Compatibility study between hydroquinone and the excipients used in semi-solid pharmaceutical forms by thermal and non-thermal techniques

Thermal techniques, such as differential scanning calorimetry (DSC), thermogravimetry (TG), derivate of TG curve, differential thermal analysis, and non-thermal techniques such as fourier transform infrared (FTIR) spectroscopy and X-ray diffractometry (XRD) were used to evaluate the possible interactions between hydroquinone (HQ) and excipients commonly used in semi-solid pharmaceutical forms. The DSC curve of HQ showed a sharp endothermic event between 173 and 179 A degrees C indicating melting point. No evidence of interaction was observed between HQ and cetyl alcohol (CA), cetostearyl alcohol (CTA), disodium ethylenediaminetetraacetate , and decyl oleate. However, based on the thermoanalytical trials, a physical interaction was suspected between HQ and dipropylene glycol (DPG), glycerin (GLY), hydroxypropyl methylcellulose (HPMC), imidazolidinyl urea (IMD), methylparaben (MTP), and propylparaben (PPP). The FTIR results show that for DPG, GLY, HPMC, MTP, and PPP, there were no chemical interactions with HQ at room temperature, but the heating promotes interaction between HQ and HPMC. The FTIR spectra of HQ/IMD show the chemical interaction at room temperature, which was also observed with heating. The XRD results of mixtures between HQ and DPG, HPMC, IMD, MTP, and PPP indicate no interaction between these substances at room temperature, but the heating modifies the HQ crystallinity in these mixtures. All of these methods showed incompatibility between HQ and the excipient IMD.