Thermodynamic Studies of Ionic Interactions for the Drug Ranitidine Hydrochloride in Aqueous Solutions at 298.15 K

In the present communication, we report the activities and activity coefficients of a model drug, ranitidine hydrochloride (RT center dot HCl) in aqueous solutions at 298.15 K. These calculations are based on the experimental measurements of densities and osmotic coefficients of RT center dot HCl in aqueous solutions as a function of concentration of the drug at 298.15 K. The density data have been used to obtain apparent and partial molar volumes at finite concentrations as well at infinite dilution and the experimental osmotic coefficient data are used to determine the activity and mean ionic activity coefficients of solute and solvent, respectively. The activity data have been processed to obtain the mixing and excess Gibbs energy changes as well as to obtain the osmotic pressure and osmotic virial coefficient of the drug. The variation of osmotic coefficient (I center dot) and ln gamma (+/-) as functions of concentration are studied; we observed a negative deviation from the limiting law in solution. The values of critical micelle concentration (cmc) and aggregation number (n) are estimated by applying the pseudo-phase separation model to the I center dot data. The data have been examined by applying the McMillan-Mayer theory of solutions and Pitzer approach for aqueous ionic solutions. The results are interpreted in terms of hydrophobic stacking interactions, structure making/breaking effects (kosmotropic/chaotropic effects) and cation-cation, cation-anion-cation attractions in aqueous solutions.