Monitoring of ion diffusion in Langmuir-Blodgett multilayers by a variable observation angle fluorescence method

Abstract

[9406][9407][9408][9409][9410] adsorption the contribution of double-layer charging may be removed via background subtraction where the background is estimated from the same voltammetric response prior to the onset of the faradaic current. This eliminates the need for additional experiments to determine the contribution of double-layer capacitance.Furthermore, the potential of semiintegral analysis to determine surface concentration in the presence of fast surface phenomena suggests that its use may be extended to other areas of electrochemical interest. The thin-layer model discussed herein is one such example which can be extended to the study of thin films where the concentration of redox species within the film is affected by partitioning into or out of the film. Of interest would be the use of semiintegral analysis for the identification and quantitation of strong adsorption involving more complex isotherms which take into consideration interactions between adsorbed species, and the use of semiintegration analysis for kinetic and mechanistic studies in the presence of weak adsorption and thin films. Acknowledgment.The spatial monitoring of ion diffusion into an ultrathin film sample was demonstrated by our depth profiling method using variable observation angle fluorescence spectroscopy under normal incidence conditions. A 143-nm-thick LB multilayer model sample was prepared in which the fluorophore rhodamine B was homogeneously distributed in each layer. The model sample was then immersed into an aqueous solution of sodium tungstate, where the tungstate ion diffused into it and r e a d with rhodamine B to form a nofluorescent complex. Because of this chemical quenching effect, the homogeneous concentration distribution of rhodamine B was changed by the tungstate ion diffusion. Changes in concentration depth profiles of rhodamine B with diffusion time were measured by the variable observation angle fluorescence depth profiling method based on the reciprocal theory, which has a spatial resolution of the monolayer thickness level in the depth direction. As a result, the diffusion behavior of tungstate ion was monitored, and its diffusion coefficient in LB multilayers was obtained as (1.2 -1.7)x 10-l' cm2/s.

Solvent-Independent Electrode Potentials of Solids Undergoing Insertion Electrochemical Reactions: Part II. Experimental Data for Alkynyl–diphosphine Dinuclear Au(I) Complexes Undergoing Electron Exchange Coupled to Anion Exchange

“…Synthesis and characterization of L1, L2 complexes was carried out as previously described. 27 Electrochemical measurements were performed at 298 K using L-modified paraffinimpregnated graphite electrodes and gold and platinum electrodes. Nanopure water and MeOH, EtOH, MeCN, DMF, NM, and DMSO (Carlo Erba) were used as solvents.…”

Solvent-Independent Electrode Potentials of Solids Undergoing Insertion Electrochemical Reactions: Part III. Experimental Data for Prussian Blue Undergoing Electron Exchange Coupled to Cation Exchange

“…It is subsequently assumed that semiinfinite boundary conditions apply so that the diffusion problem can be treated as the diffusion of K + ions in one direction through two (solution and solid) phases. The proposed model incorporates the consideration of both the exchange reaction and the binding equilibrium in the solid represented, respectively, in terms of the models from Bard et al and Wu et al for redox polymers. Then, solving the diffusion problem leads to the following expression for the CA current obtained upon application of a potential sufficiently negative to ensure diffusion control: where S is the surface area of the electrode, c K solution + is the concentration of K + ions in the electrolyte, D K solid + and D K solution + , the diffusion coefficients of K + ion through the solid and in the solution, respectively, and δ is the thickness of the layer.…”

Solvent-Independent Electrode Potentials of Solids Undergoing Insertion Electrochemical Reactions: Part I. Theory

“…Additionally, it will be assumed that a binding reaction between the immobile redox centers and the cations, equivalent to the process represented by eq , takes place in the solid, as proposed by Wu et al for the protonation processes in redox polymers. Now eq must be replaced by: Where c LM ( z , t ) represents the concentration of the cation–redox center adduct, LM.…”

“…The model only requires Nernstian electrochemical behavior and solving the diffusion problem in terms of Fick’s laws. This formulation derives from those for the electrochemistry of ion-insertion solids − and redox polymers − with account of partition and binding equilibria (for details concerning the thermodynamics of ion partition equilibrium between two phases, see , ). Application of this method to define solvent-independent potentials involves two extra-thermodynamic assumptions: (i) there is no accumulation of net charge in the solid complex/electrolyte boundary and (ii) the structures of the solid and the ion binding to the solid are not affected by the solvent.…”