wackyvorlon
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Making Aerogel?
I am somewhat new to this board, having found out about it through the recent Wired article. That article, combined with finally picking up and
reading Uncle Tungsten, have reignited an interest in chemistry that has laid dormant for many years.
I have been wondering about this one for a while. Can anyone describe the process for making a silica aerogel?
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12AX7
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Welcome. 
As I recall, you first make a silica gel, by doing something to a chemical that would rather be silica- something like, er, tetraethyl silicate I
think I've heard of. This has four ethyl groups, CH3CH2-, bound to a SiO4 core, which silicon really likes to form (most of the Earth's crust is made
up of SiO4 groups!). So you have this hairy (CH3-CH2-O-)4Si thing. Then you add water, H-O-H, which breaks up the oxygen bonds, adding H and OH to
parts: both the CH2- and the Si want oxygen, so the Si keeps the four O's it has, and four water molecules come in and break off the ethyls, forming
CH3-CH2-OH, ethyl alcohol, and Si(OH)4, silicic acid. Some of these will bond to each other (condensation reaction), releasing some water and forming
larger molecules. Of course, a fully assembled formation of such silica groups would be something like quartz, which comes out to SiO2 (because the
SiO4 groups each share an oxygen atom). But quartz forms painfully slow at nasty temperatures and pressures, so it doesn't get anywhere near quartz.
Instead it's more like a polymer, with no real structure and haphazard bonding.
This stuff, when rolled into balls and dried, is what you aren't supposed to eat inside the "silica gel" packets you get with stuff. When dried, it
shrinks a lot, but there's still a lot of open space left. As such, all the internal surface area makes a sticky surface for molecules like water.
If, instead of drying it and letting it shrink and crack and stuff, you suck the water and ethanol out with special equipment (usually supercritical
solvents), it can retain its shape with little shrinkage. Since it's a solid with a bunch of air inbetween, it's called an aerogel, in analogy to a
gel, which is a solid with liquid trapped inside it. (As opposed to a colloid (a sol), which is a liquid with solid particles suspended, or an
aerosol (there's the "aero" prefix again), which is a gas with liquid (fog) or solid particles suspended. Yes, confusing... it's all in the
structure.  )
Tim
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solo
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Synthesis of silica aerogels and their application delivery system
Irina Smirnova
http://rapidshare.de/files/22414077/Synthesis_of_silica_aero...
Summary
This work deals with the production of silica aerogels and their potential applications in the pharmaceutical industry.
Synthesis of silica aerogels is rather expensive. A lot of efforts have been made to find a way to decrease the cost of aerogel production. An
important factor of the synthesis of low density aerogels is gelation time. Gelation time can be very long, especially in the case of low density
aerogels. Different types of catalysts were used in order to accelerate the gelation.
In this work, it is suggested to use CO2 to enhance the gelation process. It is shown, that the addition of CO2 during the sol-gel process leads to
the fast gelation. The influence of different factors, like temperature, component ratio, and solvent nature on this process is also described.
Special attention is given to the experimental determination of the optimal CO2 concentration needed for gelation at different process conditions. For
the evaluation of this process, physical properties of the aerogels synthesized by this method are determined and
compared with those obtained by conventional methods.
The second part of this work deals with the application of silica aerogels as a drug delivery system (DDS). Being environmentally benign and
non-toxic, silica aerogels can be used in the pharmaceutical industry. Their large surface area and open pore structure make them to an ideal
potential carrier material.
In this work, it is suggested the use of hydrophilic silica aerogels as a carrier material for pharmaceuticals. The loading of silica aerogels with
pharmaceuticals can be made by different processes, in particular by adsorption. Silica aerogel are loaded with active compounds (drugs) by the
adsorption from the supercritical CO2 solution. The characteristics of the corresponding aerogel-drug formulations, like drug concentration and
stability, are studied in order to prove whether the process suggested in this work is suitable for the pharmaceuticals. Also, the release of the
corresponding drugs from such formulations is investigated. It is expected that the drug release from an aerogel-drug formulation is faster then that
of crystalline drugs. Pharmaceuticals adsorbed on silica aerogels get a larger surface area, which
should lead to faster dissolution of the compound in water. Moreover, the easily collapsing aerogel structure in water also favors a faster release.
Based on these experiments, the application of silica aerogels as a drug delivery system is
discussed.
[Edited on 7-6-2006 by solo]
It's better to die on your feet, than live on your knees....Emiliano Zapata.
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DrP
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Just to add to 12AX7s post -
QUOTE
"If, instead of drying it and letting it shrink and crack and stuff, you suck the water and ethanol out with special equipment (usually supercritical
solvents),"
- the gels do have a habbit of collapsing (99 % air) once the solvent has been removed. Apparently the sol-gel reaction is done inside an autoclave
which helps the aerogel hold together when the solvent is driven out and replaced by gas.
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wackyvorlon
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My plan is to attempt making the basic alcogel first, once I have the details of that ironed out, I'm going to see about building the apparatus to
perform the supercritical drying. For the drying, is it necessary to have a pump recirculate the liquid CO2? Or can it just soak?
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alyks
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From wikipedia:
Silica aerogel is made by drying a hydrogel composed of colloidal silica in an extreme environment. Specifically, the process starts with a liquid
alcohol like ethanol which is mixed with a silicon alkoxide precursor to form a silicon dioxide sol gel (silica gel). Then, through a process called
supercritical drying, the alcohol is removed from the gel. This is typically done by exchanging the ethanol for liquid acetone, allowing a better
miscibility gradient, and then onto liquid carbon dioxide and then bringing the carbon dioxide above its critical point. The end result removes all
liquid from the gel and replaces it with gas, without allowing the gel structure to collapse or lose volume.
Resorcinol-formaldehyde aerogel (RF aerogel) is made in a way similar to production of silica aerogel.
Carbon aerogel is made from a resorcinol-formaldehyde aerogel by its pyrolysis in inert gas atmosphere, leaving a matrix of carbon. It is commercially
available as solid shapes, powders, or composite paper.
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