Adsorption at Organic Surfaces V—A Study of the Adsorption of Dyes and Other Organic Solutes by Cellulose and Chitin

Adsorption experiments with cellulose and chitin and a variety of solutes, including non‐ionic hydroxy compounds, aromatic sulphonic acids, and azo dyes, are described. In non‐aqueous solvents the non‐ionic compounds are adsorbed by both substrates, but in water they are adsorbed only by chitin. Almost all the sulphonated compounds are adsorbed by both substrates, but benzenesulphonic acid is not adsorbed by cellulose. It is concluded that cellulose adsorbs by hydrogen bond formation with the solute in absence of water, but in presence of water by van der Waals attraction alone. Chitin can adsorb by both of these mechanisms and also by ion exchange. In chitin the active centres for both hydrogen‐bond and ion‐exchange adsorption are the acetamido and ammo groups; the hydroxy groups in both cellulose and chitin are believed to be strongly solvated in water, and unable to adsorb solutes by hydrogen bonding. The affinity for cellulose of the aromatic solutes and of vat dyes for which published data are available increases linearly with the logarithm of the length of the conjugate chain in the molecule. This appears to confirm that the affinity represents the attraction between the cellulose chain and the 77‐electron system of the solute molecules, which must be planar to have high substantivity. The effect on affinity of the amido group is discussed.