Chemistry of Mixing

Dr. Darren Williams, Sam Houston State University, Guest author,
with Barbara Kanegsberg and Ed Kanegsberg, BFK Solutions

You cannot clean unless you can get the soil to mix into the cleaning solution. You can do this through brute force with rapidly moving spray, impingement, turbulence, or microjets from collapsing ultrasonic cavities. But you can also do this through temperature and chemistry.

As a Professor of Physical Chemistry and Leader of the Cleaning Research Group, it is my mission to pass on my knowledge of the “art and science” of critical cleaning. Don’t worry. I’ll keep it short and to-the-point so you can quickly understand the role chemistry plays in your cleaning process.

Downhill
Nature is inherently “lazy”. A ball on a hill has gravitational potential energy. Likewise, soil on a surface has chemical potential energy. Because nature is lazy; the ball always “wants” to go downhill, towards a state of lower energy.

This is the equation we use in chemistry to describe the changes in chemical potential energy.

ΔG = ΔH –TΔS

Where ΔG change in potential energy

ΔH = change in energy (absorbed or released)

T = temperature

ΔS = change in entropy

Don’t run! I will explain in a very simple way how each of these parts apply to your cleaning process.

In this equation, “G” stands for Gibbs free energy; and the ΔG is the overall change in chemical potential energy. We want this change to be negative, meaning the chemical “ball” is “rolling downhill”. For our soil on a surface, “rolling downhill” means the soil is mixing into our cleaning solution.

Like Dissolves Like
The ΔH is the energy absorbed or released when the soil leaves the surface. This is the “like dissolves like” piece of the puzzle. If it takes too much energy (big ΔH) to get the soil to leave the surface, then it will not be removed.

If the soil likes the solution as much as it likes the surface (small or zero ΔH) then it will be removed. Oil and pure water do not mix because of a large ΔH, meaning it takes too much energy to get oil off the surface and into pure water. Adding a surfactant to water, makes ΔH much smaller.

Give Me Options
To remove soil from surfaces spontaneously, Delta G must be negative. If we want ΔG to be negative, then the negative piece (–TΔS) is our greatest friend. The T is the temperature of our process, and the ΔS is the change in the entropy.

Entropy can be a confusing concept. Entropy can be understood as the number of options one has for preparing a system. If there is only one way to prepare the system, the entropy is minimal. The higher the number of ways to prepare, the higher the entropy.

Let me describe it with a simple illustration

There once was a mom who liked to sort her candy by color. She would carefully place her candy in a mason jar in layers by color. This was a very low state of entropy because the number of options for preparing this mason jar of ordered candy was very limited – the candy had to be in layers by color; all the pieces of candy of a particular color were in the same layer.

Then, her rambunctious son grabbed the jar and shook it. She was very upset! All the layers of color were mixed up spontaneously. This was a very high state of entropy because there were many options for preparing a random mixture of candy colors.

Changing from the layered color jar to the random colored jar showed a large increase in entropy (large ΔS). In fact, almost all mixing has a positive ΔS.

Applying this to your cleaning process, a soil on the surface is like the candy jar in layers. A soil mixed into the cleaning solution is like the candy jar after it has been shaken. At the molecular level, shaking the jar represents the temperature (T) driving the change in entropy. This is why temperature has such a strong effect on removing soils from surfaces.

Barbara thinks of entropy as the inexorable trend of the surface of Ed’s desk toward a lower energy state. The desk surface is initially made up of small stacks of papers, orderly rows of pens and scratch pads, and an assortment of coasters. Seemingly within hours, papers are scattered randomly; and the number of papers have multiplied. Don’t bother to sort and stack the papers; it’s easier to just set the papers down. It takes more energy to move the papers to the trash bin. Coffee mugs are placed between coasters. It takes energy to move the mug near the coaster and to take it all the way from the desk up to the surface of the coaster. It would take an immense level of energy to move the mug all the way to the sink. Ed’s desk has a low delta G and a high delta S.

The power of entropy
Entropy is the thermodynamic secret to “spontaneous” soil removal. How does a soil mix into the cleaning solution?

ΔG = ΔH –TΔS

1. As temperature goes up, the entropy term (–TΔS) gets more negative.
2. This negative term cancels any positive energy of association that a soil has to the surface (positive ΔH).
3. This causes the overall chemical potential energy to “roll downhill” (negative ΔG), allowing the soil to mix into the cleaning solution.
4. If you make the cleaning solution more like the soil (like dissolves like), you can reduce the energy barrier (ΔH) and can remove the soil at lower temperatures.

If you want to keep your candy sorted by color, or if you want a clean uncluttered desk, entropy is awful. If you want to remove soil from surfaces, entropy is your best friend. Stay Clean!

Darren L. Williams, Ph.D. is a Professor of Physical Chemistry and the Leader of the Cleaning Research Group at Sam Houston State University. Darren teams with Barbara and Ed Kanegsberg of BFK Solutions to teach Product Quality Cleaning Workshops and Webinars (PQCW). For more cleaning resources from the BFK Solutions and Cleaning Research Group Team, visit our Product Quality Cleaning Workshops and Webinars site www.shsu.edu/pqcw. For more details and questions about upcoming and online workshops contact

Dr. Darren Williams (936)294-1529   williams@shsu.edu

Barbara and Ed Kanegsberg : (310)344-2061 info@bfksolutions.com