Electrochemical studies of the lateral diffusion of TEMPO in the aqueous liquid/vapor interfacial region.

Surface partitioning and lateral mobility of TEMPO (2,2,6,6-tetramethyl-1-piperidynyloxy free radical) in the aqueous liquid/gas interfacial region were investigated electrochemically with 100 nm wide, 1.0 cm long microband electrodes positioned at the air/water interface. For redox active amphiphiles such as TEMPO, the electrochemical current is the sum of the surface and solution components representing the diffusive transport of TEMPO in both domains as well as the dynamics of equilibration at the air/water interface. Interpretation of the recorded current-voltage curves was aided by a FEMLAB simulation code developed to analyze transport processes in this class of systems. TEMPO and TEMPO(+) partition constants (K(T), K(T)+) and solution diffusivities (D(sol), equal for the two species) were obtained experimentally yielding K(T) = 5.0 +/- 0.7 x 10(2) M(-1), K(T+) = 41 +/- 3 M(-1), and D(sol) = 7.7 +/- 0.35 x 10(-6) cm(2)/s. In view of the low value of K(T)+, TEMPO(+) was assumed not to partition to the air/water interface. We further assumed that the desorption rate constants (k(des)) of both TEMPO and TEMPO(+) were the same. Good fits between the recorded and simulated cyclic voltammograms were obtained using two correlated, adjustable parameters, k(des) and the TEMPO lateral, surface diffusion constant (D(surf)). Detailed analysis of the behavior of this class of systems was obtained for a broad range of D(surf) and k(des) values. In addition, a calibration curve of k(des) versus D(surf) was obtained. Assuming that TEMPO k(des) is in a likely range of 10-100 s(-1), its lateral diffusion constant is in the range of 7.9-3.6 x 10(-5) cm(2)/s. In view of our earlier work (Wu, D. G.; Malec, A. D.; Head-Gordon, M.; Majda, M. J. Am. Chem. Soc. 2005, 127, 4490-4496) showing that at the air/water interface TEMPO is unimmersed, and that its interactions with water are limited to hydrogen bonding with one or two water molecules, D(surf) can be related to the viscosity of the aqueous interfacial region.