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

The heat capacity per unit volume ${C}_{p}$ and density \ensuremath{\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 \ensuremath{\alpha} is found to be 0.110\ifmmode\pm\else\textpm\fi{}0.014---and hence consistent with the universal accepted value---and the critical amplitude ${A}^{+}=0.0606\ifmmode\pm\else\textpm\fi{}0.0006{\mathrm{J}\mathrm{}\mathrm{K}}^{\ensuremath{-}1}{\mathrm{cm}}^{\ensuremath{-}3}.$ \ensuremath{\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 ${C}_{1}^{+}=\ensuremath{-}0.017\ifmmode\pm\else\textpm\fi{}0.003{\mathrm{g}\mathrm{}\mathrm{cm}}^{\ensuremath{-}3},$ 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 ${\mathrm{dT}}_{c}/dp.$ The results of this analysis were consistent with previous work on this matter.

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