MARGARET M. ALLINGHAM, c. H . GILES AND E . L. NEUSTXDTER 105 GENERAL DISCUSSION Dr. P. Larose (Nat. Res. Council, Ottawa) said: As supporting evidence for the view that the rate of diffusion measured was determined by diffusion through the fibres and not through a liquid layer surrounding the fibres, Dr. Hudson cites the more rapid rate of sorption which followed when the fabric was re- immersed in the solution after interrupting the sorption by removing the fabric from the solution for a few minutes and partially removing the excess liquid in a mangle. Since a higher diffusion rate would be expected under such circum- stances even for a liquid-controlled process, the evidence is not very conclusive. One could question also the use of a diffusion equation worked out by Wilson for linear absorption.Whether one favours the Gilbert and Rideal explanation for the sorption of acids by wool or the application of the Donnan equilibrium to such a system, the relation between acid sorbed and concentration is not linear. Dr. B. Olofsson (Swedish Inst. Text. Res.) said: I agree with Dr. Hudson that the a-value to be used should correspond to the radius of the single fibres, not of the yarn. When working with fabrics the same formulae are applicable, although the diffusion is appreciably slowed down, because of the structure of the material. However, if we use a neutralizing agent in equivalent amount for washing out acid, the bath being considered of infinite volume, we get the same magnitude of /3 as for free fibres, i.e. a should have the same value.But I am not sure that it is necessary to account for the low value of D, found by Hudson, by introducing the Donnan quotient k. The value in the absorption of HC1 in my paper is D == 2a2/60 N 14 x 10-8 cm2/sec and if the real absorption isotherm is used instead of the linear approximation, this value seems to increase appreciably. Also the k-values in table 1 are remarkably low at the small acid concentrations used and do not vary with concentration. Dr. R. F. Hudson (Queen Mary Coll., E.1) (communicated) : I appreciate that the application of the Wilson equation to the present system can only be regarded as an approximation, but the close similarity between theoretical and experimental rate curves is probably due to the following characteristics.Firstly, the form of the theoretical rate curve is not highIy dependent on the value of a, and secondly the changes in a near the fibre surface are not large during the absorption (table 4). It may well be therefore that the true form of the rate curve is similar to that106 GENERAL DISCUSSION assuming a linear isotherm, owing to the logarithmic form of the acid absorption curve. The actual values of D obtaincd in this way may be in considerablc error, but it seems unlilcely that the very low valucs (- 10-8 cm2/sec), which agree closely with values given by Lindberg 1 arc due entirely to the adoption of this approximate treatmcnt. The low values are morc likely attributed to the low mobility of Hi- within the fibre.2 The low mobility is due to the low concentration of free hydrogen ions within the swollen fibre, an estimation of which may be obtained by applying the Donnan membrane treatment.Prof. R. M. Barrer (Aberdeen) said : Interruption tests of the kind described by Dr. Hudson may bc intcrpreted in several ways. One may suppose first that the film mechanism is operative ; in this event the concentration gradients before interrupting the experiment will be of the form shown in fig. la. Upon interrupt- ing the experiment and then re-immersing the fibre in the dye-bath one will have 4 distance FIG. 1. condition (b), if all adhering solution was removed from the fibre of fig. l a when the experiment was interrupted. If, however, a film of water (or solution necessarily much depleted in dye) adhered to the fibre, on re-immersing the fibre the concentration-distance curve would initially have the character of fig.lc. Clearly, a distribution like that in fig. l b would give an initial acceleration after the interruption (as observed) while fig. l c would give an initially slower rate of uptake of dye. Alternatively it may be that fibre diffusion is rate-controlling. In this case the three concentration-distance curves corresponding to fig. la, b and c will be distancc (4 solution solution FIG. 2. ._ _..___- soh! tion respectively those seen in fig. 2a, b and c. Again initial acceleration will follow the interruption experiment for the distribution of fig. 2b, while a bricf slowing- down would result from the distribution of fig. 2c. Evidently therefore little con- 1 Lindberg, Textile Res.J., 1950, 22, 381. 2 Wright, Trans. Faraday SUC., 1953, 49, 95.GENERAL DISCUSSION 107 R - \ ..%> .*.> --\, - '>\ ><,,2,\ b; .o Sh 4.: '*, '\ ; 2 B 6 \ $11 B A! A I > , , ( ' , , 1 ' '\ , I :I .= 5 , 1 ' , , I I * , , I * I , I * 1 , \ ( I , \- - A &\ $,, -, 0: '5 \\ :: ; '\ :: ', ' 1 6 , ', \ :,$ ',,, '*\. a; , s+~:, - I t , . i ',\ 'tt y * *. ', ', , . ' I $ * . . \ . . (6 1 FIG. 1. solution, and has neglected the influence of the diffusion layer at the fibre surface. The rapid rate of penetration which Prof. Barrer deduced from fig. 1 (b), which refers to a film-controlling mechanism, is accompanied by a rapid change in the diffusion gradient within the layer represented in fig. 1 (6) above. The first effect on separating the two phases completely (if this is possible) will be an instantaneous decrease in concentration in solution due to thc equilibration of the concentration difference across AB.If the quantity of solute in the difusion layer is small in comparison with the quantity in solution (as in the present case), this decrease A B (6 1 FIG. 2. .A 0 is negligible. As the diffusion gradient changes from 1-4 (fig. 1 (b)) the rate of transport across the plane AA' is zero so that the concentration in solution (which is experimentally mcasured in the present case) will not change until the original diffusion gradicnt is set up. Thc rate of establishment of this gradient will be rapid compared with the original rate of diffusion, i.e., if the amount of solute within the diffusion layer is small the time-lag will be negligible. The rate of change of concentration in solution on re-immersion is therefore approximately equal to the rate immediately prior to interruption.If the rate is fibrc diffusion controlled, the diffusion gradient will be as in fig. 2(a), not as given by Prof. Barrer. For the sake of simplicity in constructing the108 GENERAL DISCUSSION diagram a distribution coefficient K of unity is assumed. In the present case, however, it should be remembered that the solubility in the fibre is very high and K is of the order of 102. On re-immersion the instantaneous rate across the plane AA’ will be zero, although the rate of penetration into the fibre is infinite. Again, the changes in the diffusion gradient 1-3 will be extremely rapid until a gradient across AB is established. As the solubility in the fibre is high, the concentration of solute at the fibre surface will be much lower than in 2 (a) so that the diffusion gradient across AB is now considerable (gradient 3 of 2 (b)).The rate of diffusion across AA’ therefore remains high until the diffusion gradient within the fibre is of the same order of magnitude as in (a). This will take a considerable time (as diffusion within thc fibre is rate determining) and the concentration in solution will therefore change considerably as shown in fig. 2 (b‘), at an increased rate. The case (c) discussed by Prof. Barrer is a very real one and is probably of more importance experimentally than case (b), owing to the difficulty of achieving a complete separation at BB‘.In either case 1 or 2, an instantaneous decrease in