Sunday, June 7, 2009

C4: Critical Cleaning For Contamination Control: Equilibrium vs Kinetics: Is the Difference Important to You?


We often take measurements. We want to know how much of something we have or don’t have. It’s what we technologists do.

Along with our human belief that more is always better, we also believe that if we use a proper measuring instrument and proper analytical technique, then our measurements are worth making — they are meaningful to someone.

This column speaks about why our measurements may not be meaningful to anyone. The reason is that some measurements don’t represent equilibrium conditions, and we don’t know that after the measurements were made.

This word reeks of stability. We don’t want our organization, our co-workers, our relationships, or ourselves to be at equilibrium. Mostly, we value change and the growth which it can produce.

And we generally believe, without evaluation, that the measurements we take are equilibrium measurements — also reeking of stability. That’s what we want to happen.

Equilibrium means that the forward and reverse chemical reactions are operating at the same rate, that net diffusion rates are zero, that net transfer of thermal energy via any mechanism is zero, and that thermodynamic equations of state relating extensive quantities are valid.

Kinetics are the processes by which we get to or establish equilibrium, using the word kinetic to mean motion or change.

Equilibrium can’t happen unless all change has been completed or dampened out.

In other words, our measurements of anything (other than rates of change) are meaningless/worthless if they are not taken of an equilibrium situation.

With a current (still) client, I neglected to remember the distinction between equilibrium and kinetics. The project involved a battery of solubility tests aimed to select candidate solvents to remove photoresist soils. Using Hansen Solubility Parameters, we selected both “good” and “bad” solvents for test.

The results were unexpected, and temporarily confusing. Some “bad” solvents performed well; a few “good” solvents performed poorly. Both flaws were due to my not remembering how the process of chemical solution works — that it involves diffusion of solvents through the matrix of polymeric photoresist — and that diffusion rates are a strong function of the size (molar volume) of the diffusing solvent.

  • Glycol ether solvents, with large molar volumes, performed poorer than expected. Our experimental protocol stopped testing after a prescribed and short time interval. These solvents never fully diffused throughout the polymer matrix in the allotted time, and so equilibrium solution never occurred.
  • Acetone and methylene chloride have relatively small molar volumes. So, their diffusion rate was rapid, and solutioning fairly good — not because of their compatibility with the photoresist soil, but because they rapidly swelled the polymer and made it easily removable.

Surface forces can aid or retard diffusion. The hydrogen bonding forces of water shrink it’s molar volume which aids diffusion through other solvents; but, raise it’s surface tension which retards diffusion through porous media.

Aqueous cleaning usually involves formation of some “complex” or coordination product, such as a micelle. Assuming good contact between the aqueous phase (containing the two-headed surfactant) and the oil phase (the soil), micelles can form in a few seconds to a minute or so.

But, achieving fluid contact is a process of mass transfer, dominated by mechanical mixing or the firstorder rate process of diffusion. Surface films, oil thickened by an operating temperature being too low, or limited agitation can all limit diffusion rates.

Slow diffusion rates in water will fool users. They won’t know if the low removal rate of soil was due to an inappropriate choice of surfactants or kinetic factors.

The point of this column is all about design and scale-up of cleaning systems— solvent or aqueous — from laboratory data. If the data aren’t equilibrium data, AND the kinetic factors aren’t quantified, there will be one or two unfavorable outcomes : (1) the choice of cleaning chemistry will be unfortunate, and (2) the contact stage between parts and cleaning fluid will be inadequate to the task.

Is this important to you?

John Durkee is the author of the book Management of Industrial Cleaning Technology and Processes, published by Elsevier (ISBN 0- 0804-48887). He is the author of the forthcoming book Solvent Cleaning for the 21st Century, also to be published by Elsevier, and is an independent consultant specializing in critical cleaning. You can contact him at PO Box 847, Hunt, TX 78024 or 122 Ridge Road West, Hunt, TX 78024; 830-238-7610; Fax 612-677-3170

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