Ask A Scientist Chemistry Archive

name         Adam M.
age          17

Question -   Hi,
Look im completing my studies in chemistry a high school level and am
currently working with the reasoning behind why liquids are
miscible and immiscible. I am aware and have been taught about polar and
nonpolar bonds and the general consensus that like dissolve like and the
whole intermolecular forces scene. However I have done some reading into
thermodynamics and am extremely interested as to how the second law of
thermodynamics relates to miscibility/immiscibility of liquids. Obviously
a system desires a higher level of entropy and will change to that if
possible but can someone please explain to me why separating two liquids
therefore creating two layers (immiscible) is more orderly or of higher
entropy than dispersing the two liquids molecules throughout (being
miscible).
-----------------
Adam,

You said, " ... please explain to me why separating two liquids therefore
creating two layers (immiscible) is more orderly or of higher entropy than
dispersing the two liquids molecules throughout (being miscible)."

Your statement is not quite correct. Here's a corrected version: " ...
separating two liquids, thereby creating two immiscible layers, is more
orderly and of LOWER entropy than dispersing the two liquid's molecules
throughout each other in a miscible system."

The separated (two layer) liquid system is more orderly because it has two
discrete (separable) parts. When liquids mix, their entropy (state of
disorder) increases. The separated layers represent a system of greater
order and lower (not higher) entropy.

Entropy is a measure of system disorder. High entropy represents great
disorder -- chaos. Low entropy represents a state of organization and great
order.

Regards,
ProfHoff 661
=====================================================
Well, obviously you have noted that solubility and miscibility is a rather
complicated phenomenon.  You are correct in thinking that all spontaneous
processes require an overall entropy increase.  Entropy is related to the
number of micro-states available to a system; when two liquids mix, the
entire volume of the container is accessible to every molecule of both
liquids, which corresponds to a lot more micro-states than if each liquid
had its own distinct volume from which it could not escape.  So why, you
wonder, would two liquids ever not want to mix?


It comes down to the difference in entropy between the separate liquid
phases and a combined single phase.  It is actually possible for a mixed
phase, containing molecules of more than one component, to be more orderly
(have less entropy) than two phases, one rich in one of the phases and the
other phase rich in the other component.  There are two basic ways this can
happen.


The first is for the molecules of one component to form highly stabilizing
interactions with each other, which they cannot duplicate with the other
component.  Take, for example, a polar liquid.  The polar molecules can
align with each other so that positive charges are near negative charges,
and vice versa.  This constitutes increased order, and thus decreased
entropy, right?  Not necessarily.  Bringing opposite electric charges near
each other lowers the potential energy of a system.  Since energy is
conserved, this means that the kinetic energy of the system rises by the
same amount that the potential energy
goes down.  Molecular kinetic energy is heat.  So when the potential energy
goes down, the temperature goes up, making more disorder (entropy) in the
system.  Which effect dominates - the increased ordering from biasing the
molecular positions and orientations, or the decreased ordering from the
higher temperature?  It turns out that different substances can give
different answers.


What does that have to do with miscibility?  Well, if the polar liquid mixes
with a non-polar liquid, on average any two molecules of the polar liquid
become farther apart.  This means that their stabilizing interactions become
less, hence less potential energy decrease, thus less kinetic energy
increase, thus less entropy.


The second way that mixing two liquids can result in less entropy than when
they are dissociated into separate phases is for the mixing to cause
increased ordering on a microscopic scale.  Imagine, for instance, that when
molecules of substance A are introduced into liquid B, the molecules of
liquid B form an organized "shell" around the molecules of A.  Although the
molecules of A now can occupy many more positions than they could before
they were introduced into the liquid, the molecules of the liquid now are
much more constrained in how they can move about.  These two effects oppose
each other, and whether the entropy goes up or down depends on which effect
is larger.


Surprisingly enough, water seems to behave in the second way when nonpolar
substances are introduced.


If this seems confusing, it is because entropy ("disorder") can be manifested
in several different ways, and the same process can increase entropy in one
way while decreasing it in another.


Richard E. Barrans Jr., Ph.D.
PG Research Foundation, Darien, Illinois
=====================================================