Hydrophobe

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For other uses, see Hydrophobia.

Hydrophobe (from the Greek (hydros) "water" and (phobos) "fear") in chemistry refers to the physical property of a molecule that is repelled from a mass of water.

Hydrophobic molecules tend to be nonpolar and thus prefer other neutral molecules and nonpolar solvents. Hydrophobic molecules in water often cluster together. 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". However, the two terms are not synonymous. While hydrophobic substances are usually lipophilic, there are exceptions — the silicones, for instance.

[edit] Chemical background

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 polarised, 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 incorrectly named as the energetic force comes from the hydrophilic molecules.<ref>Goss, K. U. and R. P. Schwarzenbach (2003): "Rules of Thumb for Assessing Equilibrium Partitioning of Organic Compounds: Successes and Pitfalls." JOURNAL OF CHEMICAL EDUCATION 80(4): 450-455. [1]</ref> 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.

[edit] Superhydrophobic

Superhydrophobic materials have surfaces that are extremely difficult to wet with water contact angles in excess of 150°. Many of these very hydrophobic materials found in nature rely on Cassie's law and are biphasic on the submicrometer level with one component air. The Lotus effect is based on this principle. An example of a biomimetic superhydrophobic material in nanotechnology is nanopin film.

[edit] Anthropomorphic

The term hydropobe is anthropomorphic, ascribing motives to molecules, it also describes reality badly. You can wash greasy pans in water only. If you bicycle in the rain your gears end up stripped of grease.

If you put a drop of oil on a flat surface of water it spreads; it does not remain a drop of oil, so it is more attracted to water than to oil so it is not really "hydrophobic". Indeed a classic experiment to estimate Avogadro's number is made in this way. As an illustrative term frequently used in education it should perhaps be deprecated in favour of descriptions of how molecules behave in reality.