Hyperhydrophobicity of the Water‐Air Interface

Abstract

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°, which is exceedingly close to the (albeit unattainable) maximum of 180°. 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.

Keywords:

• Water‐air interface
• hydrophobicity
• hyperhydrophobicity
• air bubbles

Acknowledgments

This review article is based on and, with some requisite modifications, reprinted with permission from Chapter XI of Interfacial Forces in Aqueous Media, 2^nd edition, by C. J. van Oss, Marcel Dekker/Taylor and Francis/CRC Press (2006). Copyright CRC Press, Boca Raton, Fla.

Notes

*Sugars such as glucose and sucrose are quite soluble in water, but their aqueous solubility is nonetheless finite, i.e., 47.7% for glucose and 66.7% for sucrose (Stephen and Stephen 1963). Thus their ΔG_iwi value is negative, which makes these sugars (by definition) actually hydrophobic (van Oss and Giesc 1995), although they are probably best designated as near‐hydrophilic.

**It is noteworthy that there always is an AB attraction between these hydrophilic or near‐hydrophilic compounds and the water‐air interface, which is, however, counter balanced by a stronger LW repulsion. This is a consequence of the fact that LW forces acting between two different materials, 1 and 2, immersed in a third liquid, L, give rise to negative Hamaker coefficients (A_1L2), i.e., to a repulsion, when A₁>A_L>A₂ or when A₁<A_L< A₂, where A_i is directly proportional to γ _i ^LW . With poly‐, oligo‐, or mono‐saccharides this happens often, because usually the γ^LW of carbohydrates is of the order of 40 mJ/m², while γ^LW for water is 21.8 mJ/m² and γ^LW for air is zero. Thus Δ G _1WA ^LW >0 (van Oss, 1994).