it gives a general indication of what magnitude is to be expected.It is probable that the degree of cross- linking between the chains in the polymer whether by direct bonding or partial crystallization has a profound influence on the value of the diffusion coefficient; the magnitudc of the water sorption itself also indicates the degree of cross-linking. Ultk 15 has suggested that the epicuticle of the wool fibre is a protein similar to keratin but with a higher degree of cross-linking. This would account for the lower diffusion coefficient in the outer layers of the wool fibre than in the interior. For the ions of simple salts there is little modification of the magnitude of the diffusion coefficient due to absorption effects.If, however, there is strong inter- action between an ion species and adsorption sites in the polymer, then for those polymers which do not swell much the magnitude of the diffusion coefficient may be quite different from what might be expected on the basis of the water sorption analogy. The less the polymer swells the greater the distinction it makes between two ionic species having different absorption properties. This was demonstrated for the keratin -1- HBr system in previous work 4 when it was found that at low concentrations the self-diffusion coefficient of the hydrogen ion was much smaller than that of the bromide ions although the value for the bromide ions was itself 1000 times smaller than in aqueous solution.The hydrogen ion mobility varied strongly with concentration and when the keratin was saturated with acid, thc hydrogen ion diffusion coefficient had increased 700 times to have a value greater than that for the bromide ions. These results werc explained in terms of the strong a&i.ity of the hydrogen ion for the charged carboxyl groups in the fibre, so that for low concentrations, the proportion of mobile or frec hydrogen ions at any one time is small. As the keratin becomes saturated with acid the proportion of mobile ions increases giving a higher value for the observed or “overall” diffusion coefficient. The intention of the present work is to investigate the behaviour of an anion which has a strong affinity for the polymer, i.e.much greater than that of the halide ions used previously. The free acid of the dye was chosen rather than the sodium salt since the effect of pH changes can be investigated without the addition of another ionic species. The dye chosen (Orange 11) is a typical example of the level acid dyes (i.e. dyes which have oiily a moderate af5nity for keratin). The wool titration data for this dye already exist 16 and there is also little evidence of aggregation in solution of the dye at the temperature of these experiments.M. L. WRIGHT 63 Mcasurements involving keratin in dilute dye solutions are dominated by the high values obtained for the solubility or distribution coefficient. For a moderate affinity dye such as Orange 11 free acid, the solubility at pH 3.5 is approximately 2000, i.e.1 g of wool can practically exhaust 2000 ml of dye solution at this pH. For dyes of higher affinity such as the milling acid type, this effect will be much enhanced; e.g. for Coomassie Milling Scarlet the solubility value is about 105 at pH 6. As can be seen from the titration curves 16 the effect bccomes smaller as the dye solution concentration is increased. Although these values are for equilibrium sorption, the magnitude of the solubility cocfficient can play a very significant part in diffusion studies. Since the diffusion and permeability co- efficients are related by the solubility we find that although the diffusion coefficient may be low for such a system, the flux may be quite high.For example, consider a membrane experiment in which radioactive tracers are used; then the radio- active solution becomes exhausted of active ions while the side originally non- radioactive rapidly becomes radioactive and this reduces the concentration gradient. This may occur before the steady state is established as a low diffusion coefficient indicates that the equilibrium is only slowly established. A second effect is the maintenance of equilibrium between the dye solution and the membrane during diffusion due to the presence of a still layer of unstirred solution at the membrane surface. This phenomenon has been examined by Alexander, Gough and Hudson for the reaction between wool and chlorine 17 and for dyeing experiments ; 18 the magnitude of this effect under the conditions used in the present experiment has been shown to be negligible (see appendix).The problem in both cases, is one of supplying dye to the solution/keratin interface rapidly enough to ensure that diffusion in the polymer is rate controlling; in the present series of expcri- ments to ensure this, it was found necessary to lower the temperature to 60" C to reduce the diffusion coefficient. Both the above effects are more important with wool fibre masses or fabrics than with membranes, due to the difficulties in circulating the solution between the fibres. Also when rates of absorption are measured rather than the steady-state flow the effect is enhanced, as it is de- pendent on the flow rate. For a membrane, thickness 3 x 10-3 cm and D == 10-11 cmz sec-1, it can be shown assuming a " square root of time " law, that the uptake of dye in the first minute is 100 times that passing through per minute in the steady state for equal surface concentrations.The results obtained for the anion self-diffusion coefficient (table 1) indicate a rapid incrcase with concentration especialiy when it is appreciated that they are for only a very limited concentration range near saturation. This is a similar effect to that found for the hydroken ions in the HBr system at saturation. Measurements have also bcen made of the electrical conductivity K of the mem- brane in dye solutions at 60" C at the same concentrations as used in the diffusion measurements, these are shown in table 3. The diffusion cell used for the dye diffusion is of the same diameter as the conductivity apparatus described previously 4 TABLE 3 dye sorption conductivity calculated % PH (Lister and Peters), (ohms-1 cm-1) anion contribution 3.5 0.68 2-9 x 10-7 7-1 % (mmo W) 2 5 0.88 2.2 x 10-6 2 4 % 1.5 (0.9) 2.3 x 10-5 0-8 % and so the diffusion and conductivity results are directly comparable.The mobil- ity u of an ion can be calculated from the sclf-diffusion coefficient using the Einstein relation (u = FD/RT) and thercfore values for the dye anion mobility uD in keratin can be calculated. Also we can write formally K r= I*'8(uH + uD), where 8 is the ionic concentration in the polymer. An estimate of the hydrogen ion mobility ua can therefore be obtained from the membranc conductivity when64 SELF-DIFFUSION OF A DYE taken in conjunction with data for the anion self-diffusion coefficicnt at the same concentration and temperature.This calculation shows that the hydrogen ion mobility is much greater than for the dye anions (see table 3). Although it is seen that the hydrogen ion mobility increases greatly with concentration, the results are not directly comparable with those obtained in the HBr system, after allowing for the difference in temperature between the two sets of experiments. This may be due to the fact that any slight difference in the sorption isotherms would tend to influence strongly the diffusion results in Ihe region of saturation. A conductivity value has also been obtained at 25” C for one concentration, viz. pH 2-5. The conductivity results at 25” C and 60” C indicate that the activation energy is about 10 kcal/mole in this temperature range for the diffusion of hydro- gen ions.As the dye anion contribution is only a small fraction of the con- ductivity, the activation energy for the diffusion of the anion cannot be obtained by this method. (In the present work no direct measurements have yet been made for the anion diffusion at various temperatures.) In a previous paper dealing with the HBr + keratin system, the variation of hydrogen ion mobility with concentration was explained in terms of two energy states for thc hydrogen ions, the majority of ions being strongly bound to sites (8”) and the others existing in a more mobile state (8’). A similar treatment may be applied to the dye anions which gives 8;/(W - 0;) = 0; exp (- AA/RT).Also, the total concentration sorbed is 8 = 8’ $- 8”. The difference in chemical potential at unit activity for the anion in the two states is and @at is the concentration of bound ions at saturation. Then, as before, we assume that the overall mobility of the anions is due to the relatively small “ mobile ” fraction and therefore OUD = 8’$& where uD is the overall mobility and u t D is the mobility of the mobile fraction. Similarly O D D = 6’D’D, where OD and DtD are the cor- responding self-diffusion coefficients and therefore Taking the known value 19 for the affinity of the anion of Orange I1 for keratin as a measure of &’D then a value for D‘,, can be calculated. This value is con- siderably lower than that found from the diffusion of Orange I1 in solution and we must conclude that the steric hindrance of the keratin to diffusion is con- siderable.(The results for D’D are approximately 1000 times smaller than in aqueous solution.) Since the results in this paper are for solution concentrations which give almost saturation acid combination, the value of (8s.t - 8) tends to depend critically on the value chosen for fW. Mobility values will have to be obtained for lower ionic concentrations in the polymer before the theory can be satisfactorily tested for the dye anion. The values obtained for the self- diffusion coefficient, shown in table 1, are not much larger than estimates made for the diffusion coefficient in wool fibres.19 This is surprising because it might have been expected that the results for the wool fibres would be controlled to a great extent by the low diffusion coefficient in the surface layers.The results so far obtained for wool fibres are for the chemical diffusion coefficient of the dye as a whole (zHD). As D$ > D& therefore zHD m 20& since to a first ap- proximation the chemical diffusion coefficient is given by the harmonic mean of the self-diffusion coefficients. In practice the results will be strongly influenced by the concentration dependence of the diffusion coefficient. In the HBr system4 measurements of the membrane potential were uscd to confirm the hydrogen ion mobility value obtained from the self-diffusion and conductivity results. The membrane potential AE is directly related to the mobility ratio in a two-component system, i.c., D~ = (exp A p t D / R T ) D ‘ D / ( O S a * - 8).M.L. WRIGHT 65 Preliminary results for solutions of the dye show that the weaker solution becomes positive confirming that the hydrogen ion mobility is greater than that of the anion. APPENDn<.-The eflect of the unstirred layer may be investigated quantitatively as follows. When the system is at equilibrium consider the steady state flow of radioactive ions through the still layer and the membrane given by (1) where c, 6 refer to the various radioactive concentrations in solution and menlbrane (see fig. 2) and Ds, DM arc the diffusion coefficients in solution and membrane respectively. If the radioactive ions are removed from the right- hand side of the membrane as fast as they arrive then 82 m 0.From the titration curve it is seen that a linear relation between pH and dye ab- sorbed is approximately true in the range pH 3 to 4 and this is given by Ds(C4 - C3)/d = &(dl - &)/AX, FIG. 2. (2) Stirred el 0.33 loglo (c3/3 x 10-6)- At pH 3.5 for a membrane thickness 3 x 10-3 cm, taking Ds = 5 x 10-6 and DM = 10-11 cm2 sec-1 and assuming d = 0.1 cm, graphical solution of eqn. (1) and (2) gives c3 = pH 3.6. The dye con- centration sorbed at the surface would be 064 mmolelg compared with 0-67 mmole/g at equi- librium at pH 3.5. This is a relatively small error under the worst conditions encountered in this series of experiments (viz. S = 2000). In practice the still layer is probably much less than 0.1 cm at 60" C (Alexander and Hudson18 give a value of 0.03 cm at 25" C for stirring at 50 rev/min ; a smaller value still would be obtained at 60" C due to the reduced viscosity). so/uhon I wish to thank Dr. A. B. D. Cassie, Director of Research, for encouragement in this work and the Council of the Wool Industries Research Association for permission to publish. I would like to thank Mr. G. King for discussion and Mr. F. Bond and Miss M. M. Hargreaves for assistance with the experimental work. 1 Speakman and Smith, J. SOC. Dyers Col., 1936, 52, 121. 2 Millson and Turl, Amer. Dyestuf Reporter, 1950, 39, 647. 3 Lindberg, Textile Res. J., 1950, 20, 381. 4 Wright, Trans. Faraday Soc., 1953, 49, 95. 5 Wright, in press. 6 Kressman, J. Physic. Chem., 1952, 56, 118. 7Durrum, J. Amer. Chem. Soc., 1951, 73,4875. 8 Kunkel and Tiselius, J. Gen. Physiul., 1951, 35, 89. 9 Barrer, Difusion in and through Solids (The University Press, Cambridge, 1941), 10 Wright, J . Physic. Chem., in press. 11 Wright, to be published. 12 Lodge, Brit. Assoc. Report, 1886, 389. 13 Bauman and Eichhorn, J. Amer. Chem. Suc., 1947, 69,2830. 14 Sumner, private communication. 15 UltCe, Schooneveldt and Schuringa, Biuchim. Biophys. Acta, 1953, 10, 590. 16 Vickerstaff, Physical Chemistry of Dyeing (Oliver and Boyd, London, 1950). 17 Alexander, Gough and Hudson, Trans. Faraday Sac., 1949,45, 1058, 1109. 18 Alexander and Hudson, Textile Res. J., 1950, 20,48 1. 19 Lister and Peters, this Discussion. p. 18. C

