Molecules that repel water because they are nonpolar molecules are described with the adjective hydrophobic. Hydrophobic molecules such as alkanes, oils, fats, and greases do not dissolve in water. This characteristic is called hydrophobicity. In chemistry, we call hydrophobic molecules hydrophobes. Their opposites, hydrophilic molecules, dissolve in water.
Water, , is a polar molecule, that is, it has polarity, which is an uneven distribution of electron density among its atoms. The oxygen side of any water molecule is slightly negative, while the hydrogen side is slightly positive. Polar water does not bond with nonpolar or hydrophobic molecules.
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The adjective hydrophobic has its roots in a Greek word, hydrophobos, which means “dreading or fearing water.” Hydrophobic molecules have the quality of hydrophobicity, meaning they repel water, do not dissolve in water, or are themselves repelled by water molecules.
Because hydrophobic molecules occur across a wide range of named substances, we cannot lump all water-repelling substances under a single characteristic. Some overlapping characteristics emerge, however. Hydrophobic molecules tend to:
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Substances with extremely high contact angles (greater than ) are said to be superhydrophobic and have superhydrophobic surfaces. These substances, such as nanopin film and self-assembled monolayers (SAMs), may be useful in biomedicine, nanotechnology, energy efficiency, and improving thermodynamics in power plants.
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Water forms hydrogen bonds, notable for their weakness and short lives. These weak bonds account for water’s ability to readily bond to other polar molecules, making water the “universal solvent.” But water is not a universal solvent; it cannot bond with a long list of nonpolar, hydrophobic substances:
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All of these hydrophobic substances do not dissolve in water. Most of them repel water, as seen with car wax causing rainwater to bead on your car’s metal surfaces. You can use waxes to create hydrophobic surfaces.
Lipophilic substances will dissolve in lipids or fats, and they can absorb water. Hydrophobic substances cannot absorb water or be wetted by water.
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