Solvent-cosolvent attraction is sufficient to induce polymer collapse in good solvent mixtures

Abstract: Cononsolvency occurs when two miscible, competing good solvents for a polymer are mixed, resulting in a loss of solubility. In this study, we demonstrate through simulations, supported by theory, that cononsolvency can be driven solely by solvent-cosolvent attraction ($\epsilon_{sc}$). The primary mechanism underlying this behavior is the emergent depletion effect, which is amplified by solvent-cosolvent interactions. The polymer reaches a compact state when the solvent and cosolvent fractions are equal ($x_s = x_c = 0.5$), a finding that aligns with predictions from Flory-Huggins theory and the random phase approximation. We show that this cononsolvency behavior is observed for different cosolvent sizes, provided the cosolvent density remains below the depletion threshold and the sizes of solvent and cosolvent particles are not smaller than the monomer size. Additionally, we investigate the role of temperature and find that cononsolvency weakens as temperature increases, due to a reduction in the depletion effect. Finally, we show that when preferential cosolvent attraction is introduced in this simple model, it leads to cononsolvency driven by bridging interactions, occurring at lower cosolvent fractions ($x_c < 0.5$).