Viscoelastic Properties of Dilute Aqueous Solutions of Sodium Alginate

Abstract

The viscoelastic properties of dilute aqueous solutions of sodium alginate (Alg) at ultrasonic frequencies were investigated by means of a torsional method with crystal resonators, The new type resonators, such as a huge crystal resonator and a jointed resonator used in this study, enabled us to cover the frequency range from 26 kHz to 131 kHz.Since the association of water molecules, which are dissociated in the presence of Na+ ions, influences the affective electric resistance in a dilnte aqueous solution of polyelectrolyte, this effect was taken into consideration in the calculation of the mechanical impedance.In order to examine the configuration of polymers in solution, the complex intrinsic rigidity which was obtained experimentally by extrapolation to zero concentraion should be quantitatively compared with these calculated from the existing molecular theories at infinite dilution. However, the G /c, (G - 5)/c, and (n0-na)/c values of the dilute aqueous solution of A1g changed nonlinearly and abruptly with dilution, so that the Fuoss-Strauss equation was used to obtain the intrinsic values.In a dilute aqueous solution of Alg, polymer chains are assumed to take an extended stringlike configuration by the electrostatic repulsion between molecular chains, therefore the measured values and their frequency dependences could not be predicted by any of the existing theories. However, when the electrostatic repulsion was depressed by the addition of NaCl, the experimental values approached the theoretical curves according to the Rouse theory for a random coil polymer.