THE PHYSICAL CHEMISTRY OF PROCESSES AT HIGH PRESSURES GENERAL INTRODUCTION BY A. R. UBBELOHDE Dept. of Chemical Engineering, Imperial College, S.W. 7 For condensed states of matter, the pressure variable has been much less fully explored than the temperature variable. However, high-pressure techniques are becoming easier'to develop, at any rate up to about 5000 atm. Increased acces- sibility of operations at high pressures has resulted in a broadening of the range of physico-chemical measurements available for interpretation. For still higher static pressures equipment is at present too specialised and costly to favour its widespread dissemination in many laboratories, but results important for the thermodynamics of condensed phases are being obtained in a few centres. The use of transient high pressures up to 105 atm or more, as developed in shock and detonation waves in solids, was first applied to the measurement of thermodynamic parameters during the war.Increased attention is being paid to the development of such techniques at the present time, though there are still serious difficulties of interpretation particularly in systems containing a mixture of solids. Industrial interest in high pressures adds to the timeliness of the present Discussion ; the highly successful polymerizatiom of ethylene and the suggestive results in the polymerization of acetylene are but two examples of possibilities of applying high pressures industrially. Systematic theory has not wholly kept pacewith all these developments, but some of the papers in the present Discussion clarify important theoretical problems.The widespread influence of repulsive forces in almost all high-pressure phenomena makes the survey by Cottrell particularly timely. This contribution brings out the difficulties in finding high-pressure parameters that are at the same time simple to calculate theoretically and are experimentally sensitive to repulsive force fields. It seems quite definite that for close molecular overlap a simple inverse power function of molecular separations fails to represent repulsions successfully ; an exponential function may be more hopeful. However, the anisotropy of repulsive force fields, and additional complications where repulsions play a part in a many-body problem, have hitherto blocked theoretical progress. One promise of progress in theory stems from the much greater diversity of experimental phenomena where measure- ments are becoming available from which repulsive force fields might in principle be calculated, as is illustrated by many of the experimental contributions to this Discussion.Another broad group of theoretical problems arises from the interpretation of the transport properties of dense gases ; these are of very considerable practical interest. The outcome of many statistical-mechanical calculations which can be formulated is much too complicated to be applied conveniently to experimental examples. For this reason the devising and testing of simple models which permit some theoretical insight into transport processes in dense gases offers special opportunities for progress at the present time.In this direction, there are significant contributions to the present Discussion, by Longuet-Higgins and Valleau, and by Whalley. 78 GENERAL INTRODUCTION A third type of theoretical calculation refers to properties of compressed gases whose variation with pressure can be attributed to attractive forces. Measurement of such properties as a function of pressure, and particularly a combination of several types of measurements, promises to give information about details of inter- molecular attractions which it would be difficult to obtain in other ways. The contribution by Pople and Buckingham permits a useful extension of the range of properties whose dependence on pressure can be made to yield information about molecular force fields. Advances in techniques of working at high pressures are not specifically included in this Discussion, but the increasing range of phenomena which can now be studied conveniently is illustrated by the diversity of experimental studies presented.Thus effects of pressure on the infra-red and electronic resonance spectra of gases studied by Dr. Vodar and his colleagues contribute novel additions to our general knowledge of molecular force fields. Benson and Drickamer make the interesting suggestion that the pressure coefficient of a vibrational frequency should be em- ployed systematically to supplement the actual numerical value of the frequency itself, in correlating spectral energy levels with molecular structure. When chemical reactivity at higher pressures is considered, it is usual to separate the influence of pressure on the structure of the solvent, and its influence on various activation complexes.From observed changes in the dielectric constant of metha- nol, discussed by Hamann and Strauss, there appears to be a striking influence of pressure on the structure of this solvent. Presumably this is connected with the open net-work structure of hydrogen bonds in liquid methanol. It would be interesting and important to know if such an influence is observed generally in hydrogen-bonded solvents. So far as the activation complex is concerned, relatively simple concepts as discussed by Laidler suffice to account for much of the observed influence of pressure on the free energy of activation.The influence of pressure on reaction rate is quite striking for certain fairly complex molecules as in the cases reported by Hamann and Teplitzky where a 500-fold increase in rate accompanies the increase in pressure up to 15,OOO atm. However, expectations that the steric hindrance in specific reaction mechanisms might be reduced or even eliminated as a result of bond deformation at high pressures proved not to be fuKlled, at any rate in the examples studied by Weale. Other striking influences of increases of pressure arise in polymerization reactions. In the polymerization of styrene, rates of propagation and of chain termination have been shown by Nicholson and Norrish to depend in a marked way on the degree of compression of the solvent. In consequence the mean molecular weight is affected by pressure. It would be interesting to determine how far chain branching for a given size of macromolecule is affected by the pressure at which it has been formed.In the polymerization of ethylene studied by Laird, Morrell and Seed the reaction rate likewise depends in a complex way on the pressure, owing to the various ways in which nearest neighbours influence chain reactions as the packing becomes closer. The group of investigations on physico-chemical process in detonation and shock waves, presented for this Discussion, deals with systems of widely differing com- plexity. In heterogeneous polycrystalline mixtures, a complex diversity of pheno- mena can occur which is by no means fully cleared up. This applies, for example, to the studies reported by Deffet and Boucart, by Dempster, and by Taylor.The mixtures of explosive substances they describe can be termed " solid " only by convention; phenomena such as grain erosion in the reaction zone are of dominant importance and depend on the intercrystalline free space in a complex way. When single crystals are studied individually, as in the work reported by Bowden and his colleagues, high initial pressures are found to have only a slight effect on the " runaway temperature " in the self-heating of expolsives. Experiments on the homogeneous detonation of acetylene should in principle be simpler to interpret ; some interesting experimental abnormalities are presented by Penny. The kinetics of pyrolysis of acetylene in shock waves to give carbon asA . R. UBBELOHDE 9 reported by Atin, Tonnies and Greene offers a further instance of reactions at transient high pressures, which presents complications due to the formation of solid products of variable physical structure and chemical composition. Extensions of such kinetics studies promise to cover ranges of high pressure and high tempera- ture not conveniently obtainable in other ways. However, as in other studies of transient phenomena, relaxation effects may complicate any interpretation of the results on the basis of the thermodynamics of high pressure.