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Studies on the Donnan Potential in Systems Containing Cellulose and Aqueous Electrolyte Solution
Abstract
Donnan potentials in undyed and dyed regenerated cellulose-aqueous potassium chloride solution systems have been investigated. Eight direct dyes of different structures were used to dye regenerated-cellulose sheets over a wide range of concentrations of adsorbed dye. The measured Donnan potential depends solely on the value of the total surface charge and is independent of the structure of the dyes used. The contribution to the surface charge by the intrinsic ionised carboxyl and the sulphonic acid groups (of the adsorbed dye) depends only on the total number of such groups present and not on whether the adsorbed dye is a mono- or poly-sulphonated dye. Only approximately 25 % of the total ionisable surface groups [carboxyl groups or the sulphonic acid groups (or both)] from adsorbed dye are dissociated. Direct experimental evidence has been obtained from these measurements of potential to support the use of a variable-volume term, instead of a constant-volume term, to describe the volume of the internal charged cellulose phase. The nature of this variation indicates that the total volume of the internal Donnan phase is a function of the thickness of the electrical double layer, as suggested in a theory of dyeing recently proposed.
... Neale et al. [31] proposed the existence of the Donnan equilibrium. The idea of the Donnan equilibrium was employed for the modern electrochemical theories of the dyeing of cellulose from aqueous solutions of direct dyes [32]. ...
Dyeing auxiliaries play an important role in the determination of the final dyeing quality. The formation of additional complexes with dyes and auxiliary agents enhances the exhaustion of dyes on textile substrates. For aqueous-based dyeing, dye auxiliaries such as chelating agents, dispersing agents, leveling agents, electrolyte, pH control agents, and surfactants form complexes with the dye on natural and synthetic fibers. A growing awareness of the impact of industrial pollution on the environment became crucial for the dyeing industry in the closing decades of the twentieth century. These chapters discuss the characteristics of dyeing chemicals and how auxiliary substances can assist in achieving outstanding dyeing performance.
An improved method for measuring Donnan potentials in textile polymer–aqueous electrolyte solution systems has been developed. Contrary to previous methods, the measured potentials remain constant over a period of hours and can be applied to textile polymers both in film form and as fibre bundles. The method can also be used as a new technique for studying adsorption and desorption kinetics for ionic dye–textile polymer systems.
Zeta-potential studies of cotton and viscose rayon fibres in aqueous sodium and potassium chloride solutions of different concentrations have been carried out using the streaming-potential method. The Eversole and Boardman equation, which relates the measured zeta potential to the thickness of the electrical double layer, has been found to be valid in all cases. Using the surface-potential values calculated from this equation, the surface-charge densities in these fibres have been estimated. They are in good agreement with the theoretical surface-charge densities calculated from data on the carboxyl-group content of these fibres and the BET nitrogen surface areas for the water-swollen uncollapsed fibres. Two alternative explanations for the apparent increase in surface-charge density with decrease in the zeta potential have been given, without assuming that the anions are absorbed. The variation in zeta potential with increasing carboxyl-group content in samples of oxycellulose has been explained in terms of changes in the degree of dissociation of carboxyl groups with increasing carboxyl content, and an increase in the surface area of the cellulose fibre on oxidation. Studies of the zeta potentials in Form-W and Form-D formaldehyde-crosslinked cotton fibres have indicated that the zeta potential increases with increase in formaldehyde content with Form-D fibres. This result can be explained in terms of the decrease in the surface area of Form-D cottons with increase in the content of bound formaldehyde.
The effects of varying the concentration of salt in the dyebath on the patterns and intensities of staining of tissue sections by basic dyes were studied. Eleven basic dyes of various types were investigated, each at several salt concentrations, usually in acidic aqueous dyebaths but in some instances also in alcoholic or alkaline aqueous baths. The dyes used included most of those previously studied by others in critical electrolyte concentration investigations involving model systems.
