The kinetics of adsorption processes

In any process of adsorption, dynamic equilibrium is reached when the number of molecules condensing in unit time on the adsorbing surface is equal to the molecules evaporating. Langmuir obtained the well-known relation between the mass adsorbed and the pressurem =k 1 p /(k 2 p +b ) by writing the number of molecules evaporating as proportional to the number already adsorbed, and the number condensing as equal tona 0 kp , wherep is the pressure,n the fraction of the adsorbing space unoccupied,k a constant derived from the kinetic theory of gases, anda 0 an accommodation coefficient, which was supposed to differ very little from unity. Using these assumptions, adsorption equilibria should be reached so rapidly as to be practically instantaneous. If we regard the process of adsorption as analogous to a chemical reaction, involving changes in behaviour of the valency electrons of the adsorbed molecules and the adsorbent, the assumption thata 0 differs little from unity, corresponds with the assumption that every collision is fruitful in a gas reaction. H. S. Taylor has pointed out in a recent paper that for some adsorption processes it is apparently necessary to assume quite large energies of activation. Instead of all the molecules colliding with “unoccupied” spaces being adsorbed, only a fractione -E/RT of the impinging molecules change over into adsorbed molecules.