concentration due to mixing will occur if the aqueous film removed with the fibres (represented by CC’) is wider than the diffusion layer. This is followed by a very rapid reestablishment of the diffusion gradient across AB shown in l(c). When the original gradient (4) is set up, the rate is approxirnatcly equal to the rate prior to interruption, if the mixing effect does not change the concentration in solution appreciably. If it does, the subsequent rate will be slower. In case (2c) given by Prof. Barrer, a similar series of processes will occur, followed by the increased sorption rate given in 2(b’). In both cases, therefore, an immediate decrease in concentration will be observed followed in case 1 by il resumption of the original rate and in case 2 by an increased rate.In the experi- ment given in my paper, the volume of water adhering to the fibres was determined by weighing a mangled sample, after interruption. The decrease in concentration due to mixing on re-immersion was calculated, assuming the concentration in the liquid film to equal that in solution, and assuming that all the solute in this film is transferred to the fibre during the interruption. The calculated decreasc was subtracted from the observed concentration decreases on re-immersion in con- structing the graph in fig. 4 in the paper. In addition, it was established that no rate increase is caused by interruption under conditions of slow stirring (ca. 80 revlmin), when the mechanism is controlled by film diffusion.Similar observa- tions have been made by Kressman and Kitchener 1 on the rate of exchange on synthetic resins. Dr. K. H. Gustavsom (Sweden) (contributed) said : As a contribution to the problem of the forces involved in binding of polyvalent anions (colour acids) by proteins (keratin), which is touched upon in the first two papers, some unpublished results from investigations of the irreversible fixation of some polysulphonic aromatic acids, so-called synthetic tannins, by collagen will be mentioned. The collagen lattice is apparently able to accommodate large anions more easily than the keratin structure with its large crystalline regions inaccessible to heavy anions. Hence, the stereochemical complications are less conspicuous with collagen as the substrate.The governing effect of the affinity of the anions of polyacids for the cationic protein groups in such reactions is now generally realized. The continuous line of the curve in fig. 1 shows the irreversible uptake of a rnonosulphonic acid of a condensed phenol (Novolak) at final pH values 1-9. The dashed curve gives the percentage of the basic groups of collagen inactivated by the fixed sulpho-acid. The fixation of the Novolak is not very much influenced by the hydrogen ion concentration of the system. Particular attention should be given to the degree of inactivation of the cationic protein groups, which amounts to ca. 90 % in the pH range < 3. Furthermore, at the pH corresponding to the isoelectric point of collagen, or pH 5, not less than two-thirds of the total number 1 Kressman and Kitchener, Faraduy SOC.Discussions, 1949, 7, 90.109 of cationic protein groups have been inactivated. Even at pH values above the isoelectric point, up to pH 10, a large fixation of the Novolak occurs, more than a third of the basic groups of collagen entering into the reaction. This is a rather convincing proof of a direct interaction of the SO3--groups with the cationic sites of collagen, even at pH > 5, with subsequent discharge. Ovcr the whole PH range covered, the hydrogen bonding of the phenolic groups of the sulphonated GENERAL DISCUSSION FIG. 1.-Fixation of Novolak-sulphonic acid by collagen as a function of the final hydrogen ion concentration and the degree of inactivation of the cationic protein groups by the suipho-acid.Novolak on the protein chains (probably to the keto-imide groups) has been shown to occur. The rapid drop in fixation at pH > 10 is probably due to the smaller number of hydrogen bonding sites (OH groups) resulting from the ionization of the OH groups at the high pH values. A third type of binding may be due to electrostatic attraction of dipoles in the aromatic rings to the resonating keto-imide links. Particularly Otto 1 has emphasized the important I I FIG. 2.-Fixation of trinaphthalene-dimethane-disulphonic acid by collagen as a function of the final hydrogen ion concentration, and the degrees of inactivation of the cationic protein groups by the sulpho-acid. function of the >C-H group adjacent to the substituent in the aromatic nucleus as a dipole (mdectrons) in the reaction of dyestuffs and synthetic tannins with fibrous proteins.Some support for the suggestion of the participation of such auxiliary forces is given by data from the fixation of trinaphthalene-dimethane-disulphonic acid by collagen, represented graphically in fig. 2. The sole reactive groups of this compound are the terminal sulphonic acid groups. Accordingly, it would be 1 Otto, Leder, 1953, 4, 193.110 GENERAL DISCUSSION expected that collagen would not bind this fairly strong acid at final pH values above 5, the isoelectric point of ordinary limed hide powder. On the contrary, a marked fixation takes place in the pH range 5-10. Further, it is important to note from the curve showing the extent of inactivation of the cationic protein groups that the binding of the naphthalene compound at pH values above 7 does not result in any inactivation of the basic protein groups.Since no co- ordination centres are present in this compound, the only valency forces available for the binding are those associated with the resonating )CH group and the -CH2 link connecting the naphthalene rings. Whether the binding of the naphthalene sulphonate is connected with different electron density or due to other factors must be left open. In any case, these data form additional evidence that the polyvalent sulpho-acid anions are held in the protein structure by various forces, the relative importance of the various types of forces depending on the structure of the anions and the experimental conditions. Dr.G. A. Gilbert (Birmingham University) said : Perhaps the result most calling for comment in the paper by Peters and Lister is the value they find for the heat of reaction of wool with Orange 11, C.I. 151. This is ca. -0-8 kcal/mole in the temperature range 60-80" C and leads, when combined with the standard free energy of adsorption, to a calculatcd entropy of adsorption of + 10-20 cal/deg. mole more than the corresponding value for the adsorption of HCI. Lemin and Vickerstaff, on the other hand, in ref. (3) quoted by Peters and Lister, estimate the heat of reaction for the same dye (Orange G, C.I. 151) to be -7 kcal/mole, and the entropy change on adsorption compared with HCl to be - 8 cal/deg. mole. Since detailed arguments are put forward by Peters and Lister which depend upon the sign and magnitude of this entropy change, some inquiry seems necessary into the origin and meaning of the difference between these two sets of experi-- mental results.Can it be that in one or the other cases there has been lack of equilibrium, or can aggregation of the dye solution possibly be affecting the results of Peters and Lister more than those of Lemin and Vickerstaff, because their technique requires concentrations of dye much higher than those used by Lemin and Vickerstaff ? Is it not perhaps significant that the heat of dilution of Orange G is found by Derbyshire and Marshall (this Discussion) to be 6.0 kcal/mole, and that this figure corresponds almost exactly in sign and magnitude to the one required to reconcile the two results ? These differences in sign emphasize how desirable it is to avoid introducing confusion by making unnecessary changes of