Instrument development and a total scattering investigation of aqueous magnesium sulfate

Abstract

Non-crystalline materials, such as amorphous and glassy phases, and aqueous systems, remain an understudied yet important class of condensed materials. Many techniques can be used to enable structure elucidation of these materials, but few can match the application of total scattering and subsequent fitting of the observed data with atomistic models. This work will examine the feasibility of using a laboratory-based single crystal diffractometer for total scattering experiments that previously would often have required synchrotron radiation. The instrument has a rather unique curved image-plate detector that is ideal for collecting very low noise data over a large scattering angle. When a silver tube is fitted this enables a theoretical Qmax of 22.4 Å^-1 to be reached and data collected rapidly. Suitable data collection strategies and data reduction processes were evaluated through the study of a number of standard materials ranging from crystalline and nanocrystalline to amorphous and aqueous. The quality of the data and refined models were then evaluated. Using X-ray data from this novel setup, in combination with neutron scattering data, it was possible to use an empirical potential structure refinement (EPSR) approach to model the structural features of an aqueous sample of magnesium sulfate. Combining the isotopically substituted neutron data with the high quality in-house X-ray data enabled an extensive study of the structural features that exist within this aqueous salt. The structural features observed via this methodology highlight both the short-range ordering (first shell), similar to that found in previous studies, but also a distinct medium-range order describing the intermolecular structure not previously discussed in the literature. This highlights both the power of the refinement technique but also the capabilities of the laboratory X-ray scattering instrument. With this initial study as a foundation, it was possible to use just X-ray data to confidently study changes to the structure induced by changing conditions. Firstly, the effects of varying temperature on a constant concentration solution were investigated and found to have minimal effect. Secondly varying temperature was used as a method of stabilising higher concentrations above the ambient saturation limit to begin weaving a narrative of how the structure changes as a function of concentration with the aim of probing conditions as the crystallisation point is approached. In these studies, large, more structured features were observed that resembled those found in the various crystalline phases of hydrated magnesium sulfate. EPSR has proven to be a very powerful technique, and in combination with the unique laboratory X-ray scattering instrument, has examined an aqueous structure in more detail than previous studies.

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ORCID for Mark Light: orcid.org/0000-0002-0585-0843

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