Electrochemistry of TEMPO: an assessment of the water diffusion constant in the aqueous liquid/vapor interfacial region

Abstract

TEMPO, 2,2,6,6-tetramethylpiperidnyl-1-oxy, is a weak surfactant exhibiting a reversible redox activity: a one-electron oxidation to its oxonium cation. In the course of our earlier work (ref. Wu et al. in J Am Chem Soc 127:4490–4496, 2005; Glandut et al. in J Phys Chem B 110:6101–6109, 2006; Glandut et al. in Langmuir 22:10697–10704, 2006), we developed a full understanding of TEMPO’s electrochemistry at line microband electrodes. In these experiments TEMPO diffuses to the line electrode residing in the plane of the air/water interface in two coupled media, bulk aqueous phase with D = 7.7 × 10−6 cm2/s and along the 2D air/water interface with at least an order of magnitude greater surface diffusion constant, Dsurf. The magnitude of the TEMPO oxidation current depends jointly on Dsurf and on the rate of surface partitioning expressed by the desorption rate constant, kdes. The population of TEMPO partitioned to the air/water interface is largely unsolvated and couples to the aqueous solution by hydrogen bonding to predominately one water molecule. Our experimental methodology allows us to simultaneously determine Dsurf and kdes by recording TEMPO voltammetric curves with line microband and barrier microband electrodes. In this report, we present a new methodology of producing and characterizing barrier microband electrodes using vapor-deposited SiO and introduce additional measures such as aspiration of the air/water interface designed to substantially reduce if not eliminate negative error due to surface impurities. These investigations generated a more accurate value of Dsurf of 1.0 ± 0.3 × 10−4 cm2/s which we discuss in terms of the dynamic properties of water in the air/water interfacial region.