convention? and it is to be hoped that energies of adsorption and not desorption will continue to be used as hitherto.The question as to whether simple anionic dyes react preferentially with the positively charged groups of wool may receive its most direct answer from studies with dissolved proteins. There the hydrogen ion no longer plays such a domin- ating role and the independent adsorption of anions is possible. Some of the extensive evidence which already exists in this field was reviewed by Klotz and Ayers recently.1 Mr. G. King (Wool Industries Res. Assn., Leeds) (said) : I would like to apply Gurney's theory 2 of proton transfer to Dr.Peters' results. Gurney writes (1) where A and B are constants, and E is the dielectric constant of the solvent medium. The first term represents the free energy contribution due to quantum forces, and the second is the electrostatic contribution. The former is largely independent of temperature while the latter will vary in accordance with the dielectric constant against temperature relation. Assuming the solvent to be water, the above relation gives a good fit with AF" = - R T h K = A + B/E, 1 Klotz and Ayers, Faraday Soc. Discussions, 1953, 13, 189. 2 Gurney, J . Chem Physics, 1938, 6, 499.GENERAL DISCUSSION 111 Peters’ curve for the temperature dependence of pK for HCl, and values are ob- tained for A and B, viz., A = 2.9 kcal/mole, B/E = 2.9 kcal/mole (e = 80). Now relation (1) also gives the change in pK value with change in dielectric constant of the solvent and predicts that the afinity of HC1 for keratin should increase as the dielectric constant falls.This is in agreement with the results of Daniel 1 for HC1 absorption by gelatin from alcohol 4- water mixtures. Ac- cording to her results, the mid-point of thc titration curve increases in pH by about one unit on changing from water to a mixture containing 60 % ethyl alcohol. Assuming the values already determined for A and B to hold for the gelatin system, and taking E for the alcohol mixture to be 44, the pK in the alcohol solution turns out to be 6.0 as compared with 4.3 for water. This corresponds to a shift in pH of 0.85 unit which is near to the value measured.Prof. R. M. Barrer (Aberdeen) said : Dr. Peters and Dr. Lister make use of a quantity which they describe as the “ standard free energy ” of sorption as a measurc of the “ affinity ” of a solute for the fibre. The “ standard free energy ” obtained varies according to the amount of solute at equilibrium in fibre and in solution, and cannot therefore be the thermodynamic standard free energy. It is as well to recall how the true standard free energy is derived, and how it is related to the “ standard free energy ” of Peters and Lister. Consider the transfer of a solute from a solution where it has an arbitrarily chosen activity cis to the fibre where it has an arbitrarily chosen activity af. The free energy AG of the transfer is given by the van’t Hoff isotherm as (1) If each of the arbitrarily chosen activities af and as refers to the standard state of unit activity, then a f = a, = 1 and (2) where AGO is the true standard frce energy and (af/as)equ2e is the thermodynamic equilibrium constant.Whatever the individual concentration values (CJ)~~,,~. and (Cs)equil. in the quotient (C’/Cs)equil. the thermodynamic equilibrium con- stant (af/aJeqUi~~ will be the same, and so only one thermodynamic standard free energy is possible. Peters and Lister have used activities as given by the model in the Gilbert- Rideal theory to evaluate a so-called equilibrium constant and a so-called “ standard free energy ” AG’o such that AG = - RT In (aflaJequil. + RT In (ajlas).AG = AGO = - RTln (af/aJequil., If the Gilbert-Rideal theory is imperfect the expression in the brackets in the right hand side of eqn. (3) will not give (q/aS),,,il. correctly, and the left-hand side will thus not give the true thermodynamic standard free energy. In particular the right-hand side of eqn. (3), and therefore AG’o, will probably vary according to the absolute values of the concentrations and (CJequil. This is just what Peters and Lister have observed, and the observation thus amounts to a demonstration of quantitative imperfection in the Gilbert-Rideal theory. One should not in these circumstances use AG’o as a quantitative measure of the affinity of the solute for the fibre. When using mixtures of HC1 and H2SO4 a standard free energy term for the anions may be writtcn as Dr.B. Olofsson (Swedish Inst. Text. Res.) said: 1 Daniel, J. Gen. Physiology, 1933, 16, 457.112 GENERAL DISCUSSION If experimental values are substituted we get AGO < 0; and we get about the same AGO value if instcad of using " Gilbert-Rideal activities " we calculate internal activities as " Donnan concentrations " a,& and as&, where a is the amount absorbed and v the corresponding volume of the swelling water. How- ever, if v is calculated as the total swelled fibre volume, the absolute value of AGO decreases appreciably and is probably not significantly different from 0. Mr. D. M. G. Armstrong (Royal Veterinary COX, London) said : In connection with the controversy as to the applicability of the Donnan, Steinhardt and Harris, and Gilbert and Rideal theories, it seems to me that the theory chosen should be such that it applies equally well to the titration of keratin, collagen and ion- exchange resins since these form very similar systems.It would appear that neither the Steinhardt-Harris nor the Gilbert-Rideal theories apply to collagen when in contact with acid and salt solutions since a two-phase system consisting of swollen collagen and solution, of differing ionic molalities, is established. 1 Dr. C . H. Giles (Royal Tech. COIL, Glasgow) said : The straight-line isotherms for disperse dyes sorbed on cellulose acetate (and polyethylene terephthalate) 2 are not necessarily inconsistent with a mechanism of sorption at specific sites. The tangent of the angle, relative to they-axis, of any portion of an isotherm, represents the increment of equilibrium concentration of solute (or vapour) in the external phase, necessary to maintain a constant increment of concentration at equilibrium in the substrate.The complement of this angle, i.e. the slope of the isotherm, at any point may thus be regarded as a measure of the ease with which bombarding solute molecules can find vacant sites in the sorbate. At the origin of a normal sorption isotherm (B.E.T., type 1) the slope is high, all sites being vacant. At the saturation point the slope is nil, for no vacant sites remain. Between these points, the steadily diminishing slope represents the progressive filling up of the sites. A straight-line isotherm is therefore consistent with conditions in which the number of equal-energy sites does not diminish as sorption progresses.Such conditions might arise in one of two ways : (i) the solute, but not the solvent, is a swelling agent for the substrate. As each solute molecule becomes attached, it forces apart molecules of the substrate and exposes fresh sites. The fibres under discussion are hydrophobic, so that the use of water as the solvent medium, and dyes with specific attraction for the fibre, would satisfy the required conditions. (If the solvent can swell the fibre, maximum swelling will occur even in solutions of zero concentration, and the process of sorption of solute cannot then expose fresh sites. Thus, e.g., sorption isotherms of dyes on cellulose from aqueous solutions are normal (type l).) (ii) An alternative, but less likely, explanation is that the balance of affinities between solvent + solute, solute + solute, and solute + substrate is such that a multilayer of solute molecules can readily form on the substrate surface; i.e.a bombarding solute molecule will become attached to one of its own kind, pre- viously fixed, as readily as to a vacant site in the substrate. Thus the number of available sites does not diminish as the solute is progressively sorbed. To account for the abrupt change of slope of the isotherm at the satura- tion value it is necessary to suppose there is a sudden decrease in available sites, as e.g., might occur if sorbed molecules blocked access to intermolecular pores. A sorption process of type (i) above can still be considered as a form of solid solution, even though specific solute + substrate interactions are involved.The partition of a solute between two immiscible solvents is a special case of condition (i) above. Immiscible solvents can be considered as mutually non-swelling, and 1 Armstrong, The Nature and Structure of Collagen (Butterworths, London). 