The effect of surface charge and partition coefficient on the chemical stability of solutes in O/W emulsions.

Methylparaben (MP) was the model solute used to study the effect of surface charge on the rate of degradation in oil-in-water emulsions. The surface charge was varied by adding small amounts of phosphatidylglycerol (anionic) or stearylamine (cationic) to a standard intravenous lipid emulsion stabilized by egg phospholipid. The rates of hydrolysis at pH 8.0 in the water phase, oil phase, interface, and aqueous micellar phase were determined by application of a four-phase kinetic model. The rate of hydrolysis in the aqueous phase was dependent on the zeta potential. This was attributed to the effect of surface charge on the pH of the microenvironment of the oil drops through the phenomena known as surface acidity. MP in the aqueous phase hydrolyzed at a rate associated with the pH of the microenvironment, not the pH of the bulk. The effect of the partition coefficient of the solute was studied by substituting ethylparaben (EP), propylparaben (PP), and butylparaben (BP) for MP in the emulsions used to study the effect of surface charge. The rate of hydrolysis was inversely related to the partition coefficient. The effect of surface charge on the rate of hydrolysis was evident in the emulsions containing MP and EP. Partitioning had the greatest effect on the emulsions containing PP and BP. In general, the effect of surface charge predominated when the partition coefficient was small. The partition coefficient had a greater effect than surface charge when the partition coefficient was large. Copyright 2002 Wiley-Liss, Inc. and the American Pharmaceutical Association J Pharm Sci 91:559-570, 2002