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Thesis defense [before December 2013]
Licentiate thesis: Structural Information on Liquid Water from X-ray and Neutron Scattering
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Europe/Stockholm
122:026
122:026
Description
This thesis presents studies on the structure of liquid water, at ambient temperature and pressure, from the viewpoint of x-ray and neutron scattering techniques. As part of a larger effort dedicated to understanding the structure of water, involving a multitude of experimental and theoretical techniques, the current work has been important in establishing the extent to which x-ray and neutron diffraction data contain structural information that can discriminate between different structure models of liquid water.
For this purpose the reverse Monte Carlo structure modeling method has been used. By fitting structure models simultaneously to available diffraction data and various hydrogen bonding constraints based on a geometrical criterion, we have shown that diffraction data in fact put very loose constraints on the local coordination in water. However, diffraction data do indicate that typical molecular dynamics (MD) models exhibit too sharp and well defined peaks in the partial pair-correlation functions, as evidenced by detailed comparisons in q-space to the experimental data.
Small-angle x-ray scattering data, recently measured on liquid water, are also discussed and analyzed with the aid of large MD simulations. While the experimentally observed scattering intensity enhancement at low q indicates the existence of density heterogeneities with a characteristic length scale of around 1 nm, the simulations lack density heterogeneities of sufficient magnitude to reproduce experiment. The simulated water structures are analyzed in terms of percolation behavior and local density variations to shed light on this finding.
Together with results from x-ray absorption and emission spectroscopic methods, the small-angle scattering data support a mixture model type of structure for liquid water where strongly bonded, less dense regions are spatially separated from hydrogen bond distorted, denser regions. Recent conjectures on the possible coexistence of structurally different high-density and low-density liquid states at supercooled temperatures, associated with a hypothesized critical point at -45°C, support this picture. This type of two-component mixture model is not contradicted by diffraction data, and it can potentially provide physical explanations to water's many thermodynamical anomalies.
For this purpose the reverse Monte Carlo structure modeling method has been used. By fitting structure models simultaneously to available diffraction data and various hydrogen bonding constraints based on a geometrical criterion, we have shown that diffraction data in fact put very loose constraints on the local coordination in water. However, diffraction data do indicate that typical molecular dynamics (MD) models exhibit too sharp and well defined peaks in the partial pair-correlation functions, as evidenced by detailed comparisons in q-space to the experimental data.
Small-angle x-ray scattering data, recently measured on liquid water, are also discussed and analyzed with the aid of large MD simulations. While the experimentally observed scattering intensity enhancement at low q indicates the existence of density heterogeneities with a characteristic length scale of around 1 nm, the simulations lack density heterogeneities of sufficient magnitude to reproduce experiment. The simulated water structures are analyzed in terms of percolation behavior and local density variations to shed light on this finding.
Together with results from x-ray absorption and emission spectroscopic methods, the small-angle scattering data support a mixture model type of structure for liquid water where strongly bonded, less dense regions are spatially separated from hydrogen bond distorted, denser regions. Recent conjectures on the possible coexistence of structurally different high-density and low-density liquid states at supercooled temperatures, associated with a hypothesized critical point at -45°C, support this picture. This type of two-component mixture model is not contradicted by diffraction data, and it can potentially provide physical explanations to water's many thermodynamical anomalies.