2 Schuler and Remington, this Discussion.GENERAL DISCUSSION 113 the solute can be considered to be a swelling agent for either. The interface is mobile and fresh sites are continually exposed. The general question of isotherm shape in relation to sorption mechanism, briefly discussed here, will be dealt with in more detail elsewhere. Dr. H. E. Schroeder (du Pont de Nemours Co., Delaware) said: Our work with synthetic polymers shows clearly that in cases where a specific concentration of ionic sites is introduced in the polymer, subsequent saturation and equilibrium dyeing studies point to an excellent agreement between the concentration of dye ions and number of sites.For example, in our paper on polyacrylonitrile containing pyridine cations or co-ordinated cuprous cations, the dye anion con- centration extrapolates very closely to the site concentration. With polyamides, the work of Remington and Gladding 1 shows that dyeing occurs on terminal amino groups as NH3+. The limiting concentration of dye anion is equivalent to the number of amino sites. Further application of dye at low pH leads to chain scission. This method is in fact sufficiently precise to permit measurement of the concentration of amino chain ends.There appears to be no reason for using the Donnan concept in these cases. Dr. J. L. Horner (Wool Industries Res. Assn., Leeds) said : With regard to the presence of a small excess of carboxyl groups over proton acceptor groups (i.e. imidazole, terminal and side-chain amino and guanidine groups), referred to in the paper of Lister and Peters, reference to the works of Steinhardt et al. shows that they purified their wool samples by low temperature solvent extraction, followed by repeated washing with distilled water. No occasion arises, there- fore, for hydrolysis of amide groups during preparation. I have obtained a theoretical titration curve for the combination of alkali with wool protein 2 which, when compared with the experimental curve for virgin root wool, gives definite indication of the presence of free carboxyl groups in the isoelectric protein.Thus, although the theoretical curve agrees precisely with experiment above pH 11, perfect agreement between the two at pH 6 to 11 can only be obtained if the presence of 0.05 mequiv. of excess carboxyl group per g of dry protein is postulated. Recent analyses of the amino-acid composition of wool protein by Corfield and Robson,3 who used partition chromatography coupled with a tracer technique, give a total for the basic side-chains of 0.835 mequiv./g, which, with terminal amino groups 4 yield the value of 0.852 mequiv./g for basic groups. This may be compared with approximately 0.90 mequiv./g for the total of carboxyl groups, leading to the conclusion that there is a small excess of carboxyl groups in the native protein.Dr. K. H. Gustavson (Stockholm) said : The technique described by Armstrong which enables continuous estimation of the rate of diffusion of vegetable tannins through skin, will likely be very useful for the study of the progress of tanning processes in situ. The selection of mimosa tannin for the main investigation was wise, and even fortunate, since the tanning mechanism is much simpler with this tannin than with tannic acid, a fact also stressed by the author. This observation is supported by results from an entirely different approach, i.e., by investigating the reactions of the two tannins with collagen and other substrates. In view of the fact that the paper under discussion is the only one bearing on the vegetable tannage, some data from unpublished work, directly related to the present subject, are deemed to be of sufficient importance for con- sideration on this occasion.In the experiments which form the basis of the results shown graphically in fig. 1 and 2, solutions of mimosa extract, a typical condensed tannin, and of 1 Remington and Gladding, J. Amer. Chem. SOC., 1950, 72, 2553. 2 Horner, to be published. 3 Corfield and Robson, private communication. 4 Middlebrook, Thesis (University of Leeds, 1949). Tibbs, Thesis (University of Leeds, 1951).114 GENERAL DISCUSSION tannic acid, a representative of the hydrolyzable class of tannins, were interacted with intact and modified specimens of hide powder, as well as with the modified polyamide, mentioned in my paper.Its main reactive sites are the -CO . NH links. A certain degree of irregularity is imposed upon the polyamide structure a I $ f T 6 7 4 2 /e ~ FIG, 1 .-Mimosa extract. 1: hide powdcr ; 11: hide powder + 3w/v % NaCl ; 111: methylated hidc powder ; IV: polyamide. FIG. 2.-Tannic acid. I: hide powder; 11: hide powder + 3w/v % NaCl; HI: mcthylated hide powder; IV: polyamide. by copolymerization (Nylon-66 with caprolactame), which enables the ac- commodation of molecules of the size of vegetable tannins in the interchain spaces and further makes free co-ordination sites (for hydrogen bonding) available on some of the keto-imide links (hydrated). The curves show the irreversible fixation of the two tannins by ordinary hide powder, esterified hide powder and the hydrated polyamide from solutions covering the pH range 2-9.Firstly, on examining the curves for the fixation of mimosaGENERAL DISCUSSION 115 tannin (fig. l), it is to be noted that its binding by the polyamide is independent of the Hf ion concentration of the solutions over the whole pH range, large quantities of tannin being fixed. This is an experimental proof of the ability of the keto-imide group to bind polyphenolic tannins, probably by a mechanism of hydrogen bonding. The reactivity of the peptide link would be expected to be independent of pH. The sharp drop of the fixation of tannins by the polyamide and coiiagen substrates at pH > 9 is most likely due to the ionization of the phenolic groups at pH > 9, denoting a smaller number of OH groups available for hydrogen bonding.The intact hide powder shows the typical pH function in binding of vegetable tannins, with a minimum in the isoelectric zone of collagen, a marked maximum at pH - 2 and a rapid decline of the curve in the range of high pH values. The general trend of the fixation curves reflects the number of available reactive protein groups, non-ionic as well as cationic ones, for the fixation of the tannins, which is a function of the degree of swelling of the substrate. If the swelling is eliminated by the addition of salt, the curve of the fixation of the mimosa tannin is independent of the pH of the system in the range 2-8. These data indicate the dominant role of the keto-hide group as the site for fixation of tannins of the condensed type.It will be evident from data to be presented in the following that their fixation by collagen also involves its ionic groups. The corresponding fixation of tannic acid, which contains strong acidic groups (carboxylic) besides numerous weakly acid groups (phenolic) is more complicated (fig. 2). The