In situ characterisation of soft matter systems in environmental cells by synchrotron X-ray scattering in the hard and medium X-ray ranges are already well-established techniques. In contrast, X-ray spectroscopy in the soft X-ray region (i.e., at photon energies below 2 keV) has been confined to studies under high vacuum conditions, despite the fact that XPS and XAS have for some time been considered to be part of the core instrumentarium for studies of biomaterials and interfaces in medical research . In-situ studies under practical conditions in the soft X-ray region have been scarce, even though the important K-edges of the ‘soft matter elements’ of the second period (especially C, N, O, F) but also third period elements such as Na, Mg, Al, Si, S, P lie in this energy range. The pioneering NEXAFS and XPS work on liquids at BESSY in the last few years [2-8] has shown how structural studies of solution species by soft XAS can be performed with windowed flow cell systems, while laminar-flow liquid jet systems permit XPS, XAS and XES studies. The ability to probe phases containing these elements in situ and/or operando as liquids or in environmental cells under control of environmental conditions is extremely beneficial in the context of applications of contemporary interest, including biomolecular and pharmaceutical systems, organic and organometallic solution state chemistry and multicomponent products and formulations. The high pressure branch of VERSOX seeks to establish such a facility, world-wide for the first time for routine XAS measurements (ultimately permitting service measurements) while the main scanned microfocus XPS/XAS facility will provide more incisive insight into the physical properties of those systems for which a deep understanding is desired.
Recent work with XPS has indicated that core level spectroscopy of organic materials provides remarkably detailed and clear-cut insight into the composition and local interactions in the organic solid state, complementing and enhancing structural information that is traditionally provided through X-ray crystallography and solid-state NMR measurements [9-11]. These studies have opened up an avenue towards reliably detecting and assessing the influence of H-bonding and proton transfer on structure formation in the organic solid state in considerably more detail than is hitherto possible in the absence of neutron scattering analysis and solid state NMR measurements.
The process of phase transformation and the resulting creation of crystalline materials from liquid phase precursors are central to the science and process engineering of materials in their broadest sense. The classic kinetic models of a first order phase transition involving crystal nucleation assume spontaneous formation of molecular clusters (embryos) having a range of sizes between 1 and 10 nm whose viability is size-dependent. The concept of a critical size at which the gain in bulk free energy is balanced by the penalty of the surface free energy is well known as a model that describes the major macroscopic kinetic features of the nucleation process. In the context of current concepts of crystallisation as a supra-molecular assembly process this visualisation of the nucleation event is limited and void of structural considerations. Thus, the nature and importance of intermolecular interactions within supersaturated solutions as well as the existence and structural nature of pre-crystalline transition states do not form part of conventional considerations of nucleation theory. In the broad context of materials chemistry and especially in the field of crystal engineering, the ability to control the molecular assembly process from solution will be crucial in developing the molecular-scale process design capabilities needed for the future. Exemplars of this, as highlighted by the setting up of a new ‘Directed Assembly of Extended Structures with Targeted Properties’ (DAESTP) network as part of the Chemical Sciences and Engineering Grand Challenges, include design and control of the physico-chemical properties of condensed materials such as oily dispersions, liquid crystals, amorphous, nano-crystalline or micro-crystalline states. Elucidating the fundamental physics and chemistry that govern the structure of the nucleation transition state remains one of the truly unresolved ‘grand challenges’ of the physical sciences. The spectroscopies provided by VERSOX, would provide, for the first time in the UK, an incisive XAS/XPS measurement capability for following the chemical and physical state of molecules across short-range molecular interaction scales. Combined with complementary longer-range X-ray scattering measurements it will facilitate following the structural evolution from individual molecules in solution through molecular clusters to embryonic crystals in situ. This approach to studying nucleation phenomena is the subject of a new Manchester/Leeds EPSRC critical mass grant involving BESSY and the NSLS. The arrival of VERSOX as a UK facility for continuing such work in the future would therefore be extremely timely.
The ambient pressure environment and liquid handling capability of VERSOX provides a route to studying soft matter and multicomponent systems of technological relevance. The element specificity of soft X-ray techniques will permit structural characterisation in complex multi-component systems when one element is associated with only a single component. An example would be studying the salvation structure around surfactant head groups and the organophosphorus species used as capping agents and phase transfer ligands in liquid-liquid extractions and reactions. Dryfe and Schroeder are currently pioneering this approach in an NSF/EPSRC-sponsored programme of liquid-liquid interface investigations using hard X-ray absorption spectroscopy  Liquid-liquid and liquid-polymer interfaces are also the basis for biointerfaces and many consumer products in which the microstructure determines, usually in a complex fashion, the macroscopic properties. The local structure provided by core level spectroscopies complements the long-range information accessible through established X-ray scattering techniques.
Diamond Light Source is the UK's national synchrotron science facility, located at the Harwell Science and Innovation Campus in Oxfordshire.
Copyright © 2018 Diamond Light Source
Diamond Light Source Ltd
Harwell Science & Innovation Campus