Heat Capacity Anomaly in a Nitrobenzene+Dodecane Mixture

Nathan Utt


The isobaric heat capacity anomaly of a nitrobenzene+dodecane mixture at the critical concentration was measured using adiabatic calorimetry. Data were collected using the step technique to be able to quantify any heat leaks that occurred in the calorimeter. The temperature differences between steps were analyzed using the method of linear projections where the small slopes in the flat part of the temperature step were forced to vary smoothly. This technique produced heat capacity data that were not affected by temperature drifts in the calorimeter. Three data runs were collected at different scan rates to prove reproducibility. The heat capacity data from one run was accurately fitted to the theory with the critical exponent α set equal to 0.11 and the amplitude ratio forced to be 0.53. The critical temperature was determined to be 28.889 ± 0.001°C and the one-phase amplitude was 0.0102 ± 0.0003 JK-1cm-3. Two-scale factor universality was used to determine a correlation length amplitude of 0.294 ± 0.003 nm, which is significantly different than a previously published value. This implies that either the published value is incorrect or two-scale factor universality is not valid. A similar discrepancy, however, has been previously observed in other correlation length amplitudes reported by this same group, causing us to suspect the former of the two possibilities. Because of the unreliable value for correlation length amplitude we were unable to test the value of the two-scale factor universality constant X.