Taravillo, M; Caceres, M; Nunez, J; Baonza, VG
In a previous work, we analysed some regularities found in the behaviour of the thermal expansion coefficient, alpha(p), in compressed liquids. We confirmed that a given liquid presents a characteristic pressure range in which the condition (partial derivative alpha(p)/partial derivative T)(p) = 0 is fulfilled within a narrow range of reduced densities. We also found that the density at which the condition (partial derivative B/partial derivative T)rho = 0 is satisfied, rho(alpha), decreases with temperature, a key feature not described before. Earlier studies by other authors suggested that similar regularities are expected for the reduced bulk modulus, B. We present here a detailed analysis of the temperature and density dependence of B from existing experimental results at high pressures. Several liquids have been analysed: argon, krypton, xenon, ethylene, tetrafluoromethane, trifluoromethane, carbon dioxide, carbon disulfide, n-butane, n-hexane, toluene, ethanol, 1-hexanol, m-cresol, and quinoline. We locate that the density rho(B) that fulfils the condition (partial derivative B/partial derivative T)rho = 0 occurs at a particular region of the phase diagram, between 3.4 and 2.4 times the critical density of each liquid. Interestingly, the previously found density rho(alpha) is close to rho(B), in a similar region of the reduced phase diagram. However, we note that rho(B) typically decreases to a lesser extent with temperature than rho(alpha). In addition, we have found that rho(B)(T) behaves in a parallel fashion for the different liquids, showing larger values of rho(B) as the complexity of the molecules increases. These findings provide a strong basis for developing general equation of state models to describe the behaviour of liquids in the high-pressure regime.