Self-association and complex formation in alcohol-unsaturated hydrocarbon systems - Heat capacities of linear alcohols mixed with alkenes and alkynes

Apparent molar heat capacities, C-m(app),at dilute alcohol concentrations a nd excess molar heat capacities, C-p(E) throughout the concentration range were determined at 25 degrees C for the following systems: methanol, ethano l, propan-1-ol, hexan-1-ol and decan-1-ol mixed with n-octane, oct-1-ene an d 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-l-en e and an equimolar mixture of n-octane + oct-1-yne. The experimental C-m(ap p) 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, c yclohexene or benzene which act as weak proton accepters, forming complexes or cross-associated species with the alcohol molecules. The present data c learly indicate that there are alcohol-alkene complexes in solution, which are weaker than the alcohol-alkyne ones, but detectable through heat capaci ty measurements. The C-m(app) data for alkan-1-ols when plotted against psi (1), the concentration of hydroxyl groups in the mixture, follow a single c orresponding states curve for each of the solvents. For all alkan-1-ols, C- p(E) display the following behaviour: C-p(E) (oct-1-yne) < C-p(E) (oct-1-en e) < C-p(E) (n-octane) at low alcohol concentrations and C-p(E) (oct-1-yne) > C-p(E) (oct-1-ene) > C-p(E) (n-octane) at higher alcohol concentrations, the cross-over point being between 0.1 and 0.2 alcohol mole fraction. To i nterpret the data, the Treszcanowicz-Kehiaian (TK) model for associated liq uids has been used. The parameters of the model, i.e. volumetric equilibriu m constants and enthalpies of formation for alcohol-unsaturated hydrocarbon 1 :1 complexes have been fitted to the dilute alcohol data. With these par ameters, the TK model is able to give correct qualitative predictions of th e C-p(E) results throughout the concentration range. Using the Flory lattic e model, the volumetric equilibrium constants were transformed into a uniqu e or intrinsic equilibrium constant, which is independent of molecular size and describes the alcohol-alkene and alcohol-alkyne association. A detaile d analysis of the data for hexan-1-ol + oct-1-yne and hexan-1-ol + oct-4-yn e indicates that the dominating interaction in the formation of the alcohol -unsaturated hydrocarbon complex is that occurring between the proton of th e hydroxy group of the alcohol and the negative electron density in the dou ble or triple bond, producing what can be termed a H-bond. Using the parame ters obtained when analyzing excess volumes V-E and excess enthalpies H-E f or Similar and common systems (T. M. Letcher et at., Fluid Phase Equilib., 1995, 112, 131), the ERAS model was used to predict C-p(E), finding that it is unable to give a satisfactory rendering of the present heat capacity da ta.