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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.
Felipe et al. (Sat,) studied this question.