A new expanded solubility parameter approach

The partial or Hansen solubility parameters (HSP) are important properties of the various substances and very useful tools for the selection of their solvents or the prediction of their behaviour in numerous applications. Their design and evaluation relies on the basic rule of “similarity matching” for solubility. The present work attempts to enhance the capacity of HSPs by incorporating into their evaluation the other basic rule of solubility, namely, the rule of “complementarity matching”. This is done in a simple and straightforward manner by splitting the hydrogen bonding HSP into its acidic or proton donor component and its basic or proton acceptor one. The splitting is based on the third σ-moments of the screening charge distributions or sigma profiles of the quantum-mechanics based COSMO-RS theory. The whole development and application does not involve any sophisticated calculations or any strong specific background. The new method has been applied to a variety of solubility data for systems of pharmaceutical interest in order to verify the significant improvement over the classical HSP approach. The application of the new method requires, of course, the knowledge of the HSPs. For this reason, in Appendix A is presented an updated version of a robust and reliable group-contribution method for the calculation of the HSPs. The key features of this combined tool are critically discussed.