The Silica Gel Surface
In summary, from our present state of knowledge, it would appear that there are, indeed, three sources of water desorption from the surface of silica gel. The first source, probably that of physically adsorbed water, is mostly lost below 200oC whereas the third source, resulting from the condensation of silanol groups to siloxane bonds is not complete until a temperature in excess of 900oC is reached. The second source of water loss, between 200oC and 400oC is particularly interesting. This could be due to either the loss of strongly held water or the condensation of Geminal silanol groups as suggested by Sindorf and Maciel (although they also expressed some concern about the possible presence of water). It would appear from the results described that the major loss of water in the second desorption process is from strongly held or hydrogen bonded water.
The work on silica gel gives an excellent example of the value of using thermoanalytical techniques in explorative chemistry. The use of heat to change the physical and chemical properties of a substance can be extremely informative and, as has been shown, can be a relatively simple procedure and can be accomplished with relatively simple and inexpensive apparatus. Nevertheless, the more sophisticated sensitive, accurate and versatile instrumentation that is now available can provide tremendous advantages over the simple primitive systems, particularly in providing speed and increased throughput of samples.