Ballistic Evaporation and Solvation of Helium Atoms at the Surfaces of Protic and Hydrocarbon Liquids

Abstract

Atomic and molecular solutes evaporate and dissolve by traversing an atomically thin boundary separating liquid and gas. Most solutes spend only short times in this interfacial region, making them difficult to observe. Experiments that monitor the velocities of evaporating species, however, can capture their final interactions with surface solvent molecules. We find that polarizable gases such as N2 and Ar evaporate from protic and hydrocarbon liquids with Maxwell-Boltzmann speed distributions. Surprisingly, the weakly interacting helium atom emerges from these liquids at high kinetic energies, exceeding the expected energy of evaporation from salty water by 70%. This super-Maxwellian evaporation implies in reverse that He atoms preferentially dissolve when they strike the surface at high energies, as if ballistically penetrating into the solvent. The evaporation energies increase with solvent surface tension, suggesting that He atoms require extra kinetic energy to navigate increasingly tortuous paths between surface molecules.

MD simulations of He evaporating from dodecane

“…It has been found that the He atoms' velocity distribution is shifted to fractionally faster velocities as compared to a Maxwell-Boltzmann velocity distribution of He atoms at the temperature of the liquid surface. This result has qualitatively also been obtained in the experimental studies by Nathanson and co-workers [8], but the effect is more pronounced in the liquid jet experiments than in the simulations. We have also established that faster He atoms have a slight preference to evaporate along the surface normal, i.e.…”

“…These fits consistently yielded distributions of evaporated He atoms which are fractionally faster than a thermal 298 K distribution, with an average translational energy of (1.05 ± 0.03) × 2RT. The obtained average translation energies are a convenient measure to compare our data to the experimental work by Nathanson and co-workers, who find an average kinetic energy of He atoms evaporating from 295 K liquid dodecane of 1.14 × 2RT [8], albeit these data are for He atoms evaporating from a cylindrical jet. To allow for a better comparison, we have weighed our data by sin(Â) −1 where  is the polar angle of the evaporating He atoms with respect to the surface normal.…”

“…In this work, however, we report MD simulations which we performed to establish the kinetic energy distribution of evaporating atoms [7], and which can be directly compared with recent experimental work by Nathanson and co-workers who studied the evaporation of He atoms from dodecane in a liquid jet [8]. Using a mass-spectrometer mounted in the plane perpendicular to a liquid jet in vacuum, they measured by means of time-of-flight methods the velocity distribution of He atoms which are initially dissolved in the dodecane liquid but evaporate when the liquid jet travels through vacuum.…”

“…While much work has been done to study the structure and surface properties of liquid interfaces (described by Knudsen zones and capillary waves), and MD simulations have been used to investigate the structure of liquid interfaces, this work focussed on the molecular level description of the dynamics of evaporation of He atoms from liquid dodecane, a relatively simple system as Hebeing atomic -lacks the rotational and vibrational degrees of freedom that can add complexity [17]; these simulations are directly relevant to recent experimental studies on the evaporation of atoms and small molecules from liquids [8].…”

Probing Gas–Liquid Interfacial Dynamics by Helium Evaporation from Hydrocarbon Liquids and Jet Fuels

“…This behavior stands in contrast to larger and more polarizable species such as Ar, N 2 , O 2 , H 2 O, CO 2 , HCl, and HNO 3 , which evaporate with flux-weighted average energies of 2RT liq consistent with a MB energy distribution for a liquid at temperature T liq . 6,9 We recently found that the He speed distributions depend on the nature of the solvent molecules, with average kinetic energies ranging from 1.14 × 2RT liq for He evaporation from dodecane to 1.7 × 2RT liq from a 7.5 M LiBr/ H 2 O solution. 6 Detailed balancing of the incoming and outgoing fluxes implies that He atoms must preferentially dissolve at higher kinetic energies as well.…”