Silica aerogels contain primary particles of 2-5 nm in diameter. Silica particles of such a small size have an extraordinarily large surface-to-volume ratio (~2 x 109 m-1) and a corresponding high specific surface area (~900 m2/g). It is not surprising, therefore, that the chemistry of the interior surface of an aerogel plays a dominant role in its chemical and physical behavior. It is this property that makes aerogels attractive materials for use as catalysts, catalyst substrates, and adsorbents.
The nature of the surface groups of a silica aerogel are strongly dependent on the conditions used in its preparation. For example, if an aerogel is prepared using the supercritical alcohol drying process, its surface may consist primarily of alkoxy- (-OR) groups. On the other hand, with the carbon dioxide drying process the surface is almost exclusively covered with hydroxyl (-OH) groups. The extent of hydroxyl- coverage is ~5 -OH/nm2, a value consistent with other forms of silica. This value, combined with their high specific surface area, means that silica aerogels present an extremely large number of accessible hydroxyl groups. Silica aerogels are therefore a somewhat acidic material. A more striking effect of the hydroxyl surface is seen the physical behavior of silica aerogels.
As with most hydroxyl surfaces, the surface of silica aerogels can show strong hydrogen-bonding effects. Because of this, silica aerogels with hydroxyl surface are extremely hygroscopic. Dry silica aerogels will absorb water directly from moist air, with mass increases of up to 20%. This absorption has no visible effect on the aerogel, and is completely reversible. Simply heating the material to 100-120 degrees C will completely dry the material in about an hour (or longer, depending on thickness). As the sample cools, water will reabsorb quickly (mass increases can be seen almost immediately).
While the adsorption of water vapor does not harm silica aerogels, contact with liquid water has disastrous results. The strong attractive forces that the hydroxyl surface exerts on water vapor also attracts liquid water. However, when liquid water enters a nanometer-scale pore, the surface tension of water exerts capillary forces strong enough to fracture the solid silica backbone. The net effect is a complete collapse of the aerogel monolith. The material changes from a transparent solid with a definite shape to a fine white powder. The powder has the same mass and total surface area as the original aerogel, but has lost its solid integrity. Silica aerogels with fully hydroxylated surfaces are, therefore, classified as “hydrophilic”.
This would appear to pose a significant problem to using silica aerogels in exposed environments. Fortunately, this problem can be easily circumvented by converting the surface hydroxyl (-OH) groups to a non-polar (-OR) group. This is effective when R is one of many possible aliphatic groups, although trimethylsilyl- groups are the most common. The derivitization can be performed before (on the wet gel) or after (on the aerogel) supercritical drying. This completely protects the aerogel from damage by liquid water by eliminating the attractive forces between water and the silica surface. In fact, silica aerogels treated in this way can not be wet by water, and will float on its surface indefinitely. Silica aerogels that have been derivitized in this way are classified as “hydrophobic”.
The illustrations below demonstrate the interaction of water with the pore structure and solid backbone of silica aerogels.