Validity and Breakdown of Superposition Principles in the Viscoelasticity of Chitosan–Gum Arabic Complex Coacervates

Abstract

Complex coacervates of chitosan and gum arabic, two naturally abundant weak polyelectrolytes, were investigated by oscillatory shear rheology at varying temperatures, salt concentrations, salt types, pH, and polyelectrolyte mass ratios. The complex shear moduli are shown to scale with temperature, salt concentration, and pH, implying that the concepts of time–temperature superposition (TTSP), time–salt superposition (TSSP), and time–pH superposition (TpHSP) are applicable. However, in the case of TSSP and TpHSP, a superposition breakdown occurs in the low-frequency regime toward high salt concentrations (>300 mM) and pH values near the pKa of chitosan (≈6.5). Since the shear moduli G′ and G″ are observed to increase and become less frequency-dependent at high salt concentration and pH, the superposition breakdown is ascribed to gelation, presumably due to the associative interaction of chitosan chains and the formation of a chitosan network.