The Uptake and Reactivity of Organic and Atmospheric Gases in Salty and Surfactant-coated Water Microjets PDF Download

Author: Thomas Brian Sobyra
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Languages : en
Pages : 0

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Book Description
This thesis describes investigations of the solvation and reactions of gaseous molecules interacting with salty water and surfactant-coated solutions using gas-microjet scattering. We probed (i) the entry probability of organic molecules with diverse functional groups, including carboxylic acid dimers, into salty solutions, (ii) the reactivity of N2O5 with Br- ions in solutions with and without surfactants present, and (iii) the reactive uptake of HNO3 in solutions with and without a partial surfactant monolayer. The microjet technique enables us to suppress collisions between the solute gas and evaporating water molecules by employing a thin stream of water in vacuum (35 micron diameter in our experiments) whose size is smaller than the mean free path in the vapor cloud surrounding the jet. We first prepared microjets of two aqueous solutions, 8 molal LiBr/H2O and ~4 m K2SO3/H2O at 253 K, and exposed them to seven organic gases representing different functional groups to measure their entry probability. These gases comprise weak acids (formic and acetic monomer and dimer), weak bases (dimethylamine and piperidine), and an alcohol, ether, and ester (ethanol, dimethyl ether, and methyl formate). The entry probability of each molecule is shown to track the molecule's physical solubility, reflecting both the attractive forces between the gas and solvent water molecules and the finite solvation time of the gas in solution with respect to the 100 microsec observation time of the fast-moving microjet. Organic acids and bases were lost to solution following nearly every collision, whereas molecules with less hydrogen bonding capacity desorbed shortly after thermalization. Even dimerization of the weak acids did not prevent the interfacial water molecules from solvating and permanently capturing these nominally hydrophobic dimers, likely trapped by hydrogen-bond formation between H atoms of surface water molecules and available O atom sites on the dimer. The experiments demonstrate that it is possible to the measure the entry probability of reactive gases into salty water solutions without interference from gas-phase collisions. We next utilized 35 [lowercase mu]m jets of 8 m LiBr/H2O jet at 240 K and 6 m LiBr/H2O jet at 263 K to investigate the uptake of N2O5 and its reaction with aqueous Br- ions. N2O5 was chosen for these experiments because it is a nighttime reservoir for nitrogen oxides in the atmosphere, whose hydrolysis and reaction with halide ions impacts global concentrations of O3, OH, and CH4. The presence of the highly reactive impurity HNO3 in the N2O5 molecular beam complicated the determination of the N2O5 reactive uptake, and our measurements yielded a small negative uptake of -0.05 +/- 0.10. However, evaporating Br2 product from the oxidation-reduction reaction N2O5(g) + 2 Br-(aq) 2!Br2(g) + NO3-(aq) + NO2+(aq), a model for the nighttime reaction of N2O5 with halide ions in aerosol droplets, can be readily detected and controlled by the presence of surfactants. Addition of a non-ionic surfactant, 1-butanol, that covers the surface with ~40% of a compact monolayer reduces Br2 production by 35%. Remarkably, covering the surface with 9% or 58% of a monolayer of the cationic surfactant tetrabutylammonium bromide (TBA+/Br- ) reduces the Br2 signal by 85% and ~100%, respectively. A detailed analysis suggests that TBA+ efficiently suppresses Br2 evaporation because it tightly bonds to the Br3- intermediate formed in the highly concentrated Br- solution, and thereby hinders the rapid release and evaporation of Br2. In order to determine the impact of the nitric acid impurity on our N2O5 uptake measurements, we measured the reactive uptake of HNO3 itself into an 8 m LiBr/H2O microjet at 240 K. An uptake of less than 1 for HNO3 means that the N2O5 signal (recorded at the common NO2+ ion mass) from the microjet contains desorbing HNO3 and explains the negative uptake value acquired. Literature studies have reported the uptake of HNO3 to be between 0.2 and 0.7 for concentrated NaCl solutions. Using our liquid microjet technique we measured the uptake of HNO3 to be 0.59 +/- 0.06, which changes our previously measured N2O5 uptake value from -0.05 +/- 0.10 to 0.036 +/- 0.11, a number in accord with other uptake measurements. Organic surfactants at the surface of aerosol droplets are quite common, so we choose to investigate if a surfactant layer could block this strongly hydrogen bonding molecule from interacting with the aqueous subphase. Initial measurements show that a 58% monolayer of TBABr reduces the reactive uptake of HNO3 from 0.29 +/- 0.07 into a bare 6 m LiBr/H2O at 240 K to an uptake of -0.06 into 0.050 m TBABr + 6 m LiBr/H2O.