Liquid Microjet Measurements of the Entry of Organic Acids and Bases into Salty Water

Abstract

We explore collisions of hydrogen-bonding molecules with salty water using gas–microjet scattering experiments. Two aqueous solutions, 8 molal (m) LiBr/H2O and ∼4 m K2SO3/H2O at 253 K were exposed to seven organic gases representing different functional groups. These gases comprise weak acids (formic and acetic), weak bases (dimethylamine and piperidine), and an alcohol, ether, and ester (ethanol, dimethyl ether, and methyl formate). The scattering experiments are used to monitor the disappearance of each gas into the aqueous solutions over a ∼100 μs observation time. They demonstrate that formic acid and piperidine disappear into both solutions on almost every collision. Dimers of formic and acetic acid are also captured by the solutions on every collision, despite their pre-existing double hydrogen bonds. The methylene ring of piperidine, (CH2)5NH, also does not interfere with uptake. At the opposite extreme, methyl formate and dimethyl ether are so weakly soluble that they evaporate completely within the observation window, precluding the measurement of their entry probability. Ethanol and dimethylamine represent intermediate cases in which dimethylamine interacts more strongly with dissolved Li+ ions than K+ ions. Collectively, the experiments imply that organic acids and bases reach hydrogen-bonding configurations following nearly every collision, enabling them to be captured by surface water molecules.

Ab initio molecular dynamics studies of formic acid dimer colliding with liquid water

“…These two analyses can be compared to the impulsive energy transfer of FA dimers of structure I directly after a collision, i.e., the kinetic energy of directly scattering FA dimers relative to the initial kinetic energy of the incoming FA dimer molecular beam. The experimental impulsive energy transfer was measured to be 10%, 17 which is in good agreement with both the qualitative outlook of our simulations and with our quantitative computational result. We also note that in the trajectories, in the case of less added kinetic energies and the ones having structure II dimer, the direct scattering from the surface was absent because there is not enough kinetic energy retained for the dimer to escape from the surface.…”

“…Eight trajectories of structure I FA dimers were calculated in high-energy collisions with the water slab at 300 K. A kinetic FA dimer center of mass energy of 68k B T (170 kJ mol À1 ) was selected to mimic recent experiments. 17 We estimated the escape probability of the directly scattering FA dimer by using two points of views. First, we qualitatively counted the number of trajectories that qualify as scattering events.…”

“…[3][4][5][6][7][8][9][10][11][12][13][14][15][16] Recently, the dynamics of interactions between FA and liquid water have been investigated both experimentally and computationally. In scattering experiments, FA and FA dimer were collided with a liquid water surface and each found to undergo nearly a complete uptake into salty water at 253 K. 17 In order to yield detailed information about the molecular processes of FA at the air-water interface, ab initio molecular dynamics (AIMD) and metadynamics simulations were used to calculate collision trajectories. 18 The simulations showed formation of hydrogen bonds between FA and water molecules which served to explain the disappearance of FA in the scattering experiments.…”

“…18 The simulations showed formation of hydrogen bonds between FA and water molecules which served to explain the disappearance of FA in the scattering experiments. 17 Other fundamental dynamical events were also simulated, such as deprotonation of FA occurring during the 8-50 ps timeframe. On ice surfaces at the temperature of 250 K, earlier molecular dynamics simulations showed that FA and acetic acid form hydrogen bonds with two surface water molecules via the carbonyl group resulting in trapping of the acid molecule on the ice surface.…”

Concerted Pair Motion Due to Double Hydrogen Bonding: The Formic Acid Dimer Case

“…It has not yet explored the power of isotope substitution 80 , which provides valuable independent checks from the experimental side and sometimes even surprising phenomena such as counterintuitive isotope effects 14 which have to be modelled by quantum treatments of the nuclear dynamics. Alternatives to the relatively insensitive linear FTIR and Raman spectroscopy for jet-cooled species 81 have to be further developed in different spectral ranges, either as linear 15,82,83 or as action spectroscopy 84,85 tools, also extending into different environments [85][86][87][88][89] .…”