Hyperhydrophobicity of the water-air interface

The air side of the water-air interface is the most hydrophobic surface known. In quantitative terms the water-air interface is about 30% more hydrophobic than the surfaces of nonpolar condensed-phase compounds or materials such as octane or Teflon. The hyperhydrophobicity of the air side of the water-air interface is the main cause of the large increase in contact angle of drops of water deposited upon rough surfaces of apolar materials, as compared with the water contact angle on smooth surfaces of the same materials. A, water drop supported on a very porous fractal surface, encountering only about 1% solid support and 99% air, can reach a contact angle of 174 degrees, which is exceedingly close to the (albeit unattainable) maximum of 180 degrees. The water-air interface hydrophobically attracts completely apolar molecules, as well as the apolar side of amphiphilic molecules (such as surfactants). Thus, for instance, dissolved surfactant molecules aggregate at a high concentration at the water-air interface when dissolved in water. On the other hand, the water-air interface repels dissolved hydrophilic (or near-hydrophilic) solutes, such as sugars and polysaccharides, mainly via net repulsive van der Waals forces. Thus, the water-air interface is depleted of such hydrophilic (or near-hydrophilic) solutes, leaving a significantly higher concentration of these solutes in the bulk of the aqueous medium than at its air interface. As both of these contrasting phenomena result in strongly anisotropic concentration distributions in liquid drops and as contact angle determinations depend on a known and homogeneous free energy of cohesion of the liquid throughout the drop, one should never measure contact angles on solid surfaces for the purpose of measuring their surface thermodynamic properties by using aqueous solutions, mixtures, or solutions in or mixtures of other polar or partly polar liquids. Finally, the peculiar properties of the water-air interface give! rise to what at first sight appears to be paradoxical behavior of air bubbles in water: in pure deionized water, air bubbles attract one another and coalesce. On the other hand, upon the addition of salt (e.g., NaCl), air bubbles repel each other and thus do not coalesce, all in apparent contradiction of the classical rules governing the stability or instability of colloidal suspensions in water.