Growth kinetics of bubbles electrogenerated at microelectrodes

Abstract

The growth kinetics of electrogenerated hydrogen, oxygen and chlorine gas bubbles formed at microelectrodes, were determined photographically and fitted by regression analysis to the equation;r(t)=βt ^x, wherer(t) is the bubble radius at timet after nucleation,β the ‘growth coefficient”, andx the ‘time coefficient’. The coefficientx was found to decrease from a short time (< 10 ms) value near unity, typical of inertia controlled growth, through 0.5, characteristic of diffusional control, to 0.3, expected for Faradaic growth, at long times (\s> 100 ms). The current efficiency for bubble growth increased with bubble lifetime, reflecting the decrease in local dissolved gas supersaturation. The pH dependency of the bubble departure diameter indicated that, in surfactant-free electrolytes, double layer interaction forces between the negatively charged hydrogen evolving cathode or positively charged oxygen/chlorine evolving anode and positively (pH \s< 2) or negatively (pH \s> 3) charged bubbles, were the determining factor. The effect of addition of an increasing concentration of cationic (DoTAB) or anionic (SDoS) surfactant was to progressively reduce the pH effect on departure diameter, due to surfactant adsorption on the bubble and, to a lesser extent, on the electrode.