Master thesis defense by Martin Hoffmann Petersen

Molecular dynamic analysis of quasi-elastic neutron scattering from bulk and confined water

One of the greatest challenges presented by bulk water is to describe how the molecules move within the picosecond time scale despite having strong H-bonds arranged in a 3Dnetwork. This question becomes harder if water is confined, where a dramatic slowing down of the molecules is observed. By probing single-molecule dynamics using quasielastic neutron scattering (QENS), correlations on how the water molecules move in time and space can be experimentally obtained, while results from molecular dynamic (MD) simulation of the system allows us to further interpret the experimental data. To this end, in this work bulkwater as well as confined water in the clay minerals montmorillonite (Mt) and hectorite (Ht) were measured at 300K using the backscattering spectrometer IRIS (ISIS, UK) and the time of flight spectrometer AMATERAS (J-PARC, Japan) and simulated using MD. MD simulation obtained using the SPCE force field of bulk-water has been analysed using the commonly known phenomenological approach, describing the water dynamics with a sum of Lorentzians, as well as a recently proposed ”minimal model approach”, describing the multi-scale relaxation of the water molecules with only three parameters, both of which
were used to compare the MD simulation with the QENS data. The MD simulation was directly compared with the QENS data in the reciprocal space and time and with supporting analysis using the two approached, it was showed that the MD simulation describes the QENS spectra well, except for when localized hydrogen motions dominate. Furthermore, the analysis also showed that a minimal model approach reproduce dynamical parameters of the phenomenological approach in the limit of bulk-water. The MD simulation obtained using CLAYFF and SPCE force field of Mt and Ht were analysed using a minimal model approach, which was used to compare the MD simulation with the QENS data. In regards to Ht, the comparison between experimental and simulated data shows that the MD simulation box had a higher hydration compared to the measured sample resulting in an extra dominating water population in the interlayer. For Mt the MD simulation reproduce the data well showing that the diffusive motions of the confined water in the MD simulation depict well the behavior of the measured QENS data. To conclude, this thesis shows that MD is a unique complementary technique for the analysis of QENS data, which can be used to improve our understanding of the measured system as well as empirically verify and further improve the MD simulations of the system