ISSN:0366-9033    

DOI:10.1039/DF9541600058

出版商:RSC    

年代:1954

数据来源: RSC

摘要:

THE ABSORPTION OF SODIUM SULPHATE AND SULPHURIC ACID BY HAIR BY D. L. UNDERWOOD AND H. J. WHITE, JR. Rick Chemical Laboratory, Princeton University Textile Research Institute, Princeton, New Jersey, U.S.A. Received 2nd July, 1953 A new tcchniquc involving the use of radioactive tracers has been applied to the measurement of the absorption of H2SO4 and Na~S04 by human hair. Both equilibrium measurements and rate measuremcnts have been made. Good agreement has been ob- tained between the tracer method and thc more conventional change of titre method with respect to the mcasurement of the H2SO4 uptake. Na2S04 has been found to be absorbed by hair in appreciable quantities in a manner which suggests absorption on to bonding sites within the fibre. The ratcs of absorption and desorption processes involving single fibres have also been followed.Keratin fibres absorb acids from solutions of pH lower than about 6. This absorption of acids is the basis of the acid dyeing of wool and has been studied extensively as a result of its technical importance. Among other things the up- take-pH curves have been determined for a number of acids using wool as a sub- strate. In particular the uptake by wool of sulphuric acid, which is one of the materials discussed in this paper, has been measured by Speakman and Stott,l Steinhardt, Fugitt and Harris,2 LaFleur,3 Donovan and Larose,4 and Oloff~on.5 One factor which affects the amount of acid taken up at a given pH of the external bath is the ionic strength of the external bath. In general the greater the ionic strength the more acid absorbed at a given pH.The uptake of sulphuric acid by wool at a given pH from solutions containing various amounts of sodium sulphate has been discussed by Donovan and Larose 4 and Oloff~on.5 Although Donovan and Larose found essentially no change in the amount of acid absorbed at a given pH with increasing ionic strength, Oloffson showed that these results were true only for the limited concentration range studied by Donovan and Larose and that at other concentrations of acid marked increases in the amount absorbed could be found. Turning from the absorption of acid and considering the absorption of salts, little or no data are available on the amount of salt taken up by keratin fibres from aqueous solutions. Such data would be useful in understanding the neutral dyeing of wool and possibly certain physiological processes. This lack of data results primarily from experimental difficulties. The uptake of acid by wool fibres is usually obtained by measuring the initial and final concentrations of the treating bath.The volume of the treating bath is assumed to remain constant so that the change of titre of the bath is a measure of the amount absorbed by the fibres. This method defines the amount absorbed as the relative increase in concentration inside the fibres of the acid with respect to the water. For the amount absorbed defined in this way to be an accurate estimate of the amount of acid within the geometrical boundaries of the fibres of the sample, it is necessary that the acid be strongly absorbed with respect to the water.It is also necessary that the solution be so dilute that water absorbed by the fibre does not cause a change in concentration of the cxternal solution comparable to that caused by the absorption of the acid. These conditions arc met over a considerable concentration range with acids but not with salts. It is thus necessary to determine the amount of 66D . L . UNDERWOOD A N D H . J . WHITE, JR. 67 salt absorbed by a sampIe directly rather than by change of titre of the treating solution. This introduces the problem of washing or cleaning the sample. A sample of a convenient size contains many hundreds of fibres closeIy intermeshed and hence is capable of entraining large quantities of solution on removal from the bath.This entrained solution cannot be removed by washing the fibre mesh without removing salt present in the interior of the fibre unless the salt is strongly absorbed. The centrifuge is often used to remove entrained material. However, Preston, Nimkar and Gundavda 6 have shown that capillary water (or solution) is always an appreciable portion of the amount absorbed as determined by using the centrifuge. Only approximate corrections for the amount of capillary water held can be made at the present time. There is another way t o minimize errors caused by entrainment of solution. If a single fibre is uscd as a sample, capillary water can only occur around the hook holding the fibre, and entrained droplets are fewer because of the more regular shape of the sample.As a result washing of the sample becomes more fcasible. It will be shown in this paper that a satisfactory washing technique can be found for human hair which has been treated with sodium sulphatc solutions. The washing of individual fibres, which must necessarily be of a short length for casy handling, would make the method slow and tedious if large samples were used. Hence it is necessary to use microanalytical techniques since the most convenient sample size is a few hundred micrograms. The method uscd for the work reported in this papcr involves radioactive tracers. Similar methods have bcen uscd by Barnard, Palm, Stam, Underwood and White 7 to study the uptake of alkali halides by hair. Data are given on the absorption of sodium sulphate by hair under conditions which preclude the absorption of sulphuric acid.Data are also given on the absorption of sulphuric acid by hair and on the absorption of salt and acid from salt -I- acid mixtures. Finally, some experiments are discussed on the rates of absorption and of desorption under different conditions. EXPERIMENTAL MATERIALS.-The samples used were 10-cm Iengths of blonde unmedullated human hair. The hairs were washed in water and then extracted in a Soxhlet extractor with methanol, ethyl ether, and again with methanol. Each extraction lasted at least 1 h. The extractcd hairs were then washed with distilled watcr for several hours at room temperature. The hairs were dried over magnesium perchlorate in a vacuum desiccator at room temperature for at least 12 h and then brought to equilibrium in a conditioned room. The purpose of the initial drying was to makc certain that the hairs were on the absorption cycle with respect to moisture content.The hairs were then weighed on a microbalance or in some cases using a vibroscope.8 The dry weight of the hairs was determined using the moisture content against relative humidity data for hair taken by Chamberlain and Speakman.9 It is estimated that the dry weight can be determined to better than 2 % in this way. The chief uncertainty is the possible fibre-to-fibre variation in moisture content at a given relative humidity. The radioactive tracer solutions were prepared as folIows: HzS3504: one drop of a 4-ml stock solution containing 10 mc of H2S3504 in 1.46 N HCl* was added to 10 ml of a 0.1 N H2SO4 solution and evaporated nearly to dryness to remove HCI.15 ml of 0.1 N H2SO4 were added and the resulting solution diIuted to 25 ml at a pH of 1.27. Other solutions were formed similarly or by dilution. Na2S3504; H2S3504 was formed as above and titrated with NaOH solution to give the required Na2S3504. Either inactive H2SO4 or inactivc Na2SO4 was used to obtain the desired specific activity. If inactive H2SO4 was used it was finally neutralized with NaOH. * obtained from Oak Ridge National Laboratory with the permission of the United Stales Atomic Energy Commission.68 ABSORPTION BY HAIR NaPS04 : A stock soIution of 1 mc of Na22CI in weak HC1* was diluted to 25 ml. A 3-ml aliquot was diluted to 20 ml and titrated with 0.01 N NaOH solution giving 0.05 mequiv.of NaCI. 26 mequiv. of NazS04 were then added and the solution diluted to the desired concentration. The chloride ion which was present to the extent of 0.2 % on an equivalent wcight basis was ncglected. TREATMENT.-Each fibre was mounted on an individual spring clamp and placed in a test-tube. The hairs were then conditioned for 4-2 h with air which was previously bubbled through distilled water to minimize the effects of swelling changes on the rates of absorption. After this the desired solution was run into the test-tube. No more than four fibres were prcsent simultaneously in one test-tube. There were roughly 950 pg of hair in 10-15 ml solution. The solutions were not shaken. An average value for at least four hairs is given for points representing equilibrium data.Points on a rate curve represent data for one or two hairs. WASHING.-The fibres were removed from the treating solution, shaken vigorously to remove visible entrained droplcts, and washed for 1-2 sec in distillcd water. A rate of leaching curve is shown in fig. 1. Hairs, brought to equilibrium in Na2S3504 solution, 0 n 0 0 - 0 - 0.05 - upfoke 0 0 - Y mmo/e / / h e , sec / D /oo moo FIG. 1.-Rate of desorptioii of S 5 0 4 from hair by 0 washing with distilled water, 0 exchange with inactive solution, - - - - - unwashed hair. have been washed in distilled water for varying periods of time and the amount of salt in the fibies plotted against the logarithm of the time. The existence of a flat stcp, or induction period, in the early portion of the leaching curve is taken to mean that no appreciable amount of salt has yet becn removed from the interior of the fibre.The amount of salt carried by an unwashed hair is shown also. Crystallites of salt are visible under the microscope on dried unwashed hairs showing that a washing is necessary. Although no data will be given for the rate of leaching of NaPS04 from hair, preliminary experiments have shown that a 1-2 sec wash with distilled water does not remove the Na22 from within the fibre. It has been assumed that the same method of washing could be applied to H2S3504 in hair. Some data on the rate of exchange of sulphate ions between thc fibre and the external solution are also shown in fig. 1. The data were taken by immersing the treated fibres in a solution made up to have the same concentration as the original treating solution but without any radioactive sulphate.The subsequent ion exchange process was followed by measuring the activity remaining on the immcrsed hairs as a function of time of immersion. The hairs used for the washing and exchange experiments were dissolved using HN03 before they were counted. The reason for this will become evident in the next section and in the discussion of the results. C'ouNTrxG.---The hairs were coiled loosely on tantalum discs which had been smearcd with a very thin layer of albumcn fixative to hold the hairs in place. The discs were then * see previous footnote.D . L . UNDERWOOD AND 1-1. J . WHITE, J R . 69 heated to harden the albumen and dry the hair. Most of the hairs were then counted directly in a Nucleometer, an internal flow counter.The counts from the hairs were then comparcd with standards formed by evaporating 1 ml of a solution of known con- centration on a disc which had been smeared with albumen fixative. All radioactive molecules were counted without change, since a check showed no loss of activity for an HzS3504 sample dried directly on to a disc as comparcd with an Na2S3504 sample made by neutralizing the H2S3504 solution. All standards were designed to have an average thickness well below 1 mg cm-2. The sample and the standard were quite different and a considerable correction for self-absorption had to be made. To obtain this correction, certain discs were treated with concentrated HNO3 which dissolved the hair, heated to expel the nitric acid and Ieave a thin film of protein, and counted.The counts were then compared with those from a standard on which a hair had been spread using HNO3. A correction factor (4-5 % decrease) for the change in activity of the standard on undergoing this spreading process was also obtained. In this way a curve for the self-absorption correction factor (true activity divided by apparent activity) as a function of fibre weight was obtained for each isotope. As is to be expected, the self-absorption increases with fibre weight and is larger for S35 which emits a less energetic radiation than N@. These curves are only valid for this particular experimental technique and can bc cxpcctcd to change considerably with changing conditions.These curves are shown in fig. 2. corrpc fio n factor t 5 FIG. 2.-Self-absorption correction factor as a function of fibre weight (1) s35, (2) Na22. CHANGE OF TITRE EXPERIMENTS.-The uptake of &So4 by hair was also measured using the change of titre technique. Titration with NaOH was used to determine the initial and final concentration of acid. 0.5 g samples were used in 100 ml of acid solution. The equilibrium samples were left in the solution for 72 h. RESULTS The uptake of H2SO4 by hair as a function of pH at room temperature (- 22O C ) is shown in fig. 3. Data taken using the change of titre method and data taken using the tracer method are both shown. The line shows the uptake of sulphuric acid by wool. It is a visual estimate of the most probable uptake values for wool taken from a plot of the existing data.1-5 In fig.4 the rate of uptake of acid from a solution having a pH of 2.10 is shown. The data were taken using the tracer method with the hairs precon- ditioned as mentioned earlier. It has been shown 7 that in some cases such preconditioning is not sufficient to prevent large and complex swelling changes from occurring during the absorption process. However, these cases occur with more concentrated soIutions than were used in this experiment, and the swelling probably changed only slightly during the absorption of this acid. The fibres were not dissolved but were corrected for self- absorption using fig. 2. Since the self-absorption correction factor was determined using hairs which were brought to equilibrium with the external solution, there is an undetermined error resulting from the non-uniform distribution of acid throughout70 ABSORPTION BY HAIR the fibre during the rate measurements. This error is grcatest for the earliest rate points but is probably small even at thesc points.In table 1 and fig. 5 data are givcn on thc uptake of N a ~ S 0 4 by hair. The pH of tlic treating solutions is shown in the third column. The pH values givcn are the final pH values of the treating solutions. In some cases the pH of the solution changed during the cxperimcnt usually toward a inore acid pH. Such changcs could be attributcd to 1 I I -0.8 FIG. 3.