Principles of Adsorption

Adsorption is a physical phenomenon where specific molecules bind to the surface of an appropriate material and accumulate there. Desiccants are especially designed to accumulate water molecules.

Since adsorption is a physical phenomenon, desiccants must possess the relevant features and  properties  most suitable for maximising these effects. Therefore as many cavities as possible are required in the interior and exterior surfaces of the desiccants. This results in a large surface area where moisture can accumulate.

Functionality

Adsorbents such as silica gel or molecular sieve use physical forces that bind moisture by accumulating water vapor in cavities that extend throughout their exterior and interior surfaces.

The adsorption capacity of molecular sieve is approx. 20% of its own weight and does not depend on relative humidity.

With silica gel, however, the adsorption capacity depends on ambient humidity. The higher the level of relative humidity (up to approx. 80%), the higher the adsorption capacity.

Test method to determine residual moisture

1. Silica gel

The percentage of moisture in a desiccant is called residual moisture. To determine residual moisture, you weigh a 5 to 10 g sample of silica gel into the dish of a moisture analyzer. Then you dry the silica gel at 120° C to a constant weight. The weight difference, which equals the loss of moisture, will be indicated in the analyzer display. The residual moisture in silica gel may not exceed 3%. *

2. Molecular sieves

To determine residual moisture, you weigh approx. 5 g of molecular sieves, to the nearest 1 mg, into a tared crucible (approx. 30 ml). After weighing, you put the crucible into a muffle furnace preheated to 950° C and leave it for one hour. Then you cool it to room temperature in a desiccator and re-weigh it. The weight difference equals the loss of moisture and is expressed as a percentage. The residual moisture in molecular sieves may not exceed 6%.*

* (Cf. statement in Defect Evaluation List for Plastic Plugs, Drying Capsules and Stick Caps, vol. 18, publisher: Edito Cantor Verlag – Aulendorf 1995)

Maximum humidity load in desiccant mixtures

In the Defect Evaluation List, there is no guideline on mixtures of desiccants featuring different adsorption levels. If you perform the test in a muffle furnace preheated to 950° C – in accordance with the Defect Evaluation List guidelines for humidity tests on molecular sieves – the maximum admissible values are calculated from the percentage of the desiccant type and the maximum admissible humidity default values stated in the Defect Evaluation List for this type.

Please note that using this test method, approx. 5 % of combined water (crystal water), related to the percentage of the desiccant, will escape from the silica gel. This percentage has to be added to the calculated maximum value or either to be subtracted from the determined overall humidity.

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