LAUSR

Since its initial discovery more than fifteen years ago, the thermodynamical scaling of the dynamics of supercooled liquids has been used to provide many new important insights in the physics of liquids, particularly on the link between dynamics and intermolecular potential. A question that has long been discussed is whether the scaling exponent γS is a constant or does it depends on pressure. An alternative definition of the scaling parameter, γI = ∂ ln T/∂ ln ρ|X has been presented in the literature, and has been erroneously considered equivalent to γS. Here we offer a simple method to determine the pressure dependence of γI using only the pressure dependence of the glass transition and the equation of state. Using this new method we find that for the six nonassociated liquids investigated, γI always decreases with increasing pressure. Importantly in all cases the value of γI remains always larger than 4. Liquids having γI closer to 4 at low pressure show a smaller change in γI with pressure. We argue that this result has very important consequences for the experimental determination of the functional form of the repulsive part of the potential in liquids. Comparing the pressure and temperature dependence of γS and γI we find, contrary to what has been assumed in the literature to date, that these two parameters are not equivalent and have very different pressure and temperature dependences.