Anomalous solubility of oxygen in acetonitrile/water mixture containing tetra-n-butylammonium perchlorate supporting electrolyte; the solubility and diffusion coefficient of oxygen in anhydrous acetonitrile and aqueous mixtures

We report the oxygen electro-reduction in air-saturated acetonitrile (MeCN) solution supported by 0.1M tetra-n-butylammonium perchlorate salt under two experimental conditions: (1) anhydrous MeCN at elevated temperatures from 298.5 to 313.0K on platinum electrodes; (2) water-mixed MeCN solution (mole fraction of MeCN: 0.72<χMₑCN<1) at 298.5K on glassy carbon electrodes. The measurements of diffusion coefficients and concentrations were determined via single and double potential step chronoamperometry at microdisc electrodes. The experimental results are analysed by Shoup and Szabo equation, or where necessary via simulation. Under the first experimental condition, the diffusion coefficients of oxygen, D(O₂), and superoxide radical anion, D(O₂ ⁻), at 298.5K are determined to be (9.20±0.36)×10⁻⁹m²s⁻¹ and (2.73±0.35)×10⁻⁹m²s⁻¹ respectively. Over the studied temperature range, the diffusional activation energies of oxygen (Eₐ ,O₂) and superoxide (Eₐ ,O₂₋) are reported for the first time to be (2.07±0.05)kJmol⁻¹ and (8.01±2.24)kJmol⁻¹. The significant small value of Eₐ ,O₂ suggests the diffusional behaviour of molecular oxygen cannot be described by the Stokes–Einstein relationship; whereas that of the radical anion can be. This further confirms that the importance of the size of diffusional species must be at least comparable in size to that of the solvent molecules in order for this macroscopic theory to apply. Further, the concentration of dissolved oxygen, c(O₂), is obtained as (1.26±0.05)mM, and the oxygen dissolution is quantitatively shown to be an endothermic process. The standard Gibbs energy of solvation (ΔGSₒₗᵥ ᵉ) and the standard entropy change of solvation (ΔGSₒₗᵥ ᵉ) are experimentally calculated to be +19.9 (error: +0.12/−0.11)kJmol⁻¹ and −65.5 (error: +15.25/−12.62)Jmol⁻¹K⁻¹ respectively at 298.5K. Although the ΔGSₒₗᵥ ᵉ for oxygen in water is much more positive than that in MeCN solution, an anomalous increase in oxygen solubility after the initial water-mixing with MeCN (0.94<χMₑCN<1), under experimental condition (2), was observed. Over the same range of χMₑCN, the D(O₂) is found to drop in significant amount compared with its predicted value. Such behaviour can be tentatively explained by the preferential solvation of molecular oxygen by water molecules via hydrogen bonding at lower water content. Consequently, the thermodynamic terms of mixing may be altered, and therefore favour the dissolution of oxygen.