Determination of ibuprofen solubility in wax: A comparison of microscopic, thermal and release rate techniques

The solubility of ibuprofen in Witepsol H15, a semi-solid wax matrix, was determined using microscopy, Higuchi release rates and HyperDSC (high speed differential scanning calorimetry); solubility values of 15–20%, 18.6% and 12.7% w/w resulted from the three techniques, respectively. Microscopy was useful in additionally examining crystal size, shape and homogeneity. Release rate experiments showed that release from these formulations followed Higuchi kinetics with an inflection in release rate constant at the drug loading corresponding to drug solubility. HyperDSC not only measured solubility but also determined the melting point of the formulation. The results from these three techniques correlated well, suggesting that the simpler techniques of microscopy or HyperDSC are appropriate to determine the solubility in semi-solid pharmaceutical formulations.