Everything about Hydrophobicity totally explained
In
chemistry,
hydrophobicity (from the combining form of water in
Attic Greek hydro- and for fear
phobos) refers to the physical property of a
molecule (known as a
hydrophobe) that's repelled from a mass of
water .
Hydrophobic molecules tend to be
non-polar and thus prefer other neutral molecules and nonpolar
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 — the
silicones, for instance.
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 polarized, and because they're 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. 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.
Superhydrophobicity
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. In one study a
vanadium pentoxide surface is presented that can switch reversibly between superhydrophobicity and
superhydrophilicity under the influence of UV radiation. According to the study any surface can be modified to this effect by application of a
suspension of rose-like V
2O
5 particles for instance with an
inkjet printer. Once again hydrophobicity is induced by interlaminar air pockets (separated by 2.1
nm distances). The UV effect is also explained. UV light creates
electron-hole pairs, with the holes reacting with lattice oxygen creating surface oxygen vacancies while the electrons reduce V
5+ to V
3+. The oxygen vacancies are met by water and this water absorbency by the vanadium surface makes it hydrophilic. By extended storage in the dark, water is replaced by oxygen and
hydrophilicity is once again lost.
Further Information
Get more info on 'Hydrophobicity'.
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