No one knew very much about the surfaces of the respirator charcoals that those in the PCL at the time had striven so hard to improve. The activation by steam, whereby some 30% of the carbon is burnt away, develops activity, and the resulting structure was believed to consist of porous aggregates of graphite crystallites, with exposed surfaces which are heterogeneous, with active sites and with some surface oxide. Why one briquetted coal should give a more active charcoal than another was not understood. Douglas Everett retained an interest in these problems and attacked the question of the thermodynamics of the adsorption process and of hysteresis. Following a conversation with Victor Goldsmidt, in which it was suggested that dielectric measurements might throw light on the interactions between water and clays, Leslie Sutton carried out experiments first on brucite and later on kaolinite. There seemed to be a dipole rotation that was hindered to varying degrees as concentration and temperature were varied, but no simple model sufficed and interpretation was inevitably made difficult by sample heterogeneity. It was not until some years later that the chemistry of surfaces was taken up again in the PCL, this time by Robert Gasser, who used ultra-high vacua to study the adsorption of gases on the surfaces of pure metals.
The most important breakthrough in the study of gases adsorbed on solids was the development of ultra-high vacuum technique because it made it possible to study clean, and therefore better defined, surfaces. This had become available when Gasser started his surface work in the early sixties. He used a combination of three techniques to study the fate of species adsorbed on clean surfaces, flash desorption, analysis of desorbed species by mass spectrometry, and isotopic labelling. In flash desorption the surface is heated rapidly after material has been adsorbed on it and different surface species are released over narrow temperature, and therefore narrow time, ranges. By combining this with mass spectrometry the desorbing species could usually be identified with some certainty. A precise picture could therefore be built up of the different species formed on the surface and of some of the kinetic processes involved. Isotopic labelling adds further power to the analysis in that different isotopic species are readily distinguished in a mass spectrometer and therefore such processes as the dissociation of N2 could be followed by 15N/14N exchange. It is interesting that the combination of flash desorption and mass spectrometry used by Gasser over a period of some 15 years is now being used again in the department by John Foord to study chemical precursors at the gas/solid interface. In 1985, some time after he had moved out of research, Robert Gasser wrote a book, An Introduction to Chemisorption and Catalysis by Metals (OUP, 1985) which is widely read by chemistry undergraduates.
One of the most exciting developments in the sixties was the application of neutron scattering methods to the study of condensed matter, including surfaces. John White was one of the pioneers in this field and he showed how neutron scattering could be applied to understand processes in a wide variety of systems. The early experiments were done at Harwell, only twenty miles from Oxford. Although these facilities have recently closed, the fruit of the research in the sixties, to which White made a substantial contribution, led to the participation of the U.K. in the joint research reactor, the Institut Laue-Langevin at Grenoble, and to the later development of the pulsed neutron source ISIS at Chilton, near Oxford. The two facilities are the best of their type in the world. John White spent from 1975-80 as the British director at the Institut Laue-Langevin.
The power of neutron scattering is that it can be used to determine structure at a molecular level and to study dynamic processes ranging from slow diffusive motions on surfaces to the high frequencies of intramolecular vibrations. John White's neutron scattering experiments covered a wide range of systems. He studied vibrational excitations in polymers, as well as diffusion in liquid crystals and in liquids. Two noteworthy experiments on diffusion were the motion of water in clays, especially as influenced by the surface of the clay, and a study of the contribution of proton transfer to the overall diffusion of protons in acid solutions. John White also made some of the first applications of neutron scattering to the study of surface species, using neutrons to study the structure and motion of small molecules physisorbed on graphite and in zeolites. One of the most interesting ideas developed by him was to apply small angle neutron scattering to the study of colloidal systems, an area which has blossomed in both fundamental and applied colloid science.
Bob Thomas, after an apprenticeship with John White, has further developed the use of neutron scattering as a surface technique. Neutrons interact only weakly with matter and therefore are not an obvious choice for investigating surfaces where the premium is on very high sensitivity. However, for gases on solids this can be overcome by making use of the widely different neutron cross sections of different atoms and isotopes. Thomas has studied the way small molecules pack together in layers on the surfaces of weakly adsorbing materials. More recently, he has pioneered the use of grazing incidence reflection techniques, which use both x-rays in the laboratory and neutrons. This technique allows the structure of wet surfaces to be probed for the first time and is now being widely used. A further application of neutron scattering, developing from earlier work by John White, has been the study of the swelling of clays and its relation to the theory of colloid stability.
The X-ray diffractometer in Bob Thomas' laboratory
John Foord joined the PCL from the Inorganic Chemistry Laboratory in 1990, although his group will not move into the Laboratory until the autumn of 1992. He is concerned with the problems of chemical reactions at solid surfaces, and areas of current interest are: the design and fabrication of new solid state materials, with particular emphasis on the role of gas phase chemical precursors at the growing gas-solid interface; photochemistry of adsorbed layers; heterogeneous catalysis - metal multilayer structures which are chemically etched give materials like real catalysts, but which are sufficiently well characterised to be used in studies of the link betwen structure and catalytic activity; and, reaction dynamics and surface scattering.
The study of surface chemistry at the PCL will be strengthened by the arrival, in 1991, of Colin Bain from Cambridge. His current interests centre on the use of high power, tunable, pulsed infrared lasers to obtain vibrational spectra of organic molecules at interfaces - particularly `wet' interfaces that are difficult to study by other means. The principal technique exploited in this work is sum-frequency generation, a non-linear optical effect that allows, in principle, the determination of the nature, concentration and orientation of molecules adsorbed at a surface. Photoacoustic and photothermal techniques are also being developed.
Laser induced desorption experiments link the themes of energy transfer and of surface chemistry, and such experiments are being pursued by Stephen Simpson and his colleagues.