The interface between gases and liquids is found everywhere in nature. It is also important for many industrial processes. To improve understanding of the gas-liquid interface, researchers developed an apparatus to study reactions between gas molecules and highly volatile liquids in new levels of detail. It uses a molecular beam that is directed onto a flat liquid surface. As the beam scatters, a detector collects data on the speed, direction, and mass of the molecules in the scattered beam. This allows researchers to infer changes related to the interaction of gas and liquid. To assess the feasibility of this new approach, the researchers studied the interaction between the rare gas neon and liquid dodecane.
The interface between the gas phase and the liquid phase is a unique chemical environment. It is important to understand the chemical reactions in the Earth’s atmosphere and how carbon moves between the air and the sea surface. In industrial environments, this interface affects how air and fuel mix in internal combustion engines and other applications. The new flat jet scattering apparatus opens up new opportunities for gas-liquid interface studies of volatile liquids. Scientists can now study the reactions of molecules on the surface of liquid water with resolution at the molecular level. The researchers plan to use this method to study the formation of acid rain and molecules related to air pollution.
This research reports the first results of a newly designed flat jet diffusion device. The researchers, including scientists from the University of California at Berkeley; Lawrence Berkeley National Laboratory; the Fritz Haber Institute of the Max Planck Society; the Leibniz Institute for Surface Engineering; and the University of Leipzig, demonstrated the feasibility of the device by studying the neon liquid dodecane scattering system. They started by measuring the molecular evaporation of a neon dodecane flat jet. The research revealed that the evaporation follows an angular distribution which is best approximated by a cosine function for neon and dodecane molecules. Moreover, the velocity distribution of the outgoing neon molecules follows a Maxwell-Boltzmann distribution at the temperature of the liquid. This indicates undisturbed evaporation of neon. The researchers therefore used neon atoms to probe the dynamics of diffusion at the surface of liquid dodecane.
In the diffusion experiments, the team observed two main mechanisms: impulsive diffusion (IS) and thermal desorption (TD). In TD, the molecules hitting the surface thermalize completely with the liquid and then desorb. This mechanism has an imprint already known from evaporation studies. For IS, however, information about the initial beam energy and direction is partially retained. Research has exploited this condition to quantify translational energy transfer from neon to liquid. They showed that the nature of energy transfer can be modeled with a soft-sphere kinematic model. This model allowed them to estimate the effective surface mass of dodecane at 60 amu, which is much smaller than a single dodecane molecule (170 amu), thus indicating that only part of a dodecane molecule contributes to the interaction on the collision time scale. The team’s next steps include performing experiments related to protic/aprotic molecular diffusion of dodecane and reactive water diffusion.
This work is funded by the Department of Energy (DOE) Office of Science, Basic Energy Science (BES) Program, Chemical Sciences, Geosciences, and Biosciences Division. This research used the resources of the SLAC National Accelerator Laboratory, also supported by the DOE Office of Science, BES Program. The Alexander von Humboldt Foundation and the German Research Foundation provided additional support.