# Molecular Mechanisms of Gas–Ice Interfacial Transport: Size- and Charge-Dependent Fractionation during Bubble Close-off

## TL;DR

This study explores how gases move through ice, revealing that both size and charge affect how gases are trapped or escape during bubble close-off in glaciers.

## Contribution

The paper introduces a molecular-level understanding of gas-ice interactions using DFT calculations, highlighting the role of charge distribution and chemical hardness.

## Key findings

- Noble gases follow a size-dependent trend in permeation, while molecular gases show deviations due to anisotropic charge distribution.
- He and Ne are rapidly depleted from closed-off bubbles due to smaller size and weaker adsorption.
- Chemical hardness explains fractionation patterns, showing that interfacial interactions, not just size, govern transport.

## Abstract

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.