Characterization of the Ultralow Interfacial Tension in Liquid–Liquid Phase Separated Polyelectrolyte Complex Coacervates by the Deformed Drop Retraction Method

Dilute droplets form upon changing the temperature of a phase separated polyelectrolyte complex coacervate. This provides an in situ approach to measure the interfacial tension between supernatant (dilute droplet) and dense coacervate by the deformed drop retraction (DDR) method. The aqueous coacervate, formed via a model 1:1 by charge stoichiometric polyelectrolyte blend, exhibits ultralow interfacial tension with the coexisting phase. DDR finds the interfacial tension scales as γ = γ0(1 – Cs/Cs,c)μ, with μ = 1.5 ± 0.1, γ0 = 204 ± 36 μN/m, and Cs,c = 1.977 mol/L. The value of μ independently validates the classical exponent of 3/2. The scaling holds between Cs/Cs,c of 0.75 to 0.94, the closest measurements to date near the critical salt concentration (Cs,c). The temperature dependence of the interfacial tension is consistent with observed lower critical solution phase behavior and classical scaling. A detailed account of the DDR method and validation of assumptions are demonstrated.