Abstract
Positively charged helium clusters, also called 'snowballs', have been investigated within normal liquid helium. Thermodynamic state equations for ionic helium clusters in liquid helium have been developed, allowing us to discern the 'hydrodynamic' radius for a wide range of hydrostatic pressures and temperatures. The mobilities derived from the cluster sizes using stokes law match experimental data with unsurpassed accuracy. For low pressures the compressibility of the cluster ions was found to be distinctly larger than the compressibility of solid helium suggesting that in this pressure range clusters are fully or partially liquid.
Highlights
The investigation of ions in liquids is of considerable interest for the understanding of interactions in bulk liquids [1,2] as well as for applications, for examples ionic liquids [3,4,5] or chemical analysis [6,7]
For the hard-sphere radius we find a good fit for a = 0.74A, a result that is further supported from an investigation of positive ion mobility in supercritical helium which will be presented in a forthcoming publication
For temperatures at 4.2 K this turnaround is more shallow and lies at 9 MPa. We attribute this increase to a sensitivity of our state equations to the liquid-solid phase transition, which is affected by the presence of ions 26
Summary
The investigation of ions in liquids is of considerable interest for the understanding of interactions in bulk liquids [1,2] as well as for applications, for examples ionic liquids [3,4,5] or chemical analysis [6,7]. Attractive interaction between ions and the solvent liquid gives rise to the formation of clusters whose size can be elucidated by measuring their mobility. The modelling of ion mobility is a great challenge since fluid properties have to be taken into account [8,9]. Liquid helium has played a special role in this context because it can serve in many respects as a model. Compared to other solvents it is practically free of foreign impurities. It is non-polar, consists of atoms with few electrons and appealing for theory. Liquid helium exhibits quantum effects which can be explored through solute-solvent interaction at the nanoscale
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