Permeant

Permeant is a term used to describe a substance that can pass through a barrier by diffusion or other transport processes. In membrane science, materials engineering, pharmacology, and related fields, a permeant is contrasted with the material that remains on the original side of the barrier.

The rate at which a permeant passes through a barrier is governed by properties of both the permeant (size, polarity, solubility) and the barrier (thickness, free volume, affinity). Permeability is quantified by coefficients such as the permeability coefficient P and the diffusion coefficient D; flux J is often described by J = -P(dC/dx). Temperature, pressure, and the chemical nature of the barrier also influence permeation.

Common permeants include water, oxygen, and carbon dioxide through polymer films in packaging, or drug molecules in transdermal delivery. In gas separation and filtration, the selectivity between different permeants determines the efficiency and feasibility of a membrane process.

Permeant is distinct from permeate; the permeant is the substance capable of permeating, while the permeate is the portion that has passed through the barrier. Retentate refers to material left behind on the feed side. The term is used across disciplines to discuss transport through barriers, highlighting the difference between potential permeants and the material that actually passes through.