Molecular Sieve 3A Ethanol for Drying

 Molecular sieve 3a is an alkali metal alumina-silicate. It is of type 3A that means it is the potassium form of the type a crystal structure; it contains the effective pore opening of about 3 angstroms (0.3nm). This opening is large enough to allow the moisture inside it, but excludes molecules such as unsaturated hydrocarbons (unsaturated hydrocarbons can potentially form polymers) and this maximizes lifetime when dehydrating such molecules.

These molecular sieves are typically zeolite compounds, these compounds effectively adsorb water and have carefully controlled pore sizes. Both the solvent and the water will adsorb strongly to the molecular sieve surfaces. Smaller water molecules get access to the large surface area within the pores, so that they are successfully removed from the solvent. We can see from the table mentioned below that the water (1.93A) will enter the 3A pore size while acetone (3.08A) will mostly be excluded. Water will occupy the large surface area inside the pores and therefore be removed.  But if the solvent also enter the pores, it would struggle with water for the surface area and the removal of water becomes impossible. So Type 3A molecular sieve is effectively suitable for drying ethanol, methanol, or acetone. But type 4A sieve can fail to do the same since the pore size does not exclude these solvents.