Role of hydrogen bonded associates on the nanoscale dynamics of liquid and supercooled 2-propanol

Monohydroxy alcohols are good model systems for studying the impact of hydrogen bonding on the structure and dynamics of liquids and on the macroscopic transport properties such as viscosity. We investigated 2-propanol by static and quasielastic neutron scattering experiments supported by molecular dynamics simulations on a series of partially and fully deuterated samples at temperatures ranging from the liquid, the deeply supercooled, to the glassy state. The results indicate that the macroscopic shear viscosity has the same temperature dependence as the dynamics at the pre-peak correlated with the H-bonded associates, which highlights the fundamental role played by these structures in defining the macroscopic rheological properties of the system. Importantly, the characteristic relaxation time at the pre-peak follows an Arrhenius temperature dependence whereas at the structure peak exhibits a non-Arrhenius behavior on approaching the supercooled state. The origin of this differing behavior is attributed to an increased structuring of the hydrophobic domains of 2-propanol accommodating a more and more encompassing H-bond network, and a consequent set in of dynamic cooperativity.