-Uptake of H2S04 by hair and wool - wool, change of titre method. 0 hair, tracer method, 0 hair, change of titre method, uptake of S3504 and Na22 by hair from acid solutions.For the first three solutions a weighed amount of salt was added to a known volume of acid of a known concentration. For the last three solutions equal aliquots of a solution of known Na2SO4 content were brought to different acidities by adding equal volumes of acid solutions of different strengths. The sodium ion concentration is the same for all thrce solutions. The sulphate concentration was determined directly by analysis for the last sol~ition. For the other two solutions the sulphale concentrations could be roughly determined from theD . L. UNDERWOOD AND H. J . WHITE, JR. 71 pH and the sodium ion concentration. However, it is not necessary for the purposes of the experiment. TABLE 1.-TNE UP'TAKE 01' Na2S04 BY HAIR FROM NEUTRAL SOLUTIONS concentration mmole/ml 0.104 0.236 0.00264 0.01 12 0.0112 0.097 0.104 0.52 0.69 0.80 1.39 1 *43 0.00264 0.246 isotope tagged Na S S S S S Na Nil S S S S S S PH 5.55 5.79 6-32 6-19 6.43 6.18 6.20 6-50 6-27 (6-0)* 6-30 6.10 6.60 6.55 uptake inmole/g 0.0285 0.0419 0.0026 0.0132 0.00443 0-0204 0.0204 0.080 0.05 8 0.079 0-083 0.099 0.003 15 0.0380 * The pH of this solution was not measured directly but was known to be nearly 6.TABLE 2.-THE UPTAKE OF S3504 AND Na22 .FROM Na2S04 -b H2S04 MIXTURES BY HAIR AT ROOM TEMPERATURE concentration concentration uptake uptake sop Na+ PH SOnZ- Na+ mequiv./g mcquiv./& mequiv.lm1 mequiv./ml - 0.295 0,199 1.70 0.83 0.205 0.200 419 0.480 - 0.204 0.203 441 0.308 - - 0.185 1-24 - 0.01 10 - 0.185 2-74 - 0.0108 0.186 0.185 3.72 - 0.01 1 1 In fig. 6 the rate of absorption at room temperature of Na2S04 for a 0.80 M solution is shown.The fibres were dissolved before counting so that there is no error arising FIG. 5.-Equilibriuni uptake of Na2S04 by hair 0 Na2S3504, 0 Na222S04. from the application of the self-absorption curve to fibres in which the salt is not uni- formly distributed. As has been mentioned the hairs used in the desorption and exchange experiments shown in fig. 1 were also dissolved before they were counted.72 ABSORPTION BY HAIR DISCUSSION THE UPTAKE OF H2SO4 BY HAIR.-AS can be seen from fig. 3 results obtained using the change of titre method agree quite wcll with those obtained using the tracer method. The difference between the two is probably within the combined error of the two methods, and the agreement between the two shows the absence of any serious errcr in the tracer method.An average curve through the data for hair would fall slightly below the line for wool, except possibly at the lowest values of pH. It is quite possible that this difference is real. Speakman and Elliott 10 found an uptake of HCl by hair some 7 % lower than an average uptake for wool at pH 1-16. Any projected line through the data in this paper would not be as low at pH 1.16; however, the difference in acids would have to be considered. Finally a comparison of the chemical analyses of hair and wool shows a combined content of lysine, arginine, and histidine of 0.77 mmole/g for wool 11 and 0-72 mmole/g for hair.12 Although other invcstigators have found different values for these sums, it seems established that hair has a slightly lower theoretical acid-binding capacity than wool.FIG. 6.-Rate of absorption of Na2S3504 by hair from a 0.80 M solution 0 experimental, - Fick’s law. The solid line in fig. 4 was obtained by using Fick’s law, with a constant diffusion coefficient, for radial diffusion into an initially empty cylinder.13 The hair was assumed to have a swollen radius of 30 microns. The agrcement between the theoretical curve and the experimental data is fairly good. The diffusion coefficient, D = 3.93 x 10-10 cm2/sec is too small for an aqueous diffusion constant ( D = 10-5 to 10-6 cmalsec) and indicates that the polymer network of the hair has a great influence on diffusion.THE UPTAKE OF Na2S04 BY HAIR.-The uptake of acid by hair is known to be a special case of the back-titration of a weak acid. Thus it is known that the protons taken up are absorbed by carboxyl ions within the fibre. The data just discussed, and direct electrical measurements, show that cations and anions are taken up to the same extent so that the fibre does not become highly charged. The status of the counter anions is not clear. With dye anions it seems Iikely that strong bonding sites are available, although their exact number and nature are unknown. If the counter ions are simple inorganic anions, they are absorbcd weakly if at all. It is also known tha.t water is present within the fibre and has a marked effect on the rates of migraticn and diffusion of the other absorbed materials.It is thercfore natural to assume that ions, for which strong specific bonding siteb within the fibre are not known, are in solution in the imbibed water within the fibre. Na~S04, neither ion of which is strongly absorbed, as far as is known, affords anD. L. UNDERWOOD AND H. J . WHITE, JR. 73 opportunity to test the validity of this assumption that a solution is formed in the imbibed water within the fibres. Table 1 and fig. 5 show the amount of Na2SO4 taken up by hair from neutral solutions as a function of the concentration. The solid curve represents an estim- ate of the probable true absorption isotherm. This curve is nearly a straight line, and in view of the scatter of the data a straight line could be used with almost equal justification.It is interesting to note that a similar linear plot holds to a good approximation for data on the uptake of alkali bromides by hair.7 Accepting this curve provisionally, its slope, which is nearly constant, is im- portant. If a capillary system of constant volume is filled with increasingly concentrated solutions of a salt, a plot of the amount of salt in the capillary system against the molarity of the external solution would give a straight line through the origin. This would become a straight line with a slope of 45" when plotted on log-log paper. Thus the fact that the slope of the curve is not 45" even at the lowest concentrations means that the fibre does not constitute a capillary system of a constant volume. A plot on linear co-ordinates of the uptake against the mean activity of the salt in the external solutions gives a curve concave to the activity axis, although little can be deduced about the exact nature of the curve because of the scatter of the data.Such a curve is compatible with the absorption of salt on to specific bonding sites within the fibre. Two Facts evident in table 2 are important. The incrcase in acid absorption with incrcasing ionic strength which was shown for wool in H2SO4 1- Na2S04 mixtures specifically by Oloffs~n,s is confirmed by the experiments in which sulphate uptake was measured. The experiments in which the sodium uptake from acid solution was measured show that the sodium uptake was decreased roughly fourfold compared to the uptake from a neutral solution of the same Na~S04 content.Since the ratio of the concentration of sodium ions to protons in the external solution does not change in a similar fashion, it must be concluded that the presence of the protons inhibits the absorption of sodium ions. Again such a result would be inconsistent with the assumption of a simple solution within the fibre. Thus, although Na2S04 is not strongly absorbed by hair, it is apparent that the Na2SO4 absorbed does not have the properties of the solute in a solution imbibed in capillaries in the fibre. An alternative description for its behaviour in terms of specific absorption sites scems preferable. Similar conclusions were reached from consideration of the data on the uptake of alkali bromides.7 The question of the number and exact natures of these bonding sites must await more extcnsive and precise data.In fig. 6 the solid line results from the applicationof Fick's law to the data on the rate of uptake of Na2SO4 by hair. Again a radius of 30 microns has been used for the calculations involving the swollen hair. A diffusion constant, D = 4.3 x 10-11 cm2/sec, was used. It is evident that the theoretical curve does not follow the experimental points in thc carly portion of the absorption although agreement is good elsewhere. In the discussion of rates it is assumed that the lack of uniform stirring does not influence the observed rates in a single- fibre experiment. More information about the rate processes can be obtained from fig. 7 in which the data on the rate of absorption are plotted along with data on the rate of dcsorption, data on the rate of exchange of sulphate ions with the extcrnal solution, and the Fick's law plot used in fig.6. The absorption data are plotted as (Q, - Q)/Q, and the desorption data as Q/&, where Q is the amount absorbcd by a gram of hair at the specified time, Qa is the eventual equili- brium absorption for the absorption experiments, and Qo is the initial amount absorbcd in thc desorption experiments. If Fick's law is obeyed the absorption and desorption plots should be superimposable when plotted in this way. The absorption and exchange curvcs seem to bc superimposable over the entire time74 ABSORPTION BY HAIR range. The desorption curve leaves the common curve in the region of long times.The difference between the desorption curve and the others at long times has not been explained although Crank and Henry 14 have shown that diffusion constants which depend on the concentration within the fibre in a certain way can give a very similar relation between the absorption and desorption curves. The diffusion constant obtained from exchange experiments should be constant since the concentration within the fibre does not change during such an experiment. If the diffusion constant obtained is used as an approximate indication of the type of diffusion process at hand, it is evident, as was the case with sulphuric acid, that some process slower than an aqueous diffusion is taking place. The smaller diffusion constant for Na2SO4 as compared with &SO4 can presumably be cor- related with the larger size of the more highly hydrated sodium ion.Onc thing further can be done, again if the Fick's law curve can be accepted as a reasonable first approximation to the rate of absorption. The rate of ab- sorption of Na2SO4 from the same solution at 50" C has been followed. Thc experimental points fall closely about a Fick's law curve for which the diffusion constant is D = 26 x 10-11 cm2/sec. This value combined with the value I 0 -1 FIG. 7.-Rates of absorption, desorption, and exchange of NaZS04 by hair -- Fick's law. 0 absorption, a desorption, 0 exchange, D = 4.3 x 10-11 cm2/sec for 22" C gives an activation energy for diffusion of 12,000 cal. The equilibrium uptake at 50" C for the solution in question, 0.81 M, which has a molarity, M=0*80 at 22" C, was 0.0802 mmole/g, an apparent increase in uptake with temperature. However, because of the scatter of the equilibrium data and the possibility of loss of water by evaporation for long-term experiments at the higher temperature, no significance can be attached to this increase. It seems likely, however, because of the number of fibres involved, that the rate data are more accurate especially relative to one another than are the equilibrium data. This work was undertaken as part of the Dyeing Research Project of Textile Research Institute. The authors appreciate the guidance of the Project's Ad- visory Committee representing the textile and chemical manufacturing firms who sponsored this work. One of US, D. L. U., wishes to thank Textile Research Institute for the grant of a fellowship held during the course of this work. 1 Speakman and Stott, Trans. Faraday SOC., 1935, 31, 1425. 2 Steinhardt, Fugitt and Harris, J. Res. Naf. Bur. Sfand., 1942, 28, 201. 3 LaFleur, Amer. DyesfufSRep., 1945, 34, 443. 4 Donovan and Larose, Can. J. Res. B, 1949, 27, 879. 5 Oloffson, J. SUC. Dyers Col., 1951, 67, 57. 6 Preston, Nimkar and Gundavdn, J. Text. Inst., 1951, 42, T79.D. L. UNDERWOOD AND H. J . WHITE, J R . 75 7 Barnard, Palm, Stam, Underwood and White, Textile Res. J., in press. 8 Montgomery and Milloway, Textile Res. J., 1952, 22, 729. 9 Chainberlain and Speakman, Z. Elektroclzem., 193 1, 27, 374. 1" Speakman and Elliott, Fibrous Proteins (The Society of Dyers and Colourists, 11 Hoover, Kokes and Peterson, Textile Res. J., 1948, 7, 423. 12 Lang and Lucas, Biocliem. J., 1952, 52, 84. 1 3 Barrer, Difusion in and through Solids (Cambridge, 1951), p. 31. 14 Crank and Henry, Trans. Faraday SOC., 1949, 45, 636. Lecds, 1946), p. 116.

