Design, Multiparametric Stability, Antioxidant Activity and Release Behavior of an Oil-in-Water Cosmetic Emulsion Co-Loaded with Tyrosol and Kynurenic Acid

Tyrosol and kynurenic acid (KYNA) are natural antioxidants with complementary radical-scavenging and photo-absorbing properties, but their combined performance in a cosmetic formulation has not been reported. We developed an oil-in-water (O/W) emulsion containing tyrosol, KYNA, or a 1:1 mixture, using an unloaded emulsion as control. Formulations were evaluated for four weeks at 4, 25, and 40 °C using differential scanning calorimetry (DSC), rheology, spreadability, centrifugation, pH, DPPH antioxidant activity, HPLC quantification and release through a dialysis membrane; compositional stability was assessed by elemental (CHN) analysis. Chemical equilibrium parameters showed a consistent trend: the first week represented a maturation phase, likely reflecting redistribution of actives within the matrix, followed by a stable equilibrium state. The results are therefore discussed as initial versus equilibrated conditions. At equilibrium, the binary formulation maintained a skin-compatible pH (4.40 ± 0.04) with lower variability than single-active systems. HPLC confirmed nominal loading without degradation (0.89 ± 0.08% tyrosol, 0.97 ± 0.10% KYNA, and 1.00 ± 0.18% total in the mixture). DPPH analysis of standards revealed cooperative antioxidant enhancement in the mixture, exceeding the activity of either compound alone. In the creams, this produced a modest but reproducible antioxidant effect (9.00 ± 3.94%, 8.91 ± 3.56%, and 7.02 ± 3.59% inhibition for tyrosol, KYNA, and the mixture, respectively, versus 0.67 ± 4.80% for the control). The mixture showed a significant sub-additive interaction only at 4 °C (p = 0.014). Mechanical stability depended mainly on storage temperature: centrifugation-induced phase separation increased from 8% to 28% between weeks 2 and 4 and was largely prevented at room temperature or above. In the dialysis model, the mixture markedly increased tyrosol release while reducing KYNA release, a behavior tentatively attributed to proton-exchange/keto–enol interactions. Overall, room temperature was the optimal storage condition, and the tyrosol-KYNA combination emerged as a stable, skin-compatible antioxidant system for topical application.