Adsorption of vapors by silica gels of different structures

Abstract

1. 1. Isothermals were obtained at 25°C. for the adsorption of the vapors of methyl alcohol, ethyl alcohol, n-propyl alcohol, benzene, carbon tetrachloride, hexane, and water by silica gels of different apparent densities. For certain of these systems additional isotherms at 40 or 45°C. were obtained. 2. 2. The specific surface area of each gel was determined by three independent methods as follows: (a) by application of the BET equation to the low temperature nitrogen adsorption isotherms, (b) by the method based upon the assumption of the existence of uniform circular capillaries (i.e., a = 2Vs radius ), and (c) by treatment of the gravimetric data according to the free surface energy method of Fu and Bartell. 3. 3. For each system the values of the adhesion tension, work of adhesion, and initial spreading coefficient were calculated from the adsorption data by means of the Gibbs equation which interrelates surface area, surface tension, surface excess and chemical potential. 4. 4. It was found that the free surface energy changes which occurred when unit areas of the different porous solids were replaced by unit areas of given solid-liquid or solid-saturated vapor interfaces were independent of the apparent densities of the adsorbent, i.e., the adhesion tension values calculated for a given liquid against a series of silica gels which differed in average pore radius, pore volume, and specific surface area were practically the same. 5. 5. Inasmuch as six different organic liquids, possessing widely different surface tensions as well as widely different interaction energies against silica gave for a given gel similar values for surface area, which values were in good agreement with values obtained with other accepted methods, it appears justifiable to conclude that the free surface energy method of Fu and Bartell is generally applicable.