Relating Solvent Properties of Coexisting Aqueous Phases to Interfacial Tension*

The organization of multiple subcellular compartments is controlled by biopolymer-related liquid–liquid phase separation. Aqueous two-phase systems formed by two non-ionic polymers may serve as a simple model for phase separation in cells. Phase separation occurs with the emergence of interfacial tension. To better understand liquid–liquid phase separation mechanisms, interfacial tension was measured in aqueous two-phase systems with different additives. Interfacial tension depends on differences between the solvent properties of the coexisting phases, which were measured to quantify dipole–dipole, ion–dipole, ion–ion, and hydrogen bonding interactions. Additional measurements using FTIR showed that solvent properties are dependent on the structure of water. Based on our current measurements and literature data, we propose a mechanism for phase separation in aqueous two-phase systems.