Rotation of small diatomics in water-ethanol mixture: Multiple breakdowns of hydrodynamic predictions

We study the rotational and translational dynamics of three small important linear molecules, namely, carbon monoxide (CO), nitric oxide (NO), and cyanide ion (CN^-) in water-ethanol mixtures, at different compositions. Here, we report a detailed study of the dynamics of these diatomics in water-ethanol binary mixtures for the first time. We find multiple anomalous results, namely, (i) faster rotational motion of CO and NO than CN^-, (ii) larger translational diffusion of CO and NO in pure ethanol than in water but the reverse for CN^-, (iii) a pronounced anomaly in the composition dependence of translational-rotational dynamics at low ethanol composition, and (iv) a re-entrant type behavior in the viscosity dependence of orientational relaxation. We compare our simulation result-based observations with the existing experimental results wherever available and find that the simulation results are in reasonably good agreement with the experiments. We implement, for the first time, a calculation of the rotational binary friction following the sophisticated scheme of Evans and co-workers. We also calculate the hydrodynamic predictions for the solute molecules. On comparison with the simulation and experimental results, we find that neither the rotational binary friction nor the hydrodynamic expressions (both stick and slip) can reproduce the results. To rectify the situation, we develop a detailed mode-coupling theory and suggest that such an approach if completely implemented can provide a more reliable description than the hydrodynamic approach. Many of our results could be tested in experimental studies because these linear molecules are amenable to spectroscopic studies, such as 2D-IR.