A Generic Explanation of the Mechanism of Co-solvency

Abstract: Polymer behavior in mixed solvents often exhibits intriguing phenomena, such as cosolvency, where a polymer that collapses in two individually poor solvents becomes soluble in their mixture. In this study, we employ a combination of theoretical modeling and computer simulations using a generic polymer solution model to uncover the underlying mechanisms driving this behavior. Moving beyond conventional explanations based on solvent-cosolvent immiscibility or chemistry-specific interactions, our findings highlight the critical role of mismatches in solvophobicity and solvation strength between the polymer and the two solvent components. We demonstrate that co-solvency arises from the interplay between van der Waals interactions and specific associations, such as hydrogen bonding. The bulk phase behavior of the solution is also examined, and the resulting phase diagram, calculated using Flory-Huggins theory, shows good agreement with experimental observations. This study offers a generalized framework for understanding polymer cosolvency across diverse systems.