Study of the heterogeneous reaction of O₃ with CH₃SCH₃ using the wetted-wall flowtube technique

Abstract. This work presents the heterogeneous kinetics of the reaction of CH₃SCH₃ (dimethyl sulphide, DMS) with O₃ (ozone) in aqueous solutions of different ionic strengths (0, 0.1 and 1.0M NaCl) using the wetted-wall flowtube (WWFT) technique. Henry's law coefficients of DMS on pure water and on different concentrations of NaCl (0.1M - 4.0M) in the WWFT from UV spectrophotometric measurements of DMS in the gas phase, using a numerical transport model of phase exchange, were determined to be H ±s (M atm^-1) = 2.16±0.5 at 274.4 K, 1.47±0.3 at 283.4 K, 0.72±0.2 at 291 K, 0.57±0.1 at 303.4 K and 0.33±0.1 at 313.4 K on water, on 1.0M NaCl to be H = 1.57±0.4 at 275.7 K, 0.8±0.2 at 291 K and on 4.0M NaCl to be H = 0.44±0.1 at 275.7 K and 0.16±0.04 at 291 K, showing a significant effect of ionic strength, m, on the solubility of DMS according to the equation ln (H/M atm^-1) = 4061 T^-1 - 0.052 m² - 50.9 m T^-1 - 14.0. At concentrations of DMS_(liq) above 50 mM, UV spectrophotometry of both O_3(gas) and DMS_(gas) enables us to observe simultaneously the reactive uptake of O₃ on DMS solution and the gas-liquid equilibration of DMS along the WWFT. The uptake coefficient, g (gamma), of O₃ on aqueous solutions of DMS, varying between 1 and 15·10^-6, showed a square root-dependence on the aqueous DMS concentration (as expected for diffusive penetration into the surface film, where the reaction takes place in aqueous solution). The uptake coefficient was smaller on NaCl solution in accord with the lower solubility of O₃. The heterogeneous reaction of O_3(gas) with DMS_(liq) was evaluated from the observations of the second order rate constant (k^II) for the homogeneous aqueous reaction O_3(liq) + DMS_(liq) using a numerical model of radial diffusion and reactive penetration, leading to k^II ± D k^II (in units of 10⁸ M^-1 s^-1) = 4.1±1.2 at 291.0 K, 2.15±0.65 at 283.4 K and 1.8±0.5 at 274.4 K. Aside from the expected influence on solubility and aqueous-phase diffusion coefficient of both gases there was no significant effect of ionic strength on k^II, that was determined for 0.1M NaCl, leading to k^II ± D k^II (10⁸ M^-1 s^-1) = 3.2±1.0 at 288 K, 1.7±0.5 at 282 K and 1.3±0.4 at 276 K, and for 1.0M NaCl, leading to 3.2±1.0 at 288 K, 1.3±0.4 at 282 K and 1.2±0.4 at 276 K, where the error limits are estimated from the output of the model calculations, taking the variability of individual runs at various DMS levels into account.