A B
 A UQUID B LIOUDQUD NECKING





 0 EGE
 SQL GEL AGED GEL

 NO 0 PORE





 DRIED GEL PARTIALLY
 O ENSIFIED
 GEL

 GLASS PORE




Figure 2-6. Structural changes during the sol-gel process (adapted from Hench and West,
 1990)

 Drying. The drying step involves the removal of the solvent from within the pores

of the gel. This is very important in the determination of pore size. There are three types

of gels characterized by the solvent removal process: xerogels, aerogels, and cryogels. As

defined by Legrand (1998), a xerogel is obtained by evaporation of the liquid component

at ambient pressure and temperatures below the critical temperature of the liquid; an

aerogel is obtained from the evaporation of the liquid component above its critical

temperature; and a cryogel is obtained from the freezing and subsequent sublimation of

the liquid component. Aerogels have higher surface areas than xerogels and do not have a

material volume loss due to shrinkage. Yet, because supercritically drying gels can be

cumbrous and expensive, research has focused on making xerogels with similar