ISSN:0366-9033    

DOI:10.1039/DF9541600066

出版商:RSC    

年代:1954

数据来源: RSC

摘要:

D. L . UNDERWOOD AND H . J . WHITE, J R . 75 THE KINETICS OF ABSORPTION OF WATER AND AQUEOUS SOLUTES BY DRY VISCOSE CELLULOSE BY H. B. MANN AND T. H. MORTON CourtauIds Ltd., Textile Research Laboratory, Bocking, Braintree, Essex Received 16th June, 1953 In pad-dyeing fabrics, the final result depends on the amount of dye taken up during the very short time the fabric is immersed in the dye liquor. The rate of water and dye absorption is thus of great technical importance and interest. The kinetics of absorption by single viscose filaments havc been studied in thrce aspects : kinetics of wetting, kinetics of the absorption of simple solutes such as inorganic salts and urea on wetting the fibre in aqueous solutions, and the kinetics of dye absorption on wetting the fibre in a solution containing dye.The wetting of the fibre is a comparatively rapid process taking about 10 sec for com- pletion at 20" C . The rate of wetting is dependent both on fibre dimensions and tem- perature ; it is approximately proportional to surface area, and the rate of increase with temperature corresponds to an activation energy of about 7 kcal/mole. Simple inorganic salts, urea, glycerol, and sucrose are taken up by the fibre proportionately rather slower than water on immersion of the fibre in the corresponding aqueous solution. The absorption of a direct cotton dye on wetting the fibre in aqueous solution follows a different course. Very little dye is absorbed during the period of wetting, thc dye being presumably concentrated in solution at the interface of the swelling fibrc ; thcreafter redistribution of the dyestuff occurs by well-understood diffusion mechanisms.The foundations for a scientific study of the physical chemistry involvcd in dyeing wllulosic materials with direct cotton dyes were laid in 1933 by Neale and Stringfellow 1 and by Boulton, Delph, Fothergill and Morton.2 Since then, a great deal of further work has been carried out by a number of workers and the mechanism of this process is now iargely understood.3 Little or no systematic work has, however, been done on the mechanism of the application of these same dyes to cotton and viscose rayon by the so-called pad-dyeing process; such a study is timely, since this process is increasingly uscd commercially. The pad-dyeing process differs in a number of respects from the more conventional methods of dyeing which havc already been studied in detail.In the conventional methods the fibres arc immersed at a relatively high tem- perature in a dyebath comprising a large volume of a relatively dilute dye solution containing salt and the dye allowed to diffusc slowly from the solution into the bulk of thc fibres : a commercial dyeing may take some hours. In pad-dyeing, however, thc fibres (in fabric form) are passed rapidly (1-5 sec) through a relatively concentrated dyc solution at a comparativcly low temperature and thc excess dyc solution removed immediatcly by passing the fabric through a mangle, dye solution corresponding approximately to the imbibition of the fibre only being76 KINETICS OF ABSORPTION retained.The impregnated fabric is then given a short after-treatment-c.g. steaming-during which the dye diffuses from the periphery into the interior of the individual fibres. In order to investigate thc various mechanisms involved it is convenient to consider the process in a number of stages : (i) absorption of solvent (normally water), (ii) absorption of solutes present (other than dye), (iii) absorption of dye, (iv) penetration of dye into the fibre during stcaming. During the prescnt investigation, the kinetics of the first three of thesc processes have been considercd. As in all the previous investigations into the physical chemistry of dyeing processes, the experimental conditions were chosen for ease of interpretation of the results rather than for their similarity to the conditions which are operative in commercial practice.In the present work, the substrate (viscose rayon) has been studied in the form of regenerated cellulose sheet and as single ends of con- tinuous filament yarn of low twist; the results represent as far as possible the kinetics of absorption by a simple cellulose lamina or filament rather than the absorption by a cornplicatcd aggregate of fibres, where mechanical retention of solvent and solutes may involve considerable complications. All fibres are composed of long polymolecular chains whose axes correspond approximatcly to the length of the fibre. There are regions-the crystallites- in which the molecular chains are in a state of comparatively high order and in which the intermolecular forces are sufficiently high to prevent penetration by water molecules.The molecular chains in the remaining amorphous regions are, however, much more randomly arranged, the intermolecular forces being cor- respondingly reduced. In cellulosic fibres in particular, the molecular chains are composed of large numbers of glucose units, the chains themselves being largely held together by H-bonding between the hydroxyl groups of these units; when these fibres are immersed in water, the osmotic forces are sufficiently large to overcome the weaker intermolecular forces in the amorphous regions and cause the molecular chains to move apart, thus permitting the entry of water molecules ; X-ray techniques, however, show that the more ordered regions of the fibre are not penetrated by water molecules.It is this swelling of the amorphous regions of the fibre in the aqueous dyebath which produces capillary spaces between the molecular chains sufficiently large for the entry of the dye molecules during dyeing. The majority of dyc molcculcs are far too large to penetrate the fibre in the absencc of this swelling. Previously it has been assumed that the swelling of thc fibre in water occurs practically instantaneously and this is probably true for viscosc rayon at the tcmperatures used in normal dyeing (- 95" C). In pad-dyeing, however, where the temperature of the dyebath is much lower and the time of immersion very small, the rate of wctting of the fibre must clearly be considered in any proposed mechan- ism for the absorption of solutes from the padding bath.Since no results on the ratc of wctting of cellulosic fibres with liquid water are available, an invcstigation into the kinetics of this process was clearly desirable. THE KINETICS OF ABSORPTION OF LIQUID WATER BY VISCOSE RAYON EXPERIMENTAL MATERIALS.-Regenerated cellulose sheet (standard " 600 " quality Cellophane), of thickness 4 x 10-3 cm ; continuous filament viscose yarns-current production of 150/72, 100/40, 150/40, 200/40, 150/27, 300/50. (A viscose yarn, 9000 m of which weigh 150 g and which is composed of 72 filaments, is briefly described as 150/72 viscose. The twist in thesc yarns is vcry low, ca. 1 t.p.i. so that the individual filaments are rclativcly free.)H . B . MANN AND T . H . MORTON 77 Since one would expect the rate of wetting to be very sensitive to the presence of spinning lubricants, etc., great care was taken to remove any such contaminant froin the yarns before carrying out the wetting experiments.Similar precautions were taken to wash the regenerated cellulose sheet free from the hygroscopic plasticizing glycerine. All the materials used were conditioned (at 65 % r.h.) and had, therefore, an initial moisture regain of ca. 13 %. PROCEDURE.-FOr the yarns, the experimental procedure adopted for the rate-of- wetting determinations consisted of padding, mangling, collecting, and determining the weight of watcr taken up. This process was carried out on a single end of yarn and it is convenient to describe the experimental details of the three operations separately.(i) Padding.-The padding bath comprised a trough containing a hollow glass roller, 2 in. in diameter, partially submerged in the padding liquor. The yarn passed through a scrics of guides, round the glass roller a specified number of times, and thence to the manglc. The wetting time, taken as the time between the yarn entering the padding bath and passing through the mangle nip, was varied by changing the number of wraps of the yarn on the glass roller; it can readily be computed from the mangle speed and the length of wetted yarn. During the operation, the roller was rotated manually, so that the tension in the yarn was virtually zero, since thc observed water absorption is, to some extent, dcpcndcnt on yarn tension. (ii) Mmgling.