Study on the release properties and stability of o/w emulsions containing salicylic acid and zinc oxide

o/w emulsions with zinc oxide and salicylic acid were prepared at a pH between 4.0 and 5.2 in order to determine the best pH at which both molecules are simultaneously present in the system. The emulsions structure and the presence of zinc oxide crystals were investigated by optical microscopy. Zinc oxide caused a slight increase in pH of the emulsions over time. The emulsion that had the most stable pH was that at pH 4.6. Salicylic acid diffusion from these systems, studied as a Junction of pH and the amount of zinc oxide crystals, was maximal at pH 4.6 and was found to be unrelated to the apparent partition coefficient or to the un-dissociated molar fraction of salicylic acid. Zinc oxide decreased the diffusion of salicylic acid from all emulsions.