ABSTRACT
We have measured the elastic moduli of liquid and glassy n-propanol and propylene glycol (PG) under pressure by ultrasonic techniques and have recalculated similar characteristics for glycerol from the previous experiment. All three substances form a ternary homologous family with the common formula C3H8-n(OH)n (n = 1, 2, 3), where the number of hydrogen bonds per molecule increases with the number of oxygen atoms approximately as ≈2n. In turn, the enhancement of hydrogen bonding results in an increase in elastic moduli (bulk modulus for liquids or bulk and shear moduli for glasses) from n-propanol to glycerol at all pressures, while the volume per molecule Vm shows the opposite trend at atmospheric pressure in spite of an increase in the molecular size. Nevertheless, the ratios between the Vm values at pressure P > 0.05 GPa are inverted in liquids and tend to the ratios of molecule volumes which indicates a decrease of the relative contribution of hydrogen bonds to the repulsive intermolecular forces with increasing pressure regardless of increase or decrease in the number of hydrogen bonds and their strength. A similar volume behavior is observed for glasses at T = 77 K. We have also established that the relative difference between corresponding moduli of liquid or glassy n-propanol and PG is remarkably less than that between corresponding values for PG and glycerol. We explain this property by the formation of a three-dimensional network of hydrogen bonds in glycerol, where the number of hydrogen bonds per molecule is close to six.
ABSTRACT
2-Ethyl-1-hexanol monoalcohol is a well-known molecular glassformer, which for a long time attracts attention of researchers. As in all other monohydroxy alcohols, its dielectric relaxation reveals two distinct relaxation processes attributed to the structural relaxation and another more intense process, which gives rise to a low-frequency Debye-like relaxation. In this monoalcohol, the frequency separation between these two processes reaches an extremely high value of 3 orders of magnitude, which makes this substance a rather convenient object for studies of mechanisms (supposedly common to all monoalcohols) leading to vitrification of this type of liquids. In this work, we apply two experimental techniques, dielectric spectroscopy and ultrasonic measurements (in both longitudinal and transverse polarizations) at high pressure, to study interference between different relaxation mechanisms occurring in this liquid, which could shed light on both structural and dielectric relaxation processes observed in a supercooled liquid and a glass state. Application of high pressure in this case leads to the simplification of the frequency spectrum of dielectric relaxation, where only one asymmetric feature is observed. Nonetheless, the maximum attenuation of the longitudinal wave in ultrasonic experiments at high pressure is observed at temperatures ≈50 K above the corresponding temperature for the transverse wave. This might indicate different mechanisms of structural relaxation in shear and bulk elasticities in this liquid.