-The mangle was of the small laboratory type, power-driven at a constant speed of 4.3 in./scc ; the loading of the 4-in.diam. rolls (covered with rubber of 80 Shore hardness) was adjusted by weighting to 50 Win. width of the rolls. In order to obtain efficient removal of surplus water from the padded yarn it was nccessary to cover the mangle rolls with two thicknesses of a spun viscose rayon fabric and to in- corporate a wetting agcnt (Teepol, 2 ml/l.) in the padding liquor. The wetting agent served the dual purpose of assisting the removal of surplus water from the yarn by wetting- out the spun rayon fabric covering the rolls and of ensuring adequate wetting of the yarn in its passage through the padding bath. The first of these functions of the wetting agent was found to be the most important, since the equilibrium water take-up was lower in the presence of wetting agent than without it.The apparent rate of wetting too was some- what lower in the presence of wetting agent, although the lowcring of the rate of wetting was less marked than that of the equilibrium absorption. Both of these observations indicate that, in the absence of the wetting agent, the yarn was still being wetted by un- removed surplus water after mangling. Under the conditions specified, the " complete wetting " value approximated, for all except the finest filament yarn, to the normal im- bibition figure found by the standard centrifuge method ; the absorptions of the finest yarns tended to be higher, due, no doubt, to inefficient removal of surplus water under the mangling conditions used in these experiments.(iii) Collection of yam-In order to avoid errors due to drying of the padded and mangled material during collection, the yarn was taken up on a bobbin composed of two discs, 2 in. in diameter, machined from 8 in. Perspex sheet, mounted on a glass axle (1 in. diam.). The axle was rotated manually to provide the variation in speed necessary as the yarn builds up between the discs. The separation of the discs was adjusted to approximately the diameter of the wet yarn under test. In this way, each layer of yarn betwecn the discs was only able to air-dry for a very small fraction of a second before being covered by the succeeding layer of wet yarn. The distance between the take-up mechanim and the mangle was reduced as far as practicable (to approximately 1 in.) and undcr these conditions the total drying time was very small.Experiments in which the distance between the mangle and the collection apparatus was greatly increased (up to about 3 ft) confirined that errors due to drying under the former conditions could be neglected. When approximately 0.3-0.4 g of yarn had accumulated between the Perspex discs, the mangle was stopped, the yarn broken and the discs held over an open weighing bottle. Withdrawal of the glass axle caused the small coil of yarn to fall into the bottle which was immediately restoppered and wcighed. This procedure was again found to be vcry effcctivc in preventing drying errors. The yarn was then dried at 105°C and reweighed, and the watcr up-take (as g/103 dry cellulose) calculated and plotted against the time of wetting.With regenerated cellulose sheet, the ratc of wetting was much slower than with yarn ; samplcs of the shcet could therefore be imniersed in the padding bath for a selected period ; thc surface water removed by careful blotting between two sheets of blotting paper ; and the wet samplc transfcrrcd immcdiately to a weighing bottle, weighed, dried and reweighed as before.78 KINETICS OF ABSORPTION RESULTS ABSORPTION OF WATER AT 20" C.-Thc results for a selection of the yarns and for thc regenerated cellulose sheet are presented graphically in fig. 1. These and other experi- mental data permit the conclusion that, in the early stages of wetting, the amount of water absorbed is approximately proportioiial to the square root of time of wetting ; this COII- clusion is consistent with the hypothesis that the water sorption process is a diffusion, of comparatively uncomplicated nature, characterized by a diffusion coefficiciit of the order of 10-6-10-7 cm2/sec. hme t (sec) I---- 0 10 20 FIG.1 .-Dcpendence of sorption on A 1501'72 1 ;!?:\:: viscose rayon, D 300/50 i E ccllulose sheet ; temp. 20" C time. The tinies of half-wetting of thc six yarns and the regenerated cellulose sheet, obtained by interpolation from graphs of amount of water present against time, are givcn in table 1. In calculating the values for timc of half-wetting givcn in table 1, due allowance was made for the initial moisture content. As an approximation, it is concluded that the rate of wetting of normal viscose ccllulose is proportional to the specific surface area of the fibre or film.TABLE l.-RATE OF WETTING AT 20" c material 150/72 100/40 150/40 200/40 150127 300/50 regenerated cellulose sheet approx. surface nrca (103 cmz/g) 5.0 4.5 3.5 3.2 3.0 2.8 0.4 time of half-wetting (sec) 0.6 0-7 1.0 1.2 1.1 1.2 5.9 EFFECT OF TEMPERATURE.-The regenerated cellulose sheet and yarns 1 OO/40, 150/40, 300/50 were selected for these experiments. The experimental procedure used was the same as that adopted in the previous work, cxccpt that the temperaturc of the padding water was controlled a1 either 0" or 40" C, the former by the addition of crushed ice to the bath as required. Results are given in table 2.H. B . MANN AND T. H. MORTON TABLE 2.-EFFECT OF TEMPERATURE ON THE RATE OF WETTING OF VJSCOSE CELLULOSE time of activation matcrial temp."C half-wett ing energy 100/40 0 1.7 (S=) (kcal/rnole) 20 0.7 7.1 40 0.25 150/40 0 3.0 20 1.0 7.8 40 0.6 300/50 0 3.3 20 1.2 7.4 40 0.6 regenerated 0 15.9 40 3.1 cellulose sheet 20 5.9 6.9 79 It is found that the plot of the logarithm of the time of half-wetting against the reciprocal of the absolute temperature is linear-within the limits of accuracy of the experiments. The integrated form of the Arrhenius equation may be applied to calculate an energy of activation of the wetting process : whcre t 2 and tl are the times of half-wetting at the absolute temperatures T2 and T1 and AE is thc activation energy. Values of AE are given in table 2. King and Cassie4 have examined the rate of absorption of water vapour by wool fibres and have attributed the low ratcs of absorption which they observed to the nccessity of dissipating the hcat of absorption (some 750 cal/g of dry wool). It is not thought, how- ever, that the present unexpectedly low rates of wetting can be attributed to heating effects, since these experiments have almost certainly been carried out under isothcrmal conditions, and it is concluded that the measured rates of water absorption do, in fact, depend on the rate of diffusion of Iiquid water through regenerated viscose cellulose.THE KINETICS OF ABSORPTION OF SOME SIMPLE SOLUTES PROM AQUEOUS SOLUTION Before carrying out the main work of this section it was ascertained that none of the simpler solutes listed in table 3 had any significant eflect upon the rate TABLE 3.-RATE OF PENETRATION OF SOLU'TES INTO REGENERATED CELLULOSE SHEET AT 0" c solute (water) NaCl Na2S04 Na2C03 CaC12 Na3P04 AIC13 urea glycerine sucrose time or half- COIIC.g/f . pcnetration (see) I 50 50 50 50 50 50 100 100 100 (16) 20 26 30 27 26 32 18 30 40 of water absorption, although some changes in the equilibrium absorpiion were found, espccially when working with solutions of high concentration. The rates of penetration at 0" C into regenerated cellulose shect of a numbcr of simple solutes have been measured. The experimental procedure was similar to that detailed above, cxcepting that the amount of solute absorbed was also determined (the inorganic salts conductimetrically, urca by Kjeldahl analysis for nitrogen, and glycerine and swrose80 KINETICS OF ABSORPTION by weighing).Graphs were again constructed of mass of solute absorbcd (per unit mass of cellulose) against timc, and the timc of half-penetration intcrpolated as before. Rcsults arc given in table 3. THE KINETICS OF ABSORPTION OF A DIRECT COlTON DYE FROM AQUEOUS SOLUTION The rates of absorption of a direct cotton dye were measured under various conditions, both on regenerated cellulose sheet and on somc of the viscosc yarns mentioned above. The experimental procedure was analogous to that detaiIed in the previous sections, the dye absorbed being estimated colorimetrically after stripping from the viscose sub- strate with aqueous pyridine. Two dyes were investigated : Fast Red K (Colour Index 5 no. 278).Sky Blue FF (Colour Index 5 no. 518). NH2 OH CH3O OCH3 OHNH2 N=N (->- -Nn?O,Na Na03sG3 Na03S They were chosen because of their well-established constitutions and because they are known to possess very dissimilar diffusion coefficients in viscose rayon under conven- A. Fast Red K B. Sky Blue FF 0.2 % dye, 0.5 % NaCl at 20" C ; regenerated cellulose shcet. FIG. 2.-Absorption of Fast Red K and Sky Blue FF. tional dyeing conditions.6 Both dyes were obtained in an electrolyte-free state by aqueous ethanol extraction followed by further purification in an ion-exchange column ; their subsequent purity was checked by determination of sulphated-ash content and con- ductimetric titration for chloride. The effect of a number of variables on the kinetics of dye take-up by dry ccllulose on immersion in the dye solution has been investigated and consequently a very largeH .B . MANN AND T. H. MORTON 81 number of results have been obtained ; for the sake of clarity a selection of the data will bc presented here in graphical form. Before considering in detail the effect of such variables as dye concentration, sodium chloride concentration, temperature, and yarn gg 1 viscose rayon C regenerated celhlose 0.2 % Sky Blue FF, 0.5 %NaCI at 20" C sheet FIG. 3.