Rheological Properties of Hydrogels Agar-Agar With Sodium Chloride and Citric Acid

The rheological properties of composite hydrogels of agar-agar with sodium chloride (0.03 wt.%), citric acid (0.3 wt.%), and their mixture (at the same concentrations of additives) were studied. The hydrogels were obtained by heating the initial solutions with and without a constant magnetic field (created by a ferrite permanent magnet with a magnetic flux of at least 0.02 Mwb built into the magnetic stirrer), and then cooling them at room temperature in a Rheotest RV2 cylinder. For hydrogels obtained by heating without a magnetic field, it was shown that the addition of NaCl leads to an increase in viscosity over the entire range of shear stresses and an increase in yield strength from 14.3 Pa to 42.8 Pa. Adding citric acid reduces the viscosity and yield strength to 11.4 Pa. When a mixture of citric acid and NaCl is added to the gel, its viscosity is higher in almost the entire range of shear stresses (up to about 25 Pa) than the gel containing only agar-agar, which has a yield strength of 14.3 Pa. The flow curves of all gels contain maxima, indicating the non-equilibrium nature of the system and the presence of large aggregates in the structure within a certain range of shear rates. These aggregates break down into individual macromolecules of the polymer components of agar-agar with a further increase in shear rate. The degree of syneresis during freezing and thawing was determined gravimetrically for samples frozen at −18°C and thawed at room temperature. Introducing additives leads increases hydrogel syneresis from 46% to 90%, depending on the gels' composition and the number of freeze-thaw cycles. The worst results were observed for the gel with the addition of citric acid. It was shown that hydrogels obtained using a magnetic field have lower viscosity for all additives, and their viscosity is significantly lower than that of gels obtained without a magnetic field. Maxima on the flow curves are present only for the agar-agar gel and the gel with the addition of NaCl. The yield strength is 11.8, 11.9, and 1.6 Pa for gels with agar-agar, with the addition of NaCl, and citric acid, respectively. When a mixture of NaCl and citric acid is added, the gel does not exhibit solid-state properties, and no yield strength is observed.