Dehydrated films, structural studies

Especially in conjunction with the detection of water or OH species, ex situ XPS measurements have been critizised because of possible changes occurring during transfer and exposure of the sample to UHV. Kuroda et al. have demonstrated that structural changes of the passive film indeed occur when electron diffraction studies are performed in a hydrated and subsequently in a dehydrated environment. Structural changes, however, do not necessarily cause changes in elemental composition as determined by XPS. 

One of the main sources of controversy is that many of the structural studies performed have been on dried films and, as pointed out by O Grady,69 this results in the determination of the structure of dehydrated films whose structure can be significantly different from that of hydrated ones. 

Cyclic voltammetry offers a special possibility to study the permeability of thin clay films using clay-modified electrodes. The kinetics of the break-in/leach-out (hydration-dehydration, swelling-shrinking) processes depends on the structure of the porous aerogel hydrogel and on the layer thickness of the film. The rate constants of structure-dependent macrodiffusion and microdiffusion can be calculated, and to the best of our knowledge - this cyclic voltammetric determination of rate constants of the elementary processes is unique in the literature. 

---