Heat capacities of aqueous polar aromatic compounds over a wide range of conditions. Part II: dihydroxybenzenes, aminophenols, diaminobenzenes

The heat capacities of dilute aqueous solutions of o-, m-, p-dihydroxybenzenes, o-, m-, p-aminophenols and o-, p-diaminobenzenes (phenylenediamines) were determined using a modified flow Picker type high temperature calorimeter. The measurements were performed in the temperature range from 303 K up to the upper temperature limit, between 373 and 573 K, depending on the thermal stability of individual compounds. The data were obtained typically at pressure of 0.1 MPa at the lowest temperature and at a pressure near 2 MPa at higher temperatures to 473 K. For m-aminophenol and o-diaminobenzene the measurements were also performed at higher temperatures at pressures to 30 MPa. The standard heat capacities of aqueous solutes were obtained by extrapolation of the experimental data to infinite dilution. The new data were combined with the results on standard heat capacities of phenol, aniline, cresols and toluidines, published in our previous paper. This made it possible to examine the effects of the functional substituent groups and their position on the aromatic ring. The changes in the temperature dependence of the heat capacity with the polarity of the functional groups were compared with the evolution of standard volumes. The different trend was observed for the solutes containing two polar groups in comparison with those with one polar group only. Our data suggest that the standard derivative properties (partial molar properties at infinite dilution) of aromatic solutes with multiple polar groups tend towards minus infinity when the critical temperature of water is approached from below. Finally the results show also that a simple group contribution concept can be used to calculate the standard heat capacities for a given class of compounds. (C) 2004 Elsevier Ltd. All rights reserved.