Thermodynamic study of complex formation by hydrogen bonding in halogenoalkane–oxygenated solvent mixtures. Halothane with propyl ether, isopropyl ether, 1,4-dioxane and 2,5-dioxahexane

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Abstract
Vapour–liquid equilibria (VLE) for binary mixtures of halothane with propyl ether (PE), isopropyl ether (IPE), 1,4-dioxane (DOX) and 2,5-dioxahexane (monoglyme)(G1) have been measured at 293.15 K using a gas chromatographic headspace analysis method. Excess molar heat capacities and excess molar volumes at 298.15 K were also measured for halothane–G1. Complex-formation equilibria for these four halothane–oxygenated solvent mixture have been analysed in detail using several association models. The data reported here, together with previously published excess enthalpies and excess heat capacities, have been correlated using a multiproperty global fitting procedure. The thermodynamic properties for halothane–PE and –IPE are best correlated using the ideal association model while for halothane–DOX and –G1 the best model is the athermal solvation model where the Flory–Huggins expression for the activity coefficients is considered. In halothane–PE and –IPE mixtures there are only 1:1 H-bonded complexes while for halothane–DOX and –G1 the 2:1 complexes are also present. The model parameters, i.e. the equilibrium constants, enthalpies and heat capacities of complexation were found to be reliable, truly representing the halothane–oxygenated solvent H-bonded complexes. Enthalpies of complexation were also calculated using quantum mechanical methods and found to be in good agreement with those obtained from thermodynamics. Previously, we have presented an analogous thermodynamic study for mixtures of halothane with four other oxygenated solvents viz. acetone, methyl acetate, tetrahydrofuran and methyl tert-butyl ether. Here, a comparative analysis of the thermodynamic behaviour of the eight halothane–oxygenated solvent mixtures is presented. It was found that the enthalpy for the complexation reaction A + B ⇔ AB correlated with the oxygen surface area per H bond as given by PC MODEL calculations. Finally, a clear correlation between the solvent Kamlet–Taft β parameter (H-bonding acceptor ability) and equimolar GE, HE, TSE and TCE p was found: as β increases GE, HE and TSE are seen to become more negative and TCE p more positive, i.e. as the basicity of the solvent increases the halothane–oxygenated solvent H bonds are stronger and the degree of organization or order in solution increases. This correlation might be used to make rough estimates of the excess functions for other halothane–oxygenated solvent mixtures.

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