Self-association of 1,2-diols Apparent heat capacities of 1,2-diols in n-heptane and carbon tetrachloride

Abstract

Apparent molar heat capacities at constant pressure, C_m^app, have been measured for several 1,2-diols in dilute n-heptane and carbon tetrachloride solutions at 25 °C. The systems were: hexane-1,2-diol, octane-1,2-diol and decane-1,2-diol in n-heptane and butane-1,2-diol, pentane-1,2-diol, hexane-1,2-diol, octane-1,2-diol, decane-1,2-diol and dodecane-1,2-diol in carbon tetrachloride; in addition, values of C_m^app for hexan-1-ol and decan-1-ol in carbon tetrachloride were determined. The experimental values of C_m^app show a maximum against diol concentration which is larger and shifted to smaller concentrations than that for the equal carbon number 1-alcohol, indicating that the self-association capability of the 1,2-diols is greater than that of the monoalcohols. The contribution to C_m^app arising from intramolecular H-bonding is small compared to that arising from the intermolecular H-bonds. Diol and monoalcohol self-association is reduced in going from the inert n-heptane to CCl₄, which acts as a weak proton acceptor. The data for 1,2-diols when plotted against ψ₁, the concentration of hydroxy groups in the mixture, follow a single, corresponding states curve for each of the solvents. To interpret these heat capacity data, an extension of the Treszczanowicz–Kehiaian (TK) model for associated liquids has been undertaken. This extension takes into account the formation of intramolecular H-bonds. The parameters of the model, i.e. equilibrium constants and enthalpies of association, have been fitted to the data. It is concluded that at the dilute 1,2-diol concentrations used here, the heat capacity data can be well explained considering that there is only one predominant species in the solution, viz. pentamers or hexamers.