permeants

Permeants are substances that can cross a barrier, such as a polymer film, membrane, or textile. They may be gases (for example oxygen, carbon dioxide, nitrogen), water vapor, liquids, or ions. In barrier science, permeants are distinguished from the barrier material itself and from other species present in the system. The rate at which a permeant crosses a barrier depends on the properties of the permeant, the characteristics of the barrier, and the driving force, such as a pressure difference, a concentration gradient, or a chemical potential difference.

Most solid barriers follow the solution-diffusion mechanism: the permeant dissolves into the barrier, diffuses through it, and desorbs on the opposite side. The overall permeation is described by the permeability P, the product of diffusivity D and solubility S of the permeant in the material (P = D × S). Temperature, pressure, and the barrier’s free-volume and morphology influence D and S; higher temperatures generally increase permeability.

Permeability is measured using permeation cells and reported in units such as Barrer. Experimental methods like steady-state permeation or time-lag analysis help separate diffusivity and solubility. The selectivity of a barrier, a crucial performance metric in many applications, is the ratio of permeabilities for two permeants.

Applications span food and pharmaceutical packaging, where moisture and oxygen ingress must be limited, to gas separation membranes that enrich or remove specific gases, and protective coatings where solvent or vapor ingress is controlled.