The construction of thermodynamic expressions needed in the physicochemical mathematical model required for the rational quantitative evaluation of the freeze drying process of pharmaceutical solutions employing tertiary butyl alcohol as a co-solvent

A thermodynamic model employing the UNIFAC (Dortmund) method was developed to determine the currently unavailable partial vapor pressures of the binary gas mixture of water and tert-butyl alcohol (TBA) in equilibrium with their frozen solid mixtures. The results agree satisfactorily with the experimental data and indicate that TBA has higher vapor pressures which lead to higher total pressures at the moving interface that could result in larger total pressure gradients and convective mass transfer rates in the dried layer during primary drying. But the higher total pressures reduce the magnitude of the bulk diffusivity of the gas mixture and combined with the smaller Knudsen diffusivity of TBA could significantly impact the competing mass transfer mechanisms during freeze drying. Keywords: Freeze drying; Water and tert-butyl alcohol (TBA); UNIFAC (Dortmund); Partial vapor pressures; Convective flow and bulk and Knudsen diffusion