From interface towards organised network: Questioning the role of the droplets arrangements in macroscopically stable O/W emulsions composed of a conventional non-ionic surfactant, TiO2 particles, or their mixture

Abstract

This study focused on the formulation and the analysis of three emulsions stabilised by TiO2 particles (TiO210), by a conventional Steareth-2/21 surfactant (St10) and by the mixture of both (TiO25St5). Each system efficiently stabilised the oil/water interface and formed macroscopically stable and totally emulsified oil in water formulations.According to microscopic observations, Steareth emulsifier formed a well-organised network of small and monodispersed oil droplets, while the mixture (TiO25St5) favoured the polydispersity and bigger droplet sizes. Finally, the TiO2 stabilised emulsion showed a high droplet polydispersity and the presence of flocculated particles in the continuous phase.The type of emulsifying system also had an impact on droplets interaction and organisation. Oscillatory and continuous rheological tests, as well as thermal analysis, showed the Steareth-2/21 capacities to assure the stability through both: oil/water interface stabilisation and development of strongly connected and resistant droplet organisation. The analysis of TiO2/Steareth emulsion interestingly pointed out the Steareth main role in the stabilisation processes with specific properties like resistance to deformation. Meanwhile, the emulsion stabilised exclusively by TiO2 particles showed unique viscoelastic behaviour: the particles are forming an elastic emulsion at rest, but the droplet network being fragile is quickly disrupted under the solicitation.The applicative properties of the systems were studied through the textural approach (compression, extrusion, spreading and stretching tests) and confirmed the rheological results. For small solicitations, with partial destruction of the matrix, the 3D networks formed by the high presence of the Steareth-2/21 molecules (St10) are quite similar to the networks created by the flocculated TiO2 particles in the continuous phase of the emulsion (TiO210). However, their behaviour does not evolve in the same manner. Once the solicitation increases, the particles stabilised emulsion will break down, while the Steareth-2/21 containing ones will be more flexible and adaptive.