Physical stability of α-terpineol-based nanoemulsions assessed by direct and accelerated tests using photo centrifuge analysis

Maintaining the stability and integrity of colloidal systems (CSs) is crucial to ensure the quality of formulated products over time. The longevity of CSs is a critical consideration as it affects the quality of the developed product for an extended period. In this study, emulsions (EMs) and nanoemulsions (NEs) loaded with α-terpineol (α-TOH) were evaluated for their physical stability using Accelerated Shelf-Life Testing (ASLT) with LUMiSizer®. The Instability Index of the α-TOH-based-colloidal delivery system stabilized by Tween®20 (TW20) and Quillaja saponins was evaluated at different temperatures (25 °C, 45 °C) and relative centrifuge force (RCF: 581×g; 1308×g; 2325×g). At 25 °C, NEs displayed roughly 70X (at 581×g), 30X (1308×g), and 20X (2325×g) highest physical stability than EMs. On the other hand, when tested at 45 °C, NEs were 40X (at 581×g), 16X (1308×g), and 10X (2325×g) more physically stable when confronted with EMs, evidencing the crucial role of temperature on the stability of the colloidal systems. EMs showed fragile or negligible physical stability - reaching, in most cases - the highest value of the Instability Index (=1.0) scale in less than 1 min, independently of the tested temperature (25 °C, 45 °C) and RCF used. Nanoemulsions stabilized by Quillaja Saponins demonstrated high resistance to centrifugal force, possibly due to features of this anionic surfactant, such as high bending modulus and a sizeable polar head group, distinguishing it from non-ionic surfactants such as TW20. Lastly, the physical stability of α-TOH-NEs was similar when relying on accelerated stability and real-time observation analysis. Thus, these two methods may correlate.