Isomeric effects in propanol and butanol isomers quantified with microwave microfluidic spectroscopy

In alcohols, the hydrogen bonded network impacts important thermophysical properties, such as boiling point and viscosity. Hydrogen bonding in alcohols is influenced by solvent accessibility (steric hindrance) of the hydroxyl group. Quantifying hydrogen bonding of alcohols is critical to understanding their dissolution and phase separation in water and other solvents. The relaxation dynamics of this network occur on multiple timescales and length scales, and dielectric spectroscopy offers a direct path to accessing the collective dynamics of the hydrogen bonded network. Here, we used microwave microfluidic spectroscopy to quantify the effects of steric hindrance of propanol and butanol isomers at 25 and 30 °C from 100 MHz to 40 GHz. The dielectric spectra were fitted with a model that includes two relaxations: a Debye relaxation (Δɛ1) and a non-Debye (Δɛ2) relaxation. Our results suggest that the relative static dielectric constant (ɛs), the Debye peak amplitude (Δɛ1), and the relaxation time constant (τ1) are all influenced by both steric hindrance and temperature. We develop a simple metric for steric hindrance in alcohols to explain the trends in the Debye relaxation time constant (τ1). Our results can be used to develop dielectric models and inform molecular simulation studies.