Lobster‐shell chitin has been used in quantitative adsorption studies with sulphonated azo dyes in aqueous solutions, mainly under acid conditions. The extent of adsorption has been correlated with the dye structures.
The mechanism of adsorption appears to be largely one of physical association between the aromatic residues of the dye and the molecular chains of the chitin, hydrogen bonding in acid solutions being unimportant. When inorganic acid is present, one sulphonate group in the dye anion takes part in ion exchange with inorganic anions at the cationic centres in chitin. Additional sulphonate groups appear to remain uncombined with the substrate and to reduce the affinity of the dye.
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.
The adsorption by graphite of a variety of organic solutes, mainly acid and basic dyes, from water, has been studied.Adsorption occurs rapidly and only at the outer surface of the graphite particles. The adsorptive forces are coulombic (ion exchange) and non-polar Van der Waals' attraction. Basic dyes are adsorbed by ion exchange, the process being independent of temperature, but dependent on the surface charge on the graphite. Sulphonated dyes are adsorbed by van der Waals' (dispersion) forces; their affinity increases linearly with the area of the adsorbed molecule in contact with the substrate, but it falls with increase in the number of sulphonate groups, because of increase of water attraction.In the early stages of adsorption they are probably oriented flat on the surface, but in the completed monolayer they are oriented with ionic groups as far away as possible from the graphite surface.From solutions of high concentration, multilayers of single molecules or monolayers of micelles appear to be adsorbed. Glucose and sucrosc are adsorbed to a much greater extent than is accounted for by monolayer adsorption, but the reason for this is obscure.The shapes of the isotherms are shown to be related to the affinity of the solutes for the graphite surface. The rate-controlling factor in adsorption of dyes is believed to be the proportion of bombarding ions which have a favourable orientation relative to the graphite surface at the moment of impact.From solutions of low concentration most solutes are adsorbed as monolayers.
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