Molecular Sieve
Molecular sieve is the best desiccant based on technical performance characteristics. Its ability to adsorb moisture, in this case water vapor, is so pronounced that it can remove trapped H20 molecules from a fully saturated silica gel bead, which in turn changes the silica gel back to its original Cobalt blue color.
Molecular sieves are synthetic porous crystalline aluminosilicates which have been engineered to have a very strong affinity for specifically sized molecules. The definitive feature of the molecular sieve structure, as compared to other desiccant medias, is the uniformity of the pore size openings.
There is no pore size distribution with molecular sieves, as part of the manufacturing process the pore size on the molecular sieve particles can be controlled. The most commonly used pore size is 4 angstroms (4A) although 3 angstroms (3A), 5 angstroms (5A) and 10 angstroms (13X) are available. This distinctive feature allows for the selection of a molecular sieve product which can adsorb water vapor yet exclude most other molecules such as volatile organic compounds (VOCs) which may or may not be present in the package.
For example: 3A molecular sieve's structure, which if you will remember has a 3 angstrom pore opening, allows water vapor adsorption but excludes most hydrocarbons. 3A is good for ammonia (NH3), water vapor (H2O) and polar liquids. 4A molecular sieve has a slightly higher water vapor capacity but adsorbs molecules as large as butane. 4A is good for water vapor (H2O); carbon dioxide (CO2); sulfur dioxide (SO2); hydrogen sulfide (H2S); ethylene (C2H4); ethane (C2H6); propene (C3H6) and ethanol (C2H6O). 5A molecular sieve adsorbs normal (linear) hydrocarbons to n-C4H10, alcohols to C4HgOH, mercaptans to C4HgSH. 5 angstrom molecular sieve will not adsorb iso compounds or rings greater than C4. 10A molecular sieve (13X) adsorbs di-n-butylamine (not tri-n-butylamine) and is useful for drying HPMA.
The selective adsorption characteristics of molecular sieves can be useful when it is necessary to dry a package or environment without removing other desirable compounds within the system. Molecular sieve can trap water vapor to temperatures well past 225°C in some cases, and due to its high affinity for water vapor, molecular sieve is able to bring the relative humidity (RH) in environments down to as low as 1% RH.
In Europe molecular sieve is used in pharmaceutical applications. In the United States it is often the desiccant of choice for pharmaceuticals and bio-chemicals and/or reagents. Although molecular sieve is sligthtly higher in cost per unit due to its extremely large range of adsorptive capabilities and high capacity at low relative humiddity it is often the best value.
Lack of government approval for the molecular sieves in food and drug packaging has unfortunately limited its more widespread use. Independent testing suggests that molecular sieves meet or exceed government requirements. Presumably, however, the industry has been unwilling to fund the expensive and time consuming testing that is required for government approval.