Effect of End-Group Chemistry on Interfacial Tension in Short-Chain Complex Coacervates

Complex coacervation, a liquid–liquid phase separation phenomenon typically driven by electrostatic interactions between two or more oppositely charged polyelectrolytes, has attracted significant attention as a promising platform for bioinspired adhesive and soft material applications. Interfacial tension is a key parameter governing wetting behavior, capillary bridge formation, and adhesion performance, and is generally known to increase with increasing degree of polymerization (DP). In this study, we investigated coacervates synthesized from poly(allyl glycidyl ether) backbones functionalized with amine and sulfonate groups across a range of DPs (20–136). Using a surface forces apparatus (SFA), capillary forces were measured at various salt concentrations and converted into interfacial tension through the Young-Laplace equation. While ionic strength-dependent electrostatic screening significantly influenced the measured interfacial properties, the DP20 coacervate exhibited an unexpectedly higher interfacial tension compared to other coacervates. This unusual behavior seems to arise from the relatively high density of terminal phenyl groups in the short-chain architecture, introducing substantial non-electrostatic contributions such as hydrophobic or aromatic interactions in addition to conventional electrostatic effects. These findings suggest that coacervate interfacial properties can be strategically tuned by balancing polymer chain length and end-group chemistry, providing new molecular design principles for engineering high-performance adhesive interfaces and functional coacervate-based soft materials.

Keywords: complex coacervate, degree of polymerization, interfacial tension, surface forces apparatus