Effect of Hydrated Structure of Zwitterionic Polymer Network on Ice and Frost Formation from the Chilled Surfaces

Frost refers to an ice layer formed by the condensation of water vapor from air on a chilled surface. Frosting occurs at temperatures below the dew point, which causes water vapor to solidify onto chilled surfaces. After the initial formation of the ice phase, the adjacent water vapor is sufficiently saturated and cooled owing to the reduction in temperature below the dew point. Consequently, water vapor directly undergoes desublimation to form frost on chilled ice or material surfaces, contributing to the growth of frost in the atmosphere. Therefore, unlike ice, whose entire structure features densely packed crystallized water molecules, frost exhibits a porous dendritic configuration of ice crystals and air spaces. However, the porous frost layer on the surfaces of chilled materials diminishes their thermal conductivity, thereby deteriorating the energy efficiency of infrastructural facilities. For example, in transportation vehicles, such as airplanes, ships, and automobiles, the surface-frost-induced increase in air resistance or body weight can reduce fuel efficiency. Moreover, the heat exchangers typically used in heating, ventilation, and air conditioning (HVAC) systems can experience reduced heat-transfer and fluid-flow rates owing to frost on their surfaces, leading to a significant decrease in energy efficiency. Therefore, anti-icing/frosting technologies have been actively researched in recent years, with emphasis on achieving sustainability.<br/>Various surface treatment technologies have been developed, such as superhydrophobic surface to repel water droplets or hydrophilic polymer surface as a slippery surface for ice, to design anti-icing/frosting materials; however, few have been able to fully satisfy the fundamental requirements of an ideal anti-icing/frosting material. Recently, hydrogel materials have opened up new possibilities in the industrially important field of anti-icing/frosting material design because they exert anti-icing/frosting effects via intermolecular interactions between water and the material (that is, hydration). Therefore, comprehensive explorations of these intermolecular interactions are imperative to fully understanding and harnessing the potential of hydrogels as anti-icing/frosting materials. However, theoretical and fundamental approaches for inducing and tweaking the anti-icing/frosting effects based on the hydration states have rarely been reported. Therefore, the present study was aimed at elucidating the manner in which the state of water of hydration within the hydrogels affected their anti-icing/frosting properties. To that end, hydrogels comprising four different hydrophilic polymers showing different electrically charged nature, i.e. cationic, anionic, neutral, and zwitterionic, were prepared, and their hydration states and anti-icing/frosting performances were investigated. The results showed that the zwitterionic hydrogel exhibited a significantly low freezing temperature (−33 °C) and the highest activation energies for ice formation within the hydrogels and frost growth on the surfaces (9.44 and 4.32 kJ/mol, respectively). Furthermore, they demonstrate that the thermodynamic analysis of hydrogel materials based on an understanding of hydration states can aid significantly in addressing icing/frosting-related problems.