Type 3A molecular sieves should be used to dry acetone, ethanol and methanol.
Molecular sieves are typically zeolite compounds that strongly adsorb water and have carefully controlled pore sizes. While both the solvent and the water will adsorb strongly to the molecular sieve surfaces, the large surface area within the pores is only accessible to the smaller water molecules, so they are effectively removed from the solvent. water (1.93A) will enter that 3A pore size while acetone (3.08A) will largely be excluded. Water will be able to occupy the large surface area inside the pores and thus be removed. If the solvent could also enter the pores, it would compete with water for the surface area and there would be little or no removal of the water from the bulk solvent. Type 4A molecular sieve is not suitable for drying ethanol, methanol, or acetone since the pore size does not exclude these solvents
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