  The role of ionic interactions between sodium dodecyl sulfate, SDS, and
sapphire surfaces have been studied using specular neutron reflection to
determine the structure and composition of adsorbed surfactant layers.
Increasing the pH of the solution from 3 to 9 reduces the adsorption by
reversing the charge of the alumina. This occurs at lower pH for the R-plane (1
-1 0 2) than the C-plane (0 0 0 1), corresponding to the different points of
zero charge. The largest surface excess is about 6.5 micromol m-2, the
thickness of the adsorbed layer is about 24 Angstrom and it contains roughly
20% water. The hydrocarbon tails of the surfactant molecules clearly
interpenetrate rather than form an ordered bilayer. The structure is similar in
either pure water or in 0.1 M NaCl when the surfactant is at the respective
critical micelle concentration. Different structures were seen with lithium and
cesium dodecyl sulfate. The CsDS forms dense layers with little or no hydration
and a surface excess of about 10.5 micromol m-2. The metal cation strongly
influences the hydration of the adsorbed surfactant. An overall picture of
'flattened micelles' for the structure of the adsorbed layer is observed
