What's a 'Wetting Index' and Why Is It Important In Cleaning?

A “wetting Index” is a measure of a fluid’s ability to clean complex shapes. Water has a very poor wetting index, which is a function of its inherent molecular structure while modern planet-friendly solvents do a much better job cleaning small, delicate or intricate shapes. Here’s why:

Aqueous or water-based cleaners have challenges that many engineers just did not anticipate. The basic chemical and physical properties of water are just not conducive to cleaning the complexity and miniaturization of a lot manufacturing processes today. The surface tension or “wettability” of water is the highest of any cleaning agent available, which means it will not clean well. As good chemists, we can add a surfactant to lower the surface tension. But even with surfactants, the surface tension will still be twice that of any solvent.

Wetting index of modern cleaning fluids
Wetting index of modern cleaning fluids

The viscosity of water and IPA also plays against good cleaning. Higher viscosities mean the solvent will not flow into tight spaces easily, and if you get the water into those nooks and crannies it probably won’t come out. That works against quality cleaning.

Water is heavy, but modern solvents are heavier — typically 20-30% heavier than water, and 50%+ heavier than alcohol. This means solvents can get under particulate, dissipate any static charge and lift particulate off the surface. Water lacks this capability and IPA is even worse.

Savvy engineers can combine all these factors into a scaleless index to compare the cleaning performance of different fluids. This score is called the “wetting index.” The higher the index the more likely the product will deliver excellent cleaning. Here’s a chart showing the wetting index of today’s popular cleaning choices:

Another factor in good cleaning is solubility, which normally is measured with pH for water-based cleaners and Kb value for solvents. Solvents can be tailored to have a wide range of Kb values, which gives them very versatile cleaning performance.  Other factors include drying speed, aroma, toxicity, flammability and other concerns. Put it all together,  and more often than not modern, planet-friendly solvents deliver better cleaning at lower cost.

(Note: For details about the mathematics behind a wetting index, contact MicroCare.)