-Effect of fiIament size. dimensions, it is convenient to summarize the main conclusions which can be drawn from the results obtained : (i) The absorption of a direct cotton dye follows a quite different course from that observed with the simple solutes; a relatively small amount of dye is initially absorbed 150/40 viscose rayon at 20" C A 0.2 %, B 0.1 Sky Blue FF %, NaCl/dye ratio, 2.5.C 0.05 %, D 0.025 % FIG. 4.-Effect of concentration. very rapidly by the cellulose, the amount being apparently independent of the amount of water absorbcd, but a relatively long period elapses before a secondary absorption of dye occurs. (ii) The amount of dye absorbed in this initial stage is very much less than that which would be contained in a quantity of solution equivalent to the quantity of water absorbed.82 KINETICS OF ABSORPTION EFFECT OF Dn-Since the dyes used in this work differ in their respective salt- sensitivities, any comparisons made between them at identical concentrations-both of dye and of salt-are of doubtful value. As a generalization, however, it may be noted that, whilst the secondary absorptions of the two dyes differ considerably, the initial absorptions are very similar (fig.2). Regenerated cellulose sheet, 0.2 % Fast Red K, 0.5 % NaC1. A, 40" C; B, 30" C; C, 0" C FIG. 6.-Effect of temperature, EFFECT OF DENIER.-h fig. 3 are shown the results for two yarns of widely differing denier and for regenerated cellulose sheet. The rates of penetration vary considerably as expected, higher absorptions, both initial and secondary, being associatcd with larger specific surface areas of the substrate. concentration upon the rates of penetration is difficult to assess without a knowledge EFFECT OF DYE CONCENTRATION AT CONSTANT SALT/DYE RA'rro.-The cffect of dyeH. D. MA" AND T. H. MORTON 83 of the equilibrium value of the absorption of dye, but the extent of the initial absorptions varies considerably (fig.4). DF= k(Ds)n, wherc DF =- concentration of dye on fibre, Ds =- concentration of dye in bath. k and 11 are constants, IZ having the value 0.76 for Sky Blue FF under the chosen conditions. The effect of sodium chloride concentration (fig. 5) is seen to be unexpectedly complex in that, at low concentrations, the amount of dye absorbed actually decreases with time over the first minute. EFFECT OF TEMPERATURE.-The effect of temperature upon the extent of the initial dye absorption is small ; the secondary dye absorption, however, is considerably faster at the higher tcn~peratures (fig. 6). Thesc initial absorption values obey the relationship : EFFECT OF SODIUM CHLORIDE CONCENTRATION AT CONSTANT DYE CONCENTRATION.- DISCUSSION The various effects found when a dry cellulosic fibre is immcrsed in water or in a solution of simple solute or dye comprise a group of phenomena more complicated than was anticipated at the beginning of the experimental work.All the observed effects can be unified, and explained at least qualitatively, by a simple working hypothesis. Imagine a dry filament of circular cross-section aaa which is immersed in water. Water and cellulose diffuse into each other, so that within a short time equilibrium is achieved, to give a swollen filament of cross-section bbb. If now the water is replaced by an aqueous solution, the water and cellulose will interact as before, but, unless the solute enters the fibre at the same rate as the solvent, some or all of the solute contained in the region bbb less aau will be " filtered " and left in the form of a highly / - , / ' ,6 FIG. 7.conccntrated solution aiong the periphery 666 ;- this high concentration of solute will tend to equilibrium by diffusion into the yarn or into the bulk of the solution. In general, therefore, an interpretation of the phenomena depends on assigning reasonable approximate values to the diffusion constants characterizing the various aspects of this process. (i) The kinetics of the absorption of water by cellulose (or, alternatively, the diffusion of thc cellulose into water) at 20" C can be characterized by a diffusion coefficient of the order of 10-6-10-7 cmZ/sec. For comparison, the diffusion cocfiicicnts ( x 106) into water may be noted : urea, 8 ; glycerol, 4 ; sucrose, 3 ; and sodium chloride, 9.(ii) If the same diffusion coefficient characterized each of the celluloses, then the rate of wetting should be proportional to the square of the specific surface arca; since approximate proportionality to the first power is found (table l), the diffusion coefficients corresponding to the finer filaments must be rather smaller -i.e. the cellulose must be tougher-than for the coarse filaments and regenerated sheet-a difference corresponding to one already well established for the diffusion of dyes in cellulose. (iii) The effect of temperature on the rate of wetting is about that expected for the mutual interdifiusion of water and a close-packed structure such as swollen ccllulosc ; about half the activation energy of the process-7 kcal/mole--can be attributed to the decrease in the viscosity of water with temperature.(iv) The rate of absorption of water from simple solutions is virtually inde- pendent of the solute and its concentration, as might be cxpected if the rate-deter- mining factor were the interdiffusion of cellulose and water ; the equilibrium value of thc swelling is, howcver, to some extent dependent on the solute and its concentration.84 KINETlCS OF ABSORPTION (v) The rate of absorption of a solute is rather slower than of the water in which it is dissolved (table 3), and it can be assumed that the solutc is rather slower than water in diffusing through swollen cellulose. The relative rates for sodium chloride, urea, glycerol and sucrose are closely paralleled by the respective diffusion Coefficients of these substances in water, quoted above.(vj) The initial absorption of dye is very much less than the amount present in the water absorbed. It is probable that most of the dye retained is absorbed by molecular entanglement in the surface layers of the cellulosc; the data of fig. 3 show that the extent of the surface is an important factor ; and the known value of the diffusion cocfficicnts of the dyes in water or dilute salt solutions, ca. 10-6 cni2/sec, indicates that any concentrated dye solution near the surface of the fibre should come into equilibrium with the bulk of the solution at about the same rate as the fibres swell in water.Thus, persistence of the initial dye absorption for the periods noted in fig. 2 and 3 is good evidence for actual fixation of some dye in the surface layers of the fibre. The drop in the initial absorption with time in a salt-free system (fig 5) is an indication that this surfax-fixed dye may, under certain conditions, diffuse back into the solution as well as into thc bulk of the fibre. (vii) The initial dye absorption increases logarithmically, not linearly, with the dye concentration in the solution; the relationship is, in form, identical with that of the Freundlich adsorption isotherm. The dye molecules swept by the absorbed water into the surface pores of the fibre set up a surface change on the fibre (cf. Crank7) which will tend to reduce the probability of later absorption of succeeding dye, so that a falling-off from proportionality of absorption with concentration is not unexpected.(viii) The secondary stage of the dye absorption is clearly the onset of the normal dycing process, whereby dye molecules are absorbed from solution on to the cellulose surface and from there diffuse into the bulk of the Cellophane. For the two dyes employed, the normal dyeing difyusion coefficients at 90" C in shect cellulose are Fast Red K Sky Blue FF 5 x 10-9 cm2/scc, 1.3 x 10-9 cm2/sec. At 20" C the dye diffusion constants may be expected to be smaller by a factor of ca. 102. These dyeing diffusion coefficients agree generally with the data of fig. 2 so far as the secondary dye absorption is concerned. (ix) The initial dye absorption is virtually independent of temperature (fig. 6) as might be expected if it is due to molecular entrapment of the dye molecules on the fibre surface. The secondary absorption is, however, very dependent on temperature. The data of fig. 6 are consistent with an energy of activation of 17 kcal/mole for the secondary diffusion process ; this value is in good agreement with the generally accepted mean value 3 of 14 kcal/mole for the energy of activa- tion of the diffusion of direct cotton dyes in viscose cellulose. (x) In the preceding paragraphs the working hypothesis has been found to conform qualitatively with each successive aspect of the data considered. It is believed that the hypothesis provides an adequate physical representation of the mechanisms and is capable, at a later stage, of more quantitative treatment ; further, it is proposed to apply this conception to the analysis of the pad-dyeing of fibre aggregates. 1 Neale and Stringfellow, Trans. Fararlay Soc., 1933, 29, 1167. 2 Boulton, Delph, Fothergill and Morton, J . Text. Inst., 1933, 24, P113. 3 Vickerstaff, The Physical Chemistry of Dyeing (Oliver and Boyd, London and 4 King and Cassie, Trans. Faraday SOC., 1940, 36, 445. 5 Colour Index (SOC. Dyers and Col., Bradford, 1st ed., 1924). 6 Neale, J. SOC. Dyers Col., 1936, 52, 252. 7 Crank, J . SOC. Dyers Col., 1947, 63, 412. Edinburgh, 1950).

