Thermodynamic study of complex formation by hydrogen bonding in halogenoalkane–oxygenated solvent mixtures: halothane with acetone, methyl acetate, tetrahydrofuran and methyl tert-butyl ether

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Abstract
Complex-formation equilibria in binary mixtures of 2-bromo-2-chloro-1,1,1-trifluoroethane (halothane) with acetone (AC), methyl acetate (MA), tetrahydrofuran (THF) and methyl tert-butyl ether (MBE) have been analysed in detail using several association models. Previously reported vapour–liquid equilibria, limiting activity coefficients, excess enthalpy and excess heat capacity data for these mixtures have been correlated using a multiproperty global fitting procedure. The thermodynamic properties for halothane–AC and –MA are best correlated using the ideal association model while for halothane–THF and –MBE the best model is an athermal solvation model where the Flory–Huggins expression for the species activity coefficients is considered. In halothane–THF and –MBE mixtures there are only 1 : 1 H-bonded complexes while for halothane–AC and –MA the 2 : 1 complexes are also present. An explanation at the molecular-level for this behaviour is given by PC MODEL calculations which indicated that the oxygen surface area available for H-bonding is twice as large in AC and MA than in THF and MBE. 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. Using quantum-mechanical methods, enthalpies of complexation for the present mixtures were calculated and found to be in excellent agreement with those obtained thermodynamically. Other models, where the physical contributions to the excess properties of the present systems are given by the properties of (halothane–homomorph-of-solvent) and (homomorph-of-halothane–oxygenated solvent) mixtures, have been tested. These extensions of the ideal association model do not represent improvements of the simple models.

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