METHYL SALICYLATE MICROEMULSION BASED SYSTEMS. PART I: EFFECT OF ADDITIVES ON THE PHYSICOCHEMICAL PROPERTIES

Ojectives: The objective was to determine the use of methyl salicylate, the oily active substance, as oil phase in a given surfactant system of Tween 20-IPA(1:1) in microemulsion preparation and also to investigate the effects of additives in the oil phase as well as their amounts on the physicochemical properties of such microemulsions. Methods: The pseudoternary phase diagram of methyl salicylate/Tween 20-IPA(1:1)/water system was first constructed as control. Then, other additive-containing systems were investigated to illustrate their phase behaviors. Each additive was incorporated at three different weight ratios of methyl salicylate to additive varying as 3:1, 2:1, 1:1. Afterwards the single phase liquid systems composed of 15% oil phase in the absence and the presence of additives, 50% the Tween 20-IPA (1:1) mixture, and 35% water were prepared and characterized. Results: Methyl salicylate was successfully used as oil phase in microemulsion preparation. All obtained formulations in the absence and the presence of additives were o/w microemulsions. They showed acceptable physicochemical properties of microemulsion characteristics, which had low viscosity with Newtonian flow. After long term stability for 1 year, the methyl salicylate microemulsion showed good physical stability. The systems containing menthol, isopropyl palmitate or isopropyl myristate as additive at the methyl salicylate-additive weight ratios of 3:1 and 2:1 also showed good physical appearance. Conclusions: Menthol, IPP and IPM were proper additives in this study at the methyl salicylate-additive weight ratios of 3:1 and 2: