Effect of Surface Tension on Multilayer Gas Adsorption at Moderate Pressures

Taking into account the variation in the particle density and the normal component of stress in the surface layer inside the adsorption layer of a dilute real gas near the plane boundary of the adsorbent, the amount of adsorption, the spreading pressure, and the two-dimensional second virial coefficient are calculated. The corrections due to the surface layer are found to be proportional to the surface-tension coefficient of the gas, and are of the second degree in the pressure. It is also shown that the same expressions can be derived thermodynamically, if the partition function is defined appropriately to include the effect of the surface layer. Simplified approximate expressions are obtained in the cases of monolayer and multilayer adsorption. The correction in the amount of adsorption is evaluated numerically for neon and xenon on graphitized carbon blacks for a wide range of temperatures, and is found to be of the order of one-tenth of the deviations of the amount of adsorption from the corresponding values for the ideal gas.