Thermodynamic study of the solubility of sulfapyridine in some ethanol plus water mixtures

The solubility of sulfapyridine (SP) in ethanol + water cosolvent mixtures was measured at temperatures from 293.15 to 313.15 K in all the polarity range provided by aqueous mixtures conformed by ethanol and water (solubility parameter, delta(mix)=26.5-47.8 MPa1/2). The solubility was maximal in the mixture with 0.80 in mass fraction of ethanol (delta=30.0 MPa1/2) and very low in pure water (delta = 47.8 MPa1/2) at all the temperatures. The thermodynamic functions Gibbs energy, enthalpy, and entropy of solution were obtained from these solubility data by using the van't Hoff and Gibbs equations. Thermodynamic quantities of mixing were also calculated by using calorimetric values related to the drug fusion process. A nonlinear enthalpy-entropy relationship was observed in a plot of enthalpy vs. Gibbs energy of solution. The plot of Delta H-soIn degrees Vs. Delta(soIn)G degrees shows two different trends according to the slope obtained; that is, i. one with negative slope from pure water up to 0.20 in mass fraction of ethanol and another one from the mixture with 0.80 in mass fraction of ethanol up to pure ethanol; and ii. one with positive slope from the mixture with 020 in mass fraction of ethanol up to the mixture with 0.80 in mass fraction of ethanol. Accordingly, the driving mechanism for SP solubility in water-rich and ethanol-rich mixtures is the entropy, whereas, between 0.20 and 0.80 mass fractions of ethanol the driving mechanism is the enthalpy. (C) 2012 Elsevier B.V. All rights reserved.