Rheology and Thermotropic Gelation of Aqueous Sodium Alginate Solutions

Abstract

In this work, we present the evaluation of the thermotropic gelling ability and steady-state rheology of aqueous sodium alginate (NaAlg) solutions. NaAlg is been considered due to its gelling capabilities and to determine its suitability as a film-forming agent with the potential capacity for immobilization of very potent, solid, and hydrophobic drugs. The existence of a thermally induced gelation of 1.0 to 2.5 wt.% sodium alginate solutions between 13°C to 22°C was demonstrated through three independent rheological tests: the constant-stress temperature-ramp viscosity, dynamic viscoelastic moduli, and thixotropy tests. Oscillatory dynamic tests also showed the thermoreversible nature of the NaAlg gels, but a large hysteresis between the gel formation and the gel melting was observed. The steady-state viscosity was experimentally determined as a function of shear rate for NaAlg concentrations between 1.0 to 2.5 wt.% and for temperatures from 20°C (or above Tgel) to 90°C. A viscosity master curve was constructed by applying time–temperature superposition and a semi-empirical shift factor for concentration. A power-law fluid model (\( \eta = k{{\dot{\gamma }}^n} \)) was adjusted to the data from 20°C to 60°C and found to provide good agreement with fitting parameters k = 4.81 and n = −0.449 ± 0.003.