Transfer of chemical species:

The word permeation refers to a much more common phenomenon of transfer of chemical species from one region to another under different types of driving forces, concentration gradient, electrical gradient, namely, pressure gradient, or even temperature gradient.

If the membrane is viewed as a porous body within that solvent but not the solute is adsorbed on the pore walls, this adsorbed solvent might so fill the pores such which there are no room for the passage of solute molecules. Solvent molecules may pass through by successive transfer from one adsorption site to the next. Because the related of the adsorbed solvent molecules along with the adsorbing site needs not be completely broken down during the procedure, it needs associatively little energy, compared to that needed for a solute molecule to invade the mass of adsorbed solvent molecules. In that case of desalination membranes, the adsorption of solvent molecules is brought out through hydrogen bonding. Such membranes are known to permit permeation of those solutes such as ammonia, phenol, etc. that have hydrogen bonding tendencies same to that of water.