In Search of the Perfect Solvent
In the real world of critical cleaning, the “perfect” cleaning solvent would have certain qualities. It would be inexpensive, effective, versatile, and safe. Given the limitations of chemistry, however, can the perfect solvent exist?
Table 1 shows our wish list of characteristics of the perfect solvent. A generation ago CFC-113 or trichloroethane fulfilled these goals. But today manufacture of both is banned because both flunk the ozone depletion (ODP) goal.
A generation ago, a table such as Table 1 would have contained only physical properties (A) and chemical solvency (B). Since the 1980s, however, ESH considerations (C) have become as or more important than chemical solvency. Today, the so-called “designer solvents” are expensive (D) because of development cost, uniqueness, and composition. The “solvency” category (B) recognizes that solvents are chosen to match the soils to be removed. The column titled “Approach” is an over-simplification of reality. Not all solvents so described will have the given characteristic, but the approach listed is the most straightforward way of providing it. The table is certainly incomplete; inevitably, some readers have situations not covered.
The advantages of halogenated solvents are clear: nonflammability, rapid drying, and good penetration of small spaces. The advantages of simple and small molecules containing only H and C atoms are also obvious: environmental security and specific solvency. Complex structures containing C, H, and O atoms, such as n-methyl pyrrolidone, are in the minority of solvents useful for cleaning polymeric residues and removing some paints. Azeotropic solvents, blends, and co-solvents are often chosen to provide these competing characteristics.
Does it look like all those goals can be achieved by a single solvent, or a few multiple solvents? Is it obvious why some say that water is the “perfect solvent?” (Water, of course, presents a separate set of problems.)
So unless our requirements are very simple, our chances of getting the “perfect solvent” are zero. This analysis also shows why it is so difficult to find “drop-in” replacements for banned solvents.
So what are users to do? That which we must do all our lives: compromise. We must accept the limitations of the possible. Decide which characteristics are of most value in our applications. Painfully identify solvents whose failure to have desirable characteristics can be tolerated because they have the essential ones. If a solvent which provides that compromise isn’t available, consider aqueous cleaning or changing the problem constraints.