As the salt concentration in the dyebaths rose, the usual effect was that the staining intensities of some structures decreased whilst that of others increased. Consequently, the overall staining patterns were complicated and were often quite unlike the simple critical electrolyte concentration patterns previously reported for the staining of model systems by the same dyes. It is important, and unfortunate, for histochemical analysis that the nature of the anionic tissue functional group (i.e. carboxylate, phosphate or sulphate) did not appear to be the crucial factor in determining the staining patterns observed.
The electrochemical behavior of a cation-transfer membrane with respect to cations of various charge was investigated. Measurements were made of capacity, water content, Donnan sorption, conductance, water transfer, and transport numbers. Results suggest that ionic mobilities are lower in the membrane than in free solution, indicating a high degree of association between the cations and the charge centers of the membrane.
Heats of dyeing of Chrysophenine G, Chlorazol Sky Blue FF, and Congo Red on cotton, viscose, and cuprammonium rayon have been determined. It is shown that, except for the initial low concentration of adsorbed dye, the values of heat of dyeing remain more or less the same over a fairly wide range of concentration of adsorbed dye, and that beyond a critical dye concentration, the values tend to decrease continuously when the proportion of the adsorbed dye is increased. Values for the critical concentration of adsorbed dye beyond which the values of heat of dyeing tend to decrease agree closely with values for dye adsorption at which maximum volume contraction in the substrate takes place, as observed from density measurements, and also with values of dye concentrations where maximum extent of oxidation of cellulose takes place during catalytic oxidation of the fiber substance by sodium hypochlorite in presence of the dye. A new thermodynamic approach has been developed to express the activity of the dye in a cellulosic fiber substance in terms of the fraction of the total sites occupied at a particular stage of dyeing. Based on this thermodynamic approach, affinity values of the three direct dyes for the three cellulosic fibers under different experimental conditions have been determined; the validity of the treatment is discussed.
The cationic and anionic distribution ratios for some simple salts between cellulose and water have been measured by chemical analysis. In an attempt to throw light on the origin of the cation binding power of cellulose, oxycelluloses and dyed celluloses have also been employed. The ionic distribution is in general agreement with the Donnan theory if water is regarded as the solvent in both phases. In concentrated solutions it may however be necessary to take account of a specific affinity between the salt and cellulose, and when large numbers of anionic groups are present in the cellulose there is evidence of reduced activity of the counter ions.
Investigations were carried out to study the adsorption of Chlorazol Sky Blue FF on cotton fiber from aqueous solutions of 0.1 N LiCl, NaCl, KCl, RbCl, and CsCl. The dye uptake both at 50° and 60° increased markedly with increasing size of the alkali metal cation. The equilibrium dye adsorption could be described by a Langmuir-type equation. The calculated standard affinity of dye adsorption showed a fairly constant value, but the exothermic heat of dyeing and the entropy of dyeing, calculated from the individual isotherms, increased as the dye adsorption increased in the presence of either higher electrolyte concentrations or equivalent concentrations of alkali metal cations of larger size. Possible changes in the orientation of adsorbed dye ions on the cellulose surface can account for the increased -ΔH° and -ΔS° values. It is pointed out that, apart from the usual well known effects of electrolyte cations, an ,important contributory factor to the adsorption process arises from the disturbance caused by the cations to the structure of water hydrogen-bonded to the cellulose surface and "iceberg" water around the hydrophobic parts of dye molecules. This effect, which will increase with the size of the alkali metal cation, explains the results adequately.
Measurements have been made of the electrical potential difference (Donnan potential) between dyed Cellophane and aqueous solutions of alkali-metal chlorides. The presence of adsorbed coloured ions of negative electric charge increases the negative potential of cellulose, whereas positively charged coloured ions may reverse the sign of the charge. The measured potentials, whilst smaller than those predicted by the application of the Donnan theory, after making the usual simplifying assumptions, are in good general agreement with modern views of the mechanism of direct dyeing.
