Researchers developed an apparatus to analyze reactions between gas molecules and extremely volatile liquids using a molecular beam directed onto a flat liquid surface. To assess the feasibility of this new technique, the interaction of the noble gas neon with liquid dodecane was explored.
Gas-Liquid Interface Applications and Experiments Using Molecular Beam Apparatus
The gas-liquid interface is a unique chemical environment. Therefore, understanding chemical interactions in the Earth's atmosphere and how carbon flows between the air and the sea's surface is critical. The interface also significantly impacts industrial operations, atmospheric chemistry, and environmental science. The applications include air-fuel mixing in internal combustion engines, acid rain generation, tropospheric aerosol surface chemistry, and CO2 uptake at the ocean-air interface.
To explore interactions at gas-liquid interfaces for volatile liquids, a crossed molecular beam apparatus capable of measuring the angular and translational energy distributions of scattered products has been modified for liquid jet scattering. A microfluidic chip is employed to create a stable flat liquid sheet inside a vacuum. It is where scattering occurs. Both evaporation and scattering from the sheet are described using a rotatable mass spectrometer, which can measure product time-of-flight (TOF) distributions.
Gas-liquid Interface and Flat Jet Scattering Equipment
The study published in The Journal of Physical Chemistry presents the preliminary results of a newly constructed flat jet scattering equipment. The researchers studied the neonliquid dodecane scattering system. They began by monitoring molecule evaporation from a flat jet of neon-doped dodecane.
For neon and dodecane molecules, the researchers discovered that evaporation follows an angular distribution best described by a cosine function. At liquid temperature, the velocity distribution of the exiting neon molecules follows a MaxwellBoltzmann distribution. This suggests that the evaporation of neon is unaffected.
TOF measurements demonstrate that the dodecane translational energy distributions are super-Maxwellian near the surface normal and become more Maxwellian at increasing scattering angles. This effect is due to scattering between evaporating molecules, similar to what occurs during a supersonic expansion. As a result, the researchers utilized neon atoms to investigate the scattering dynamics at the liquid dodecane surface.
In the scattering experiment, the scientists identified two key mechanisms: impulsive scattering (IS) and thermal desorption (TD). In TD, molecules collide with the surface, fully thermalize with the liquid and then desorb. This mechanism has already been identified through evaporation research. On the other hand, IS partially conserves information about the initial beam energy and direction. The study used this condition to quantify the translational energy transfer from neon to liquid. They demonstrated that a softsphere kinematic model could be used to model the nature of energy transmission.
The model allowed the researchers to calculate the effective surface mass of dodecane to be 60 amu, which is substantially smaller than the effective surface mass of a single dodecane molecule (170 amu), implying that just a portion of a dodecane molecule contributes to the interaction at the collision timescale. The next step for the researchers is to conduct studies on protic/aprotic molecular scattering off dodecane and reactive scattering off the water.
Future research will concentrate on more complex scattering partners and volatile solvents. The researchers are especially interested in conducting evaporation and scattering tests on flat water jets to investigate the critical gas-liquid interface and sort out the consequences of the higher vapor density surrounding such a jet.
The experiments are currently underway and will be reported on shortly.
Chin Lee from the University of California, Berkeley, led the study. Lee is joined by scientists from the Lawrence Berkeley National Laboratory, the Fritz Haber Institute of the Max Planck Society, the Leibniz Institute of Surface Engineering, and the University of Leipzig.
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