Temperature and pressure dependence of the alpha relaxation and configurational entropy of a prototype glass-former

The a relaxation process of the fragile glass former salol is investigated in the T-P domain by means of photon correlation spectroscopy. We find that a time-pressure superposition principle is obeyed for the relax- ation function in addition to the time-temperature superposition. The behavior of the relaxation time is studied by using an extended version of the Adam-Gibbs model including the pressure dependence. The excellent conformity of the equation describing the bidimensional surface t (T,P) to the experimental data provides a positive check for this model, here verified on photon correlation measurements. The same model gives a rationale of the phenomenological expressions recently introduced to describe the changes in the slow dynam- ics induced by varying both temperature and pressure. These findings suggest that the reduction of configura- tional entropy actually guides the liquid toward the glass transition.