Studies in Adsorption

Recent quantitative data for adsorption of disperse dyes on cellulose acetates and the relations between adsorption and dye structure are considered. The dye probably enters the fiber partly in monodisperse and partly in associated form, and penetrates between the fiber chains in regions inaccessible to water. This type of adsorption produces the characteristic linear adsorption isotherms. Several properties of the dye molecule determine the maximum adsorption of dye. Size. The smaller the molecule the easier it can penetrate into the fiber structure, hence the higher the maximum adsorption. Number of hydrogen donor atoms. The more donor atoms there are the higher is the adsorption, in certain cases; the dye forms —OH... and —NH... bonds with the fiber. Number of hydrogen acceptor atoms. The dye forms —CH...O, —CH...N, and —CH...Cl bonds with the CH ₈ groups of the acetyl residues in the fiber.' Thus, the —CH...C1 bond between ethyl acetate and chlorobenzene in aqueous ethanol solution has an energy of about —3 kcal./mole and chlorobenzene itself is strongly adsorbed by cellulose triacetate from this solvent. In some cases adsorption rises with the number of acceptor atoms; in some cases it falls because of opposing electronic effects of other substituent groups. Steric effects. Functionality. There is a tendency for wide spacing (e.g. 1:4 disub stitution) of bonding atoms to reduce adsorption, probably by promoting dye-dye asso ciation rather than dye-substrate association. Nylon and polyethylene terephthalate fibers appear to adsorb by a similar mechanism, polyethylene terephthalate showing additional (aromatic nucleus) dye-fiber association.