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?
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...
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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Update: June 2012