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

Abstract

We have monitored the speeds of evaporating helium atoms dissolved in liquid octane, isooctane, 1-methylnaphthalene, dodecane, squalane, ethylene glycol, and two jet fuels. In all cases, the average kinetic energies of the evaporating He atoms exceed the Maxwellian value of 2RT. The energies roughly track solvent surface tensions; this correlation may reflect the tighter packing and attractions of interfacial solvent molecules that restrict the gaps through which He atoms escape. Mixtures of dodecane, squalane, and 1-methylnaphthalene generate He evaporation energies that lie between the pure liquid values. We find, however, that He atoms evaporate from pure 1-methylnaphthalene with kinetic energies lower than expected based on its high surface tension, perhaps because the sideways packing of the aromatic rings provides more direct channels for the escaping He atoms. Additionally, He evaporates from two complex fuel mixtures, Jet A and JP-8, with nearly identical energies, implying that the extra additives in JP-8 do not segregate to the surface in ways that alter the dynamics of evaporation. ■ INTRODUCTIONHelium atom scattering is an enormously powerful tool to elucidate the surface structure and surface vibrations of crystalline solids, organic monolayers, and even polymer films. 1−5 The diffraction features that make this technique so useful are unfortunately lost for room-temperature liquids because of long-range disorder and extensive thermal motions that promote energy transfer between the He and surface atoms. However, it may still be possible to glean insights into interfacial motions and packing by replacing He atom scattering with He atom evaporation from liquids. 6 We continue our studies here by measuring the energy distributions of He atoms evaporating from a wide range of linear and branched hydrocarbon liquids, including mixtures, jet fuels, and ethylene glycol (as a model for a deicing additive). The experiments are performed by dissolving He atoms into each liquid, injecting the liquids into a vacuum as microjets, 7,8 and monitoring the velocities of the evaporating He atoms.The evaporation of He atoms dissolved in hydrocarbons, alcohols, and salty water appears to be special: these small and weakly attractive atoms evaporate at speeds that are faster than predicted by a Maxwell−Boltzmann (MB) distribution. 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. 10 In this timereversed picture, the attractive...

When Liquid Rays Become Gas Rays: Can Evaporation Ever Be Non-Maxwellian?

“…For hydrogen-bonding gases, the attractive forces are also very strong and lead to significant trapping. We also find, however, that even Ar and N 2 evaporating from salty water evaporate in Maxwellian distributions (within our signal to noise) [42,43,50]. By detailed balancing, this Maxwellian evaporation implies that collisions of Ar and N 2 at energies populated in a Maxwellian distribution must thermally equilibrate upon collision.…”

“…During 30 years of observation, we have monitored the vacuum evaporation of liquids such as glycerol, ethylene glycol, alkanes and aromatics, fluorinated ethers, and water from sulfuric acid and pure and salty water itself [39][40][41][42][43]. [39,40,42,[44][45][46][47][48]. We observed Maxwellian speed distributions in every case (except when the vapor pressure is so high that the gas expands supersonically [41,49]).…”

Microjets and coated wheels: versatile tools for exploring collisions and reactions at gas–liquid interfaces

“…(This distribution is expected when the evaporation coefficient is near one or is independent of collision energy. 39,40 ) The observed spectrum is slightly narrower, most likely reflecting a weak supersonic expansion arising from the few collisions that do occur near the jet surface. As calculated above, however, this jet breaks up into a string of droplets less than 1 mm from the nozzle, and N coll likely falls between the cylinder and isolated sphere values of 0.1 and 0.04.…”

“…Helium, which is often used as the backing gas, has the lowest solubility among all gases due to its very low polarizability, and this trait leads to remarkable consequences. 40,41,[53][54][55] Fig. 11b shows the TOF spectrum of evaporating He atoms dissolved in a cold salty water jet in comparison with a Maxwell-Boltzmann distribution.…”

Atomic and Molecular Collisions at Liquid Surfaces

“…The existence of surface-specific chemical and physical phenomena (15) suggests that the gas-liquid interface can function as a reaction medium distinct from vacuum and from bulk liquid. Although much remains to be learned about the surfaces of pure liquids, many current gas-liquid scattering studies are building on the groundwork of previous studies and targeting complex liquid mixtures (16,17).…”