Determining Degree of Polarization
Date: April 2006
Which is more polar water or methanol? How do you
know? How can you find out?
Water is more polar than methanol, and I believe is THE most polar solvent.
Polarity of a solvent is proportional to the overal dipole
of the molecule and dielectric constant, which can be found in the
CRC Handbook, or charts like the one listed on this website:
Unfortunately, "polarity" is one of those chemical terms that
is often used carelessly and/or ambiguously. For example, liquid
'A' has a higher boiling (melting) point than liquid 'B'.
Therefore, 'A' is more polar than 'B'. In reality boiling
(melting) points are a function of many factors. To confuse
matters more, "polarity" is sometimes confused with the term
"polarizability" which resembles the term "polarity" but is
entirely different. First, I will describe "polarizability"
although that is not what you asked about, but we need to get it
"out of the way".
Polarizability describes the response a molecule experiences
when subjected to an external electric field. It pops up in the
description of many physical phenomena -- index or refraction,
dielectric constant, long range electronic interactions such as van
der Waals constants, Raleigh scattering -- is a short list.
Qualitatively, it measures "how squishy" the electrons in an atom
or molecule are. So for example SCS (carbon disulfide) and OCO
(carbon dioxide) are both linear molecules so neither have an
electric dipole moment, but the polarizability of SCS and OCO is
8.74 and 2.911, respectively. The units of polarizability have the
curious units of 10^-24 cm^3 -- the same units as volume. But that
is a result of some fortuitous cancellation of units. Carbon
disulfide is a lot more "squishy" than carbon dioxide. The
polarizability of water and methanol is 1.45 and 3.29 in the same
curious units, respectively. So methanol is more electronically
"squishy" than water. Even atoms have "polarizabilities" even
though they are spherically symmetric. For example, He and Xe have
polarizabilities of 0.2 and 4.0 respectively (again in units of 10^-24 cm^3).
Now on to your question. More commonly, the term "polarity"
refers to the permanent asymmetrical distribution of electric
charge in a molecule. Linear and/or spherically symmetric molecules
have zero dipole moment, as a result of their high symmetry. So
atoms and linear molecules have zero permanent electric dipole
moments. In the case of water and methanol the dipole moments are
1.855 and 1.70 respectively. The units here are in Debye units. In
SI units 1 D = 3.336x10^-30 Coulomb*meters (qualitatively "charge"
x "distance). The conversion is so one sided because the Coulomb is
a large amount of charge relatively speaking and the meter is a
large distance on the molecular scale. So water has the larger
permanent electric dipole moment but the average electron cloud is
more rigid than methanol as reflected by the smaller polarizability
of water vs. methanol.
The CRC Handbook of Chemistry and Physics has rather large
tables of both electric dipole moments and polarizabilities. In
addition, a "Google" search on those terms will lead you to
discussions from the very basic to the very complicated, depending
upon how deeply you wish to probe the topics.
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Update: June 2012