Tailoring the Stability of W/O/W Liquid Foundations: A Comparative Study of Polyglycerol-Based W/O Emulsifiers

Water-in-oil-in-water (W/O/W) multiple emulsions are thermodynamically unstable systems characterized by high interfacial free energy and complex thermodynamic driving force promoting internal droplet coalescence or Laplace pressure-driven water migration. In color cosmetics such as liquid foundations, this intrinsic instability is further exacerbated by the incorporation of inorganic pigments (e.g., titanium dioxide (TiO₂) and iron oxides), which perturb the delicate interfacial equilibrium and trigger premature phase separation. Utilizing polyglyceryl-10 stearate (PG10S) as a fixed oil-in-water (O/W) emulsifier, this study systematically investigated the stability of these challenging systems by assessing four polyglycerol-based water-in-oil (W/O) emulsifiers with distinct molecular architectures: polyglyceryl-3 polydimethylsiloxyethyl dimethicone (KF-6106), polyglyceryl-6 polyricinoleate (PR-15), polyglyceryl-3 polyricinoleate (PG3PR), and polyglyceryl-2 dipolyhydroxystearate (PGPH). Through a combination of confocal laser scanning microscopy (CLSM), rheometry, differential scanning calorimetry (DSC), surface/interfacial tension analysis, and accelerated stability testing, the structure-property relationships governing the integrity of the multiple-layered structure under both intrinsic and pigment-induced stresses were elucidated. The results demonstrate that the silicone-modified emulsifier (KF-6106) exhibited superior stabilizing efficacy, achieving a high thixotropic recovery of 91.16%. This performance is attributed to its ability to form a resilient, "self-healing" interfacial film with high segmental mobility, which effectively suppresses the spontaneous merging of internal droplets and maintains a robust barrier against the destabilizing effects of pigment particles. These findings establish a mechanistic framework for optimizing emulsifier selection and provide theoretical guidance for the rational design of stable, high-performance W/O/W multiple emulsions in complex, particle-filled systems.