Effect of Surface Modification on the Interlayer Chemistry of Iron in a Smectite Clay

A hydrophobic nanostructured composite has been created through the addition of an organic monolayer to the external surface of a smectite clay. This film inhibits the free exchange of water in to and out of the interlayer located between the aluminosilicate sheets of the clay. The effect of the presence of such a monolayer, which is formed from octadecyltrimethoxysilane, on iron(III) cations exchanged into the clay interlayer has been examined using X-ray absorption spectroscopy. Mild hydrothermal conditions have been used to increase the rate of potential reactions involving the ferric ions. The addition of the organic coating results in partial reduction of the iron to the ferrous state, a process that is furthered by hydrothermal treatment of the hydrophobic clay. In contrast, the iron in the simple ion-exchanged clay aggregates into small iron−oxygen clusters but is not reduced. These results are discussed in the context of the reduction potential of iron(III) and the susceptibility of that iron to hydrolysis in aqueous solution.