Sticky Minerals

mica sheet on silicon block support

Figure illustrating the very thin mica sheet on the silicon block support. The Data shows the distinctive ‘double critical angle’ indicative of the mica/D2O interface (the two ‘steps’ at low Q) and the changes on adsorbing a layer of AOT at the mica surface, clearly evidence at high Q. using data such as this we can identify and structurally characterise the layers at the mica surface.

Mica is a phyllosilicate mineral which is formed of stacks of negatively charged layers, with interlayer cations, mainly potassium. These layers cleave perfectly to give a molecularly smooth surface making mica the ideal substrate for many surface experiments. There is particular interest in adsorption on molecular species from solution, such as surfactants and other molecules.

Neutron reflection is an important surface technique as, unlike most other surface techniques, it is non-invasive and so does not perturb the delicate adsorbed layer system. It does however require a large, atomically flat and defect free substrate, which has previously hindered the use of mica as a substrate for this method. Through a novel mounting technique using thin freshly cleaved mica sheets glued onto a silicon wafer support, we have been able to study the behaviour of various molecules at the mica-water interface using neutron reflection.