polyamide fixes very large amounts of tannic acid in a restricted, very acid range. A tremendous drop in fixation is already evident at pH 4-5. Augmented molecular weight of tannic acid by aggregation at low pH values, and participation of the -COOH group of the molecule of tannic acid in hydrogen bonding on the polyamide are factors which have to be considered in any attempt to explain the peculiar pH function of the fixation.The pH function of the intact hide powder resembles that of the mimosa tannin. However, it should be noticed that the swelling-depressing action of salt hardly effects the curve of the fixation of tannic acid, and further that the fixation curve of esterified collagen for tannic acid is entirely different from that of the mimosa tannin. The curve which represents the system tannic acid + polyamide proves that the keto-imide group plays a subordinate role compared to that in the mimosa + polyamide system. It is also seen that the participation of ionic protein groups in the fixation of tannic acid, and of hydrolyzable tannins generally by collagen has to be included in the tanning equation. Further information on the mode of fixation of the two tannins by collagen has been obtained by comparative studies of their fixation by hide powder (through ionic and non-ionic groups) and by modified hide powder, in which the ionic groups have been inactivated by irreversibly fixed condensed naphthalene di- sulphonic acid.The latter does not appreciably interfere with the reactivity of the non-ionic groups of collagen. Thc modified collagcn should mainly react by means of non-ionic valency forces (keto-imide groups). Some relative values obtained after 24 h tannage show the ratio of ionic to non-ionic binding of mimosa tannin to be about 1/1, whereas the corresponding value for tannic acid is of the order of 3/1. There are certain indications that the ionic protein groups are involved simul- taneously with the non-ionic (hydrogen bonding) groups in the fixation of a part of the mimosa tannins1 That would imply a multipoint fixation of the tannin molecule by means of the two principal types of valency forces.In the initial fixation of tannic acid, with ionic protein groups dominating, a considerable part of the tannins held in irreversible binding with collagen would be expected to have reacted with the cationic protein groups. The marked shift of the pH corresponding to the isoelectric point of collagen towards low pH values which 1 Page, J. SOC. Leather Trades Chent., 1953, 37, 183.116 GENERAL DISCUSSION is produced by the binding of tannic acid by collagen and the convincing experi- ment of the late F. C. Thompson 1 demonstrating complete displacement of hydrochloric acid held by gelatin by the addition of tannic acid, may be cited as proofs of this hypothesis.Finally, fig. 3 and 4 give an idea of the rate of fixation of tannic acid and mimosa tannin by the two types of protein groups involved in The initial, very rapid fixation which has been shown to impart their fixation, hydrothermal FIG. 3.-Fixation of tannic acid by ionic and non-ionic protein groups (sulpho-acid inactivated). FIG. 4.-Fixation of mimosa tannins by ionic and non-ionic protein groups (sulpho- acid inactivated). stability to the collagen lattice 2 appcars to be due mainly to the participation of ionic protein groups. The gradual uptake of tannin in the later stage of the pro- cess is shown to be hydrogen bonding on non-ionic protein groups. It is clcar from the data reviewed that mimosa tannin (condensed) and tannic acid (hydrolyzable) show cntirely different types of reaction with collagen.1 Thompson, J. Int. SOC. Leather Trades Cliem., 1934, 18, 175. 2 Gustavson and Nestvold, Leder, 1951, 2, 121.GENERAL DISCUSSION 117 Armstrong’s observation that the reactions of stages I1 and 111, in his designation, probably occur simultaneously in the tannic acid system may be due to the different types of reaction which have taken place on the fixation of tannic acid by collagen, while the tannins of mimosa show a more normal behaviour towards collagen. It is also clear from these investigations, and these facts are of sufficient importance to merit restatement, that the mechanism of the reaction of hydro- lyzable tannins with collagen involves ionic protein groups to a great extent besides non-ionic protein sites, and thus that it differs markedly from the re- action of the condensed type of vegetable tannins with hide protein, in which tannage the non-ionic groups of collagen are primarily concerned, but not, however, to the complete exclusion of other groups.Dr. B. Olofsson (Swedish Inst. Text. Res.) said: I would ask Dr. Armstrong if his moving boundary is really sharp. This is, however, a very general question as such boundaries are formed not only in tanning but also when dye or vapour penetrates fibres. Only if this boundary is sharp is the theory of Hill-Hennans strictly applicable; if not a theory involving the sorption isotherm as devised by Wilson-Crank and myself must be used. Mi.D. M. G. Armstrong (Royal Veterinary College, London) (communicated) : In reply to Dr. Olofsson’s query, Stather 1 observed that the boundary between the tanned and untanned region of skin was very sharp with the following tannin materials ; mimosa, quebracho, mangrove, myrobalans, sumac and pine bark. With sulphited quebracho, chestnut, algarobilla, valonia and gambier, the boundary was not very sharp. With mimosa, the sharpness of the boundary can be seen in sections, unstained with dichromate, over the whole of the first stage of tanning. With tannic acid, the boundary is difficult to see since the tanned region is almost the same colour as the untanned in unstained sections: in crudely stained sections it does not appear very sharp. Thus the use of the Hill-Hermans theory is justified in many systems.In those where it may not be strictly applicable, its simplicity and the unavailability of any valid sorption data justify its use for practical purposes where approximate estimates of rates and states of tanning are better than none at all. Prof. R. M. Barrer (Aberdeen) said : The term self-diffusion originally referred to the diffusion of a substance in itself, e.g. Pb in Pb ; Au in Au ; H20 in H20 ; H2 in H2, and so on. The use which has been made of the term self-diffusion in the paper by Wright is an extension of the term in a direction which cannot be wholly justifiable. The dyestuff tagged by radio-active atoms is not diffusing in a crystal of itself, but is surrounded by and is diffusing in the fabric of the polymer.The polymer, not the dye, is the diffusion medium. It seems desirable to limit the term self-diffusion to its original use, and to employ some other term for a description of the process discussed by Dr. Wright. Dr. Andre Parisot (Inst. Text. de France) said: I t would seem that in all the papers presented insufficient account has been taken of the architecture peculiar to the wool fibre. It has been proved that the actual surface of the fibre presents a physical structure differing from that of the layers situated immediately below. Whether one regards the epicuticle as being a distinct morphological membrane, or as being a superficial state of the cuticle, it remains none the less true that this epicuticle governs all the phenomena of diffusion in the wool fibre.Since its slight thickness makes it somewhat fragile, the treatments to which the fibre is subjected may destroy or considcrably modify the epicuticle. The result of this is that the subsequent behaviour of the reagents on passing from the sur- rounding solution into the fibre will depend on the treatments to which the fibre has been previously subjected. The diffusion of the reagent, in particular, is one of the phenomena most liable to be affected by this factor. 