Understanding the Effects of Particle Size and Anisotropy on Microstructural and Rheological Properties of Suspoemulsions
Podium 14
Presented by: M. Tümerkan Kesim
UV radiation can cause serious problems such as skin aging, cancer and suppression of immune system. Traditionally used chemical/organic UV filters in sunscreens or cosmetics with UV protection raises health concerns due to potential allergic effects, risk of skin penetration and entering bloodstream. Mineral/physical metal oxide UV filters such as titanium dioxide and zinc oxide (ZnO) are safer alternatives to chemical filters and offer higher UV protection. However, there can be difficulties in formulating emulsion systems with mineral filters. While organic filters are typically dissolved in water and/or oil phases and give reliable and consistent SPF values per mass of filter used, mineral filters are incorporated as an additional phase in the emulsion suspended in water and/or oil phases forming a suspoemulsion system. This two-phase existence adds complexity to the stability and UV protection performance of the emulsion which is already thermodynamically unstable by its nature. Therefore, novel formulation design strategies with mineral UV filters are necessary to obtain reliable UV protection taking into account the complex interplay of emulsion structure and formulation ingredients with metal oxide particles. In this study, the changes in structural and physicochemical properties of O/W and W/O emulsions as a result of the interaction of emollients and emulsifiers with different mineral filters (size and morphology) were investigated.
O/W and W/O emulsions with different emollients, emulsifiers and mineral filters were prepared using a mechanical stirrer, water bath and a homogenizer. The parameters used in mixing and homogenization were kept constant. Optical microscopy was employed for microstructural imaging and quantitative determination of particle and droplet diameters in the final emulsion. Viscosity profiles, elastic/viscoelastic and rheological characteristics of emulsions were determined with a rheometer. In-vitro SPF & UVAPF of emulsions were measured on PMMA plates using a spectrophotometer with an integrating sphere following Colipa guidelines. Homogeneity and dispersion state of powders in emulsions were investigated with a bar-coater where necessary.
Results show that high polarity emollients and nonionic emulsifiers in emulsions are beneficial for better dispersion of mineral filters and increasing stability of emulsions. To achieve higher film coverage and hence high efficacy, homogeneous dispersion of mineral filters is required. Resultant emulsion characteristics are highly sensitive to the type of emulsion system, choice of emollients, amount of emulsifier, particle size and morphology of mineral filters. Anisotropic, micron-sized platelet-shaped particles in W/O emulsions exhibit higher zero shear viscosity compared to conventional spherical nanoparticles due to more efficient entrapment of oil phase between individual plates resisting flow. Upon application of shear force, platelets align towards the direction of shear, therefore decreasing viscosity more compared to emulsions with spherical nano-sized filters. This finding also suggests that spreadability and initial storage stability is enhanced for formulations containing anisotropic platy particles. It is known that inorganic particles tend to stabilize emulsions by decreasing the interfacial tension between dispersed and continuous phase in Pickering emulsions, similar to how emulsifiers function in emulsion systems. It is found that for O/W emulsions, ZnO particles tend to locate themselves at the droplet interface reducing the droplet size. On the other hand, particles are homogeneously dispersed in the continuous oil phase in W/O emulsions with much less affinity towards phase interface.
Overall, the performance of inorganic filters in cosmetic formulations with UV protection depends on the complex interplay of formulation ingredients and processing conditions. UV protection performance is directly related to film-forming ability of applied formulation and dispersion/agglomeration state of inorganic filters which in turn determine the efficacy of the prepared emulsion. Chemically identical inorganic filters with different size and shape can exhibit vastly different properties in emulsion enabling the formulator to exploit morphological differences to design high performance and stable products with good sensory profile.
O/W and W/O emulsions with different emollients, emulsifiers and mineral filters were prepared using a mechanical stirrer, water bath and a homogenizer. The parameters used in mixing and homogenization were kept constant. Optical microscopy was employed for microstructural imaging and quantitative determination of particle and droplet diameters in the final emulsion. Viscosity profiles, elastic/viscoelastic and rheological characteristics of emulsions were determined with a rheometer. In-vitro SPF & UVAPF of emulsions were measured on PMMA plates using a spectrophotometer with an integrating sphere following Colipa guidelines. Homogeneity and dispersion state of powders in emulsions were investigated with a bar-coater where necessary.
Results show that high polarity emollients and nonionic emulsifiers in emulsions are beneficial for better dispersion of mineral filters and increasing stability of emulsions. To achieve higher film coverage and hence high efficacy, homogeneous dispersion of mineral filters is required. Resultant emulsion characteristics are highly sensitive to the type of emulsion system, choice of emollients, amount of emulsifier, particle size and morphology of mineral filters. Anisotropic, micron-sized platelet-shaped particles in W/O emulsions exhibit higher zero shear viscosity compared to conventional spherical nanoparticles due to more efficient entrapment of oil phase between individual plates resisting flow. Upon application of shear force, platelets align towards the direction of shear, therefore decreasing viscosity more compared to emulsions with spherical nano-sized filters. This finding also suggests that spreadability and initial storage stability is enhanced for formulations containing anisotropic platy particles. It is known that inorganic particles tend to stabilize emulsions by decreasing the interfacial tension between dispersed and continuous phase in Pickering emulsions, similar to how emulsifiers function in emulsion systems. It is found that for O/W emulsions, ZnO particles tend to locate themselves at the droplet interface reducing the droplet size. On the other hand, particles are homogeneously dispersed in the continuous oil phase in W/O emulsions with much less affinity towards phase interface.
Overall, the performance of inorganic filters in cosmetic formulations with UV protection depends on the complex interplay of formulation ingredients and processing conditions. UV protection performance is directly related to film-forming ability of applied formulation and dispersion/agglomeration state of inorganic filters which in turn determine the efficacy of the prepared emulsion. Chemically identical inorganic filters with different size and shape can exhibit vastly different properties in emulsion enabling the formulator to exploit morphological differences to design high performance and stable products with good sensory profile.