Voltammetric determination of concentrations of ferrocene-included nitrobenzene droplets in water

When a nitrobenzene (NB) phase came in quiescent contact with a water phase, water-droplets were formed spontaneously near the oil|water interfaces (Electrochem. Commn. 11 (2009) 239). We reported here quantitative data of the NB-droplets by use of UV-absorbance, dynamic light scattering (DLS) and voltammetry with a help of ferrocene. The supernatant separated spontaneously from the oil-water mixture contained 15.5 mmol dm evaluated by UV spectra, whereas the centrifuged supernatant did 9.3 mM. The difference suggested the presence of NB-droplets, the diameter of which ranged 0.15-0.5 μm obtained by DLS. Ferrocene was dissolved deliberately in the aqueous solution and the NB solution up to saturation. The voltammograms in the ultrasonicated supernatant exhibited diffusion-controlled redox peaks of ferrocene, which should be supplied from both dissolved ferrocene and ferrocene-dissolved NB droplets. The former was 1/3.6 times of the latter by the comparison with the current of ferrocene-saturated aqueous solution. Applying the expression for the diffusion-controlled peak current of a big particle, we estimated the number concentration of NB droplet to be 1.1×10 dm. This value is equivalent to the average distance, L = 2.1 μm, between neighboring two droplets, corresponding to the diffusional traveling time, L/D = 1.3 s, which would be long enough for collision of droplets to stabilize the emulsions. key words: spontaneous emulsification; oil|water interface; size of droplets; voltammetry of big particles * Corresponding author, e-mail: [email protected]