Solution-Diffusion:

Solution-diffusion is frequent cited as the probable mechanism to explain selective permeation of a chemical substance by membrane that is nonporous, from a feed mixture whereas no appreciable size differences exist. For any chemical species to permeate from one side of the membrane to the other side, it has to dissolve or absorb in the membrane and subsequently diffuse by the membrane and get desorbed on the other side. Therefore, certain components in the feed combination get sorbed on the membrane in preference to other components due to chemical similarity or chemical preference which could give the needed selective permeation property to the membrane.

On the other hand, when, in those cases where no important changes exist in the sorption behaviour of the elements of the feed mixture, selective permeation still can arise because of differences in their diffusion rates through the membranes. Chemical substances with higher molecular mass diffuse slowly by the membranes as compared to substances along with lower molecular mass. The word permeation and diffusion is on the other hand used to describe the transport of chemical species across the membranes. Therefore, the distinction among them requires to be clearly understood. The word diffusion specifically refers to random molecular motion of a species yielding to a net transfer of which species from a region of higher concentration to a region of lower concentration.