Gels of cysteine/Ag-based dilute colloid systems and their rheological properties

Abstract

The rheology of various systems consisting in L-cysteine, Ag, and aqueous solutions of different electrolytes has been studied. These systems can form gels at very low concentration of cysteine not exceeding 0.036 wt%. In periodic low amplitude oscillations, the gels behave as linear elastic media with frequency independent modulus and low mechanical losses in a wide frequency range. The modulus values lie in the range of 10–100 Pa. Meanwhile these systems demonstrate the dualism in the rheological behavior. In the stress-controlled shearing, “gels” behave not like elastic but like visco-plastic media. At low stresses, they flow with Newtonian viscosity of the order of 10³–10⁵ Pa s. However at some critical stress (“the yield stress”), the apparent viscosity falls by 6–7 decimal orders. Special rheological experiments demonstrated that this is not a bulk effect but most likely a consequence of the stream separation (“shear banding”). This effect is reversible and the thixotropic formation of the initial structure and properties takes place in the time-scale of the order of minutes. It was argued that the structural coagulation network is formed by the Ag nano-particles stabilized against sedimentation by L-cystine appeared as a result of the cysteine oxidation.