SOME IMPROVEMENTS AND EXTENSIONS OF THE APPLICATION OF SPECULAR NEUTRON REFLECTION TO THE STUDY OF INTERFACES

The level of the background scattering in neutron reflection experimen ts limits the range of momentum transfer over which a specular reflect ivity profile may be measured and also makes it difficult to observe o ff-specular scattering. A further limitation of neutron reflection, wh ich applies to studies of the solid/liquid interface, restricts access ibility to a small range of solid surfaces. The restriction of specula r reflection measurements to below a relatively small value of K-max m eans that the intrinsic resolution of specular neutron reflection is l ow. We show how selective isotopic labelling can be used to enhance th e resolution beyond the limit determined by K-max and illustrate the m ethod using the structure of hexadecyl trimethyl ammonium bromide as a n example. For the range of coverages typical of a surfactant monolaye r at the air/water interface the structure can be resolved to the leve l of a group of three to four carbon atoms. The difficulty of extendin g neutron reflection at the solid/liquid interface to different solids is that the large size of solid sample and the requirement that the s olid usually be crystalline makes the cost of the solid material prohi bitively high except for a small range of materials which include sili con, amorphous silica and crystalline quartz. We give examples that sh ow how self-assembled monolayers with different chemical functionality on their outer surfaces can be used to create a wide variety of diffe rent surfaces with quite different adsorptive behaviour. This should g reatly extend the range of neutron reflection experiments on the impor tant solid/liquid interface. The lamellar phase of the surfactant Aero sol-OT adsorbs preferentially from a mixture of lamellar phase and iso tropic solution at both air/liquid and solid/liquid interfaces. Strong off-specular scattering from fluctuations in the lamellae is observed which is enhanced by Bragg diffraction from the multilayer structure. This indicates that there must be partial correlation of the fluctuat ions in different lamellae. (C) 1998 Elsevier Science B.V. All rights reserved.