Abstract

Further examination of the minor proportion of wattle “solvent resistant” tans indicates that these are comprised of high molecular weight material, structurally similar to the main constituents. The existence of similar “solvent resistant” fractions has been confirmed in the case of a number of vegetable tannin extracts from current production. Examina tion of the effect of p1-I level in solvent extraction shows that, in the case of the “hydrolysable” tannins particularly, the residual affinity phenome non is absent at pH 7 and the tannages are completely solvent reversible. It is concluded that residual affinity effects in solvent extraction are attributable to orientation of high molecular weight constituents on the protein surface so as to achieve a high degree of multiple hydrogen bonding which renders this fraction resistant to the action of polar organic solvents. The presence of dissociating groups imparting anionic character to tannin constituents serves to reduce affinity. Thus, at acid p1-1. where dissociation is reduced, the binding of the “hydrolysable” tannins resembles that of the “condensed” tannins with characteristic residual affinity effects being observed, while at neutral pH, increased solvation and charge repulsion result in complete reversibility in solvent extraction. The binding of cresol sulphonic acid synlan is similar to that of the “hydrolysable” tannins which it resembles with respect to anionic character and content of phenolic hydroxyl groups, while the affinity characteristics shown by naphthalene sulphonic acid syntars in extraction were consistent with previous worl relating to the binding of structured anions with aromatic character. The affinity effects observed are readily explicable in terms of ionic binding, dipole attraction and hydrogen bond formation in conjunction with factors such as molecular size and dimensional compatibility, and the evidence excludes the formation of covalent auxiliary bonds in normal vegetable tannage.