1 Stather, Collegium, 1933, 326.118 GENERAL DISCUSSION The morphology of the fibre is aIso a factor, especially its degree of crimping or curliness. Certain dyes are more readily absorbed in the concave areas; and we have found that the bubbles or blisters of the Allworden reaction, consisting of the action of bromine water or chlorine water on the wool, more readily appear on the convex portions of the crimps.Dr. L. Valentine (Leeds University) said: I should like to take this oppor- tunity to present some results on a diffusion-controlled reaction involving wool fibres, although neither dyeing nor tanning is actually involved. Lipson and Speakman 1 showed that considerable amounts of vinyl monomers, e.g. methacrylic acid, could be polymerized within wool fibres by first impregnating them with Fez-!- ions, and then immersing them in an acid aqueous solution of the monomer and H202. The H202 and monomer diffuse into the wool, whereupon the H202 reacts with the Fez+ to liberate OH radicals which initiate the polymerization ; it can be shown that the polymerization takes place within, not on the surface of, the fibres.A study of the polymerization of acrylonitrile within loose wool at 25", using a slightly modified version of Lipson and Speakman's technique, indicates that the percentage increase in weight Wof the wool due to polymerization is a linear r------' ' ' -I 1- 80 I FIG. 1. function of the square root of the time t up to a value of W e 4 0 %. The reaction mixture, which was not agitated, consisted of acrylonitrile 3.8 %, H202 0.003 %, H2SO4 0.01 N, liquor/wool ratio, lOS/l. As shown in the fig. 1, the weight increase reached a saturation value at W = 75 %. The W against t+ relationship is similar to that found for a diffusion process followed by rapid adsorption, and it scems probable that here the polymerization is rapid compared to the diffusion of reactants, which then becomes the rate-determining step. The apparent diffusion constant D has been calculated on this assumption by two slightly different methods which yicld concordant results : (i) Using Hill's 2 graph of the average degree of saturation as a function of D r p , where r is the radius of the fibre (24 p), the following values were obtained : av. degree of saturation 0.15 0.30 0.50 0.75 0.90 D (cm2/sec x 1011) 1.02 0.99 1.14 1-15 0.85 (ii) Using the initial lincar W against t t plot, D was calculatcd from Hill's equation for the amount Q diffused into a semi-infinite solid, viz., Q = 2 c ( D t / ~ ) 4 1 Lipson and Speakman, J.SOC. Dyers Col., 1949, 65, 390. 2 Hill, Proc. Roy. SUC.B, 1928, 104, 39.GENERAL DISCUSSION 119 where c is the concentration determining diffusion. There is some ambiguity as to the meaning to be assigned to c in this reaction, but reasonable values would seem to be either (a) the saturation concentration of monomer units in the wool + polymer system, taking the volume to be the total volume of the wool 4- polymer (0.58 Q cm-3), or (6) the concentration of monomer units in (i) poly- acrylonitrile, i.e. its density (1.14 g cm-3), or (ii) acrylonitrile monomer (0.80 g cm-3). These assumptions lead to values of D of 2.3, 0.6, and 1.2 X 10-11 cm2/sec, respectively, the last figure being in good agreement with that calculated from the saturation increase in weight. It is rather striking that the reaction can apparently be described adequately by a single diffusion constant up to a weight increase of 70 % at 25" ; preliminary experiments at 37" indicate that a similar position holds up to weight increases of 100 %.Compensatory processes are probably at work, since the increase in fibre diameter would counteract the expected diminution in D as polymer fills the fibre. The absolute value of D is perhaps somewhat lower than might have been expected for the diffusion of acrylonitrile into swollen fibres at pH 2 ; Hudson 1 finds values of ca. 10-9 cm2/sec for picric acid and C6HsS03H for well-agitated solutions. (Extrapolation to zero rate of stirring suggests values of ca. 3 x 10-10 cm2/sec.) The activation energy has been found to be 7.7 kcal/mole, compared with the value of 6.3 kcal/mole for C6H5S03H found by Hudson.However, both the absolute value of D and its activation energy are in harmony with the view that D is a true diffusion constant, probably that of acrylonitrile into the fibre, although it cannot be claimed that this has been proved. Mr. D. M. G. Armstrong (Royal Veterinary Coll., London) (communicated) : In the paper by Underwood and White, it is not clear whether any adsorption of sodium sulphate on hair takes place although there is no doubt that there is ab- sorption of the salt into the fibre. It is unlikely that there is any positive adsorp- tion of sodium sulphate by hair since with skin collagen Eilers and Labout 2 have shown, from analyses of solutions of sodium sulphate and other saIts in contact with skin powder, that negative adsorption takes place, part of the water inside the fibres (to the extent of 0.38 g/g collagen with 0.2 M sodium sulphate) being "non-solvent " water.In the absence of any clear evidence for positive adsorption there is no justification for the suggestion that there are bonding sites in the fibre to the presence of which absorption can be ascribed. Dr. P. Larose (Nat. Res. Council, Ottawa) said: Nowhere in the paper of Underwood and White is there any evidence that a correction was made for the acid or salt that would normally be taken up by the fibre due to the free water in it. If that has not been done then the evidence for the sorption of sodium sulphate is compIeteIy destroyed as the following figures show. For acid sorption, neglect of the correction does not give rise to such large errors.Uptake (mm/g) * tcxpt. calc. expt. calc. expt. calc. expt. calc. 0.0285 0.031 0.0419 0.071 0.0026 0*0008 0,0132 0.0033 0.00443 0.0033 0.0204 0.029 0.0204 0.031 0.080 0.156 0.058 0.207 0.079 0.240 0.083 0.42 0.099 0.43 0.003 15 0.00079 0.0380 0,074 *This is the uptake that one would expect if it is assumed that the hair contained Texpt. values from column 4, table 1, in the paper by Underwood and White. 0-30 ml of free water per gram before immersion in the salt solution. Dr. M. L. Wright (Wool Industries Res. Assn., Leeds), said : In connection with the paper of Underwood and White I would like to give an account of some 1 Hudson, this Discussion. 2 Eilers and Labout, Symp. Fibrous Proteins (Soc. Dyers Col.) (Bradford, 1946), p.30.120 GENERAL DISCUSSION work on the absorption of simple salts by horn keratin (at present in the press) 1 as it is related to the work of Underwood and White. Using horn membranes in radioactive NaBr solutions exposed to the air, measurements have been made of self-diffusion, membrane conductance, membrane potential and equilibrium sorption. It was found that the sodium and bromide ions are not picked up in equal amounts, the disparity being greatest in dilute solutions (fig. 1). Hydrogen ions maintain the electroneutrality ; these can be obtained from the solutions which can absorb atmospheric carbon dioxide to give a solution at about pH 5.7. It is suggested that when keratin and similar polymers are placed in dilute salt solutions, the net result is mainly the sorption of the acid of that salt.Evidence for this can be obtained by consideration of the work of Steinhardt on the acid titration of wool keratin in the presence of salts at various ionic strengths2 From their data, the amounts of hydrogen ion combined at pH 5.7 for various KC1 concentrations can be obtained ; these are in good agreement with the values obtained, by difference, from the amounts of bromide and sodium absorbed. FIG. 1.