Adsorption at Organic Surfaces

Abstract

Adsorption of mineral and organic acids, from aqueous solution, by chitin prepared from the carapace of Nephrops norvegicus has been studied. Full elementary analyses suggest that chitin does not consist entirely of poly‐ N ‐acetylglucosamine as usually supposed, but that about one‐eighth of the amino groups are unacetylated. The material appears to be highly crystalline. Mineral acid anions are adsorbed at all the acetamido or amino groups of the substrate at pH about 2.5. At pH < 2.5 very greatly increased adsorption occurs, much beyond stoichiometric quantities. Organic acids, both aliphatic and aromatic, are much less strongly adsorbed than mineral acids, and < 10% of the total amount of the substrate appears to be accessible to them. Two opposing factors appear to determine the amount of an organic acid adsorbed at moderate acidities: ( a ) the crystallinity of the substrate, and ( b ) the length of the conjugate chain of the anion. The main source of anion affinity under acid conditions appears to be physical attraction between the anion and the water‐solvated glucosidic residues of the chitin molecular chains. Hydrogen‐bond affinity appears to be weaker than the ion–ion affinity or that due to the van der Waals attraction of large aromatic anions, and does not appear to influence the extent of adsorption under acid conditions.