Solid-liquid equilibrium of nicotinamide in water-ethanol and water-propylene glycol mixtures

Nicotinamide is an active pharmaceutical ingredient (API) with high structural complexity and one form of vitamin B3 still poorly understood. For the pharmaceutical industry, the solubility knowledge of APIs is truly relevant for their production, bioavailability, and to be careful with polymorphs formation. In this context, it was determined nicotinamide solid–liquid equilibrium in temperatures from 20 to 50 °C for two solvent systems: water–ethanol and water-propylene glycol (PG), by a refractometric method. The experimental results were compared with a correlative thermodynamic model (Apelblat), three activity coefficient models (Wilson, UNIQUAC, NRTL) with (Δcp = 0, Δcp = ΔS, Δcp = f(t)) and with literature data. The Apelblat model showed a good correlation in both solvent systems, the best predictive fit model. For the water–ethanol system was NRTL (Δcp = 0), and for the water-PG was UNIQUAC (Δcp = 0). Maximum solubility values were observed in a 25 % m/m water-PG composition at 50 °C. The results of this work were compared with the literature and indicated to be in good agreement for the water-nicotinamide and ethanol-nicotinamide data. X-ray diffraction measurements of the solids in equilibrium with the liquid indicated that the behavior of solubility curves is not associated with a change in the crystalline structure of solids, probably due to the formation of oligomers in solution.