A new approach for studying aqueous phase OH kinetics: application of Teflon waveguides

Abstract

Bimolecular rate coefficients for the reactions of the hydroxyl radical, OH, with methanol, ethanol, tetrahydrofuran, dimethylmalonate [CH 3 in aqueous solutions have been measured using a novel experimental approach. The centrepiece of the new experimental technique reported in this work is a Teflon AF 2400 liquid core waveguide. The physical properties of the Teflon AF 2400 liquid core waveguide allow for the construction of a micro-flowtube reaction photolysis cell with an extremely low volume and a very long optical pathway. Such a reaction system allows for a very sensitive detection of chemical transients in the aqueous phase. The micro-flowtube experiments involved competition kinetics of the OH radical with the organic reactant of interest and an SCN À anion, (k OH+SCN À ¼ 1.29 Â 10 10 M À1 s À1 ). 1 The (SCN) À 2 anion was detected using UV-visible spectroscopy following a medium pressure mercury lamp photolysis (l ! 366 nm) of H 2 O/H 2 O 2 /reactant/ KSCN mixtures. All experiments were carried out at room temperature. Measured rate coefficients for the reaction of the OH radical with methanol, ethanol, tetrahydrofuran, dimethylmalonate, dimethylsuccinate, dimethylcarbonate and diethylcarbonate are (units are 10 8 M À1 s À1 ):22, k OH+diethylcarbonate ¼ 7.9 AE 3.2. Uncertainties in the above expressions are AE2s and represent precision only. The reported rate coefficients for the reactions of OH with ethanol, methanol and THF agree very well with the currently recommended values. To date, there is no kinetic data reported in the literature for the OH radical reaction with dimethylmalonate, dimethylsuccinate, dimethylcarbonate and diethylcarbonate. The reaction mechanism is briefly discussed as a function of bond energies.

Rate constants for the OH reactions with oxygenated organic compounds in aqueous solution

“…Aqueous solutions of the organic reactant and the radical precursors were continuously pumped into the LCW, which was hence used as a microflowtube. In all cases, the liquid passed through the LCW in less than 0.5 s. Under these experimental conditions, steady state was obtained within the LCW and verified experimentally (see a previous study 7) and theoretically using numerical simulations of the chemistry within the LCW.…”

“…For this competition kinetics system the temperature dependent rate constant reported by Chin and Wine has been applied as a reference: ln k ( T ) = (29.614 ± 0.636)–(1900 ± 190)/ T [M −1 s −1 ] 6. The competition kinetic system based on SCN − as a competitor for the study of OH reactions in aqueous solution has already been applied and discussed in some previous studies 4,5,7,8.…”

“…Note that a recent study 9 has questioned this reference value while a second one has validated it and even extended its application to various ionic strengths 10. In this context, it has been decided to derive all rate coefficients by using the reference rate coefficient reported by Chin and Wine 6 as the latter has been widely used as mentioned before 4,5,7,8. Where data for comparison exist, the results obtained in the present work are in good agreement with the kinetic data from those previous studies and various available kinetic data from the literature demonstrate that the use of thiocyanate as a competitor and the use of the kinetic reference data set of Chin and Wine are appropriate for the experimental conditions applied in the present work.…”

“…The Teflon WP system has been detailed in two previous studies 7,8. Basically, in WP system a “windowless” reaction cell is applied, which can be created by using a liquid‐core waveguide (henceforth called LCW) made of Teflon AF‐2400.…”

A novel long path photolysis cell—application to the reactivity of selected organic compounds toward the nitrate radical (NO3)

“…In our previous paper, 5 we discussed the reactivity of the hydroxyl radical toward some oxygenates, and new rate coefficients were established for the OH þ dimethylmalonate, OH þ dimethylsuccinate, OH þ dimethylcarbonate and OH þ diethylcarbonate reactions, followed by a discussion on the H-abstraction mechanism. In the present study however, we focused on the reactivity of NO 3 toward a few oxygenates.…”