Molecular Mechanisms of Gas-Ice Interfacial Transport: Size- and Charge-Dependent Fractionation during Bubble...

Gas-ice interfacial transport phenomena are essential across diverse cryogenic environments, ranging from gas fractionation in polar glaciers to the preservation of cosmogenic noble gases on icy celestial bodies. Bubble close-off in polar glaciers is a compelling example of the complex gas-ice interactions that challenge the interpretation of paleoclimate records preserved in ice cores. While previous studies have provided valuable insights, the molecular mechanisms governing fractionation, especially those involving both geometric and electronic characteristics, remain incompletely understood. Here, using density functional theory (DFT) calculations, we determine effective permeation energy barriers (E _P) for noble gases (He, Ne, Ar, Kr, and Xe) and molecular gases (N₂, O₂, and CO₂) through a model ice layer. Our results reveal that noble gases largely follow a size-dependent trend, whereas molecular gases deviate from such a simple relationship due to more complex gas-ice interactions resulting from their anisotropic charge distribution. The exponential dependence of permeation rates on E _P accounts for the observed nonlinear depletion phenomenon. He and Ne, with their smaller sizes and weaker surface adsorption, exhibit higher permeation rates and rapid depletion from closed-off bubbles. Conversely, larger noble gases and molecular gases are preferentially retained due to increased energy barriers. Notably, molecular gases show significantly lower permeation rates than Ne despite comparable effective cross-sectional sizes, owing to stronger adsorption affinity. Chemical hardness, a descriptor reflecting electronic properties, helps reconcile the fractionation patterns observed for both gas types, indicating that interfacial interactions, not molecular size alone, govern transport through ice layers. These findings provide insights into gas preservation in diverse cryogenic environments, which is essential for the fidelity of paleoclimate reconstruction and the rational design of materials for selective transport. Discrepancies with field observations underscore the role of structural heterogeneities, such as grain boundaries, suggesting that bubble close-off fractionation involves additional pathways beyond idealized lattice permeation.