Silica is the most commonly used sorbent for flash column chromatography. When solvent is pumped through a column packed with dry silica you may notice it gets warm and sometimes down right hot!
In this post I will attempt to explain why this phenomena occurs.
Flash chromatography is a widely utilized purification methodology. Typically used with non-polar and moderately polar solvents and columns packed with silica, this technique has the ability to separate compounds with different polarities and functional groups.
With bulk silica, as well as pre-packed flash cartridges, the media is “active”, meaning it is dry. Because silica is also an adsorbent media as solvent passes through it several things happen…
- The solvent displaces the air in the silica’s pores
- The solvent adsorbs to the silica surface and generates heat
- Friction builds
It is the adsorption process that generates the initial “heat band” and the more polar the solvent, the greater the heat generated. This phenomena is known as the heat of adsorption.
Since silica has a polar surface it preferentially adsorbs polar solvents. The solvent’s polarity and heat capacity influence the amount of heat generated during the initial solvent-sorbent interaction. This is the main reason silica flash cartridges are equilibrated prior to sample injection as the generated heat will negatively influence mass-transfer kinetics and compound retention/separation.
Once fully equilibrated just the act of pumping solvent through the packed column generates friction, and therefore heat. The amount of heat generated is dependent upon media particle size, flow rate, and solvent viscosity but is typically very low in flash chromatography.
Is this a concern in your daily flash chromatography? Only if you do not equilibrate your flash cartridge/column prior to loading your sample, Figure 1. It is not a concern with reversed-phase or other bonded silicas as these media are bonded and therefore, deactivated.
Have you experienced this heating phenomena? If so, how has it impacted your purification?