Unfrozen Water in Polyelectrolyte Brushes: The Role of Counterion Association in Phase Behavior

Polyelectrolyte brushes (PEBs) are charged polymer coatings that provide a versatile platform for controlling interfacial water behavior. Their ability to attract, confine, and stabilize water molecules makes them relevant across a wide range of fields, including surface engineering, soft matter physics, cryobiology, and energy materials. Understanding how water interacts with and freezes within these systems is critical for developing technologies that operate reliably under extreme temperature conditions. In this work, the freezing behavior of water confined within poly([2-(methacryloyloxy)ethyl]trimethylammonium) (PMETA) brushes containing chloride, iodide, and sulfate counterions was investigated using a custom-built low-temperature attenuated total reflectance infrared (ATR-IR) spectroscopy system. This method enabled direct, in situ observation of changes in water structure and polymer dynamics during cooling. Results reveal that PMETA brushes retain high amounts of liquid-like water even at –60 °C, demonstrating a remarkable ability to inhibit ice crystallization through nanoscale confinement and electrostatic interactions. The extent of water retention depends on the type of counterion present, underscoring the importance of ion-specific effects in interfacial freezing. These insights not only deepen our understanding of water structuring in charged polymer networks but also bridge connections to cryoprotectant design, biological antifreeze mechanisms, and low-temperature lubrication. More broadly, this work provides molecular-level guidance for engineering adaptive coatings with applications in anti-icing surfaces, frozen storage, energy devices, and environmental protection.