The Thermogravimetric Analysis of Silica Gel

 

Briefly, the sample is suspended from the arm of a continuously recording microbalance in a temperature-controlled furnace. The sample is heated from a defined starting temperature to a specified final temperature at a designated heating rate usually given as temperature change per unit time.  As discussed above in the modern TGA instrument both the temperature and the sample weight are continuously digitized and the data stored. Depending on the software provided with the instrument, the results can then be printed out or an appropriate graph constructed relating sample mass to temperature. To help identify the desorption of different species, derivative curves can also be produced.

 

The results obtained by Odlyha et al from a sample of Matrex 20m LC silica gel taken directly from the Perkin Elmer TGA instrument is shown in figure 27.

Figure 27.  Thermogram of Silica Gel

 

It is seen from the derivative curve that there appears to be three distinctly different desorption processes. The first takes place from about 30^oC to 130^oC; the second between about 200^oC and 450^oC and the third between about 400^oC and 900^oC. The three different desorption processes (probably those discussed by Lange) are distinct and unambiguous and are similar to those previously identified by Scott and Traiman. The total loss from the sample was about 5%w/w but it would appear from the TGA curve that the condensation to siloxyl groups  was not entirely complete even at the temperature of 900^oC.

 

Odlyha et al considered that, the curve relating mass of water lost (obtained by subtracting each data point from the initial total mass of sample) against temperature in a TGA analysis, was a type of desorption isotherm that could be described by assuming three distinct and separate desorbable species on the silica surface, each evolving water over a specific temperature range. From the two TGA curves the authors proposed the following theoretical explanation.