Influence of chloride ion adsorption on the kinetics and mechanism of Ru(NH3)63+/2+ electrode reactions

Electrochemical observation of individual collision-blocking events of TX-100 nanomicelles: An accurate and universal approach for the critical micelle concentration determination of surfactants

The electrochemical collision-blocking technique, equipped with the nanoelectrode of Pt was proposed for determination of the critical micelle concentration (CMC) of non-ionic surfactant TX-100. The approach was found on detection of individual collided nanomicelles in amperometric measurements of the oxidation of K₄Fe(CN)₆ varying the titrated concentration of TX-100 whereas the formed micelles above the CMC stick on the electrode surface during collision to locally block the flux of electroactive species and further to change the faradaic current. The step-like current transients observed in i-t curves have been demonstrated corresponding to electrochemical collision events of individual TX-100 micelles and micelle aggregates by 3D COMSOL simulations. The logarithm relations between the collision frequency of micelle(s) and the concentration of TX-100 were derived by regression analysis to give the corresponding values of CMC in salt solutions. Further, an 'ideal' CMC of TX-100 without influence of additional salts was estimated to be 0.194 mM using the McDevit-Long theory. The more accurate CMC determined in this work has shown less than the previously reported, mainly due to the detection limit for micelle as low as 0.41 fM. Also, we determined the second CMC of 1.21 mM as the first observation of the collision response of micelle aggregates during TX-100 titration. Owing to its analytical characteristics in single-particle tracking and material insensitivity, the approach we proposed is potentially to be a universal tool for accurate determination of CMC of surfactants, and also for studying the formation of polymer particles at a single-particle level, which is not easily accessible using conventional ensemble measurements.

Electrochemical Investigation of Redox Processes of Labile Cu(II)/Cu(I)-Cl Complexes by Scanning Electrochemical Microscopy

The studies reported here provide detailed information on the kinetics and reaction mechanism determined by the scanning electrochemical microscopy (SECM) for Cu(II)/Cu(I) redox reactions in a system with excess chloride. In positive feedback mode, the tip voltammograms obtained consist of two partially overlapping reduction waves, not one quasi-reversible process as was always thought. The reduced species of Cu^IICl₄^2- and [Cu^IICl₃(H₂O)]^- could coexist during the time scale of SECM measurements, due to the slowness of the ligand substitution reaction of [Cu^IICl₃(H₂O)]^- to Cu^IICl₄^2-. The stepwise EC reactions of Cu^IICl₄^2- have experienced the same electron transfer kinetics as do outer-sphere reactants, while the reactions of [Cu^IICl₃(H₂O)]^- followed concerted EC pathways and much lower rate constants can be obtained, mainly due to the nonadiabatic character of electron transfer (k⁰_Pt/k⁰_Au = 1.625). The findings of this study can offer valuable insights for analyzing the electrode kinetics of labile complexes of metals in aqueous solutions.