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

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 C₃H_8−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 V_m shows the opposite trend at atmospheric pressure in spite of an increase in the molecular size. Nevertheless, the ratios between the V_m 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.