Modern engineering research focuses on designing new materials and processes at the molecular level. Statistical thermodynamics provides the formalism for understanding how molecular interactions lead to the observed collective behavior at the macroscale. This course will develop a molecular-level understanding of key thermodynamic quantities like heat, work, free energy and entropy. These concepts will be applied in understanding several important engineering and biological applications.
Module 6.2: Water: Unique properties
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Из курса от партнера Carnegie Mellon University
Statistical Thermodynamics: Molecules to Machines
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Applications: Water, Polymer and Photosynthesis
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Venkat ViswanathanAssistant Professor
Mechanical Engineering
[MUSIC]
In this video, we'll discuss a few unique properties of water,
specifically itss polarizability and hydrogen bonding.
The water molecule itself is bent.
And its bent structure is due to the presence of a lone pair
of electrons on the oxygen atom.
Now as the oxygen atom is more electronegative than hydrogen atom,
the electrons tend to localize near the oxygen atom.
This leads to a charge separation and the emergence of a dipole.
The dipole moment of water molecules results in electrostatic
interactions that tend to order water in the liquid phase.
Thus the many unique properties of water are governed by the dipole-dipole
interactions present in water.
Just to remind you,
the electrostatic energy associated with a monovalent ion in a dielectric medium
scales inversely with the distance of separation between the two charges.
Now, it depends linearly on a length scale defined as the Bjerrum length.
The Bjerrum length is simply the length scale at which
the electrostatic energy of an interaction equals the thermal energy
given by the Boltzmann constant times the temperature.
Now let's make this a little bit more complex.
Let's now consider two dipoles with orientations given by u1 and
u2, and they're separated by a distance vector r.
The valence of charge is z.
And now let's assume that they're separated by a distance l.
Now the electrostatic energy associated with these two dipoles is given by the sum
of four interactions, the repulsive interactions between the two positive
charges and the two negative charges.
And the attractive interaction between the opposite charges on the two dipoles.
Now, when this energy is expanded in powers of the distance,
given to order l square, we get.
Now, the dipole-dipole interaction emerges as a long ranged
interaction that scales as the inverse third power of distance.
As a result, the polarizability of a solution can lead to dramatic
consequences on the macroscopic scale for the physical behavior of water.
Now, let's consider the simple example of dropping salt into water.
It quickly separates into sodium plus and chloride ions in solution.
These ions will then order the dipoles around water.
That is, the sodium ions will pull the oxygen end, that is,
the negative end of the dipole of water.
While the chloride ions will pull the hydrogen end,
that is the positive end of the dipole of water.
As a result, the water shields the interaction between the sodium ions and
the chloride ions.
Now this effect is captured by the dielectric constant of the medium.
Now higher the dielectric constant,
the greater is the ability of the medium to shield the electrostatic interactions.
Now what happens if we increase the temperature?
Now as we increase the temperature,
the entropic contributions become more important.
Now the dipole ordering where the sodium ions look for
the favorable dipole ends is energetically favorable.
However, this comes at the loss of rotational entropy.
Now as you increase the temperature, the entropy begins to take over,
and more of these water molecules move away from the shielding region.
This leads to a weakening of the shielding, and
as a result, the dielectric constant drops as a function of temperature.
Another important consequence of the dipoles present in water
is the emergence of hydrogen bonds.
Now hydrogen bonds occur when the partial positive charge on the hydrogen atom
interacts with the negative charge on the oxygen atom of an adjacent water molecule.
Now, the strength of the hydrogen bonds is extremely strong, and
is 8 times the thermal energy at room temperature.
Now, just to get a sense of the hydrogen bonding energy scale,
this is about as energetic as the thermal energy associated with
a furnace sitting at 2,400 degrees Celsius.
Now, because water likes itself so much, when a substance that cannot form
hydrogen bonding is added to water, the substances are simply excluded from water.
This effect is known as the hydrophobic effect.
The most common one that we encounter in our daily lives is oil and water.
The most beautiful, biological example of hydrophobicity is the lotus effect.
The lotus effect is where the leaves of the lotus flower
repel water quite strongly, and they're strongly hydrophobic.
So when water droplets are dropped on the leaf,
they rapidly move around the lotus leaf.
While doing so, the water picks up the dirt particles.
Now this happens because the micro and nanoscopic
architecture of the surface minimizes the droplet's adhesion to that surface.
Now what practical applications do these things have?
Superhydrophobic coatings are emerging in a wide variety of applications.
Now, wouldn't it be wonderful
if you would never have to wipe your car windshield again, right?
All we have to do to enable this is somehow mimic the lotus leaf.
Now glass coatings are being developed by covering a surface with hydrophobic,
extremely water resistant and hydrophilic,
extremely water absorbent nanopolymers that can work together.
Now when a fluid lands on that surface, it is repelled from certain regions,
the hydrophobic regions, and attracted to others, the hydrophilic regions.
Now these serve to guide the flow.
How would a super-hydrophobic windshield look and work?
Well there have been traditionally very few surface coatings on windshields.
Most people just wait for
their car to get dirty, then clean the windshield with wiper fluid.
Now this can sometimes leave unsightly streak marks.
And most fluids are hopeless against greasy messes like bird droppings.
Now using super hydrophobic nano coatings on a windshield is a distinctly modern
invention where the water droplets are guided off the glass surface and
other fluids simply cannot stick.
This leaves the driver's field of vision completely clear,
leading to safer driving experience.
Now we've discussed a lot of wonderful things about water,
but water can't be all good, can it?
Certainly not, water causes a lot of structural damage.
For instance,
copper binds to water stronger than the hydrogen bonding network present in water.
As a result, eventually it forms a copper hydroxide filum and corrodes the copper.
Now of course as a result, there is a distinct classification of metals
where there is an emergence of a new class of metals known as precious metals.
These precious metals are simply those metals that bind to water
weak enough such that water prefers to be in its own environment,
that is prefers its hydrogen bonding over adhesing to these metals.
Now, if the hydrogen bonding in water was half a strong as what it is,
then water would corrode even these most precious metals, gold and platinum.
It's simply because water has a strong enough hydrogen bonding network that these
precious metals can stay precious.
To summarize in this module,
we learned about many of the unique properties of water.
And these are primarily due to its dipole-dipole interactions and
the hydrogen bonding present.
The hydrogen bonding in water on an energy scale
is about 8 times as energetic as the thermal energy at room temperature.
This leads to two classification of nearly all materials as either hydrophobic,
the ones that don't like water, and hydrophilic, ones that like water.
This is an emerging research area where new designs are being thought of
using these hydrophobic and
hydrophilic coatings that are finding a wide variety of applications.
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