Connection between dynamics and thermodynamics of liquids on the melting line

Abstract

for a large number of non-associating liquids. Rigid, spherical molecules containing no polar bonds have Γ=γ; consequently, the reduced relaxation time, viscosity and diffusion coefficient are each constant along the melting line. For other liquids γ>Γ always; i.e., the dynamics is more sensitive to volume than is the melting point, and for these liquids the dynamics at the melting point slows down with increasing T m (that is, increasing pressure). ______________________________________________________________________

Are polar liquids less simple?

“…6,  varies by a factor of two. In dielectric spectroscopy experiments, density changes exceeding 20% are rare, although for some liquids experimental viscosities and diffusion coefficients are available over a much wider volume range and have been found to conform to density scaling [46]. Limiting the range of densities for the mds data to no more than a 40% higher variation in , we obtain the density scaling plots of D* shown in Figure 7.…”

Communication: Effect of density on the physical aging of pressure-densified polymethylmethacrylate

“…A prominent example is the correlation of various properties with the time scale of molecular motions, as observed under isochronal conditions. In these studies, the primary relaxation time, τα, is maintained constant through simultaneous control of pressure and temperature [2], with consequent invariance for many liquids of the dynamic correlation length [3,4], the shape of the relaxation dispersion ("isochronal superpositioning") [5,6], the dynamic crossover [7,8] and for a few cases, the melting line [9]. An interpretation of isochronal invariance of properties comes from molecular dynamics (MD) simulations, which have shown that for a certain class of materials, the behavior, including τ, is governed by the existence of isomorphs [10,11,12].…”

Viscosity of methane to 6 GPa and 673 K

“…Again, viscosities along the line of vapor-liquid equilibrium are seen to depart from the higher-temperature data; a somewhat tighter correlation of the latter can be achieved with γ = 3.9, but only at the cost of increasing deviation along the vapor-liquid line. The interesting equation noted in [18] between γ and does not hold at higher pressures and temperatures. Indeed, is not constant over the larger range of this extended data set; as seen in the inset to Fig.…”

“…Fragiadakis and Roland [18] have shown that fluids of small, nonassociating molecules have kinetic properties which, when suitably reduced, scale to the quantity T /ρ γ with γ a constant of the system; further, for molecules approximating 2)] is plotted against the quantity (ρ/ρ crit ) 4 /(T /T crit ), here normalized to the value at the critical point. Symbols are the same as in previous figures.…”