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

The alpha 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 relaxation 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 tau(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 dynamics induced by varying both temperature and pressure. These findings suggest that the reduction of configurational entropy actually guides the liquid toward the glass transition.