Exploring the structural relaxation in 2-ethyl-1-hexanol by neutron spin echo: from intermolecular and supramolecular correlations to long-range density fluctuations

The combination of neutron spin echo experiments on protonated and deuterated samples has provided direct insight into the hydrogen (H) self (incoherent) and collective (coherent) dynamics of the monoalcohol 2-ethyl-1-hexanol (2E1H) over a wide range of scattering vector ($Q$) values. Thereby we have covered length scales from the intermolecular ones (at the structure factor peak $Q_{\text{max}}\sim 1.4$ Å${}^{-1}$) to long-range density fluctuations ($Q\sim 0.1-0.3$ Å${}^{-1}$) including also the relevant length scale for the supramolecular structures developed by H-bonding (pre-peak at $Q\approx 0.5$ Å${}^{-1}$). Away from the main structural peak, incoherent contributions to the measured echoes have been properly subtracted to isolate the coherent scattering. The structural relaxation of long-wavelength fluctuations and intermolecular correlations as well as the incoherent H-motions reveal two processes, a non-dispersive (local) one and diffusion. Their relative weight to the structural relaxation dramatically varies from low-$Q$ ($0.1\lesssim Q\lesssim 0.4$ Å${}^{-1}$) – where relaxation is dominated by the non-dispersive process – to $Q_{\text{max}}$ – where it takes place mainly by diffusion –. The opposite tendency is shown by incoherent scattering. These results corroborate for 2E1H the observations recently reported in other systems with different interactions and molecular sizes, and support the assignment of the local process as that responsible for stress relaxation. Moreover, the NSE results at the pre-peak reveal a much slower relaxation than at intermolecular level. Comparison of the NSE timescales at the different characteristic lengths investigated with those obtained by relaxation techniques (dielectric spectroscopy, oscillatory shear rheology, depolarized light scattering, NMR, and calorimetry) sheds light on the origin of these processes.