Solubility, similarity and compatibility: a general-purpose theory for the formulator

Abstract After thousands of papers on solubility science (broadly including “similarity” and “compatibility”) over the past decades, the formulator has been provided with surprisingly few tools that can help solve problems in the necessarily messy world of complex formulations. This failure of much of the academic enterprise has led to a culture of trial-and-error in formulation. Although a very few tools are of enduring value in solubility-related work, only one, currently, has a broad applicability across the range of solubility and compatibility problems faced by formulators. This tool is the Hansen Solubility Parameter (HSP) approach which, despite its many acknowledged faults and limitations is powerful and general enough to be able to guide users through formulation space with the help of on-line apps and datasets. The challenge for academics is not to fiddle with current tools, but to think big and find new tools to exceed the power and generality of HSP.