Studies on the Donnan Potential in Systems Containing Cellulose and Aqueous Electrolyte Solution

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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.

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