Neutron Reflection

Measurements at the Solid/Liquid Interface

Because silicon is relatively transparent to neutrons it is straightforward to do a neutron reflection experiment by bringing the neutron beam in and out through the silicon with reflection off the silicon/silica/water interface (there is usually an oxide layer on silicon). The refractive index of silicon is intermediate between H2O and D2O and so a good range of contrast variation is possible. The first figure shows the neutron reflectivity from two self assembled monolayers of OTS on the silica surface, one fully deuterated and the other fully protonated (the OTS forms a monolayer attached through the hydrolysed SiCl3 end group). Note that neutron reflectivity is measured as a function of the momentum transfer, k, which is defined as 4p(sinq)/l, where 2q is the scattering angle (twice the grazing angle of incidence) and l is the wavelength of the neutrons.

Measurements such as these can be used to characterize the thickness and composition of the OTS layer. There are two effective options for determining the thickness of a surfactant layer subsequently adsorbed onto the hydrophobic OTS layer. The first is to match the contrast of the aqueous solution to silicon and to use the combination of deuterated OTS layer and deuterated surfactant and this is shown below.

The second is to use D2O in combination with deuterated OTS, which has the effect of highlighting the protonated adsorbed layer. The reflectivity in the latter condition shows a negative interference dip in the reflectivity which is inversely proportional to the thickness of the composite layer, while the maximum in the reflectivity at higher k is found to be primarily sensitive to the coverage of surfactant. The advantage of this less obvious second method is that it can be adapted to determine the orientation of the surfactant, as follows.

When the chain of the surfactant is protonated and the head deuterated the contribution of the head is contrasted out when the solution is D2O and the position of the negative interference effect is now related to the thickness of the composite layer less the thickness of the head group (assuming that the surfactant orients with its chain pointing towards the hydrophobic layer).

Thus, by judicious use of deuterium labelling quite complex interfacial systems may be studied. See other examples in collaborative work with Adrian Rennie (King's College London), Steve Armes (University of Sussex), and Jeff Penfold (ISIS).