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 otherwords, 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.