Speaker
Description
Polarized neutron time-of-flight spectroscopy permits to access quasi-elastic scattering informing on picosecond time-scale diffusion. We have employed this method using LET at the ISIS Neutron and Muon Source to access the molecular structural dynamics in water-ethanol mixtures [1]. We also investigate the effect of a supramolecular gelator and of a paracetamol (PCM) drug added to these mixtures [2]. Our observations indicate an enhanced rigidity of the hydrogen bond network in the water-ethanol mixtures at the mesoscopic length scale compared to the molecular scale as the ethanol fraction increases, which is consistent with the hypothesis of small transient molecular clusters forming in the mixtures. When adding the supramolecular gelator, we find that, although this gelator significantly alters the macroscopic rheological behavior of the solvent at concentrations of 0.3 and 0.5 wt.%, it does not affect the hydrogen bond network at the microscopic level. In contrast, the addition of PCM at 5 wt.%, which does not change the macroscopic properties, modifies the structural dynamics of water–ethanol mixtures at length scales commensurate with and below the PCM–PCM correlation length in the mixture. This finding reveals the intricate interplay between solute, solvent, and gel interactions, demonstrating a lack of direct correlation between macroscopic rheological and microscopic diffusion properties in such complex systems. On the example of these hydrogen-bonded fluids, the results illustrate the benefit from unambiguously separating the self- and collective dynamics. The work with loaded drug molecules points towards a future interest in higher energy resolution at a similar range of momentum transfers.
References:
[1] R.Morbidini et al., J. Chem. Phys. vol.159, p.221103 (2023), https://doi.org/10.1063/5.0174448
[2] R.Morbidini et al., J. Chem. Phys. vol.162, p.114504 (2025), https://doi.org/10.1063/5.0252501
