Effect of high hydrostatic pressure on the dielectric relaxation in a non-crystallizable monohydroxy alcohol in its supercooled liquid and glassy states

Abstract

The complex relative permittivity of a non-crystallizable secondary alcohol, 5-methyl-2-hexanol, is measured over a wide range of temperatures and pressures up to 1750 MPa (17.5 kbar). The data at atmospheric pressure (P = 0.101 MPa) are analyzed in terms of three processes, and the results are in complete agreement with that of O. E. Kalinovskaya and J. K. Vij [J. Chem. Phys. 112, 3262 (2000)]. Process I is of the Debye type and process II is of the Davidson-Cole type, whereas process III is identified as the Johari-Goldstein relaxation process. For pressures of ∼500 MPa and higher, processes I and II are seen to merge into each other to form a single dominant process which unambiguously cannot be resolved into more than one process. The dielectric relaxation strength of process I decreases slightly initially with pressure and when the two processes have merged at elevated pressures, the total relaxation strength increases with increase in pressure. Process III is better resolvable at higher pressures especially above T g in the supercooled liquid state for the reason that the separation in the time scales between the dominant and the JG relaxation process increases at elevated pressures. Surprisingly we find a change in the slope in the plot of log τ JG vs. 1/T for P = 1750 MPa. The results for the relaxation time of alcohols are compared with the Kirkwood correlation factor, g, and it is found that higher is the g, lower is the relaxation time for process I, and it is more of the Debye type. On a reduction in g brought about by an increase in pressure at lower temperatures, the dominant process becomes non-Debye though extensive hydrogen bonding is still present. The dielectric strength of the merged processes increases with increase in pressure. The values of the steepness index, m = |d log τ /d(T g /T)| T = Tg for processes I and II are different for P = 0.1 MPa. However the value of m, for the composite process, which is a merger of processes I and II, for P = 1750 MPa is almost the same for process II at P = 0.1 MPa. From the results of the activation volume, activation enthalpy, and a comparison of the relaxation times with the g factor, we conclude that both processes I and II are significantly affected by hydrogen bonding and both contribute to the structural relaxation.

Experimental studies of Debye-like process and structural relaxation in mixtures of 2-ethyl-1-hexanol and 2-ethyl-1-hexyl bromide

“…For (2E1H) 0.5 (2E1Br) 0.5 the situation is different: At 127 K, the base temperature for the aging experiment, the extrapolated τ D /τ α is about three decades and clearly physical aging takes place on the time scale of the α-relaxation and not on that of the Debye-like process. Therefore, the notion that the structural relaxation could contribute to both processes, as recently suggested, 68 is not supported by the present data, unless this is meant to say that the α-relaxation represents the elementary step of the Debye-like process. The latter implication is part, e.g., of the transient chain model.…”

“…A recurring feature in monohydroxy alcohol systems is that nonlinear variations in various properties show up as a function of, e.g., concentration, 24,26 temperature, 25,26 pressure, 7,68,71 and solvation. 72 This very broad array of observations suggests that the hydrogen-bonded structures which exist in the undiluted bulk phase, at low temperatures, and at low pressures are what one could call "self stabilized," a notion that in studies of hydrogen-bonded systems is well known as cooperative effect.…”

Molecular dynamics changes induced by solvent in 2-ethyl-1-hexanol

“…This heterogeneity in the structure around a reference dipole leads to a broader distribution of relaxation times at high enough pressure or dilution. Moreover, the time scale of both processes I and II becomes similar and only the single, stretch relaxation process is observed [36,51].…”

Activation volume in superpressed glass-formers

“…Tests in hydrogen-bonded molecular liquids showed the case-sensitivity of the activation volume, determined via Eq. (3) to subtle features of molecular structures 38–40 . Worth stressing is also the broadly used link between the activation volume at the glass transition, and the fragility: Δ V # = m P × 2,303 R ( dT g / dP ) 8,40–42 , where the fragility is one of key ‘universality’ metrics for the glass transition phenomenon 2–5,9,10,43,44 .…”