Type 3A molecular sieves should be used to dry dehydration solvents
for
electron microscopy. Most common solvents (acetone, ethanol, and
methanol, etc.) need to be anhydrous for electron microscopy
embedding
work using epoxy resins, yet they have a tendency to pick up
atmospheric water when bottles are opened. Molecular sieves
are used to dehydrate the solvents
used in the final stages of dehydration and embedding . Molecular
sieves are typically zeolite compounds that
strongly adsorb water and have carefully controlled pore sizes.
While
both the solvent and the water will adsorb strongly to the molecular
sieve surfaces, the large surface area within the pores is only
accessible to the smaller water molecules, so they are effectively
removed from the solvent. From the table below it can be seen that
water (1.93A) will enter that 3A pore size while acetone (3.08A)
will
largely be excluded. Water will be able to occupy the large surface
area inside the pores and thus be removed. If the solvent could also
enter the pores, it would compete with water for the surface area
and
there would be little or no removal of the water from the bulk
solvent.
Type 4A molecular sieve is not suitable for drying ethanol,
methanol,
or acetone since the pore size does not exclude these solvents. The
Advanced Specialty Gas Equipment catalog lists Type 3A for drying
ethanol and methanol.
Molecular radius, nm
Water
0.193
Acetone
0.308
Table data from Alexey B. Nadykto and Fangqun Yu, JOURNAL OF
GEOPHYSICAL RESEARCH, VOL. 108, NO. D23, 4717
(http://www.albany.edu/~yfq/papers/NadyktoYuJGR03.pdf)
(For estimating molecular sizes: C = 1.85A, O = 1.60A, H =
1.00A)
Running the solvent slowly through a column of the molecular sieves
would be the most effective way to remove water but, ordinarily,
dried
sieves are just placed into the bulk solvent container (about 5%-10%
by
volume) to remove most water. The sieve material is ceramic, so be
very
careful to not stir up any fines when the solvent is withdrawn since
it
could eventually end up damaging sectioning knives.
Regenerate the molecular sieves at 250 C for 2 hr or more in a
shallow
layer. Place the container of hot, regenerated sieves on the
porcelain
plate of a glass desicator and place under vacuum while they
cool. Put dry sieves into bottles with polyethelene cap liners to
keep
dry until needed. Type 3A molecular sieves will reduce water in an
air
stream to 0.001 mg/liter air. Residual moisture is probably higher
for
sieves dumped into bulk solvent containers, but functionally it is
sufficiently effective in avoiding water contamination problems.
For more information on desiccant materials and properties:
J.T. Baker Chemical
http://www.jtbaker.com/techlib/documents/3045.html