Speciation of chitosan with low and high molecular weight carboxylates in aqueous solution

Abstract

Quantitative data on the speciation of chitosan (310 kDa) with low and high molecular weight carboxylates in aqueous solution are reported. The following carboxylic ligands were considered: monocarboxylate (butyrate); dicarboxylates (malonate, succinate, azelate); tricarboxylate (1,2,3-propa- netricarboxylate); tetracarboxylate (1,2,3,4-butanetetracarboxylate); polyacrylates (2.0 and 20 kDa); polymethacrylate (5.4 kDa). The investigation was performed by potentiometry at t 1/4 25 C, at low ionic strength (without addition of supporting electrolyte) and at I 1/4 0:15 mol L 1 (NaCl). For all the systems the formation of (chitosan)LHi species was found (L 1/4 carboxylic ligand; i 1/4 1 to 4 depending on the carboxylic ligand considered). The stability of proton – chitosan – carboxylate species depends on the number of carboxylic groups involved in the complexation, and it was possible to calculate a rough free energy value per bond DGn 1/4 15 2 kJ mol 1. By using the stability data, the quantitative seques- tering capacity of chitosan towards the carboxylates here considered [expressed as the – log(total chitosan concentration) necessary to bind 50% of carboxylate, i.e., pL50] was calculated for different pH values, at low ionic strength and at I 1/4 0:15 mol L 1. The pL50 values, ranging from 3 to 7, show that chitosan is quite a strong sequestering agent towards carboxylates. Evidences were also obtained for the different behaviour between low and high molecular weight carboxylates.