The Physics of Cleaning, Part 5: Hansen Solubility Parameters
Ed Kanegsberg
Why do we use solvents? We can easily see or visualize the brute force
of a cleaning system. Forces like the mechanical agitation of a high-pressure
spray nozzle can remove soil from a surface, no matter what liquid is used.
However, to get real power, real efficiency, real profits in cleaning,
you
have to understand solvency. Solvency provides a powerful, customizable
vector to the cleaning process. Understanding the power of the Hansen
parameters
yield information you need to design your best cleaning system.
How soils on a surface dissolve
It is possible to remove soils from a surface without agitation. It is
all a matter of relative strength at the molecular level. Two conditions
have
to occur to make soil attached to a surface dissolve. First, the forces
of attraction between the soil (the solute, the material being dissolved)
and
the solvent have to be stronger than the forces that keep the soil attached
to a surface. Second, the forces of attraction between the soil molecules
and the solvent molecules have to be equal or stronger than the forces
that hold the molecules of the liquid solvent together. In other words,
the solvent-solute
bonds are as strong or stronger than solvent-solvent bonds. When both
of these conditions exist, the soil molecules are removed from the surface
and “slip” in
between the molecules of the solvent.
The Evolution of the Hansen Parameters
Hansen Solubility Parameters are not the only quantitative measure of
solvency characteristics, but they are the most used. In 1936, Joel
Hildebrand proposed
a single solvency parameter for each solvent based on cohesive energy.
The cohesive energy is the energy “holding” the molecules of a liquid
together. Cohesive energy makes a liquid a liquid rather than a gas. These
numbers are referred to as Hildebrand Parameters.
In 1966, Charles Hansen subdivided the Hildebrand Parameters into three
separate parameters, one each for the cohesive energies associated
with polar, non-polar
(dispersive), and hydrogen bonding forces. Hansen’s separation added
more information and clarification, providing a better fit to observed conditions.
In a three-dimensional coordinate system defined by the three solubility
parameters (one for each type of force), a point can be plotted
for each substance. By comparing the differences between Hansen parameters
for
differing materials, Hansen defined a radius of affinity of a sphere
surrounding
that point. He found that empirically, the dispersion force was
more
important than the others in determining the affinity, and he therefore
gives the
dispersion
force higher weight in calculating the radius. Materials likely
to be “good” solvents
for the material of interest lie inside that sphere; materials that lie outside
the sphere are likely not to be a good solvent.
A slice of the three dimensional space can depict this good solvent/bad
solvent characteristic as shown in Fig. 1.
Figure 1. Two-dimensional Hansen plot for oils (courtesy of K. Dishart)
The circles represent the radius of affinity for common oils
used in manufacturing processes. A number of solvents are
plotted. Those
within
the circles (e.g.
TCE) are probably good solvents for these oils. Those outside
the circles (e.g. water, ethylene glycol) are not good solvents.
There
is no on/off
switch. Those near to the circle edge may be adequate even
if they may not be as
effective a solvent as one near the center of the circle.
Water and Oil Do not Mix – Even at the Molecular Level
Hansen parameters for several organic solvents and for water
are compiled in Table 1. Notice how different water is
from the other
solvents both
in terms of the balance of the forces and in terms of the
absolute numbers. That is why water itself is such a poor
solvent for
oils, that have high
non-polar and low polar or hydrogen bonding parameter values.
The forces that hold the water molecules together are too
strong for
the oil molecules
to slip in between and be dissolved.
| Compound | Non-polar (dispersive) | Polar | Hydrogen bonding |
| C7-11 Hydrocarbons, 25% aromatics | 15.8 | 0 | 0 |
| Trichloroethylene | 18.0 | 3.1 | 5.3 |
| Methylene chloride | 18.2 | 6.3 | 6.1 |
| Perchloroethylene | 19.0 | 6.5 | 2.9 |
| n-propyl bromide | 16.0 | 6.5 | 4.7 |
| HFC 43-10mee | 12.9 | 4.5 | 5.3 |
| Isopropyl alcohol | 15.8 | 6.1 | 16.4 |
| Acetone | 15.5 | 10.5 | 7.0 |
| Methyl acetate | 15.5 | 7.2 | 7.6 |
| Methyl ethyl ketone | 16.0 | 9.0 | 5.1 |
| N-Methylpyrrolidone | 18.0 | 12.2 | 7.2 |
| d-limonene | 16.6 | 0.6 | 0 |
| Water | 8.6 | 13.4 | 25.8 |
Table 1. Hansen solubility parameters for organic solvents and water
Teas Style
In the previous installment of “The Physics of Cleaning,” I discussed
the Teas Diagram, a tool that relates the ratio of the
three types of forces that characterize inter-molecular interactions, polar,
dispersive (non-polar),
and hydrogen bonding. Substances with the same ratio of
these forces are in the same location on the triangle of the Teas Diagram
and tend to have similar solvency
characteristics. The diagram is a convenient, simple way
of depicting the three types of forces, and in many cases it works fairly
well to predict solvency characteristics.
The Teas diagram is actually newer than the Hansen parameters.
The Teas diagram was introduced to portray the Hansen
parameters in a
graphically more accessible
manner. So why do we need Hansen parameters?
The Teas Diagram can depict the solvency “style.” However,
it only indicates the ratios of the three forces. Two solvents
could be in a similar
location on the diagram, but one might be an order of magnitude
more powerful for removing the soil of interest than the
other. Depending on the materials
of construction in your product, you may want the more
powerful solvent.
Wonderful new Solvents
Suppose you were using Methylene Chloride to remove
a contaminant and it was suggested that you substitute
HFC 43-10mee.
Look at the numbers
in
Table 1.
While the ratios of the forces are quite similar,
so they would be close together on
a Teas diagram, the values are lower for the HFC
43-10mee. This means that the HFC is a weaker solvent
and might
not be within
the sphere
of affinity
for the
contaminant you wish to remove. Another solvent
in the list, nPB, is just as close to Methylene Chloride
on a
Teas diagram
but also
has
very similar
Hansen
Solubility Parameters, and therefore might have
a better chance of being the replacement you could
use.
While tools such as a Teas diagram and Hansen parameters
can help provide a “short-list” of
candidates for solvents to test with your specific application,
Hansen parameters are not a perfect predictor of solvency.
Especially with solvent blends, it may
be difficult to predict the Hansen parameters of the blend.
The best judge is practical testing with your product.
Turning water into a useful solvent will be covered
in the next action-packed installment of “The Physics
of Cleaning.”