Coefficients, D, for the self-diffusion of nine cations in sulfonated polystyrene-divinylbenzene type cation exchangers were determined as a function of temperature and polymer cross-linking. A strong dependence of D on the formal cationic charge was observed at all cross-linkings: In Dowex-50 at 25° values of 2.88 × 10-7 for Na+, 2.89 × 10-8 for Zn++, 3.18 × 10-9 for Y+++ and 2.15 × 10-10 for Th++++ ion were observed. Increased cross-linking, measured by the nominal divinylbenzene (% DVB) content, resulted in a sharply lowered self-diffusion rate. The coefficient at 25° for Zn++ in 1% DVB was 1.06 × 10-6 compared with 2.63 × 10-9 cm.2 sec.-1 in a 24% DVB exchanger. Activation energies, Eact, which were temperature but not charge dependent, increased with cross-linking from ca. 4700 to 10,000 cal. mole-1. A qualitative interpretation of the variations of D was attempted based on absolute reaction rate theory modified by Barrer to explain observed temperature variations of Eact. The derived entropy of activation, ΔS≠, was regarded as the resultant of two opposing effects: (a) a positive contribution from the disturbance created in its environment by the diffusing ion; and (b) a negative contribution from the electrostriction of water accompanying the separation of charge in forming the activated complex.
When a membrane, such as a sheet or a strip of cellulose, separates one solution of an electrolyte from another of different concentration, an electrical potential difference is established across the membrane, and this potential difference can be accurately measured by suitable experimental means. When potassium chloride is used as the electrolyte, the diffusion potential within the membrane is very small, and the latter may be regarded not as an inert sieve but as a macromolecular structure, the electrochemical properties of which are due to the presence of non-diffusible ionogenic groups, such as carboxylic acid groups. The dissociation, in an aqueous medium, of a portion of these groups causes the membrane to acquire a negative potential with respect to each of the contiguous solutions. A method is described whereby the single potential difference between the membrane and a solution of potassium chloride in which it is immersed may be determined. The experimental results are in agreement with the view that the membrane may be regarded as an aqueous region of uniform potential, and that this potential is dependent on a Donnan distribution of the mobile ions. The degree of dissociation of the non-diffusible ionogenic groups depends on the concentration of neutral electrolyte solution in contact with the membrane, since, on account of the Donnan effect, the pH of the solution imbibed by the membrane differs from that of its external environment. Values for the dissociation constant of the carboxylic acid groups in Cellophane have been obtained from the present results. When hydrochloric acid is the electrolyte, and in salt solutions of low pH, the dissociation of the carboxylic acid groups is almost entirely repressed, and the Donnan potential vanishes. The results obtained with materials of artificially enhanced carboxyl content are also in agreement with expectation, and indicate that the Donnan potential is due solely to the presence of carboxylic acid groups. The variation of the potential with the water imbibition of the cellulose lends further support to the concept of the swollen membrane as an equipotential region.
Correlation of diffusion, transport number and conductivity data with the chemical equilibria considered to exist in a weakly basic anion exchange medium has been reported. The data are consistent with the assumption that the resin studied (PERMIONIC ARX-44) has the properties of a simple electrolyte solution phase in which the resin salt is partially dissociated with a dissociation constant less than 1 and greater than 0.1.
1
A new theory of direct dyeing is presented in terms of the Langmuir theory of surface adsorption, modified to allow for the effect of foreign electrolyte in the dyebath.
2
A simple formula is derived, relating the equilibrium absorption of dye to the dyebath concentration, the concentration of foreign electrolyte in the dyebath and the temperature.
3
The formula is shown to account satisfactorily for the main features of the equilibrium dyeing of cellulose by Chrysophenine G in the presence of sodium chloride.
4
The theory is used to calculate the energy of dyeing of cellulose by Chrysophenine G.
On expose une théorie quantitative de la sélectivité ionique des membranes.
- Meyer
- Schofield
- Robinson