Solubility of pharmaceuticals in water and alcohols

• Aminophylline, (−) lobeline hydrochloride, perphenazine, and indomethacin.
• The measurements of the low solubility of pharmaceuticals in water, ethanol and 1-octanol.
• The new pKa’s values in water.
• The determination of the activity coefficients from the calorimetric and the solubility data.

The aim of this study was to examine the phase equilibria in binary systems: {pharmaceutical, Ph (1) + water, or alcohol (2)}. This work was carried out for four pharmaceuticals, Phs: aminophylline and (−) lobeline hydrochloride, perphenazine and indomethacin. Solubility of tested Phs was determined in three solvents. Water, the main component of any living organism. Ethanol, substance corresponding to the transport of the Ph in the body, and 1-octanol, a model compound of human cell and skin membrane. All studied Phs have aromatic structure and functional groups typical for drugs.The differential scanning microcalorimetry technique, DSC was used to measure basic thermal properties of pure drugs, i.e., temperatures of fusion, enthalpy of fusion, glass-transition temperatures and heat capacity change at the glass-transition temperature. Molar volumes have been calculated with Barton group contribution method. The Bates–Schwarzenbach method enabled us to determinate the pKa of used Phs. These values are important because it may designate the Phs dosage and the activity at the certain pH. The pKa experimental values are slightly different than the literature data which was already published as a results of different experimental methods. In this work, altogether 12 binary systems {Ph (1) + solvent (2)} were studied with the use of dynamic and spectrophotometric method. Three GEx thermodynamic models were used to correlate the experimental data: Wilson equation, NRTL equation and UNIQUAC equation.

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