The Physics
of Cleaning, Part 2—May the Forces be With You
Ed Kanegsberg
In the previous issue of Clean
Source, I explained that as a physicist, I tend to view the processes
of cleaning, washing, rinsing, and drying, in terms of physical actions
- force
and energy. Soil (matter that is out of place) is generally held to the product
by intermolecular forces. Understanding these forces is pretty simple. Once
you understand them, you can often make simple and profitable improvements
to your industrial or critical cleaning process.
In general, three types of intermolecular forces are defined to describe
the bonding of a chemical and its solubility in another chemical.
These are polar, hydrogen bonding, and dispersive (or non-polar) forces.
Polar forces
Molecules generally are neutrally charged. That is, there are an equal
number of positively charged protons (in atomic nuclei) and negatively
charged electrons,
so that the charges cancel each other. Exceptions are ions or free
radicals that carry a net positive or negative charge.
Many molecules, however, are neutral in an overall, global sense, but
they are not neutral locally. This non-neutrality contributes to
cleaning force
at the molecular level and translates into solvency at the chemical
or macro level. Such molecules have their constituent atoms arranged
such
that the
molecule does not have a symmetric distribution of electronic charge.
One portion (side) of the molecule is more positively charged (fewer
electrons)
with another side more negatively charged (more electrons). Such
molecules are called “polar”.
The most commonly known polar chemical is water (H2O). It is polar
because the two hydrogens are not arranged in a straight line with
the Oxygen
(this would look like H-O-H) but rather form a V, with an angle
of ~105 deg and
the Oxygen at the vertex. The Oxygen end is more negative and the
Hydrogen end is more positive, giving water its polar characteristic.
For a
more complete, relatively non-technical discussion of the water
molecule, visit http://www.lsbu.ac.uk/water/mol-easy.html (a more technical version can be found at the same site). We will
also discuss properties of water in our March 2007 monthly column
in Controlled
Environments
Magazine.
Hydrogen Bonding
The hydrogen atoms in a single molecule of water are not lined
up with the oxygen atom. However, when two water molecules are
adjacent,
it
is possible
for an oxygen atom in molecule #2 to be in a straight line with
one of the sides of the “V” in molecule #1 so that in effect there is a
linear O-H-O relationship. The maximal bonding forces in this arrangement
are stronger than the normal polar forces and are known as hydrogen bonding.
Hydrogen bonding is sometimes called a different kind of polar force. In
water, it is considered a constituent of “hydrogen disloyalty,” i.e.
the tendency for hydrogen atoms to migrate from water molecule to water molecule.
The transient sharing of hydrogens also causes water to have
a ‘hermaphroditic’ quality
in the sense that it behaves like it is simultaneously both an acid and a
base. This quality contributes to the solvency of water being sensitive to
the presence of other substances. Even relatively modest changes in the make-up
of a water-based formulation can have large effects on its ability to dislodge
and remove contaminants from surfaces.
Now you have a fundamental idea of two types of solvency forces,
polar and hydrogen bonding. There is a third force that completes
the basis
of solvency
at the molecular level, so important to effective cleaning.
In the next article, I will explain this third type of intermolecular
force,
the
dispersive or
non-polar force and how the balance of forces affects the
way certain cleaning agents remove soils more readily than others.
