Unperturbed dimension, interaction parameters, zeta potential and rheology of sodium alginate in binary solvent mixtures

Abstract

The intrinsic viscosity [η] of sodium alginate having different molecular weights (high molecular weight (type A), medium molecular weight (type B) and low molecular weight (type C)) is measured at 293–303 K temperature in various mixtures of water (good solvent), acetone (ACE, poor solvent) and water-ethoxy ethanol (EE, poor solvent). The observed result particularly the Huggins constant (K H) values shows a significant variation of cosolvency as a function of solvent composition (ΦACE/ΦEE) and the temperature. Unperturbed dimensions (K θ) under non-theta condition have been calculated using various equations in different water-acetone and water-ethoxy ethanol mixtures. The value of K θ obtained from three different methods of measurements viz., Burchard-Stockmayer and Fixman (BSF), Berry and Inagaki-Suzuki-Kurata (ISK) agree well with each other except in a few compositions of solvents. The molecular extension factor (α n) and actual end-to-end distance (α n K θ) have also been computed herein. Further, the shear thinning nature of the sodium alginate solutions (representative illustration: type A) in water and water-acetone mixed systems has been investigated at increasing strain rate. Results show that the viscous modulus G″ predominates over the elastic modulus G′ and the systems behave as viscoelastic fluids under present conditions. Higher value of G′ in acetone-water mixed solvent system compared to that in pure water undoubtedly indicates enhancement of the elasticity in alginate system in the presence of acetone. Finally, zeta potential measurement demonstrates that the polymeric surface of sodium alginate (representative illustration: type A) is essentially negatively charged and the zeta potential of the polymer is more responsive to the composition of ethoxy ethanol than that of acetone in the aqueous-organic mixed solvents.