Purported fragile-to-Arrhenius crossover in squalane

Jadhao and Robbins (1) claim to have discovered a crossover from Vogel–Fulcher–Tammann (VFT) to Arrhenius in squalane occurring at a viscosity of $\mu =\mathrm{1,000}$ Pa⋅s using extrapolation of the simple Eyring equation to the limiting low-shear viscosity from nonequilibrium molecular dynamic simulation at high shear rates. Stickel et al. (2) introduced a non–model-specific derivative analysis of relaxation time and viscosity data. The Stickel function is

$$S\left(1/T\right)={\left[\frac{\partial \ln x}{\partial \left(1/T\right)}\right]}^{-1/2},$$ [1]

where x is a dynamic property of the liquid, low-shear viscosity, $\mu$, or relaxation time, $\lambda$. A temperature-Stickel plot of $S$ versus reciprocal temperature results in a horizontal straight line for an Arrhenius response and a descending straight line for a VTF response. For ambient pressure, data for viscosity (3) and relaxation time (4) are available which reach to nearly the glass transition. The Stickel functions of the experimental data of Deegan et al. (3) and Reichert et al. (4) are plotted in Fig. 1 to compare with the Arrhenius equation reported by Jadhao and Robbins (1). The experimental data are consistent with each other and do not support the purported crossover to Arrhenius at $\mu =\mathrm{1,000}$ Pa⋅s.

Fig. 1.

Stickel analysis for the temperature dependence at ambient pressure showing no crossover to Arrhenius in the experimental data.

A reference was offered by Jadhao and Robbins (1) as support for a VFT-to-Arrhenius crossover. This is the article by Mallamace et al. (5), which has been refuted (3, 6, 7). Mallamace et al. (8) responded to the criticism voiced by Elmatad (6). If such a crossover existed in the isobaric data, it should also occur in the isothermal, pressure-dependent data (9) for squalane at the same viscosity of 1,000 Pa⋅s. It clearly does not (10).