I sometimes run into wrong assumptions concerning how silica gel works. One is that silica gel in a display case keeps absorbing water until either the silica gel runs out of capacity or the the interior of the display case reaches 0% RH, which isn’t true.

Probably the most helpful thing to know is that the physics laws that govern how silica gel works are the same ones that govern the behaviour of paper, leather, wood, photographs and a great many other things found in museums. What happens when you put a piece of paper into a new environment? Depending on the condition that the paper was stored at before, it will lose or gain water until it reaches some kind of equlibrium with the new environment. If we now raise the RH, the paper will increase in water content until a new equilibrium is reached. Similarly, if we drop the RH, the paper will lose water until it reaches equilibrium with the new environment.

So this is all about an equilibrium between the water in the object and water vapor in the air around the object. One last little detail is that in all of these “dry” materials, the water is aDsorbed (with a “d”) and not aBsorbed (with a “b”) In aBsorption, the absorbate is held within the body of the absorbant. Fill a sponge with water and if was possible to cross-section the sponge without the water running out, we would see large and small holes filled with water. This is also a relatively large scale event. With aDsorption, individual molecules of adsorbate are stuck to molecule surfaces in the adsorbant like little refrigerator magnets. They stick on relatively easily and they come off relatively easily.

Adsorbed gases are considered to be in a condensed phase. More common condensed phases for water are liquid and ice. so what we have is a phase equilibrium between the adsorbed phase and the vapor phase.

Most likely we want to maintain some given RH so we’ll condition the silica gel to our desired relative humidity. We pour the silica gel into the display case and if the case condition and the silica gel are at equilibrium, then nothing happens. If they aren’t at equlibrium, then the silica gel, just like paper, will adsorb or desorb water until equilibrium is reached. This follows Le Chatelier’s principle which says that if a system is at equlibrium and we impose and change on it , (in out case, we might change the RH, temperature, or atmospheric pressure), then the equilibrium will shift in the direction that opposes the imposed change. So if the RH in the case goes down, then silica gel will desorb water and the RH will rise again (although not quite back to where it was.) If you want to put on your lab coat and safety glasses, then you can tell people that it’s obeying the first law of thermodynamics: the conservation of energy.

-Doug
Douglas Nishimura
Image Permanence Institute
Rochester institute of Technology .

This post is also available in French translated by Aurore Tisserand.