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How does a gel form from a sol?


Frequently Asked Questions

There is no strict definition of what a gel is. However, since a gel is always created when a sol (see FAQ: What is a sol) solidifies, this is a practical working description.

After a sol has been fabricated it can be destabilised (or activated), depending on the starting materials. A colloidal sol (a dispersion of a fine powder often in water) is destabilised by changing the pH. A polymeric sol is activated either by the addition of water or by modifying the pH, or both.

Once the sol has been activated the solid particulates interact, join and form larger agglomerations and a 3-D network begins to grow. Eventually the network will span the container and turn into a gel, this is often referred to as the point of gelation. One working description of a gel is that is will not flow under gravity, and so if the container is tipped upside down the gel will stay in place. If the point of gelation has not been reached then the sol will flow downwards (although this may be very slow).

The gel itself is therefore a solid network that spans the container, however it also contains significant amounts of entrapped solvent. Modification of the conditions and rate of gelation affect the structure of the solid network and therefore also that of the pores that contain this trapped liquid. Control over the gelation conditions can therefore be used to generate very specific and desirable pore architectures.

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