The microstructure characterization of meloxicam microemulsion and its influence on the solubilization capacity

Objective: The aim of this study was to characterize the microstructure of microemulsion consisting of oleic acid, Cremophor RH40, ethanol, and water (K-m = 2), and investigate the influence of microstructure on the solubilization potential of the microemulsion to meloxicam (MLX). Methods: Pseudo-ternary phase diagrams of microemulsion were constructed using the H2O titration method. The microstructures of microemulsion on dilution line N91 were identified by means of conductivity, viscosity, surface tension, and density. The freeze-fracture electron microscopy proved the specific microstructure. Differential scanning calorimetry (DSC) was used to evaluate the position of MLX in microemulsion, and the solubility of MLX in chosen microemulsions on dilution line N91 was measured. Results: The three microstructures along dilution line N91, including water-in-oil (W/O), bicontinuous (BC), and oil-in-water (O/W) microemulsion, were characterized. The solubilization capacity of W/O microemulsion is the best, compared with the other two, whereas the O/W is the weakest. A possible structure model has been applied for the explanation of difference. Conclusions: The solubilization capacity of microemulsion is closely related with its microstructure.