In chemistry, hydrophobicity (from the combining form of water in Attic Greek hydro- and for fear phobos) is the physical property of a molecule (known as a hydrophobe) that is repelled from a mass of water.
Hydrophobic molecules tend to be non-polar and thus prefer other neutral molecules and non-polar solvents. Hydrophobic molecules in water often cluster together forming micelles. Water on hydrophobic surfaces will exhibit a high contact angle.
Examples of hydrophobic molecules include the alkanes, oils, fats, and greasy substances in general. Hydrophobic materials are used for oil removal from water, the management of oil spills, and chemical separation processes to remove non-polar from polar compounds.
Hydrophobic is often used interchangeably with lipophilic, "fat loving." However, the two terms are not synonymous. While hydrophobic substances are usually lipophilic, there are exceptions—such as the silicones and fluorocarbons.
According to thermodynamics, matter seeks to be in a low-energy state, and bonding reduces chemical energy. Water is electrically polarized, and is able to form hydrogen bonds internally, which gives it many of its unique physical properties. But, since hydrophobes are not electrically polarized, and because they are unable to form hydrogen bonds, water repels hydrophobes, in favour of bonding with itself. It is this effect that causes the hydrophobic interaction—which in itself is misleadingly named as the energetic force comes from the hydrophilic molecules. Thus the two immiscible phases (hydrophilic vs. hydrophobic) will change so that their corresponding interfacial area will be minimal. This effect can be visualized in the phenomenon called phase separation.
Superhydrophobic surfaces such as the leaves of the lotus plant have surfaces that are extremely difficult to wet. The contact angles of a water droplet exceeds 150° and the roll-off angle is less than 10°. This is referred to as the Lotus effect.
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