ABSTRACT
This chapter deals with the phase behavior of fatty acid-soap-water systems, particularly the region of the phase diagram in which fatty acid/soap vesicles are formed in excess water (>95wt% water). In contrast to pH-insensitive diacyl phosphatidylcholine vesicles (conventional liposomes), which are thermodynamically and chemically stable over a relatively large pH range of 3 to 9, fatty acid/soap vesicles are pH sensitive. The thermodynamic stability of these vesicles is restricted to a rather narrow pH range that is close to pH 7, 8, or 9, depending on the fatty acid. The vesicles are characterized by a fatty acid to soap molar ratio close to 1. For the sake of simplicity, fatty acid/soap vesicles are just called fatty acid vesicles. On either side of the stable pH range, the lamellar fatty acid/soap phase (the fatty acid vesicles)
is in equilibrium with other phases; i.e., at the high pH boundary, the vesicles are in equilibrium with soap micelles, whereas at the low pH boundary they are in equilibrium with fatty acid oil droplets. This pH-sensitivity or instability is a property of fatty acid vesicles that is distinct from conventional phospholipid vesicles. Good use can be made of the limited pH stability of fatty acid vesicles. The release of compounds entrapped in the aqueous cavity of these vesicles can be readily triggered by small pH changes, e.g., by a small increase in pH that induces the vesicle-micelle transition.
