Solubility modelling and preferential solvation of paclobutrazol in co-solvent mixtures of (ethanol, n-propanol and 1,4-dioxane) + water

The equilibrium solubility of paclobutrazol in co-solvent mixtures of ethanol (1) + water (2), n-propanol (1) + water (2) and 1,4-dioxane (1) + water (2) were determined experimentally by using isothermal dissolution equilibrium method within the temperature range from (278.15 to 313.15) K under atmospheric pressure.(101.1 kPa). At the same temperature and mass fraction of co-solvent, the mole fraction solubility of paclobutrazol in the three binary co-solvent mixtures obeyed the following order from high to low: 1,4-dioxane (1) + water (2) > n-propanol (1) + water (2) > ethanol (1) + water (2). The preferential solvation parameters were derived from their thermodynamic solution properties by means of the inverse Kirkwood-Buff integrals. The preferential solvation parameters for ethanol, n-propanol or 1,4-dioxane (delta x(1,3)) were negative in water-rich mixtures but positive in compositions from 0.24 (0.19, 0.18) in mole fraction of ethanol (n-propanol or 1,4-dioxane) to pure ethanol (n-propanol or 1,4-dioxane). The preferential solvation magnitude of paclobutrazol by the co-solvent was highest in ethanol (1) + water (2) mixtures and lowest in n-propanol (1) + water (2). It was conjecturable that in the former case hydrophobic hydration around the aromatic rings played the main role in the drug's solvation. The higher solvation by ethanol (n-propanol or 1,4-dioxane) in intermediate compositions and in colsolvent-rich mixtures could be explained in terms of the higher basic behaviour of this co-solvent interacting with the Lewis acidic groups of the paclobutrazol. Besides, the drugs' solubilities were mathematically represented by using the Jouyban-Acree model, van't Hoff-Jouyban-Acree model and Apelblat-Jouyban-Acree model obtaining average relative deviations lower than 4.73% for correlative studies. (C) 2017 Published by Elsevier Ltd.