Effect of Loading Strategy on Methylene Blue Encapsulation in Ethosomes: A Comparative Study of Aqueous and Ethanol Phases

This study presents a comparative analysis of the effect of methylene blue (MB) loading strategy on the physicochemical and colloidal properties of ethosomes prepared by the cold method. Two synthesis protocols differing in the phase of introduction of the cationic hydrophilic dye were investigated: a classical approach with MB loading into the aqueous phase and an alternative approach involving MB incorporation into the ethanolic lipid phase. It is shown that the loading strategy is a critical technological factor determining vesicle size, encapsulation efficiency, loading capacity, and electrokinetic properties of the systems. The alternative method results in the formation of smaller ethosomes (Rh ≈ 78 nm) compared to the classical protocol (Rh ≈ 96 nm), but is accompanied by a lower encapsulation efficiency (EE ≈ 36% versus 48%). The results indicate that a reduction in vesicle size does not necessarily lead to higher encapsulation of hydrophilic cationic MB and may be associated with a decrease in the total internal aqueous volume as well as an increased contribution of a weakly bound surface-associated dye fraction. Spectral analysis indicates the preservation of a predominantly monomeric form of MB within ethosomes, while differences in ζ-potential suggest distinct localization of the dye within the vesicular systems. Overall, the results highlight the importance of optimizing the loading protocol in the development of ethosomal drug delivery systems for photodynamic therapy and topical applications.