Temperature dependence of self-diffusion in compressed monohydric alcohols

Abstract

The p,T-dependence of the self-diffusion coefficient D for methanol, methan(2H)ol and ethanol has been studied between 150 and 450 K at pressures up to 250 MPa. The experiments were performed in strengthened high-pressure glass cells by the application of the nuclear magnetic resonance (NMR) spin-echo technique with pulsed magnetic-field gradients. Upon cooling, molecular mobility is strongly reduced, leading to a pronounced non-Arrhenius temperature dependence of D. Applying the rough hard-sphere model (Chandler) to our data, a dramatic decrease of the A-parameter with falling temperature is observed. This behavior indicates that attractive intermolecular interactions dominate translational mobility. The best description of the data is given by the empirical Vogel–Tammann–Fulcher (VTF) equation, with ideal glass transition temperatures T0, that are in excellent agreement with those obtained from calorimetric studies. The isotope effect for self-diffusion in methanol and methan(2H)ol increases from ∼5% at high temperatures to about 40% in the supercooled region. This drastic increase is assumed to originate from a difference in hydrogen bond strength of the isotopes, as has already been found for light and heavy water.