Natural Surfactant-Based Emulsion Systems: The Influence of Common Pharmaceutical Excipients on Colloidal Structure and Physical Stability

There is a growing need for research into new skin- and environment-friendly surfactants. The aim of the study was to find out whether a combination of an alkylpolyglucoside natural surfactant with established pharmaceutical excipients could provide a solid pharmaceutical base with satisfied physical stability. The study was carried out in two phases: the first one focused on the colloidal structure of vehicles formulated with oils of different polarity and/or different costabilizer (lipophilic versus hydrophilic) and the second one evaluated vehicles' physical stability. A number of techniques were used (polarization, light, and transmission electron microscopy, pH, conductivity and thermogravimetric measurements, rheological analysis and cyclic temperature stress test). Natural surfactant's interaction with used excipients resulted in the formation of semisolid emulsion systems of different rheological profiles, stabilized predominantly by synergistic effects of lamellar liquid-crystalline (L alpha) and complex lamellar gel (L beta) phases. The type of used oil and costabilizer significantly influenced the colloidal structure of the vehicles, particularly in terms of water distribution mode and initial rheological performance as well as their physical stability. It was recommended that medium polar oils of ester type and lipophilic costabilizers, particularly long chain fatty alcohols, should be used in the formulation of stable alkylpolyglucoside-based topical vehicles.