ISSN:0366-9033    

DOI:10.1039/DF9541600075

出版商:RSC    

年代:1954

数据来源: RSC

摘要:

THEORETICAL ASPECTS OF THE DYEING OF CELLULOSE ACETATE RAYON BY c. L. BIRD, F. MANCHESTER AND MISS P. HARRIS Dept. of Colour Chemistry and Dyeing, The University, Leeds 2 Received 29th June, 1953 The aqueous solubility of a number of disperse dyes has been determined. Some have appreciable solubility at the usual dyeing temperature (SOo C). Even the most insoluble are solubilized to some extent by the dispersing agent normally added to the dyebath, and in these cases the rate of dyeing of cellulose acetate rayon is increased. The evidence now available is considered to support Clavel's view that dyeing takes place from a dilute, saturated aqueous solution. A study of the equilibrium distribution between fibre and dyebath of one of the more soluble disperse dyes has shown that thcre is a linear partition resembling that observed when a solute is distributed between two immiscible solvents.From the standard heat of dyeing it is inferred that two hydrogen bonds link this dye to the fibre. Cellulose acetate rayon, whilst superficially similar to viscose rayon, differs from it both physically and chemically. Viscose rayon is substantially pure cellulose, whereas in cellulose acetate rayon, consisting of " secondary " cellulose acetate, five out of every six hydroxyl groups have been repIaced by acetyl groups, which amount to about 40 % of the total weight. In consequence the fibre has a much reduced affinity for water and swells to a much smaller extent in aqueous solutions. This fact is illustrated by values for the volume term V, i.e. the volume in the water-swollen fibre available for the formation of a dye solution. Thus, Fowler and Michie 1 have assumed a volume term of 0.1 I./kg of dry cellulose acetate rayon, the corresponding figure for viscose rayon 2 being 0-45 l./kg.Similarly, Marsden and Urquhart 3 estimate the average pore size in acetate rayon to be only 5-10& whereas the corresponding figurc for viscose rayon given by Morton 4 is 20-30 A. The small diameter of the pores in acetate rayon limits the size of dye molecules able to penetrate the fibre. Meitner 5 has shown that some acid dycs of large particle radius will not penetrate viscose rayon, even when the fibre is steamed, although under these conditions they will penetrate cuprammonium rayon, which has a volume term 2 of 0.60 l./kg and is known to have pores of larger size than those of viscose rayon.When acetate rayon was first marketed it was found that, apart from the basic dyes (which have poor fastness properties) and a few monosulphonated acid dyes, the water-soluble dyes available had little affinity for the fibre. This difficulty was soon overcome, principally owing to the work of Holland Ellis of British Celanese Ltd., and of Baddiley and Shepherdson of British Dyestuffs Corporation Ltd. (now the Dyestuffs Division of I.C.1. Ltd.), who produced a new range, now known as the disperse dyes, especially for use on acetate rayon. In recent years it has been found that these dyes are also the most suitable for dyeing Nylon, Terylcne, and other synthetic fibres of a relatively hydrophobic character. The disperse dyes consist almost entirely of derivatives of azobenzene and of aminoanthraquinone, two typical examples being 4'-nitro-4-aminoazobenzene and 1 : 4-di(methylamino)anthraquinone.They are supplied as easily dispersible powders produced by milling the crude dye with a suitable anionic surface-active agent followed by drying. Dyeing is usually carried out at about $0" C in presence of 1-2 g/l. of dyebath of a dispersing agent, e.g. a synthetic detergent. 8586 DYEING OF RAYON Holland Ellis realized {hat successful dycings could only be obtained if these dyes were in a very fine state of dispersion, and manufacturers now ball-mill them to give a maximum particle size of about 2 p . The influence of particle size was shown by Vickerstaff and Waters,6 whose results arc given in columns 2 and 4 of table 1.The disperse dye used by these authors, however, has the lowest aqucous solubility of any that we have examined. Morc representative results are given in columns 3 and 5 of table 1, for a disperse dye of average aqueous solubility, but it is not suggested that there is no need for thorough ball-milling. THEORY OF DYEING AND MECHANISM OF THE DYEING mocEss.--It is necessary to account, on the one hand for the affinity of disperse dyes for secondary cellulose acetate, and on the other hand for their ability to transfer from aqueous dispersion to the interior of the fibre. To say that the dyes are " soluble " in the fibre leaves open the qucstion as to what are the forces of attraction between the solid solvent and the solute.Marsden and Urquharl,3 in their work on the swelling of cellulose acetate rayon by phenol, postulated hydrogen bonding between the hydroxyl group of the phenol and one of the acetyl groups of the secondary acetate. Since ncarly all TABLE EFFECT OF VARYING DEGREES OF DISPERSION ON DYE ADSORPTION treatmcnt crystalline dye suspended in Lissapol crystalline dye ground with Lissapol above, milled for 1 h in ball-mill ball-milled for 10 11 ball-milled for 48 h LS LS in a mortar rng dye adsorbed by Ig fibrc at 80OC in : 30 rnin 24 h ____- __ ~ _. ___._ 1 -mcthyl- p-nitro- 1-methyl- p-nitro- arnino-4- aniline a m i n o 4 aniline anilino- -5 anilino- -> anthra- N-ethyl- anthra- N-ethyl-N- quinone N-P-hyd ro x y- qui none &hydro xy- ethyl- cthyl- aniline aniline trace 3.3 0.05-0.1 7.5 1.0 6.6 2.7 7.6 1.2 7.2 3.6 7.7 1.2 7-0 4.3 7.3 2.7 7.0 5.4 7.5 the disperse dyes contain -NH2, -NHR (R = alkyl or aryl) or -OH, similar hydrogen bonding could take place between dye and fibre.There are, however, a few exception, e.g. 3-methoxybenzanthrone, and the obsolete dyes p-nitro-o- anisidine -+ dimethylaniline, p-nitro-o-anisidine --z diethylaniline, and 2 : 4-di- nitroaniline -> diethylaniline. Moreover, azobenzene itself will dye acetate rayon, to the extent of 0.40 % at 22" C. This figure may be compared with the saturation values (at 80" C) of disperse dyes on acetate rayon, many of which lie between 1 % and 2 % of pure dye. It therefore seems probable that azo and methoxy groups impart some affinity foAthe fibre, and for solvents such as ethanol, acetone and alkyl acetates, this affinity being strengthened by the presence of a -NH2, -NHR or -OH group.As far as the mechanism of dyeing is concerned it may bc assumed that the usual pattern is followed, i.e. that a surface film of dye is first produced on the fibre, followed by diffusion of dye into the interior. According to KartaschofT,7 who watched the dyeing of single acetate rayon fibres under the microscope, positively charged particles of suspended dye are attracted to the negatively charged surface of the fibre. Lauer,8 h.owever, found thc particles of dye to be negatively charged, which will ccrtainly be the case in normal dyeings when an anionic dispersing agent is present. Vickerstaff and Waters,G and later Millson and Turl,9 were unable to detect any attraction of dye particles by the fibre.C .L. BIRD, F . MANCHESTER AND MISS P . HARRIS 87 The reason why subsequent workers failed to confirm Kartaschoff's observation may be due to the presence of a dispersing agent in their experiments. Kartaschoff's used Celatine (B.D.C.) dyes consisting of aniinoanthraquinone derivatives in paste form with either sodium ricinoleate, or a hydrocarbon solvent or alcohol. In the last two cases no dispersing agent would be present. The effect of the presence of a little dispersing agent is shown in fig. 1. A fine dispersion of the dye, p-nitroaniline -+ diethylaniline, was obtained by grinding for 2 h in a mortar with water and a very little dispersing agent.The paste was then diluted and allowed to stand for 24 h. The resulting fine suspension was used for the experiments, which consisted of 10-min dyeings at 60" C , with addition of increasing amounts of purified Lissapol LS. In the virtual absence of dispering agent the suspension was unstable, i.e. somc of it was immediately adsorbed on to the fibre or on to any glass surface in contact with it. On adding dispersing agent, which is adsorbed both by the fibre and by the fine particles of dye, the rapid initial adsorption of dye by the fibre was eliminated, as illustrated by the left-hand portion of the curve in fig. 1. FIG. 1.-Effect of dispersing agent on rate of adsorption. In practicc the phenomenon observed by Kartaschoff will not normally occur, because a dispersing agent is almost always added to the dyebath.The problem to be solved consists of two parts, vk. (i) to determine how the dye, originally in fine suspension, reaches the fibre surface, and (ii) how the dye, in single molecules, diffuses into the fibre. In 1923, when the disperse dyes were just beginning to be used, Clavello suggested that dyeing takes place from a saturated solution, which is immediately replenished from the fine suspension as dye is removed from solution by the fibre. Clavel provided no evidence in support of his theory, and until recently the theory of Kartaschoff 7 was generally accepted. Kartaschoff observed that the particles of crystalline dye which were attracted to the surface of the wet fibre appeared to dissolve in the fibre on warming to 60" C.He concluded that dyeing should also take place if the dry rayon and finely powdered dye were left in contact, and he found that dyeing did in fact take place after 15 days at 60" C, or 4 days at 73" C. We have repeated some of Kartaschoff's experiments and find that dyeing will also take place if the rayon is suspended above the dye powder, thus showing, as Johnson11 has suggested, that under these conditions dyeing takes place through the vapour phase. We have not examined this phenomenon88 DYEING OF RAYON in any detail, but, as might be expected, it appears to depend on the volatility of the dye, and the property may be limited to those dyes which are known to sublime. The rate of dyeing, even when the fibre and dye are in contact, appears to be much slower at 60" or 100" C than that observed when dyeing from an aqueous dispersion.This is presumably accounted for by (i) the unswollen state of the dry fibre and (ii) the absence of any appreciable amount of water in the fibre to act as a carrier. Kartaschoff considered that the dyeing of acetate rayon was best explained by the solid solution theory of Witt. The X-ray evidence available at the time seemed to support this view; there appeared to be no fine structure in acctate rayon, so the fibre was described as a " solid colloid ", but it is now known that acetate rayon, like viscose rayon, does contain amorphous and crystalline regions, corresponding to varying degrees of disorder of the long chain molecules. Further circumstantial evidence for the solution theory was provided by the close parallel between the " solubility " of a dye in acetate rayon and its solubility in certain organic solvents; in fact, the solubility of a dye in ethyl acetatc or acetone is a measure of its suitability for colouring acetate rayon.12 The failure to repeat Kartaschoff's experiments, described above, rcvived interest in Clavel's theory, the great stumbling block to the acceptance of which is the apparent complete insolubility of many of the disperse dyes.We have now carried out solubility determinations at 80" C for a considerable number of purified disperse dyes and somc of the results are given in table 2, together with Corresponding values for 0.1 solutions of Lissapol LS (I.C.I.). TABLE 2 solubility at 80" C (mgll.) -~ dye water 01 % Lissapol LS p-nitroaniline --f aniline 9.5 11-5 p-nitroaniline 3 diphenylamine 0.5 4.0 p-nitroaniline + N-ethyl-N-/3-hydroxyethylaniline 7.0 19.0 1 : 4-diaminoanthraquinone 1 : 4-diamino-2-methoxyanthraquinone 1-methylamino-4-anilinoanthraquinone 17.0 20.0 11.0 15.0 < 0.2 3.0 The solubilities were determined by leaving a small quantity (ca.0.01 g) of crystalline dye in contact with 50 ml of either distilled water or a 0.1 % solution of Lissapol LS, using rubber-stoppered bottles partially immersed in a thermostat bath. The flow of water in the bath produced slight agitation of the carrier holding the bottles. After 3 days with water, and 24 h with the Lissapol LS solution, a sample of the liquid phase was removed by means of a pipette through a small plug of cotton-wool and the amount of dye present was estimated colorimetrically by means of a Hilger Spekkcr photoelectric absorptiometer.Since the cotton-wool adsorbed somc dye, the pipette was