Heat capacity of associated systems. Experimental data and application of a two-state model to pure liquids and mixtures

Abstract

The predictions from a recently reported ( J. Chem. Phys. 2004, 120, 6648) two-state association model (TSAM) have been tested against experimental data. The temperature, T, and pressure, p, dependence of the isobaric heat capacity, C-p, for three pure alcohols and the temperature dependence at atmospheric pressure of the excess heat capacity, C-p(E), for four alcohol + ester mixtures have been measured. The branched alcohols were 3-pentanol, 3-methyl-3-pentanol, and 3-ethyl-3-pentanol, and the mixtures were 1-butanol and 3-methyl-3-pentanol mixed with propyl acetate and with butyl formate. These data, together with literature data for alcohol + n-alkane and alcohol + toluene mixtures, have been analyzed using the TSAM. The model, originally formulated for the C-p of pure liquids, has been extended here to account for the C-p(E) of mixtures. To evaluate its performance, quantum mechanical ab initio calculations for the H-bond energy, which is one of the model parameters, were performed. The effect of pressure on C-p for pure liquids was elucidated, and the variety of (T) behaviors was rationalized. Furthermore, from the C-p data at various pressures, the behavior of the volume temperature derivative, (partial derivative V/partial derivative T)(p), was inferred, with the existence of a (partial derivative V/partial derivative T)(p) versus T maximum for pure associated liquids such as the branched alcohols being predicted. It is concluded that the TSAM captures the essential elements determining the behavior of the heat capacity for pure liquids and mixtures, providing insight into the macroscopic manifestation of the association phenomena occurring at the molecular level.