A variety of different isotherm equations have been proposed, some of
which have a theoretical foundation and some being of a more empirical
nature. Many of these equations are valid over small relative pressure ranges
but do not fit experimental data when tested over the full range of relative
pressures. Only those which are commonly used for the description of the
physical adsorption of gases or vapours onto the surface of porous
adsorbents will be outlined. Some of these theories, as shown later, can be
extended to describe the simultaneous adsorption of two or more components