The Nature of the Sodium Dodecylsulfate Micellar Pseudophase as Studied by Reaction Kinetics

The nature of the rate-retarding effects of anionic micelles of sodium dodecyl sulfate (SDS) on the water-catalyzed hydrolysis of a series of substituted 1-benzoyl-1,2,4-triazoles (<b>1a</b>–<b>f</b>) has been studied. We show that medium effects in the micellar Stern region of SDS can be reproduced by simple aqueous model solutions containing small-molecule mimics for the surfactant headgroups and tails, namely sodium methyl sulfate (NMS) and 1-propanol, in line with our previous kinetic studies for cationic surfactants (Buurma et al. J. Org. Chem. 2004, 69, 3899−3906). We have improved our mathematical description leading to the model solution, which has made the identification of appropriate model solutions more efficient. For the Stern region of SDS, the model solution consists of a mixture of 35.3 mol dm^–3 H₂O, corresponding to an effective water concentration of 37.0 mol dm^–3, 3.5 mol dm^–3 sodium methylsulfate (NMS) mimicking the SDS headgroups, and 1.8 mol dm^–3 1-propanol mimicking the backfolding hydrophobic tails. This model solution quantitatively reproduces the rate-retarding effects of SDS micelles found for the hydrolytic probes <b>1a</b>–<b>f</b>. In addition, the model solution accurately predicts the micropolarity of the micellar Stern region as reported by the E_T(30) solvatochromic probe. The model solution also allows the separation of the individual contributions of local water concentration (water activity), polarity and hydrophobic interactions, ionic strength and ionic interactions, and local charge to the observed local medium effects. For all of our hydrolytic probes, the dominant rate-retarding effect is caused by interactions with the surfactant headgroups, whereas the local polarity as reported by the solvatochromic E_T(30) probe and the Hammett ρ value for hydrolysis of <b>1a</b>–<b>f</b> in the Stern region of SDS micelles is mainly the result of interactions with the hydrophobic surfactant tails. Our results indicate that both a mimic for the surfactant tails (NMS) and a mimic for the surfactant headgroups (1-propanol) are required in a model solution for the micellar pseudophase to reproduce all medium effects experienced by a variety of different probes

Branching Ratios in Reactions of OH Radicals with Methylamine, Dimethylamine, and Ethylamine

The branching ratios for the reaction of the OH radical with the primary and secondary alkylamines: methylamine (MA), dimethylamine (DMA), and ethylamine (EA), have been determined using the technique of pulsed laser photolysis–laser-induced fluorescence. Titration of the carbon-centered radical, formed following the initial OH abstraction, with oxygen to give HO₂ and an imine, followed by conversion of HO₂ to OH by reaction with NO, resulted in biexponential OH decay traces on a millisecond time scale. Analysis of the biexponential curves gave the HO₂ yield, which equaled the branching ratio for abstraction at αC–H position, <i>r</i>_αC–H. The technique was validated by reproducing known branching ratios for OH abstraction for methanol and ethanol. For the amines studied in this work (all at 298 K): <i>r</i>_αC–H,MA = 0.76 ± 0.08, <i>r</i>_αC–H,DMA = 0.59 ± 0.07, and <i>r</i>_αC–H,EA = 0.49 ± 0.06 where the errors are a combination in quadrature of statistical errors at the 2σ level and an estimated 10% systematic error. The branching ratios <i>r</i>_αC–H for OH reacting with (CH₃)₂NH and CH₃CH₂NH₂ are in agreement with those obtained for the OD reaction with (CH₃)₂ND (<i>d</i>-DMA) and CH₃CH₂ND₂ (<i>d</i>-EA): <i>r</i>_{αC–H,d‑DMA} = 0.71 ± 0.12 and <i>r</i>_{αC–H,d‑EA} = 0.54 ± 0.07. A master equation analysis (using the MESMER package) based on potential energy surfaces from G4 theory was used to demonstrate that the experimental determinations are unaffected by formation of stabilized peroxy radicals and to estimate atmospheric pressure yields. The branching ratio for imine formation through the reaction of O₂ with α carbon-centered radicals at 1 atm of N₂ are estimated as <i>r</i>_CH2NH2 = 0.79 ± 0.15, <i>r</i>_CH2NHCH3 = 0.72 ± 0.19, and <i>r</i>_CH3CHNH2 = 0.50 ± 0.18. The implications of this work on the potential formation of nitrosamines and nitramines are briefly discussed

Unimolecular Kinetics of Stabilized CH₃CHOO Criegee Intermediates: <i>syn</i>-CH₃CHOO Decomposition and <i>anti</i>-CH₃CHOO Isomerization

The kinetics of the unimolecular decomposition of the stabilized Criegee intermediate syn-CH3CHOO has been investigated at temperatures between 297 and 331 K and pressures between 12 and 300 Torr using laser flash photolysis of CH3CHI2/O2/N2 gas mixtures coupled with time-resolved broadband UV absorption spectroscopy. Fits to experimental results using the Master Equation Solver for Multi-Energy well Reactions (MESMER) indicate that the barrier height to decomposition is 67.2 ± 1.3 kJ mol–1 and that there is a strong tunneling component to the decomposition reaction under atmospheric conditions. At 298 K and 760 Torr, MESMER simulations indicate a rate coefficient of 150–81+176 s–1 when tunneling effects are included but only 5–2+3 s–1 when tunneling is not considered in the model. MESMER simulations were also performed for the unimolecular isomerization of the stabilized Criegee intermediate anti-CH3CHOO to methyldioxirane, indicating a rate coefficient of 54–21+34 s–1 at 298 K and 760 Torr, which is not impacted by tunneling effects. Expressions to describe the unimolecular kinetics of syn- and anti-CH3CHOO are provided for use in atmospheric models, and atmospheric implications are discussed