Abstract Active pharmaceutical ingredients with poor solubility in water and some organic solvents are a challenge in the pharmaceutical industry. To overcome this limitation, microemulsion systems are a choice to increase the solubility of a sparingly soluble active ingredient. The purpose of this study is to introduce and compare two types of oil-in-water (O/W) and ionic liquid-in-water (IL/W) microemulsions, which were formulated to increase the solubility of celecoxib as an active pharmaceutical ingredient. The proposed formulations are composed of the same nonionic surfactant/co-surfactant of Tween-80/transcutol®P, and different oil phases of isopropyl myristate, [ BMIM][PF6] and [ OMIM][PF6]. The pseudo-ternary phase diagrams for the microemulsion systems have been determined at a surfactant-toco- surfactant mass ratio of 3:1 and 298.15 K. From the microemulsion region of the phase diagrams, four formulations was selected and their physico-chemical properties as density, viscosity, refractive index, electrical conductivity and surface tension were measured at 298.15 K. The solubilities of celecoxib in all selected formulations were also determined and compared. The results show considerable increases in solubility of the celecoxib in the ionic liquid-based microemulsion systems.
