Relationship of entropy to hydrophobic effect

2015-01-13

azim58 - Relationship of entropy to hydrophobic effect

Basically water cannot form hydrogen bonds with hydrophobic surfaces. As
a result, the water molecules are forced to reorient themselves in an
ordered fashion to form a hydration shell around the hydrophobic surface.
This hydration shell decreases the entropy of the solution which is
unfavored. When the hydrophobic molecules or surfaces aggregate together,
they reduce the surface area exposed to water and minimize the size of
the organized hydration shell that must be formed thus maximizing the
possible entropy of the solution.

here's an excerpt from the wikipedia article on the hydrophobic effect
http://en.wikipedia.org/wiki/Hydrophobic_effect#The_origin_of_hydrophobic_e
ffect

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The hydrophobic interaction is mostly an entropic effect originating from
the disruption of highly dynamic hydrogen bonds between molecules of
liquid water by the nonpolar
solute[9[http://en.wikipedia.org/wiki/Hydrophobic_effect#cite_note-8|]]. 
A hydrocarbon chain or a similar nonpolar region or a big molecule is
incapable of forming hydrogen bonds with water, introduction of such a
non-hydrogen bonding surface into water causes disruption of the hydrogen
bonding network between water molecules. The hydrogen bonds are
reoriented tangential to such a surface to minimize disruption of the
hydrogen bonded 3D network of water molecules and thus leads to a
structured water "cage" around the nonpolar surface. The water molecules
that form the "cage" (or solvation shell) have restricted mobilities. In
the solvation shell of small nonpolar particles, the restriction amounts
to some 10%, e.g. in the case of dissolved Xe at room temperature, a
mobility restriction of 30% has been
found.[10[[http://en.wikipedia.org/wiki/Hydrophobic_effect#cite_note-Xenon-
9|]]] In the case of larger nonpolar molecules the reorientational and
translational motion of the water molecules in the solvation shell may be
restricted by a factor of two to four. Thus at 25°C the reorientational
correlation time of water increases from 2 to 4-8 picoseconds. Generally,
this leads to significant losses in translational and rotational entropy
of water molecules and makes the process unfavorable in terms of free
energy of the
system.[11[http://en.wikipedia.org/wiki/Hydrophobic_effect#cite_note-10|]
] By aggregating together, nonpolar molecules reduce the surface area
exposed to waterand minimize their disruptive effect.

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