Complex formation and self-association in ternary mixtures. Apparent heat capacities for alcohol–methyl acetate–hydrocarbon and acetone–chloroform–cyclohexane

Abstract

Apparent molar heat capacities, ϕ_c, have been measured for hexan-1-ol and 2-methylcyclohexanol at 25 °C in their binary mixtures with methyl acetate (MA) and the following ternary mixtures: hexan-1-ol–(p wt % of MA–n-dodecane) where p= 2, 4.3, 10.2 and 30 and 2-methylcyclohexanol–(p wt % of MA–n-decane) where p= 1.5, 5 and 50. The measurements were made from extreme dilution up to 10 wt % of alcohol. The ternaries acetone–(p wt % of chloroform–cyclohexane) where p= 4.5 and 7.4 are also reported. Plots of ϕ_c against alcohol concentration show that the high maximum obtained for the binary alcohol–hydrocarbon mixture (OH–I) is progressively decreased for the ternary mixtures as the concentration of MA increases until, for the binary alcohol–MA mixture (OH–MA) the ϕ_c curve has been flattened. This behaviour is explained quantitatively by the Treszczanowicz–Kehiaian (TK) model for associated mixtures extended to include the possibility of complex formation between the alcohol and the proton-acceptor, MA. In a MA–hydrocarbon environment, ϕ_c drops owing to the partial destruction of hydrogen bonds between alcohol molecules and the corresponding formation of a hydrogen-bonded complex between the alcohol and MA. At low alcohol concentrations, ΔC_p of mixing is negative for the OH–I binary, turning positive when the hydrocarbon is replaced by a mixture of hydrocarbon with an increasing amount of MA. Further increase in MA concentrations decreases ΔC_p until for the binary OH–MA it is again negative. These changes are explained by the TK model in terms of the different amounts of self-associated and complex species in solution. The TK model has been applied to ϕ_c data for the mixture acetone–chloroform–cyclohexane, where only complex formation is present in solution, and found to be qualitatively successful.