A cleaning chemistry is effective due to physical and chemical properties. In the last issue of Clean Source (1), we discussed wetting of surfaces and explained that for a fluid to wet a substrate surface, the surface energy of the substrate needs to be greater than the surface tension of the fluid. However, there is more to cleaning than wetting.
All chemistry and all critical cleaning has a basis in physics. Cleaning is removing an undesired material, a soil, from the substrate. This is accomplished by a combination of chemical solvency and physical movement.
Solvency
Chemical solvency starts at the molecular level. The material to be dissolved (the solute, or in cleaning applications, the soil) is held in place by cohesive and/or adhesive forces (1). The forces are overcome due to stronger forces of the dissolving liquid (the solvent; or, in critical cleaning, the cleaning agent). The solute molecules then disperse within the molecular matrix of the solvent. In cleaning applications, the rinse steps dilute or remove the dissolved soils. In cleaning, the term ‘solvent’ frequently is reserved for organic, carbon containing, liquids. Aqueous cleaning is more complex. Aqueous cleaning can involve solvency, sequestration (2), and micelle formation (3).
The adage, “like dissolves like,” applies to chemical solvency. When the polar, hydrogen bonding, and dispersive forces associated with the solvent closely match those of the solute, we have a “like dissolves like” situation.
Polar
Polar forces result from a non-symmetry of the positive and negative charges within a molecule. For example, in the water molecule, the two positively charged hydrogen atoms on one side and the negative oxygen atom on the other create a non-symmetry. Two polar molecules have an attractive (pulling together) force when the positive side of one is adjacent to the negative side of the other.
Frozen Fractals
Hydrogen bonding is similar to polar forces. In hydrogen bonding, a hydrogen in one molecule is attracted to a negatively charged atom in an adjacent molecule. This force causes water to solidify in a hexagonal pattern – it’s responsible for snowflakes. It is considered separately from other polar forces due to the small size of a hydrogen atom and its strong affinity for a single electron. A detailed explanation of hydrogen bonding is way beyond what I’m going to discuss here.
Dispersive
Non-polar or dispersive forces occur between molecules with a normally symmetric charge distribution. As these molecules closely approach each other, the unlike charges on the two molecules are pulled toward each other while the like charges are pushed away. This causes a distortion of the charge symmetry, resulting in an attraction that tends to hold the two molecules together. Dispersive forces are weaker than polar forces but, in many cleaning applications, are the dominant forces holding a soil to a substrate surface.
Hansen Parameters
One way of consolidating these forces is by determining the Hansen Parameters, which characterize the strengths of the polar, hydrogen bonding and dispersive forces associated with a material (4). Therefore, ideally, it is preferable to choose a cleaning agent that has Hansen parameters that are similar absolute numbers and relative proportion to the soils being removed. Of course, sometimes we have mixed soils or the chemical composition of the soils is not known. Perhaps we need to do solvent substitution. In those instances, it is desirable to choose a solvent with Hansen parameters that are similar to the one we are trying to replace. Safety and environmental regulatory restrictions have limited the availability of solvents with Hansen parameters that provide aggressive, effective removal of industrial soils of interest. In such instances, Hansen parameters can be a pathway to formulating more effective blends.
In future episodes of “The Physics of Cleaning,” I will introduce other physical parameters including temperature, viscosity and momentum and discuss how these affect wetting and cleaning in small spaces such as under electronic components or crevices and holes in mechanical structures.
References
- CS Sept. 2014
- CS May 2013
- CS April 2010
- CS Jan. 2008
1 Comment
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