title: Bicontinuous Fluid Structure with Low Cohesive Energy: Molecular Basis for Exceptionally Low Interfacial Tension of Complex Coacervate Fluids.
abstract: An exceptionally low interfacial tension of a dense fluid of concentrated polyelectrolyte complexes, phase-separated from a biphasic fluid known as complex coacervates, represents a unique and highly sought-after materials property that inspires novel applications from superior coating to wet adhesion. Despite extensive studies and broad interest, the molecular and structural bases for the unique properties of complex coacervates are unclear. Here, a microphase-separated complex coacervate fluid generated by mixing a recombinant mussel foot protein-1 (mfp-1) as the polycation and hyaluronic acid (HA) as the polyanion at stoichiometric ratios was macroscopically phase-separated into a dense complex coacervate and a dilute supernatant phase to enable separate characterization of the two fluid phases. Surprisingly, despite up to 4 orders of magnitude differing density of the polyelectrolytes, the diffusivity of water in these two phases was found to be indistinguishable. The presence of unbound, bulk-like, water in the dense fluid can be reconciled with a water population that is only weakly perturbed by the polyelectrolyte interface and network. This hypothesis was experimentally validated by cryo-TEM of the macroscopically phase-separated dense complex coacervate phase that was found to be a bicontinuous and biphasic nanostructured network, in which one of the phases was confirmed by staining techniques to be water and the other polyelectrolyte complexes. We conclude that a weak cohesive energy between water-water and water-polyelectrolytes manifests itself in a bicontinuous network, and is responsible for the exceptionally low interfacial energy of this complex fluid phase with respect to virtually any surface within an aqueous medium.
authors.affiliation: Department of Chemistry and Biochemistry, University of California Santa Barbara , Santa Barbara, California 93106, United States.
authors.forename: Kuo-Ying
authors.initials: KY
authors.lastname: Huang
authors.affiliation: Division of Integrative Biosciences and Biotechnology, Pohang University of Science and Technology (POSTECH) , Pohang 37673, Republic of Korea.
authors.forename: Hee Young
authors.initials: HY
authors.lastname: Yoo
authors.affiliation: Center for Soft and Living Matter, Institute for Basic Science (IBS) , Ulsan 44919, Republic of Korea.|Asia-Pacific Center for Theoretical Physics , Pohang 37673, Republic of Korea.
authors.forename: YongSeok
authors.initials: Y
authors.lastname: Jho
authors.affiliation: Department of Chemistry and Biochemistry, University of California Santa Barbara , Santa Barbara, California 93106, United States.
authors.forename: Songi
authors.initials: S
authors.lastname: Han
authors.affiliation: Division of Integrative Biosciences and Biotechnology, Pohang University of Science and Technology (POSTECH) , Pohang 37673, Republic of Korea.|School of Environmental Science and Engineering, Pohang University of Science and Technology (POSTECH) , Pohang 37673, Republic of Korea.
authors.forename: Dong Soo
authors.initials: DS
authors.lastname: Hwang
mesh_terms: D000818:Animals; D049872:Bivalvia; D006820:Hyaluronic Acid; D013500:Surface Tension; D014867:Water / Q000737:chemistry*
chemical_list: D014867:Water; D006820:Hyaluronic Acid
keywords: Overhauser effect dynamic nuclear polarization; complex coacervate; cryo-transmission electron microscopy; hydration water; interfacial energy; polyelectrolyte complexes; water dynamics