Clean air and gases are critical for many industrial processes. Trace molecular impurities can contaminate and subvert a process. As mentioned last month, given the diverse nature of molecular contaminants, there is no universal AMC (airborne molecular contaminant) filter or purifier. However, filters may be divided into three major categories based on the mechanism of action. The categories are relevant to both AMC and to purification of pure gases.

PHYSICAL ADSORPTION
Physical adsorption filters are effective for removing non-polar contaminants, such as organic oil vapors from air or gas. Physical adsorption filters rely on intermolecular dispersion forces to trap impacting contamination molecules on a surface without chemically altering the adsorbate (the material adsorbed). Inter-molecular dispersion forces also hold molecules together as a liquid or a solid, rather than as a gas. In many cases, because the adsorption forces are weaker than those of the chemical bonds, it is possible to regenerate the filter using heat or vacuum. Adsorption filters are typically made from porous materials such as activated carbon or activated alumina to create a large surface area.

Physical adsorption filters are not restricted to hightechnology controlled environments. The humble desiccant, a porous hygroscopic material such a silica gel or clay used to remove moisture, is a physical adsorptive molecular filter. It is typically referred to as neither a filter nor a purifier.

Physical adsorption filters have limitations in that they are not very selective. With multiple contaminants, there can be competition for the absorption sites. The efficiency of adsorption depends on such parameters as molecular weight, boiling point, and vapor pressure. The adage “like dissolves like” to summarize solvency characteristics can be adapted to “like adsorbs like” to describe adsorption.¹ For example, a carbon filter has an affinity for carbon-containing organic molecules.

CHEMICAL ADSORPTION, CHEMISORPTION
While chemisorption filters, commonly used to remove acids and bases in the vapor phase, may use the same substrate material as physical adsorption filters to provide a large surface area, they more selective. They are impregnated with chemicals, that are specifically selected to more strongly bond with a target contaminate, chemically altering the contaminant. For instance, to remove acids, the filter or purifier or trap would be impregnated with a base. Because the forces may include stronger inter-molecular dipole or hydrogen bonding forces and/or chemical bonding, adhesion forces of chemisorption are stronger than those of physical adsorption and regeneration is typically not feasible.