Evidence for a pH difference controlled by thermodynamics between the interior and the exterior of a new type of vesicles in suspension

Spherical surfactant aggregates of a new type were recently discovered. Named spherulites or onions, these aggregates are droplets of lamellar phase that can also be seen as vesicles (0.1-20 mu m) with a dense multilamellar structure. In this article, we report the formation of an aqueous dispersion of Spherulites with a difference in acidity between the bull phase and the vesicles. This pH difference is achieved by the addition of fatty acids to the surfactant system. A series of fatty acids of varying chain length (from acetic acid to dodecanoic acid) is studied and is shown to induce different values of the pH shift. pH shifts up to 1.5 units are measured. This pH difference is thermodynamical in nature. A theoretical description using simple thermodynamics is proposed; it is used to build up an experimental pH determination without the use of a local probe. It is demonstrated herein that spherulites are a new type of encapsulating system that permits a thermodynamical control of acidity inside the microcapsule.