Scaling of high-pressure viscosity data in low-molecular-weight glass-forming liquids

We discuss the phenomenology related to the pressure behavior of viscosity in low-molecular-weight glass-forming liquids. We show that the effect of pressure on viscosity in two examined glass-forming systems, salol and ortho-terphenyl, can be satisfactorily described, over considerable ranges of pressure (up to 1 GPa) and temperature (300--425 K), by the two-parameter pressure law which has been proposed previously for the relaxation time shift. The presented analysis indicates that one of the parameters (the pressure counterpart of the Vogel-Fulcher-Tammann law parameter D, which accounts for the fragility of a glass former) is independent of temperature in salol and ortho-terphenyl. As a consequence, the scaling plot which normalizes data collected for different temperatures can be constructed for a given glass former. On the basis of the presented considerations, a general form of the temperature-pressure viscosity ``equation of state'' is proposed.