Weak adsorption, strong consequences: How trace gases control ice growth and quasi-liquid-layer structure

Gases in the atmosphere are often treated as passive background species in cold environments, yet their molecular roles at icy interfaces remain poorly understood. Here, we show how hydrophobic and hydrophilic trace gases can actively reorganize the ice interface and regulate freezing through interfacial stabilization. We find these gases can actively modulate both the kinetics and thermodynamics of the water-ice interface. Using molecular simulations, we find that all gases examined retard ice-front propagation relative to pure water to different extents. This slowing arises from weak but preferential adsorption at the water-ice interface, where gases occupy water-dense interfacial layers and locally suppress ice-like ordering. After freezing is complete, gases do not distribute uniformly across the frozen surface, but partition selectively among multiple adsorption regions across the ice–quasi-liquid-layer (QLL)–air interfacial landscape. Relatively hydrophobic gases exhibit a three-region adsorption pattern, whereas relatively hydrophilic gases occupy only two dominant adsorption regions. Despite these differences, both classes preferentially accumulate near the QLL-air boundary and its immediate outer region rather than the ice-QLL interface. There, gases pair with and compensate for dangling surface bonds (through a surface-passivating effect reminiscent of ligand stabilization), reducing the thermodynamic driving force for premelting and thereby thinning the QLL. These results establish a unified molecular picture in which atmospheric gases are active interfacial modulators of both ice growth and ice structure, with implications for freezing, ice-cloud microphysics, and heterogeneous atmospheric chemistry.