Why Do Alcogels Crumble During Preparation?

[alex caps]

I was trying to make some alcogels and either the solution remained un-changed or it solidified quickly and crumpled. I was not able to make the gel. Any ideas?

 

[chem_tr]

Hello, http://eande.lbl.gov/ECS/aerogels/saprep.htm mentions something about alcogels.

Alcogel (wet gel):
At the gel point, the mixture forms a rigid substance called an alcogel. The alcogel can be removed from its original container and can stand on its own. An alcogel consists of two parts, a solid part and a liquid part. The solid part is formed by the three-dimensional network of linked oxide particles. The liquid part (the original solvent of the Sol) fills the free space surrounding the solid part. The liquid and solid parts of an alcogel occupy the same apparent volume.

Edit: I have googled "alcogel preparation" and found about 250 results. These will give you a hint about your problem, I think.

 

[alex caps]

Well I am taking sodium silicate and mixing with distiled water (I have tried just about every combination of the two.) I then stur thoroughly until it is clear and add a drop of acid as a catalyst. The top of the solution immediately harders a little, but then it either disappears or just doesn't go any further.. Any ideas.

 

[chem_tr]

This involves the following reaction:

$$Na_2SiO_3 + H_2O \longrightarrow H_2SiO_3 \downarrow+ 2NaOH$$

As you see, the reaction refuses to go to the right side; dissolved sodium silicate needs a very long time to be converted to silicate acid and you must do something to consume the sodium hydroxide produced. I had added concentrated hydrochloric acid dropwise in the presence of a suitable acid indicator (phenolphthalein is the best one here) to show the gelation to the students in my previous T.A. years (sigh). The reaction had very rapidly processed the gel, and I had turned the test tube upside-down to show the formation.

Maybe this observation of mine may help you.

 

[alex caps]

It seems to be the same thing I did. I took sodium silicate and mixed it with water, then added a drop of acid as a catalyst..

I tried diluting and also using the acid pure, I tried with more water than silicate and vice versa. I don't know what went wrong, I am going to try again sometime soon..

 

[chem_tr]

More acid is needed for gelation. Don't add it as a drop; add until it 'solidifies'.

 

[alex caps]

More acid is needed for gelation. Don't add it as a drop; add until it 'solidifies'.

Well when I add too much acid it solidifies too quickly and crumbles, I think the bonds are too weak to hold it together when I add too much acid..

 

[chem_tr]

Maybe stirring is bad for these... I don't remember vigorous stirring conditions in this kind of gelation. So this may be the reason of your observed crumbling. Add the concentrated acid dropwise until you see the gelation. This must work.

 

[alex caps]

Maybe stirring is bad for these... I don't remember vigorous stirring conditions in this kind of gelation. So this may be the reason of your observed crumbling. Add the concentrated acid dropwise until you see the gelation. This must work.

Well I stir just the water and silicon dioxide so they become one substance, when I add the acid I don't stir because it aggitates the molecules and makes it impossible for bonds to form..