Broadband dynamics in neat 4-methyl-3-heptanol and in mixtures with 2-ethyl-1-hexanol.

ABSTRACT

The relatively small dielectric Debye-like process of the monohydroxy alcohol 4-methyl-3-heptanol (4M3H) was found to depend slightly on the intramolecular conformation. Proton and deuteron nuclear magnetic resonance demonstrate that the hydroxyl dynamics and the overall molecular dynamics take place on similar time scales in contrast to the situation for the structural isomer 2-ethyl-1-hexanol (2E1H) [S. Schildmann et al., J. Chem. Phys. 135, 174511 (2011)]. This indicates a very weak decoupling of Debye-like and structural relaxation which was further probed using volume expansivity experiments. Shear viscosity as well as diffusometry measurements were performed and the data were analyzed in terms of the Debye-Stokes-Einstein equations. In mixtures of 4M3H with 2E1H the Debye-like process becomes much stronger and for 2E1H mole fraction of more than 25% the behavior of this alcohol is rapidly approached. This finding is interpreted to indicate that the ring-like supramolecular structures in 4M3H become energetically unfavorable when adding 2E1H, an alcohol that tends to form chain-like molecular aggregates. The concentration dependence of the Kirkwood factor in these mixtures displays a high degree of similarity with experimental results on monohydroxy alcohols in which the pressure or the location of the OH group within the molecular structure is varied.