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Title: Super-Maxwellian helium evaporation from pure and salty water

Helium atoms evaporate from pure water and salty solutions in super-Maxwellian speed distributions, as observed experimentally and modeled theoretically. The experiments are performed by monitoring the velocities of dissolved He atoms that evaporate from microjets of pure water at 252 K and 4–8.5 molal LiCl and LiBr at 232–252 K. The average He atom energies exceed the flux-weighted Maxwell-Boltzmann average of 2RT by 30% for pure water and 70% for 8.5m LiBr. Classical molecular dynamics simulations closely reproduce the observed speed distributions and provide microscopic insight into the forces that eject the He atoms from solution. Comparisons of the density profile and He kinetic energies across the water-vacuum interface indicate that the He atoms are accelerated by He–water collisions within the top 1-2 layers of the liquid. We also find that the average He atom kinetic energy scales with the free energy of solvation of this sparingly soluble gas. This free-energy difference reflects the steeply decreasing potential of mean force on the He atoms in the interfacial region, whose gradient is the repulsive force that tends to expel the atoms. The accompanying sharp decrease in water density suppresses the He–water collisions that would otherwise maintain a Maxwell-Boltzmann distribution, allowing themore » He atom to escape at high energies. Helium is especially affected by this reduction in collisions because its weak interactions make energy transfer inefficient.« less
Authors:
; ; ; ;  [1]
  1. Department of Chemistry, University of Wisconsin-Madison, 1101 University Avenue, Madison, Wisconsin 53706 (United States)
Publication Date:
OSTI Identifier:
22493702
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Chemical Physics; Journal Volume: 144; Journal Issue: 4; Other Information: (c) 2016 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; ATOMS; BOLTZMANN STATISTICS; DENSITY; ENERGY TRANSFER; EVAPORATION; FREE ENERGY; HELIUM; INTERFACES; KINETIC ENERGY; LAYERS; LIQUIDS; LITHIUM BROMIDES; LITHIUM CHLORIDES; MOLECULAR DYNAMICS METHOD; POTENTIALS; REDUCTION; SOLUTIONS; SOLVATION; WATER