first filled through the plug and then emptied before taking a samplc for determination of dyc content. The more solubIe dyes can be applied from aqueous solution if a sufficiently Iargc volume of dye liquor is used-although this would not be practicabIe on the large scale -and it is almost certain that in these cases dyeing takes place on the lines suggested by Clavel. Probably more than 50 % of modern disperse dyes fall into this category. A satisfactory theory, however, must also explain the dyeing of acetate rayon by such highly insoluble dyes as l-methylamino-4-anilinoanthraquinone, which is used com- mercially.This dye imparts no visible coloration to water after 3 days at 80" C, but a slight coloration, indicating a solubility of 1-3 mg/l. was obtained at 80" C with a number of dispersing agents used at a concentration of 1 g/l. (cf. table 2). It is significant that this dye is the slowest dyeing of all the disperse dyes examined by Vickerstaff, a fact which would be anticipated if the dye has to pass through the aqueous phase, since the rate at which dye is adsorbed by the fibre is proportional to the concentration of dye in solution. It is also significant that this dye needs to be very finely ground in order to obtain maximum rate of dyeing (cf. table 1). Some rough experiments carried out mainly in order to determine the colour of various disperse dyes on acetatc rayon lend support to the view that dyeing takes place fromC .L. BIRD, F . MANCHESTER A N D MISS P . HARRIS 89 solution. A piece of acetate rayon was placed in a stoppered flask with some distilled water and a little crystalline dye, and the flask and contents kept at room temperature for one month. With the more soluble disperse dyes appreciable coloration of the rayon was observed after a few hours, and equilibrium appeared to have been reached after a few days, the water then being faintly coloured. At the other end of the scale, with l-methylamino-4-anilinoanthraquinone, only a pale blue was obtained after 1 month and the water was colourless. Although the crystalline dye was in actual contact with parts of the rayon, the colourings obtained were quite uniform from first to last and it was difficult to avoid the conclusion that dyeing had taken place via solution in the water.Further evidence is provided by the results of Corbikre,l3 who found that, with two anthraquinonoid disperse dyes, the rate of dyeing is increased by adding certain dispersing agents to the dyebath. We have obtained a similar result with 1 -methylami110-4-anilinoanthraquinone, but with more soluble disperse dyes, e.g. p-nitro-o-anisidine -> N-di(/%hydroxyethyl)aniline, the reverse effect was observed. This retarding effect is presumably similar to the well-known action of certain ethylene oxide condensates, e.g. Dispersol VL (I.C.I.) on leuco vat dyes, i.e. the dispersing agent combines with the dye and then releases it gradu- ally for combination with the fibre.The increased rate of dyeing observed by Corbikre, and illustrated by the right-hand portion of the curve in fig. 1, must be due to the fact that, with the least soluble disperse dyes, addition of dispersing agent results in considerably more dye being present in actual solution (cf. table 2). Consequently, the bottleneck caused by the low aqueous solubility of the dye is removed and the rate of dyeing is increased, in spite of the fwt that the dye must first be released from combination with the dispersing agent. Although there is now a considerable amount of evidence in favour of Clavel's theory, it does not provide a conclusive proof that dyeing takes place only via aqueous solution.In most cases the bulk of the dye is present in the dyebath in the form of a fine suspension, and it is probable that an equilibrium will be established rapidly between (i) the dissolved and suspended dye and (ii) the surface of the fibres. There will be a. little loosely adhering solid dye on the surface of thc fibres as well as in suspcnsion, as has in fact been observed by several workers in this field. If the unadsorbed dye is wholly in solution, as will often be the case at thc end of a dyeing, therc should be no superficial dye on the fibres. Thus it is possible for dye to be adsorbed on the fibre surfaces from solution or deposited from suspension. At the present time there is little indication as to how the dye passes from the surface to the interior of the fibre.The dye must pass between the long chains of the water-swollen fibre through pores, crevices or spaces in the network. These inlets appear to admit individual molecules only, and even these must not be too large. For example, we have failed to observe any diffusion of Lissapol LS (commercial sodium oleyl p-anisidide sulphonate) through a cellulose acetate film, after 24 h at 80" C . If fine particles of' dye ad- sorbed from suspension on to the fibre surfaces can break down into single mole- cules, it is possible that they may then be able to diffuse into the fibre, even if they are virtually insoluble in water. In this connection it is of interest to recall the azoic dyes, which are used on acetate rayon 14 as well as on cellulosic fibres.The aqueous solubility of these dyes appears to be even less than that of the least soluble of the disperse dyes, but when produced in situ within the water-swollen acetate fibre azoic dyes show a mobility similar to that observed in cellulosic fibres. This mobility, however, results in aggregation, not in the dispersion which must take place if individual molecules of dye are to penetrate the fibre. Dispersion via aqueous solution therefore seems a more probable mechanism, even with dyes of very low solubility, in which case the more soluble disperse dyes should show the highest rate of diffusion in the fibre. Meyer, Schuster and Bulow 15 found a constant partition ratio of 182 for the dis- tribution of o-nitroaniline between cellulose acetate and water at room temperature, over the concentration range in water of 0-180 mg/l.THE PARTITION OF DISPERSE DYES BETWEEN CELLULOSE ACETATE AND WATER.-90 DYEING OF RAYON Our work on aqueous solubilities showed that one dye, viz. p-nitro-o-anisidine -+ N-di(P-liydroxyethyl)aniline, had an aqueous solubility of 240 mg/l. at 80" C, which is suficicnt to enable partition experiments to be carried out over a wide range of concentrations. In these expcrimcnts 1 g samples of desized acetate rayon yarn were dyed at 80" C for 4 h with aqueous solutions of the crystalline dye of increasing concentration. Dyeings were carried out in stainless steel cages enclosed in Pyrex test-tubes fitted with a rubber bung. A vertical traverse was imparted to each cage via a stainless steel wire passing through a narrow glass tube in the centre of the bung.This apparatus replaced the usual open tubes in the Marney dycing machine. At the completion of dyeing a sample of the solution was removed for colorimetric analysis. The dyed yarn was rinsed, squeezed, dissolved in acetone and the dye estimated colorimetrically. For the desorption experiments dyed yarn was treated in the same apparatus at 80" C for 1 h with distilled water, followed by estimation of the dye present in solution and on the fibre. The results of thcse experiments arc illustrated in fig. 2, which indicates a linear, reversible partition over the whole range of normal dyeings. FIG. 2.-Adsorption and dcsorption of O ~ N / ~ - N = N - ( _ ~ N ( C , H ~ O M ) ~ . \- 0 adsorption. dcsorption.A similar straight line relationship has been obtained by Wahl, Arriould and SiInon,l6 using the normal dispersion technique with a series of dyes of general formula where R1 and R2 are H, OH, OC2H40H Or OC2H40C2H40H. Moreover, these authors show that a point is reached when the fibrc becomes saturated; the CUrvc then becomes a straight line almost parallel to the abscissae. The same type of curve has becn obtained by Daruwalla and Turner 17 for l-amino-4- hydroxyanthraquinone (Duranol Red 2B) ; in this case distribution was effected by steaming a cellulose acetate film in contact with a starch film containing the dye. Daruwalla and Turner consider that their results favour the solid solution theory. The solid solution concept has the mcrit of indicating the shape of the equi- librium isotherm, but on the other hand it implies that the dyeing of acetate rayonC.L . BIRD, F . MANCHESTER AND MISS P . HARRIS 91 is fundamentally different from other forms of dyeing. It secm more likely that the dyeing of acetate rayon with disperse dyes takes place in a manner similar to that observed with direct dyes and viscose rayon, i.e. dye niolccules are first adsorbed from solution on to the sites available at the surrace of the fibre and then diffuse (in solution) through the water-filled network of long-chain fibre molecules into the interior of the fibre, finally becoming fairly evenly distributed on sites located on micellar surfaces. With disperse dyes, however, the affinity for the aqueous phase, i.e. aqueous solubility, must be kept very low. Conse- quently, at the commencement of dyeing the bulk of the dye is normally present in the form of a suspension, but it cannot diffuse into the fibre until it has passed into solution.Solid solution or vapour phase dyeing probably plays a minor part in the dyeing of acetate rayon with disperse dyes, but it may be much more important with hydrophobic fibres such as Terylene and Qrlon. illustrated in fig. 2 may be uscd to determine the standard affinity (- Ap") of the dye, using the equation 18 (1) where Of is the concentration of dye in the fibre and Ds the concentration in the aqueous solution, expressed in moles/kg of completely dry fibre and moles/l., respectively, whilst Yis the volume term, the value used, viz. 0-1 I./kg, being that used by Fowler and Michie.1 The standard aanity at 80" C for the dye, p-nitro- o-anisidine --f N-di(/%hydroxyethyl)aniline is 5.87 kcal/mole, which is of similar magnitude to the corresponding figures for acid dyes on wool and direct dyes on cellulose.The value of Df/Ds, which is found to be constant, in accordance with eqn. (l), is 425. In order to determine the standard heat AH" of dyeing, affinity values were determined over a range of temperatures, using the Gilbert 19 desorption apparatus, as modified by Lemin.20 The results are given in table 3. TABLE 3 temp. ("K) 316 333 353 363 THE STANDARD AFFINITY AND HEAT OF DYEING OF A DISPERSE DYE.-The results - Ap0 = RT In (Df/VDs), - Apo (kcal) 6-35 6-1 6 5.87 5.76 Since AH" can be obtained by plotting Ay"/Tagainst 1/T, the slope giving AH", provided that a straight line relationship is obtained, as is here the case.The results in table 3 give a value for AH" of - 10.5 kcal/mole, a figure which is of similar magnitude to that obtained by Marshall and Peters 2 for two direct dycs on cellulose, viz. Durazol Red 2B and Chlorazol Brown M, which, out of a total of eight dyes, had the lowest heats of dyeing. If 5 kcal is taken as the value for a hydrogen bond, the figure of 10.5 kcal suggests that two hydrogen bonds are formed between secondary cellulose acetate and the dye, p-nitro-o-anisidine -+ N-di(P-hy droxyethy1)aniline. We desire to express our gratitude to Courtaulds' Scientific and Educational Trust Fund for a Scholarship which cnabled one of us (P. H.) to take part in this work. 1 Fowler and Michie in Vickerstaff's The Physical Chemistry of Dyeing, p. 276. 2 Marshall and Peters, J. Soc. Dyers Col., 1947, 63, 446. 3 Marsden and Urqiihart, J . Text. Imt., 1942, 33, T105. 4 Morton, Trans. Farachy SOC., 1935, 31, 281. 5 Meitner, J. Soc. Dyers Col., 1945, 61, 33. 6 Vickerstaff and Waters, J. SOC. Dyers Col., 1942, 58, 116.92 UNIMOLECULAR FILM BALANCE 7 Kartaschoff, Helv. chim. Acta, 1925, 8, 928. 8 Lauer, Kolloid-Z., 1932, 61, 91. 9 Millson and Turl, Text. Res. J., 1951, 21, 685. 10 Clavel and Stanisz, Rev. Gin. Mat. Col., 1923, 28, 145, 167 ; 1924, 28, 94, 158: 222. f * Johnson in Vickerstaff’s The Physical Chemistry of Dyeing, p. 260. 12 Mellor and Olpin, J. Soc. Dyers Col., 1951, 67, 622. 13 Corbihe, Teintex, 1948, 13, 433. 14 Mellor and Olpin, J. Soc. Dyers Col., 1947, 63, 396. 15 Meyer, Schuster and Biilow, Melliand Textilber., 1925, 6, 737. 16 Wahl, Amould and Simon, Teintex, 1952, 17, 288. 17 Daruwalla and Turner, J. Soc. Dyers Col., 1953, 69, 240. 18 Vickerstaff, The Physical Chemistry of Dyeing (Oliver and Boyd, 1950), p. 104. 19Gilbert’ Proc. Roy. Soc. A, 1944, 183, 167. 20 Lcmin in Vickcrstaff’s The Physical Chemirtry of Dyeing, p. 92.

ISSN:0366-9033    

DOI:10.1039/DF9541600085

出版商:RSC    

年代:1954

数据来源: RSC