gaspermeation

Gas permeation refers to the movement of gas molecules through a barrier material driven by a difference in partial pressures across the material. It is a central concept in membrane science and is relevant to fields such as gas separation, packaging, and barrier coatings. The most common framework for understanding permeation is the solution-diffusion model.

In this model, a gas first dissolves into the material at the high-pressure side (solubility, S), then diffuses through the film (diffusion coefficient, D), and finally desorbs on the low-pressure side. The permeability, P, is the product of diffusion and solubility: P = D × S. For a film of thickness l, the steady-state flux J is J = (P / l) × Δp, where Δp is the partial pressure difference across the film. The selectivity of a membrane for gas A over gas B is α = P_A / P_B (or equivalently J_A / J_B under the same driving force).

Permeability and selectivity depend on gas properties (size, condensability) and polymer characteristics (free volume, chain mobility). Temperature, pressure, and aging can alter D and S. Glassy polymers often show dual-mode sorption, and high CO2 or other penetrants can plasticize or swell the matrix, changing performance over time.

Measurement of gas permeation is typically performed in permeation cells (constant-volume or constant-pressure) and reported as permeability with corresponding selectivity. Common units include Barrers for P and dimensionless values for selectivity. Material systems range from polymers (e.g., polyimides) to inorganic and mixed-matrix membranes, each with trade-offs between permeability and selectivity.

Applications include hydrogen purification, natural gas sweetening, CO2 removal, and packaging barriers, where the goal is to maximize permeability for desired gases while maintaining adequate selectivity and stability.