Interfacial tension in polyelectrolyte systems exhibiting associative liquid-liquid phase separation

A continued interest in polyelectrolyte phase diagrams guides the study of interfacial phenomena driven by polyelectrolyte complexation. The liquid-liquid interfaces formed by associative phase separation of oppositely charged synthetic and natural polyelectrolytes provide measurement challenges addressed by force-sensitive methods and deformed droplet retraction. The ultralow interfacial tension, typical of these systems, is sensitive to salt concentration and temperature and displays universal features described by mean-field theory. Several areas of fundamental development and novel applications of charge complexation for interfacial study and examples from membraneless organelles and biomolecular condensates are described.