The mediated rheological properties of emulsions stabilized by thread-like mesoporous silica nanoparticles in combination with CTAB

Abstract

The combination of hydrophilic particles and surfactants provides a simple method to stabilize Pickering emulsions. The type and concentration of the particles and surfactants play important roles in the microstructure and rheological properties of the resulting emulsions. Herein, stable n-octane-in-water Pickering emulsions with tunable rheological properties were prepared using thread-like mesoporous silica nanoparticles (TMSNPs) and cetyltrimethylammonium bromide (CTAB) as emulsifiers. The CTAB concentration (C_CTAB) highly affected the properties of emulsions, which were divided into three regions according to the results of large-amplitude oscillatory shear responses. In the low C_CTAB range (0.03 mmol L⁻¹ ≤ C_CTAB ≤ 0.1 mmol L⁻¹), the emulsions gelled with a high storage modulus . With C_CTAB increasing, the value of emulsions, measured by the small-amplitude oscillatory shear, decreased from approximately 1000 Pa at 0.03 mmol L⁻¹ to 100 Pa at 0.3 mmol L⁻¹ and then to 40 Pa at 3 mmol L⁻¹. A three-dimensional percolation structure formed by cross-linking of TMSNPs in the emulsion continuous phase was observed via cryo-SEM in the low C_CTAB range but not in the intermediate and high C_CTAB ranges. The mechanisms showing the synergistic stability and rheological properties of these emulsions were investigated. It is attributed to the unique morphology of TMSNPs and the competitive adsorption of CTAB molecules at the oil–water interface and on the nanoparticle surface in different C_CTAB ranges. Moreover, owing to the porosity and hydrogen-bonding interactions between the TMSNPs and the confinement effect of the flocculated oil droplets, the viscoelasticity of the emulsions could be mediated by adding a trace amount of acid/base. This study provides a new strategy to regulate the rheological properties of emulsions. It also expands the Pickering emulsion systems with tunable rheological properties.