Great Enhancement in the Oxidation Ability of Dilute Nitric Acid in Nanoscale Water-Droplets of Reverse Micelle Systems

Abstract

Abstract In reverse micelle systems, a large enhancement of the oxidation ability of dilute nitric acid was discovered, and its oxidation mechanism was explored. The Br− ion in the surfactant, CTAB, was oxidized to Br2 (or Br3−) in the CHCl3/CTAB/H2O reverse micelle system with W = 1.0–4.0 by diluted nitric acid (0.25–2.5 mol dm−3 in 1.0 vol % H2O portion) at 15–40 °C where CTAB stands for cetyltrimethylammonium bromide, and the W value is the ratio of [H2O]/[surfactant]. At higher concentrations of nitric acid and temperatures, faster reactions occurred. Otherwise, long reaction times were needed, e.g., 10 h for 1.0 mol dm−3 HNO3 at 25 °C. Light or ambient oxygen did not appear to affect the reaction. The ratio of produced Br2 or (Br3−) to the initial amount of HNO3 indicated the following reaction scheme: 2HNO3 + 2Br− → Br2 + NO2− + NO3− + H2O. The nitroyl ion (or nitronium ion), NO2+, was proposed as the intermediate active species. The addition of HClO4 as a proton source caused the complete reduction of N(V) as follows: NO3− + 6H+ + 5e− → 1/2N2 + 3H2O. The hydrogen-bonding structure of H2O in the CTAB or CTAC (cetyltrimethylammonium chloride) micelle system was found to be distorted, compared to that in bulk water, by the 1H NMR chemical shift of H2O. The change in 1H NMR chemical shift also demonstrated the consumption of protons during the oxidation of Br−, but not of Cl−, by dilute HNO3.