Longmuir in 1916 derived an equation from theoretical consideration. He was the first to point out that in the chemical adsorption of a gas on a solid surface a layer single molecule in thickness is formed. He proposed that adsorption process consists of two opposing actions condensation of molecules from the gas phase on the surface and evaporation of molecules from the surface back into the body of the gas. At equilibrium the two rates become equal. He derived an equation based on the following assumptions.
(i) Each solid surface has a fixed number of adsorption points.
(ii) Each adsorption point can hold only one molecule of the adsorbate.
(iii)The absorbed gas behaves ideally in the vapours phase.
(iv) There is no interaction between the adsorbed molecules present on different adsorption points.
(v) The heat of adsorption is the same for all the adsorption points present on the surface of the adsorbent.
(vi) There exists a dynamic equilibrium between the condensation of adsorbate molecules on the adsorbent and their evaporation or desorption from its surface. A mathematical relationship can be derived from these postulates.
Consider an adsorbing surface exposed to a gas. Molecules of the gas will strike the surface and stick for an appreciable time due to condensation while other gas molecules will evaporate from the surface due to thermal agitation.