Temperature Dependence of the Enthalpy Near Critical and Tricritical Second-order and Weakly First-order Phase Transitions
The first studies of critical phenomena date back to the second half of the nineteenth century and resulted in the first half of the twentieth century in the comprehensive picture of the so called mean-field theory. As more precise experimental techniques were developed it became apparent in the middle of the last century that the behavior of real systems was inconsistent with the mean-field predictions. The anomalous specific heat capacity behavior at critical points obtained from high-resolution calorimetry played a major role in this insight. In the following decades, major progress was made and new concepts such as universality, critical exponents and the renormalization group theory emerged. In particular, universality united critical phenomena in seemingly unrelated systems. In the whole process, high-resolution calorimetry in fluid systems substantially contributed. The many phase transitions in liquid crystals constituted testing ground for the investigation of general concepts of statistical physics. In this evolution the possibility of adiabatic scanning calorimetry (ASC) to measure simultaneously the temperature dependence of the enthalpy and heat capacity played a major role in the determination of the order of transitions and the power law behavior. In this chapter, the role of ASC is exemplified by presenting results, in particular of the enthalpy, for liquid–liquid criticality and for phase transitions in liquid crystals.