A membrane is a selective barrier between two phases that allows mass transfer under the action of a driving force (e.g., gradients in pressure, temperature, concentration, or electrical potential) . The stream that passes the membrane is called 'permeate', whereas the remainder is defined as 'retentate'. The transport mechanisms across a membrane depend on the membrane's morphology, and two typical morphologies can be distinguished: porous and dense. In dense membranes, the interactions between permeate and membrane material dictate the transport rate and selectivity. Transport in such systems is described by the solution/diffusion model in which the combination of Fick's law (diffusion) and Henry's law (solubility) leads to the equation: