Adsorption properties of hydrophobic surfaces

Abstract

Completely hydrophobic adsorbents such as graphitized thermal carbon blacks, BN, and many sulfides are nonspecific, provided that the structural defects are largely eliminated. Their surfaces carry no π-bonds or groups with lone electron pairs, ions, or free radicals. The adsorption isotherms of water vapor on such surfaces are convex towards the pressure axis and the maount of adsorption is very low. Adsorbates capable of forming intermolecular hydrogen bonds are adsorbed on such surfaces at first as isolated molecules with a heat of adsorption much below the heat of condensation, but, with increasing surface coverage, they become associated, and the heat of adsorption abruptly increases. Certain porous polymers, for instance, chromosorb 102, are also fairly hydrophotic, although the presence on their surfaces of aromatic π-bonds and possibly oxygen-containing groups impart to them a slight specificity, especially in relation to adsorbate molecules containing OH or NH groups. Specific adsorbents may be made nonspecific by chemical modification, grafting saturated groups to their surface, or adsorption of a dense layer of saturated macromolecules. Conversely, the deposition on th carbon black surface of dense layer of macromolecules or high melting organic substances, containing π-bonds or functional groups with lone electron pairs, imparts specificity to the surface. the heats of adsorption on modified adsorbents are usually lower than the heats of condensation. The oxidation in liquid media of carbon blacks makes them as hydrophilic as hydroxylated silicas.