Pressure and temperature dependent viscosity of two glass forming liquids: Glycerol and dibutyl phthalate

Abstract

The pressure and temperature dependent viscosities of two glass forming liquids, glycerol and dibutyl phthalate (DBP), have been studied in the range P=0–3 GPa, T=0–125 °C, and η=101–1010 cP. These studies were made using a combination of a rolling-ball and a centrifugal-force diamond anvil cell viscometer. The majority of the results extend up to viscosities of 107 cP, with those at 22.5 °C going to 1010 cP. The overall precision of the data are approximately 10% or better throughout. This level of precision allows us to define a viscosity surface which can then be extrapolated to the glass transition along both temperature and pressure cuts. The T-dependence of viscosity is larger for glycerol than DBP but the P-dependence smaller for glycerol than for DBP, whereas the T-dependence is much more pressure sensitive for DBP. These data provide an assessment of the T-dependence of an isothermal model (free volume), the P-dependence of an isobaric model (Vogel–Tammann–Fulcher) and by extension that for isochoric conditions. Fragility parameters are evaluated for these three isometric conditions. For glycerol and (less conclusively) DBP under isobaric conditions, the fragility increases markedly at high pressure. Under isochoric conditions, the fragility for both glycerol and DBP increases with increasing density. This is dramatic for DBP, which goes from a strong to an intermediate-strength liquid. For the isothermal model, we derive a new measure of fragility. Using this, DBP shows a trend common to several liquids, a decrease in fragility with increasing temperature. Glycerol, however, becomes more fragile over the same temperature range. For glycerol, the trends towards increased fragility at elevated pressure and temperature are consistent with diminished hydrogen bonding under those conditions. The P-dependence of the glass transition is also determined over a wide range of T. The slope, dTg/dP, is positive with the pressure dependence for glycerol being considerably smaller than for DBP; both are nonlinear, tending to saturate in temperature at high pressures.

Microscopic dynamics of glycerol in its crystalline and glassy states

“…This is mostly caused by the presence in the glassy state of molecular isomers of different skeletal conformation, as can be shown by consideration of the molecular internal frequencies calculated for different atomic configurations. In fact, an estimation of the spread in frequencies arising from variations in the 1,2 angles yielded values for every mode distributed within some Ϸ1.8 THz, a result comparable with the observed shifts.…”

“…The experimental crystal structure belongs to the orthorhombic P2 1 12 where the second row atoms ͑C,O͒ are approximately coplanar, showing a shape of a flattened ''W,'' which is the most stable conformation for the isolated molecule. 11 The crystal packing shows infinite hydrogenbonded chains, each hydroxyl hydrogen participating in one hydrogen bond and each oxygen being involved in two hydrogen bonds both as a donor and acceptor, yielding a total of twelve hydrogen bonds per unit cell ͑see Fig.…”

Communication: Inflection in the pressure dependent conductivity of the protic ionic liquid C8HIM NTf2

“…Whether this feature is reflected in the pressure dependent viscosity for C 8 HIM NTf 2 is not clear so far. However, a similar behavior can be seen in η(p) for many molecular liquids 15,16 and some ionic liquids, 17 even though the concave progression at low pressures is not necessarily incorporated in the models used in these reports. This inflection might arise from a non-linearity of the volume with a change of pressure.…”

“…With rising temperature, the point shifts to higher pressures. The shift is fast initially, but slows down significantly at high T. Measurements performed by Cook et al 15 show inflection points in the viscosity of glycerol and dibutyl phthalate (DBP) over a temperature range from 273 K to 398 K. While the temperature dependence of p i for glycerol is similar to C 8 HIM NTf 2 , DBP exhibits a more linear p i (T )-behavior. It could be speculated that this observation is connected to the strong H-bonding in both glycerol and C 8 HIM NTf 2 , which is absent in dibutyl phthalate.…”

“…For the first time, we could observe a transition between slower than exponential and Vogel-Fulcher-like pressure dependence of the dc-conductivity, similar to the behavior reported in pressure dependent viscosity and relaxation times in other materials. [15][16][17] The measured data were fitted with the hybrid model suggested by Bair for η(p) (Eq. (4)) over the whole pressure range.…”