Nonpolar Surfaces

It is evident that the hydrophobic surface state is established by nonpolar surfaces which are not extensively hydrated. Now the nonpolar surface criterion for hydrophobicity is well known and has been established for some time. Such characteristics of the hy-drophobic surface state have been known since the mid-1950s. In some cases the hydrophobic surface state is due to the elemental composition of the surface; the surface is composed of elements of low polarity that do not hydrogen bond with water molecules. These elements include C, H, S, and large atoms of low polarizability. Examples include graph ite, coal, elemental sulfur, and iodyrite. Even the surfaces of metal sulfide minerals are reported to be hydrophobic in the absence of oxygen and can be considered to be intrinsically hydrophobic. Of course, exposure to even parts per billion of oxygen can lead to oxygen fixation and subsequent complex electrochemical reactions, the surface products of which may or may not be hydrophilic depending on solution chemistry and the extent of oxidation. In general, simply the fixation of oxygen at sulfide mineral surfaces can provide sufficient surface polarity to create a hydrophilic state. Nevertheless, under anaerobic conditions the sulfide surface is expected to be hydrophobic due to its limited ability to hydrogen bond with interfacial water molecules.

In addition to the elemental composition of the surface, the crystal structure and bonding influence the polarity of mineral surfaces. In some cases, specifically surfaces that are created by breakage of weak van der Waals bonds, a nonpolar surface is created even containing elements that normally would hydrogen bond and be hydrated by interfacial water molecules. Examples include pyrophyllite, talc, and boric acid. In this way it has been established that the hydrophobic nonpolar surface state can arise from the intrinsic properties of the elements of which the surface is composed and from bonding considerations associated with the crystal structure. Finally it should be noted that hydrophobic surfaces can be charged just as hydrophilic surfaces are and that generally maximum hydrophobicity is found at the isoelectric point, or the point of zero charge, of the surface.

Figure 3 Water contact angle for a sessile drop of water at the surface of a newly developed water repellant material.

Figure 3 Water contact angle for a sessile drop of water at the surface of a newly developed water repellant material.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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