4 - Effects

Adsorption is the collection of a species on a surface. In absorption, adsorbed species go on to dissolve into the bulk of the material. Absorption is dependent on the strengths of various fundamental interactions between the absorbed species and the sorbent material. In sensor applications, bulk absorption can collect more vapour to a sensor surface than surface absorption, thus offering higher senitivity. Since the principal function of a polymeric membrane is to act as a variable resistance to the passage of permeating species, its most important characteristics, to which all others must be considered secondary, are permeability and permselectivity. The former is a measure of the rate at which a given species permeates a polymeric barrier, and the latter is a measure of the rates of two or more species relative to one another. The following considerations are valid mainly for the gas permeation, but they can easily be applied for the other process types. The material current density can be expressed according to the Fick's law: where D is the diffusion coefficient; K is the partition coeffitient, t is the thickness of the membrane; J is the material current, and A is the area of the membrane surface. The permeability constant, P, which is characteristic for a permeant system, can be defined as the ratio of the flux of the gas or vapor to the concentration gradient across the thickness of the membrane: , thus, it can be expressed with the diffusion and partition coefficients: . Permselectivity can be defined as a separation factor, , that is, the ratio of permeabilities for components A and B: