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

Phase equilibria and critical behavior of square‐well fluids of variable width by Gibbs ensemble Monte Carlo simulation

The vapor–liquid phase equilibria of square†well systems with hard†sphere diameters σ, well†depths ε, and ranges λ=1.25, 1.375, 1.5, 1.75, and 2 are determined by Monte Carlo simulation. The two bulk phases in coexistence are simulated simultaneously using the Gibbs ensemble technique. Vapor–liquid coexistence curves are obtained for a series of reduced temperatures between about Tr=T/Tc=0.8 and 1, where Tc is the critical temperature. The radial pair distribution functions g(r) of the two phases are calculated during the simulation, and the results extrapolated to give the appropriate contact values g(σ), g(λσ−), and g(λσ+). These are used to calculate the vapor†pressure curves of each system and to test for equality of pressure in the coexisting vapor and liquid phases. The critical points of the square†well fluids are determined by analyzing the density†temperature coexistence data using the first term of a Wegner expansion. The dependence of the reduced critical temperature T* c=kTc/ε, pressure P* c=Pcσ3/ε, number density Ï * c=Ï cσ3, and compressibility factor Z=P/(Ï kT), on the potential range λ, is established. These results are compared with existing data obtained from perturbation theories. The shapes of the coexistence curves and the approach to criticality are described in terms of an apparent critical exponent β. The curves for the square†well systems with λ=1.25, 1.375, 1.5, and 1.75 are very nearly cubic in shape corresponding to near†universal values of β (β≊0.325). This is not the case for the system with a longer potential range; when λ=2, the coexistence curve is closer to quadratic in shape with a near†classical value of β (β≊0.5). These results seem to confirm the view that the departure of β from a mean†field or classical value for temperatures well below critical is unrelated to long†range, near†critical fluctuations

Adsorption from alkane+perfluoroalkane mixtures at fluorophobic and fluorophilic surfaces. I. Nature of the noncritical adsorption profiles

International audienceNeutron reflection has been applied to probe the nature and extent of adsorption from a mixture of (1-x)n-hexane+xperfluoro-n-hexane against silicon substrates modified with alkylsilane (fluorophobic) or fluoroalkylsilane (fluorophilic) coupled layers. For an equimolar mixture (x=0.5, 60.7 vol %) in the one-phase region at T=30 °C—removed both in temperature and composition from the upper critical point at 22.65 °C and x=0.36—the structure was resolved at both fluorophobic and fluorophilic surfaces. Liquid mixtures with three different refractive index contrasts were used to reduce model ambiguity in the ensuing analysis. For both surfaces the composition profiles of the adsorbed liquids could be represented using two-layer slab models which included interlayer Gaussian roughness. For the fluorophobic surface, the thickness of the layer closest to the substrate is ∼20 Å and composed of ∼83 vol % n-hexane, and the second, more dilute layer has a composition profile which decays smoothly into the bulk over a range of ∼100 Å. A similar result is found for the fluorophilic surface, but in this case the layer closest to the substrate is ∼15 Å thick and composed of ∼95 vol % perfluoro-n-hexane. Qualitatively similar behavior is found for adsorption from a mixture with x=0.7 against a fluorophobic substrate and for a mixture with x=0.2 against a fluorophilic substrate