RE: CS>Warning for LV CSJason wrote:
When NIST, or any other qualified PHD scientist that does not
manufacture equipment or sell products, states that TEM is not the best
analytical method for analysing the most minute particles in colloidal
suspension, then I'll take a second look. That said, I believe that any
data you can acquire through studying any substance will provide
valueable information. When you do comparative analysis of
various products using the exact same method, the
results are always enlightening.
To get the right answer you must ask the right question.
If the question concerns a colloid, with no ions present, then a TEM may
accurately show the particles. Sample preparation is very important and if
done improperly can produce images that have little to do with what is in
the colloid.
If you ask a knowledgeable TEM operator about making images of ionic
solutions, they will tell you that what you will see are the compounds
formed when the sample is desiccated.
A TEM operates under high vacuum so it is not possible to have water present
in the sample. A sample prepared for TEM viewing requires that the sample be
desiccated. When water is removed (evaporated) from an ionic solution, the
metal cations combine with the companion anions to form a solid compound.
Here is what happens when the water is evaporated from an ionic solution:
Silver ions in a solution cannot exist without water, so when the water is
evaporated the silver ions (cations) must combine with an available anion to
form a compound. The predominant anions present in a silver colloid solution
(EIS) are hydroxide and carbonate. The compounds thus formed are silver
hydroxide and silver carbonate. Silver hydroxide is unstable and reduces to
silver oxide and hydrogen. The silver carbonate will reduce to silver oxide
and carbon dioxide. The final compound that remains is silver oxide.
This process begins as a single silver ion is forced to combine with a
single anion forming a single molecule of the compound. The molecule has no
ionic charge and therefore no repulsive force. The lack of repulsion causes
the molecules to be attracted to each other by van der Waals' force of
attraction which causes them to aggregate and form small particles of the
compound. The size of the particle growth is limited by the reduced mobility
of the molecules as the water evaporates. What remains is particles of
silver oxide whose diameter is 1 - 3 nanometers. It is these particles which
predominate in TEM images made of silver colloid solutions (EIS) which have
a high ionic content compared to the silver particle content.
This is why TEM images of ionic solutions do not represent what was in the
solution.
The old saying applies "What you see ain't what you got."
Frank Key
Colloidal Science Lab.
www.colloidalsciencelab.com
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