Elastic properties of liquid and glassy propane-based alcohols under high pressure: the increasing role of hydrogen bonds in a homologous family

Polyhydric alcohols under high pressure: comparative ultrasonic study of elastic properties

We carried out an experimental ultrasonic study of polyhydric alcohols with the general chemical formula CnHn+2(OH)n with an increasing number of OH groups: glycerol (n = 3), erythritol (n = 4), xylitol (n = 5), sorbitol (n = 6). The baric and temperature dependences of the elastic characteristics of these substances in the crystalline and glassy states were studied both under isothermal compression up to 1 GPa and during the isobaric heating of 77-295 K. For glycerol, glasses were obtained at different cooling rates, glass-liquid transitions were studied at different pressures. All the studied glasses have lower elastic moduli than the same substances in the crystalline state at the same pressure-temperature conditions. We obtained a cascade of glass-supercooled liquid-crystal transitions during heating of glassy erythritol. In the series of substances with n = 3, 4, 5 the bulk moduli show a tendency to decrease with increasing n. However, sorbitol (n = 6) unexpectedly has the highest elastic moduli among the studied substances in both the glassy and crystalline states. The studied glassformers show a general tendency to increase the glass transition temperature Tg and the fragility coefficient m with increasing n.

Thermobaric history as a tool to govern properties of glasses: case of dipropylene glycol

The elastic properties of high- and low-pressure glasses of dipropylene glycol were determined for the first time under conditions of isothermal compression up to 1 GPa at 77 K and isobaric heating of 77-300 K at 0.05 GPa and 1 GPa. A strong dependence of the elastic properties of glasses on their thermobaric history has been revealed: glasses obtained at high pressure have not only higher densities (3.9%), but also noticeably higher elastic moduli. This effect is especially pronounced in the shear modulus: high-pressure glass has a 30% higher shear modulus than low-pressure glass. The behavior of elastic moduli during the glass-to-liquid transition also depends on the thermobaric history. Glass produced at low pressure but heated at high pressure has anomalous temperature dependences of the elastic moduli. Heating dipropylene glycol glasses at different pressures allowed us to refine the Tg(P) dependence.

Phase transitions in 1-bromoadamantane compared to 1-chloroadamantane: similarities and unique features

The elastic properties of 1-chloroadamantane and 1-Bromoadamantane in order-disorder and order-quasi-order phase transitions at temperatures in the range of 77-305 K and high pressures up to 1.1 GPa are studied by the ultrasonic method. The elastic moduli of halogenated adamantanes clearly indicate these transitions, demonstrating high capabilities of the ultrasonic method. Our ultrasonic studies have detected for the first time the λ-anomaly of the elastic properties and, thereby, have confirmed that the phase transition from the orientationally ordered to quasi-ordered phase (M → O) in 1-bromoadamantane is a weak first-order phase transition having some properties of a second-order phase transition. The pressure derivatives of the elastic moduli of 1-chloroadamantane and 1-bromoadamantane in the orientationally ordered phase at 77 K are dB/dP ≈ 8, which allows using the Lennard-Jones potential to theoretically describe this phase.

Comparative study of the elastic properties of adamantane and 1-chloroadamantane at high pressure and different temperatures and at order-disorder transitions

We present a comparative ultrasonic study of the elastic properties of adamantane and 1-chloroadamantane at high pressure (up to 1.4 GPa) and different temperatures (77-293 K) and at order-disorder transitions. The ultrasonic method provides complementary pictures of the order-disorder transitions in diamondoids under pressure. The equation of state of adamantane and 1-chloroadamantane was determined up to 1.4 GPa from ultrasonic measurements of bulk modulus and is in good accordance with the previous equations developed from volumetric data. We measured the bulk and shear moduli and Poisson's ratio of adamantane and 1-chloroadamantane up to 1.4 GPa. The behaviors of elastic moduli are different for adamantane and 1-chloroadamantane. This indicates that the substitution of one hydrogen atom for chlorine significantly reduces both elastic moduli, particularly the shear modulus (≈30%). Although the pressure dependences of the bulk modulus B are almost linear and its pressure derivatives for adamantane and 1-chloroadamantane are close to each other (B' ≈ 10-12), a jump is hardly observed on the pressure dependence B(P) for adamantane at the transition from the plastic to ordered phase, whereas the pressure dependence of the bulk modulus for 1-chloroadamantane exhibits a jump of almost 17%. The experimental dependences of the bulk modulus and relative changes in the volume for both materials clearly demonstrate that the compressibility of 1-chloroadamantane is much higher for both phases. The Poisson coefficient calculated from our experimental data is larger for 1-chloroadamantane, having lower both bulk and shear moduli.

Molecular heterogeneities in the thermal expansivity of polyalcohols

Density is the key quantity for nearly all the numerous theories of the (dynamic) glass transition of supercooled liquids and melts. As mean field quantity, it is used to describe correlations and heterogeneities between regions consisting of several molecules. In contrast, the question how density is created by the interactions (i.e., bonds) within a molecule and to its nearest neighbors is almost unexplored. To investigate this for the example of a homologous series of polyalcohols (glycerol, threitol, xylitol, and sorbitol), Fourier-Transform InfraRed (FTIR) spectroscopy is carried out in a wide range of temperatures from far above to far below the calorimetric glass transition T_g. This enables us to determine the potentials and hence the bond lengths of specific intramolecular and intermolecular interactions. While the former has an expansion coefficient of (∼0.1 pm/100 K) with only smooth changes, the latter shows a 30-40 times stronger response with pronounced kinks at T_g. A comparison with the overall expansion based on mass density reveals that one has to separate between strong (OH⋅⋅⋅O) and weak (CH⋅⋅⋅O) intermolecular hydrogen (H)-bridges. Despite the fact that the latter dominates glassy dynamics, their expansivity is 5 times smaller than that of the weak H-bridges. It is to be expected that such heterogeneities on intramolecular and intermolecular scales are a general phenomenon in liquids and glassy systems demonstrating especially the necessity of atomistic simulations.

Unusual Debye relaxation in 4-methyl-2-pentanol evidenced by high-pressure dielectric studies

The Debye relaxation is the main signal in the dielectric measurements of monoalcohols arising from the hydrogen-bonded superstructures, but its physics remains to be cleared. In this work, a monoalcohol of 4-methyl-2-pentanol is studied using dielectric spectroscopies recorded at high pressures. The dynamic parameters of the Debye and structural relaxations are extracted. The calculation of the Kirkwood factor of the Debye relaxation indicates chain-like H-bond molecular configurations. Remarkably, we found that both ratios of the relaxation strength and relaxation time between the Debye and structural dynamics, Δε _D/Δε _α and τ _D/τ _α , decreases upon compression, indicating a positive correlation. This is different from the results reported in primary 2-ethyl-1-hexanol and secondary 4-methyl-3-heptanol, where the two ratios are inversely correlated. The discussion and interpretation of these different results are provided.