Adsorption isotherms dyeing behaviour

Electrolytes are used to promote the exhaustion of direct or reactive dyes on cellulosic fibres they may also be similarly used with vat or sulphur dyes in their leuco forms. In the case of anionic dyes on wool or nylon, however, their role is different as they are used to facilitate levelling rather than exhaustion. In these cases, addition of electrolyte decreases dye uptake due to the competitive absorption of inorganic anions by the fibre and a decrease in ionic attraction between dye and fibre. In most discussions of the effect of electrolyte on dye sorption, attention is given only to the ionic aspects of interaction. In most cases, this does not create a problem and so most adsorption isotherms of water-soluble dyes are interpreted on the basis of Langmuir or Donnan ionic interactions only. There are, however, some observed cases of apparently anomalous behaviour of dyes with respect to electrolytes that cannot be explained by ionic interactions alone. 

The formation of dimers in solution at higher concentrations indicates an interaction between the dye molecules and so the b term in the Frumkin isotherm is not a surprise. The adsorption behaviour suggests that the dye is highly surface-active and formation of a monolayer is almost complete by the transition at ca. 6 pM. The standard free energy of adsorption derived from the isotherm is consistent with a monolayer coverage forming even at the micromolar aqueous concentrations. Once the first layer is substantially complete, subsequent adsorption takes place on this layer to form a second-ordered layer. 

---