-Ionic sorption from NaBr solution by horn keratin at pH - 5.7. Any one, or any one combination of the equilibrium sorption theories can be applied to the sorption of these three ion species, viz., Na, Br and H, but un- fortunately it does not seem possible to use the results to distinguish critically between the theories. If we apply the Gilbert and Rideal3 theory to the NaBr results we get a value for ApNaBr approximately zero, indicating little specific affinity.Using the same results we can calculate A h e r at approximately 6-7 kcal/mole which compares well with results obtained from the HBr titration curve for wool keratin. When the simple Donnan theory is used, the ratio of the ionic activity product inside to outside should be unity. If we calculate the activity inside as a concentration in the imbibed water using Peters and Speakman’s4 value of 0.285, values about 6-7 are obtained, but if we regard the keratin as a total swollen phase then the values become nearer unity. 1 Wright, Trans. Faraday SOC. (in press). 2 Steinhardt and Harris, J. Res. Nat.Bur. Stand., 1940, 24, 335. 3 Gilbert and Rideal, Proc. Roy. SOC. A, 1944, 182, 335. 4 Peters and Speakman, J. SOC. Dyers Col., 1949, 65, 63.GENERAL DISCUSSION 121 Referring to Underwood and White’s results, I would like to suggest that the low value found for DNazso4 is in fact the measurement of the small diffusion coeficient for hydrogen ions at low concentrations in the polymer1 which is controlling the process. Secondly it is seen from analysis of fig. 1 in their paper that desorption and exchange can be distinguished. Possibly this slow desorption is the result of chemical diffusion of sulphuric acid out of the fibre, which will be controlled by the hydrogen ion diffusion coefficient. Exchange, on the other hand, would correspond to the relatively rapid value of the self-diffusion coefficient for sulphate ions.Dr. Howard J. White, Jr., and Mr. Donald A. Underwood (Text. Res. Inst., Princeton, N.J.) (communicated) : In answer to Dr. Armstrong, we wish to point out that three results-the shape of the curve of uptake against concentration, the inhibition of the absorption of Na+ by H-C, and the slow rate of absorption of salt-are cited in the paper as experimental evidence difficult to reconcile with a simple “ internal solution ” theory of absorption. These results are qualitatively explainable in terms of absorption on to sites, and, while they may not supply “clear evidence ”, they are at least suggestive. Hair could well give results similar to those cited for collagen in 0.2 M solution. Thus, if the amount of water absorbed by the fibre remains roughly constant regardless of the concentration of the treating solution for a 0.2 M solution, water would be preferentially absorbed from solution by dry hair, as can be deduced, for example, from Dr.Larose’s table. In answer to Dr. Larose, no correction of the type mentioned has been made. As Dr. Larose’s table shows, “ corrected ” values would be positive at low con- centrations of treating solution and negative at higher concentrations. The physical meaning of such a result is obscure and, since the correction rests on an assumption of “ free water in the fibre ”, we prefer to abandon the assumption. In fact, one of the main purposes of the experiments with Na2S04 was to examine the validity of this assumption. It was concluded that it was untenable in its simple form.In Dr. Wright’s discussion, reference is made to the low value of D(Na2S04). The diffusion experiments are discussed. The pH of the treating solution and the fact that Na22 and S3504 are absorbed in equivalent quantities as far as the equilibrium results are concerned make it unlikely that the rate of absorption can refer to anything but Na2S04. In desorption into slightly acid distilled water, hydrolysis of absorbed Na2S04 could very possibly account for the difference between the rate of desorption and of exchange. Dr. T. Vickerstaff (I.C.I. Dyestufi), said: With reference to the difficulty of measuring the very small time intervals involved in Mann and Morton’s work, we ourselves have studied the rate of dyeing of acid dyes on gelatin film preswollen in water. Using hand agitation, rate curves were measured down to times of 2 sec.Plotted against d t t h e data gave straight lines down to the smallest observed times although a scatter of the points due to experimental error occurred below 4 sec. No evidence of a two-stage process with an initial rapid absorption followed by a slower diffusion could be detected. Is it possible that the effect observed could be due to the use of an anionic surface-active agent in the dye- bath which might be adsorbed on the outer surface of the fibre more rapidly than dye ? This would then retard the initial adsorption of dye until it had been replaced by dye. Alternatively, if the effect is real, may it not be regarded as the initial setting up of an equilibrium distribution of dye between the fibre surface and the dye- bath ? In this case is the initial dye uptake the same as that which is generally termed “ strike ” and if so may “ strike ” be defined as the concentration of dye on the fibre surface which would be in equilibrium with the initial dye-bath? 1 Wright, Trans.Faradny SOC., 1953, 49, 95.122 GENERAL DISCUSSION Dr. T. H. Morton (Courtadds Ltd., Braintree) (partly communicated) : We would have no quarrel with Dr. Vickerstaff’s findings that the rate of uptake of acid dyes on pre-swollen gelatine fiIm is linear with the square root of time. Under these conditions, it is difficult to see how a two-stage process could be set up. The absorptions with which we have dealt in the paper are not linear with the square root of time, simply because the time required to wet out the film or fila- ment is finite and further, during wetting in the dye solution, a very concentrated layer of dye is laid down in the interface which further complicates the simple dye diffusion.We do not believe that our observation of an initial dye sorption is related to the dyer’s “ strike ”. The initial absorption is roughly the same for a variety of dyes, whereas the varying “ strike ” is related to the varying speed of initial diffusion of dye from bath to yarn, that is, to the second of the two stages of thc process. The point made above on the linearity of sorption with the square root of time is well shown by the data of fig. 1. With Azo Geranine 2G (C.I. 31) the sorption is linear within experimental error on pre-swollen shcet cellulose, whereas with initially dry shcet, thc absorption, though linear over most of its range, is not linear in the very early stages.Azo Geranine is a dye almost, but not quite, without substan- 0.1 tivity towards cellulose. With Sky Blue FF absorbed on to initially dry sheet-cellulose, the absorption departs greatly from the linear relation. It will be 46 noticed that the perturbation lasts much longer than the time for fully wetting the cellulose sheet and is more probably re- latcd to the time required for the dispersal by diffusion of the con- centration of dye at the cellulose surface. The experiments of fig. 1 FIG. 1 .