The Hydrophobic Effect

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The Hydrophobic Effect
What does hydrophobic mean? What are examples of
hydrophobic molecules? When two hydrophobic
molecules are placed in water, they appear to be
strongly attracted to one another. This, however,
is a highly deceptive appearance! The vdw force
between two hydrocarbons is weaker than vdw force
between a hydrocarbon molecule and a water
molecule. Hydrophobic molecules actually like
water molecules as they can interact with water
via dipole-induced dipole interactions. These
are stronger than induced dipole-induced dipole
forces (dispersion), the only possible type of
interaction between non-polar hydrophobic
molecules.
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The Hydrophobic Effect
Each molecule of water in the solid state is
engaged in 4.0 hydrogen bonds. (accepting two and
donating two) Each molecule of water in the
liquid state is engaged in approximately 3
hydrogen bonds. What happens when a hydrophobic
molecule is added to water?
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The Hydrophobic Effect
In the example of n-butane in water at room
temperature the equation for Gibbs free energy
breaks down as follows ?G ?H - T?S 24.5
kJ/mol ?H -4.3 kJ/mol -T?S 28.7 kJ/mol It
is actually enthalpically favorable to place
n-butane in water, but it is very unfavorable
from an entropic perspective. Water molecules
which are in direct contact with a hydrophobe
have to freeze in attempt to maintain a maximum
number of H-bonds. Therefore, they have to lose
entropy. The larger the area of such contacts,
the more entropy will be lost by water
molecules. As a result, water molecules will do
whatever they can to minimize such contacts, i.e.
will cause the aggregation of hydrophobic
molecules into larger and larger aggregates.
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The Hydrophobic Effect
Because of these entropic reasons, hydrophobic
molecules are squeezed out of the aqueous
medium. Water molecules dislike hydrophobes.
In contrast, hydrophobes like water molecules
and would like to maximize the contact with H2O
molecules. However, they cannot do it. Water
almost always WINS. It is more accurate to call
water a lipophobic (oil-hating) substance. The
term hydrophobic (water-hating), which is
commonly used for hydrocarbons, is highly
misleading. This could be the origin of one very
common misconception that hydrophobic molecules
repel water. Main consequence The hydrophobic
effect is a unique organizing force, based on
repulsion of solute by the solvent instead of
attractive forces at the site of organization
Tanford, Science 1978.
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The Hydrophobic Effect
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The Hydrophobic Effect
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The Hydrophobic Effect
The hydrophobic effect can be used for creating
well-defined supramolecular assemblies. Dependin
g on the size and the volume ratio of hydrophobic
and hydrophilic blocks of an amphiphile, zero-,
one-, and two-dimensional supramolecular
nanostructures can form.
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Self-Organization of Amphiphiles Driven by the
Hydrophobic Effect
Cylindrical micelles of PS-PAA amphiphile
Spherical micelles of PS-PAA amphiphile
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Self-Organization of Amphiphiles Driven by the
Hydrophobic Effect
Vesicles of PS-PAA amphiphile
Vesicles inside vesicles
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Self-Organization of Amphiphiles Driven by the
Hydrophobic Effect
Baroclinic tubes
Tubes
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Self-Organization of Amphiphiles Driven by the
Hydrophobic Effect
Tube-walled vesicles
Starfish micelles