Shear rheology and phase behaviour of sodium oleate/water mixtures

The knowledge of structural and dynamical properties in binary sodium oleate (NaO)/water mixtures represents a first step for studying new biocompatible structures. In addition, a deeper understanding of their flow behaviour is crucial to control their mechanical properties. The purpose of this investigation was to revisit the binary NaO/water phase diagram through a combined effort of shear-rheology, NMR-diffusometry, optical polarizing microscopy and differential scanning calorimetry. The preparation of NaO/water mixtures in this work fitted with the interesting regions of the phase diagrams where most of the new systems will be prepared (i.e. from 0.01 to 30 wt.% NaO and within the temperature range 5–70 °C).Steady shear and linear viscoelastic experiments were accurately performed and the relationship between phase morphology and rheological properties was also studied. Regarding the mixtures at low surfactant concentrations, the shear flow showed: (1) a structural evolution into the liquid micellar phase (L1) as a function of composition and (2) the presence of a viscous micellar region (L1*) induced by temperature and composition changes. NaO/water mixtures with a liquid-crystalline structure (hexagonal H1) were studied by linear shear deformations and paying particular attention to frequency and temperature scans. Viscoelastic spectra of H1 phase presented a gel-like response which, although not expected, indicated the presence of a stiff liquid-crystalline lattice. Finally, a correlation between rheology and microstructural changes was conducted by the “weak-gel” model.