In the last C4 column, I started my contribution to compliance with Controlled Environment’s 2010 editorial calendar by communicating some of the basics behind cleaning science. I used two simple “laws” of cleaning, and showed how they described how cleaning works—that it is really soil management. Cleaning is essentially moving soil from where it is not wanted to where it can be accepted. In this column I will relate the two other “laws” and summarize all four.

CLEANING THERMODYNAMICS
As classical thermodynamics teaches that no process is perfectly and totally reversible, the “Third Law of Cleaning” is:

• One can never get something completely clean.

If a perfectly clean surface is contacted with a cleaning agent containing one molecule of soil, that soil molecule can be transferred (at some slow rate by diffusion) to the clean surface and infect it. Then the driving force is reversed, and the soil molecule can be transferred back (at some slow rate) to the cleaning agent. So, a perfectly soil-free surface is theoretically impossible.

Said another way, a surface can be no more clean than the cleanliness of the cleaning agents last in contact with it.

This concept of a limiting driving force explains why perfect cleaning cannot be obtained in any single stage of cleaning (or rinsing) work.

It’s not an abstract concept. It’s why vapor degreasing was developed so that surfaces could be rinsed with pure liquid solvent produced on the part surface from condensed vapor. It’s why the final contact on critical surfaces in aqueous cleaning is with the purest and most mineral-free water (DI water).