Cross-linking of highly methoxylated pectin with copper: the specific anion influence

Abstract

The aim of this work was to investigate the influence of specific anions on the cross-linking process of highly methoxylated pectin using the following copper salts CuSO₄; Cu(C₂H₃O₂)₂; CuCl₂; and Cu(NO₃)₂ wherein the initial salt concentrations were varied from 0.5 up to 10 g l⁻¹. It was found that the anions affected the sorption capacity wherein the Cu²⁺ sorption capacity from the sulphate solution was the highest, while it decreased in the presence of CH₃COO⁻, Cl⁻ and NO₃⁻ ions, respectively. This difference was mostly pronounced at the higher initial salt concentrations (c₀(Cu²⁺) > 2 g l⁻¹). The obtained beads were characterized by FTIR, AAS, SEM/EDS microanalysis and mechanical compression tests up to 80% of deformation. The sorption data were applied to the Langmuir and Freundlich isotherm models and various calculated parameters confirmed the sulphate anion supportive nature in metal ion binding. The value of the K_f parameter for the cross-linking process of pectin in the presence of acetate, chloride and nitrate was approximately the same (K_f ≈ 0.027 g g⁻¹), while it was higher in the presence of sulphate anions by more than 20% (K_f = 0.0352 g g⁻¹). The predicted 1/n values (1/n < 1, 1/n > 1, 1/n = 1 for the sulphate, nitrate and acetate, and chloride anions, respectively) were the quantitative confirmation of the specific interactions involved in the cross-linking mechanism caused by different anions. The established anion influence was in accordance with the typical ion-specific influence on macromolecules in aqueous systems proposed by Hofmeister.