Excess Volumes of Mixing and the Preparation and Properties of Aluminium Hydride Etherate

The thesis is presented in two parts. Part 1. Excess Volumes of Mixing. In recent years, there have been a number of attempts to form a complete theory of liquid mixtures which would allow the prediction of the thermodynamic properties of a given liquid mixture from a knowledge of the intermolecular forces existing within it. Among the most successful of recent theories have been those by Prigogine and his co-workers of the Brussels school. The latest theory is based upon what is termed the Average Potential model, and permits the prediction of the values of the thermodynamic excess functions of mixing of a given liquid mixture of non-electrolytes from the measured values of such physical properties of the pure, unmixed components as the density, and the heat capacity, The numerical results obtained from the detailed treatment are in good agreement with experimental measurements on systems of mixtures of molecules of nearly equal size, for example, carbon monoxide and methane. The theory is, however, not restricted to dealing with systems of mixtures of molecules of similar size and it has been extended to cover the case of mixtures of r-mer molecules. It is this aspect of the Average Potential theory which has been of principal interest in the present work. One of the most interesting predictions of the theory is that, in mixtures of r-mer molecules, the excess volume of mixing should exhibit minima when the ratio of the chain length of one species in the mixture is approximately an integral multiple of that of the other. It is also possible to calculate theoretical values of the excess volumes and its variation with temperature. Very little direct experimental evidence has been offered to confirm the validity of the extension of the Average Potential theory to mixtures of r-mer molecules, and it was the intention in the present work to obtain appropriate experimental information which would provide a test of the theory. The excess volume of mixing of a set of systems of the type cyclohexane + dicyclohexyl, was measured at various temperatures. The choice of systems was such that the molar volume of the second component was approximately twice that of the first component, which was in every case cyclohexane. The second components were all closely related chemically, e.g. dicyclohexyl and dieyclohexylmethane, and thus the variations between systems of the observed excess volume of mixing should depend principally on the differing sizes of the molecules in the systems. It was hoped that the excess volumes of the systems would show a minimum for the system for which the ratio of molar volumes of the components was closest to the integral value,two. The occurrence of such a minimum would provide a direct test of Prigogine's theory. It was also intended to compare the theoretical and experimental values of the excess volume of mixing, v e, end its variation with temperature, dv e/dT, for each system It was found that the agreement between the observed and calculated values of v e and dv e/dT was qualitative at best, with regard to magnitude and sign, but the expected minimum in the excess volume was found. The present research has confirmed that the ideas underlying the extension of the Average Potential theory to mixtures of r-mer molecules is essentially valid, but that the comparison between the observed and calculated values of the excess function is poor. Part 2. The Preparation and Properties of Aluminium Hydride Etherate. It has been shown that beryllium hydride, BeH2, prepared under conditions which produce an ether-free product, is less sensitive to moisture and to spontaneous decomposition than the etherate BeH2(C2H5). It was hoped that the same behaviour would be show by aluminium hydride and its etherate. Various methods of preparing aluminium hydride in an ether-free condition were investigated. It was found that none of the methods which have been described in the literature, was successful in producing an entirely unetherated product. The aluminium hydride which was obtained did contain less other than was found in samples obtained by evaporating ethereal solutions to dryness. The reactivity of the former material was greater than that of the latter. This has been ascribed to difference in the molecular structure of the products formed under different experimental conditions