Heat capacity and thermal expansion anomalies in the nitromethane-1-butanol mixture near its upper critical point.

Abstract

The heat capacity per unit volume C(p) and density rho of the nitromethane-1-butanol critical mixture near its upper consolute point are determined in this work. C(p) data are obtained at atmospheric pressure as a function of temperature in the one-phase and two-phase regions, using a differential scanning calorimeter. The suitability of DSC for recording C(p) as a function of T in the critical region is confirmed by measurements of the nitromethane-cyclohexane mixture, the results being quite consistent with reported data. By fitting the C(p) data in the one-phase region, the critical exponent alpha is found to be 0.110+/-0.014-and hence consistent with the universal accepted value-and the critical amplitude A(+)=0.0606+/-0.0006 J K(-1) cm(-3). Rho data were only obtained in the one-phase region, using a vibrating tube densimeter. The amplitude of the density anomaly was found to be C(+)(1)=-0.017+/-0.003 g cm(-3), which is moderately low in spite of the large difference between the densities of the pure liquids. The thermodynamic consistency of the A+ and C+1 values was examined in relation to the previously reported value for the slope of the critical line dT(c)/dp. The results of this analysis were consistent with previous work on this matter.