Neutron Reflection from Surfaces
Until recently there have been no experimental techniques for probing the structure of air/liquid and solid/liquid interfaces and consequently they are poorly understood. The second of these interfaces is particularly important in technology (colloids, detergents, biological systems, etc.). We have developed the technique of neutron specular reflection to a level where it is now one of the most effective techniques for probing structure and composition at wet interfaces, and we are now applying it to a range of different systems.
At the air/solution interface we are studying the configuration of simple amphiphiles at the surface, e.g. the orientation of the hydrocarbon chain and the penetration of the surfactant into the water surface. We are also studying mixtures of amphiphiles in an attempt to understand the factors that determine the composition of the adsorbed layer (which is nearly always different from the bulk) and the specific interactions between different amphiphiles, or between amphiphiles and polymers. Diblock copolymers are a class of compounds that may also have interesting interfacial properties, although they have been less widely exploited than simple surfactants, and neutron reflection is particularly effective for their study.
The range of behaviour of the technologically important solid/liquid interface is considerable. Thus surfactants may adsorb in either bilayers or monolayers depending on the condition of the surface, or even in surface aggregates such as micelles. Also, the chemistry of the surface may be manipulated in situ to change its adsorptive and reactive properties over a very wide range. This is currently the most active area of the research group and it is expected that the structure of the adsorbed layers of surfactants, polymers, and biopolymers (mainly proteins) will now be investigated on chemically modified surfaces. The dynamics of the interactions between different species and different types of species will also be studied, e.g. competitive and cooperative adsorption.
Neutron reflection facilities used in the work are the spallation source at ISIS (just outside Oxford), the high flux beam reactor at Grenoble, and occasionally neutron sources in the U.S.A. There is no difficulty of access to these sources. Much of the work is done in collaboration with industrial groups (Unilever, Kodak (U.K. and U.S.A.)) and with groups in other universities.
Clay minerals absorb surfactants to form intercalated binary compounds. Most clays are platelet structures and in the binary complex with a surfactant the separation of the clay platelets is considerably increased. For a typical vermiculite the clay swells by about 3 times in the direction normal to the platelets when the surfactant is adsorbed. These binary materials have the further possibility that they can absorb various oils to form a highly swollen ternary system. Even though the oil is often liquid-like the ternary system is still well ordered and can be studied by diffraction methods. These compounds offer a number of possible avenues of research; they have potential value as pollution scavengers, they may be used as precursors in the preparation of catalysts with well defined pore sizes, they may play a significant role in tertiary oil recovery, and they are of fundamental interest in the study of the factors underlying oil/surfactant interactions. We are investigating some of these avenues using X-ray and neutron diffraction to monitor the nature of the materials.