Influence of Particle Surface Charge and Shape on the Rheology and Microstructure of Worm-like Micellar Solutions

The influence of the size, shape, and surface chemistry of charged nanoparticles on the rheology and microstructure of worm-like micellar (WLM) solutions is investigated. The WLM solutions considered in this work are formulated from CTAB (cetyltrimethylammonium bromide), a cationic surfactant, and sodium nitrate (NaNO₃). The addition of negatively charged particles decreased the viscoelastic properties (zero-shear viscosity and relaxation time of the micelles) of the WLM solutions. In contrast, the incorporation of positively charged particles into the micellar networks resulted in (i) an increase in zero-shear viscosity and relaxation time and (ii) an increase in the number of entanglements of the micelles. The WLM solutions are found to undergo a transition from viscoelastic fluid-like to gel-like at sufficiently high concentrations of hematite nanospheres. The association between particles and surfactant molecules is further confirmed by dynamic light scattering (DLS) and small-angle X-ray scattering (SAXS) measurements. This work demonstrates that the degree of enhancement of the viscosity and relaxation time of the micelles is strongly influenced by the surface charge, shape, and surface chemistry of the added nanoparticles.