-Dye absorbtion by regenerated sheet c e h - show clearly that a two-stage lose at 20” C: 1 % pure A20 Geranine 2G-A, sheet process occurs even with simple previously wetted; B, sheet immersed dry: 0.2% Sky non-substantive solutes and that Blue FF and 0.5% NaC1-C, shect immersed dry.the two-stage nature of the pro- cess is even more marked when dealing with substantive dyes of comparatively slow diffusion speed. Dr. H. Zollinger (Bade University) said : Bird, Manchester and Harris deter- mined the standard heat of dyeing. They think that the calculated value of - 10.5 kcal/mole indicates two hydrogen bonds. On the other hand they showed in an excellent way that dyeing of dispersed dyes takes place from a saturated solution. It may be assumed that the dye molecules are solvated by water molecules in solution. The measured heat of dyeing therefore does not correspond to the formation of dye-fibre hydrogen bonds, but to the difference between the heat of formation of these bonds and heat of breaking the dye-water molecules hydrogen bonds.I think, thcrefore, that the value found does not prove definitely the existence of two hydrogen bonds. A similar opinion is mentioned in Robinson’s paper.1 Dr. T. Vickerstaff (I.C.I. Dyestufi) said: I would like to support the views expressed in the paper of Bird et al. that Clavel’s theory of the dyeing of cellulose acetate is worthy of reconsideration. Several points in support of this view are : constant distribution ratio of dye between fibre and dyebath in desorption 1 this Discussion. -08 ‘04 .02GENERAL DISCUSSION 123 experiments, the effect of particle size on equilibrium absorption, and finally the fact that the rate of dyeing of cellulose acetate from an infinite dyebath does vary with the concentration of dispersed dye.The latter fact suggests that under these conditions the rate-determining step in the dyeing process is the rate of solution of the solid dye. I am still of the opinion that the dye used in Kartashoff's experiments was in the form of negatively charged particles and that the ac- cumulation of dye particles at the fibre surface arose from the mechanical limita- tions of his experimental arrangement but this must remain pure conjecture. Dr. H. E. Schroeder (du Pont de Nemours Co., Delaware) said : The data of Bird et al. extend only to about one-sixth of saturation. The methods described in Schuler and Remington's paper have been extended by them to the dyeing of cellulose acetate with non-ionic dyes.Isotherms have been determined for 1-amino-4-hydroxyanthraquinonc and N1 : N4-diphenyl-3-nitrosulphaniIamide. It was clearly shown that up to saturation each colour gives a linear plot of DF against Ds. The corresponding values at fibre saturation were for the anthraquinone DF = 10 mg/g ; Ds = 0.01 15 mg/d and for the sulphanilamide DF = 32.6 mg/g ; Ds = 0.0077 mg/ml. The same results were obtained with mixtures of the two dyes, For the saturated case, the total amount of dye on the fibre was the sum of the solubilities of the individual dyes, namely, 42.6mglg. In all cases the plots of Dr: against Ds were linear. This indicates clearly that in the sorption of non-ionic dyes by cellulose acetate there is no need for a concept of a limited number of sites, i.e.of Langmuir's absorption. A solution of dye in the fibre appears the best representation of the situation. Dr. H. Zollinger (Bask University) said : Dr. Giles discussed the question of hydrogen bonds of the benzidine dyestuffs. In this connection it seems to me to be worth while clarifying a spatial problem which is mentioned in most textbooks on dyeing, dyestuffs and textile chemistry. These books presuppose that direct dyestuffs have hydrogen bond-forming groups, spaced at a distance very closely approaching the length of the cellulose unit cell, which is 10-3A. Probably the most frequently mentioned example is Congo Red.1 This view is based on an unpublished investigation of Paine and Rose, who attribute a value of l0.8A to the distance between the respective hydrogen bonding atoms.In most papers (e.g. Vickerstaff 1) the amino hydrogen atoms are the species assumed to be the hydrogen bonding atoms. Working with scale models based on recent and accurate measurements of bond lengths, I found that the distance between the amino hydrogens is much larger (fig 1). Depending whether we assume the Congo Red ion to have a trans configuration (lower formula of fig. 1) or a " symmetric " configuration (upper formula), we got values of 16.2A and 15.3A respectively, these values do not at all correspond to the length of the cellulose unit. Some rough calculations based on the effect of these two dipoles of the sul- phonic groups shows that thc trails has a somewhat lower energy.The difference in energies of these two configurations is about 1/40 of the thermal energy at room temperature, therefore it is probable that the trans configuration is favoured slightly." Coming back to the spatial problem, it does not seem likely that the other two hydrogens are involved in the hydrogen-bonding with cellulose, because they belong to the rather strong hydrogen bridge to the azo nitrogen. Opening of 1 cp. fig. 56 in Vickerstaff, The Physical Chemistry of Dyeing (London and Edinburgh * Assuming a dipole moment of 4 D for the sulphonic group and neglecting all other 1950), p. 168. moments and the influence of the surrounding medium: trans-configuration 12.2 x 10-16 erg, symmetric configuration 23-5 x IO-l6erg, thermal energy (300" K) 414 x 10-16erg. (unpublished calculations of H. Labhart, Physics Laboratory, Ciba Ltd., Bask).1 24 GENERAL DISCUSSION this bridge would cause a remarkable shift in the wavelength of the absorption peak. This is, in fact, not the case. The only distance corresponding roughly to the cellulose unit is the distance between the nitrogens attached to the diphenyl nucleus, i.e. lO.1A. But these nitrogens can hardly be hydrogen-bonded to cellulose so long as their pair of unshared electrons is involved in a hydrogen bond with the amino group. It is a well-known fact that the nitrogen atoms of an azo group are able to form only one hydrogen bond (compare complex formation, etc.). Another example is Chrysophenin G, a stilbene dyestuff. Willis, Warwicker, Standing and Urquhart 1 think that this dye is hydrogen-bonded at the phenolic oxygens. These atoms are 24-8 8, apart. The distances between the azo nitrogens are 12-2 and 13.6A respectively. Therefore, this dyestuff is another example of the non-existence of a correlation with the cellulose unit length. As a result, I think that the hypothesis of the corresponding lengths of the cellulose unit and the reactive groups in direct dyes cannot be upheld any longer. This supports the view of Dr. Giles that hydrogen bonding is not a condition sine qua non for substantivity. Congo Red 0 1 2 3 4 5 --I-- FIG. l.--Scale models of Congo Red. I emphasize that my remark concerning the correlation between distances uf groups which are able to form hydrogen bonds with the cellulose unit cell, is intended to show that it is unlikely that such a correlation is important for sub- stantivity. I do not say that it proves the non-existence of hydrogen bonds between fibre and direct dyestuffs. The models of Dr. Robinson 2 show (i) that hydroxy groups are abundant along the cellulose chain and therefore almost every distance between potential hydrogen-bonding groups in a dyestuff molecule “ fits ” the hydroxyls of the cellulose, (ii) that in amorphous cellulose the chains are curved in such a way that dyestuff adsorption is still possible. Concerning the last point we think that dichroism of dyed cellulose fibres indicates that dyeing takes place on the surface of oriented (i.e. crystallized) regions of cellulose, as shown by Boulton and Morton,3 Frey-Wyssling and Walchli 4 and others. Dyestuff ions do not penetrate crystallized regions. This does not exclude the oriented adsorption which seems to be important, although my remark shows that the mentioned correlation between distances is improbable. 1 Willis, Warwicker, Standing and Urquhart, Trans. Faraday Soc., 1945, 41, 506. 2 this Discussion, compare fig. 2a, 2b and 3 of Robinson’s paper. 3 Boulton and Morton, J. Suc. Dyers Cul., 1940, 56, 145 ; 1946, 62, 272. 4 0. Walchli, Thesis (Federal Institute of Technology, Zurich, 1945). Frey-Wyssling, J. Polymer Sci., 1947, 2, 314.