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Name: Kevin
Status: other
Grade: 6-8
Location: IL



Question:
A close friend is a grade school teacher asking me (an engineer) questions about silica and the packets you find in shipping boxes, shoe boxes, etc. for an experiment she wants to try...... "What happens to the liquid water molecules when it is absorbed by silica?" I know silica is a desiccant and assuming it is hydrophilic it changes the surface tension of the water. So does this just make it evaporate quicker? Does the water particle just stay absorbed to the silica particle and never let go? Let us say you had two similar surfaces one with very fine silica particles embedded in the surface of it and one without....what would happen if you put the same amount of water drops on each? After a few seconds, minutes, hours?


Replies:
Hi Kevin,

I think you misunderstand the function of silica gel. Silica gel is not acting like a sponge that absorbs liquid water. Silica gel is a desiccant, and the purpose of a desiccant is to absorb or adsorb gaseous water vapor. Silica gel is in fact pretty useless as an absorber of liquid water.

You asked if its being "hydrophilic" caused it to reduce the surface tension of water, and perhaps made the water evaporate faster. In fact, since liquid water is not involved, clearly surface tension or even that silica gel may or may not be hydrophilic, is irrelevant.

Silica gel is (as mentioned) a desiccant, and that means that it adsorbs water vapor molecules from the air. When this occurs in a closed environment (say, in a sealed plastic bag that contains moisture-sensitive equipment), the overall relative humidity in the environment is lowered. By adsorbing water VAPOR, the resulting lower relative humidity eliminates the possibility of water vapor condensing to liquid water that would damage the contents of the sealed bag.

Silica gel is pure silicon dioxide that is made in such a way that each particle has trillions of submicroscopic pores. The overall surface area of these pores is astounding. It has been said that a pound of silica gel particles has over an acre of total surface area. Water vapor molecules (that is, individual free molecules of water in the air) are attracted to, and are adsorbed (not absorbed!) onto the surfaces inside these particles. This adsorption continues until an equilibrium is reached. In other words, for any given temperature and air pressure, there is a limit how much water vapor can be adsorbed.

The adsorption process is reversible. If Silica gel that has reached its limit of adsorption at (say) room temperature is heated, it will begin releasing some of its water vapor. This can be a problem in some cases, since the water vapor that had been adsorbed over time will now be released if there is a sudden increase in temperature.

An example of this in double-pane sealed windows. The metal frame that surrounds the two sheets of glass contains silica gel, and is perforated with tiny holes. The silica gel is intended to adsorb the small amount of water vapor that slowly permeates past the window's seals. When the sun shines on an old window whose silica gel desiccant is nearly saturated, the heat causes the silica gel to release some of its water vapor, into the space between the panes, which then can condense to liquid water on cooler surfaces not heated by the sun. This is visible as a film of condensed water on the inside of one or both glass panes.

Hope this clears up the confusion...

Regards,
Bob Wilson


Kevin,

Those packets contain zeolites or porous silica that have a very high surface area/high porosity and water is adsorbed into these pores. Crushing the silica would speed up adsorption by allowing more pores to be available at any given time. It would not increase the amount of water adsorbed however. Your friend could test this by first activating the silica (heating in an oven at well above the boiling point of water), cooling in a desiccator, exposing pre-weighed equal amounts to water vapor (say both of them in a sealed jar with water), and then weighing over time.

Greg (Roberto Gregorius)



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