Critical Cleaning For Contamination Control: High Solubility or Low Surface Tension -- Take Your Pick
By: John Durkee, Ph.D., P.E.
May 2008

This column is about compromise — why it’s chemically impossible to have the solvent you want. In critical cleaning, we want solvents with low surface tension, say less than 15 dynes/cm, so that the liquid can penetrate between particles and the surfaces they are contaminating. In critical cleaning, or any cleaning, we want solvents which dissolve the soils about which we have concern, such as adhesives used in the manufacture of disk drives. The problem is we can’t have both. This column is about why that is so.

A TIME FOR SIMPLICITY
Briefly, one needs intermolecular forces to dissolve soils, and intermolecular forces produce surface tension forces.

THE SCIENCE OF SOLUBILITY
Ideas of Professor Joel Henry Hildebrand (1881-1983, University of California, Berkeley), and the work derived from them, allowed development of a useful system of solubility characterization. Hildebrand’s basic idea was that dissolution (or solubility) occurs when there is an energy match within a fluid.

Specifically, the attractive interaction energy of the solvent molecules must approximate the attractive intermolecular energy in the solute (soil). Hildebrand showed that these energy requirements were at a minimum if the solute (soil) and solvent exerted the same forces upon one another.

Hildebrand created a parameter, named after him, which quantifies the level of intermolecular force so that solvents can be matched with soils. Higher values of the Hildebrand Solubility Parameter (HSP) have higher values of intermolecular force — what’s often wanted in cleaning operations.

THE SCIENCE OF SURFACE TENSION
Surface tension is another effect of the intermolecular forces within solvent molecules.

It may seem trivial, but there is no surface tension in the bulk mass of a fluid. It is only at a surface where there is an unbalanced force. At a liquid surface, solvent molecules are pulled inwards by other molecules deeper inside the liquid, and much less so by the molecules in the adjacent medium (vacuum, air, a solid surface, or another liquid).

Molecules at a boundary have fewer neighbors than interior molecules. They exist in a higher state of energy. Minimization of solvent energy, at a given temperature and pressure, means minimization of the number of surface molecules. In other words, the thermodynamic state of minimum energy requires a minimum of surface area — the surface with the “smoothest” curvature.

So, solvents with high levels of intermolecular forces have high values surface tension — what’s not wanted in cleaning operations.

A TIME FOR COMPROMISE
A plot of the Hildebrand Solubility Parameter vs. surface tension is shown in Figure 1.

Solvents such as PFCs, paraffins, HFCs, HFEs, and CFCs have low levels of intermolecular force, low values of HSP, and low values of surface tension. They will therefore match up with soils which share that condition — for which no adhesives need apply.

So is the opposite true, as is shown by Figure 1. N-methyl pyrrolidone, used to remove adhesives from disk drive components, has a surface tension of 40.7 dynes/cm. Water, the “perfect” solvent (it’s not, but that’s another column), has a surface tension value of 71. dynes/cm.

Next time you want to wash particles from surfaces with water, and use no mechanical force with sonic trans ducers, or want to clean soils having high levels of hydro gen-bonding with HFEs, you’ll know why you’re unlikely to be successful doing either one.

1.       A benchmark reference about intermolecular forces and energy transfers within solvents is: Hildebrand, J and Scott, R.L., Regular Solutions, Prentice-Hall, Engle wood Cliffs, NJ, 1962.

2.       A commonly used and numerically equivalent unit for surface tension is mili-Newton/meter (mN/m).

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; or jdurkee@precisioncleaning.com