Self-association and complex formation in alcohol-unsaturated hydrocarbon systems Heat capacities of linear alcohols mixed with alkenes and alkynes
Apparent molar heat capacities, Cmapp, at dilute alcohol concentrations and excess molar heat capacities, CpE throughout the concentration range were determined at 25°C for the following systems: methanol, ethanol, propan-1-ol, hexan-1-ol and decan-1-ol mixed with n-octane, oct-1-ene and oct-1-yne; in addition, the following mixtures were also measured: hexan-1-ol with oct-4-yne, cyclohexane, cyclohexene, benzene, hex-1-ene, dec-1-ene and an equimolar mixture of n-octane+oct-1-yne. The experimental Cmapp show a maximum against alcohol concentration; this maximum is reduced in magnitude and displaced to higher alcohol concentrations when the inert n-octane is substituted by the unsaturated oct-1-ene, oct-1-yne, oct-4-yne, cyclohexene or benzene which act as weak proton acceptors, forming complexes or cross-associated species with the alcohol molecules. The present data clearly indicate that there are alcohol–alkene complexes in solution, which are weaker than the alcohol–alkyne ones, but detectable through heat capacity measurements. The Cmapp data for alkan-1-ols when plotted against ψ1, the concentration of hydroxyl groups in the mixture, follow a single corresponding states curve for each of the solvents. For all alkan-1-ols, CpE display the following behaviour: CpE (oct-1-yne)<CpE (oct-1-ene)<CpE (n-octane) at low alcohol concentrations and CpE (oct-1-yne)>CpE (oct-1-ene)>CpE (n-octane) at higher alcohol concentrations, the cross-over point being between 0.1 and 0.2 alcohol mole fraction. To interpret the data, the Treszcanowicz–Kehiaian (TK) model for associated liquids has been used. The parameters of the model, i.e. volumetric equilibrium constants and enthalpies of formation for alcohol–unsaturated hydrocarbon 1:1 complexes have been fitted to the dilute alcohol data. With these parameters, the TK model is able to give correct qualitative predictions of the CpE results throughout the concentration range. Using the Flory lattice model, the volumetric equilibrium constants were transformed into a unique or intrinsic equilibrium constant, which is independent of molecular size and describes the alcohol–alkene and alcohol–alkyne association. A detailed analysis of the data for hexan-1-ol+oct-1-yne and hexan-1-ol+oct-4-yne indicates that the dominating interaction in the formation of the alcohol–unsaturated hydrocarbon complex is that occurring between the proton of the hydroxy group of the alcohol and the negative electron density in the double or triple bond, producing what can be termed a H-bond. Using the parameters obtained when analyzing excess volumes VE and excess enthalpies HE for similar and common systems (T. M. Letcher etal., FluidPhaseEquilib., 1995, 112, 131), the ERAS model was used to predict CpE, finding that it is unable to give a satisfactory rendering of the present heat capacity data.