What is it about?
This paper reports mobility measurements of positive ions in supercritical helium close to the critical point. Using Stokes law the hydrodynamical radius was derived from the mobility allowing insight into the size of solutes in supercritical gases. Cluster growth and compression as well as the change from liquid to solid phase can be observed. The paper also presents a model to simulate the radius and mobility over a large pressure ranges covering the the Knudsen and the Stokes-flow regimes.
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Why is it important?
The paper reports important fundamental benchmark data and novel insight into the structure and phase of clusters formed in supercritical gases. It reports for the first time direct size information of clusters grown in supercritical helium, an important model system due to relative simplicity of the solvent atoms and high purity, allowing for very precise data. The clusters are free and also in thermodynamic equilibrium: pressure and temperature are well defined. In regions where the clusters grow the clusters were found to be liquid. Upon increase of pressure they solidify. We modeled the mobility and hydrodynamic radius for several isotherms over a large pressure range covering the Knudsen and the Stokes flow regime.
Perspectives
This is one of my nicest pieces of work that emerged from a joint project with Nelly Bonifaci and Frederic Aitken when I stayed in Grenoble as a visiting professor. The paper relates to solution chemistry in supercritical gases and so-called snowballs which are frequently studied He-ion in helium clusters. Contrary to their name they are often liquid, particularly at lower pressure in the supercritical phase of helium.
Dr Klaus von Haeften
University of Leicester
Read the Original
This page is a summary of: Formation of Positively Charged Liquid Helium Clusters in Supercritical Helium and their Solidification upon Compression, The Journal of Physical Chemistry Letters, August 2015, American Chemical Society (ACS),
DOI: 10.1021/acs.jpclett.5b01159.
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