A Kinetic Model for Adosorption and Transfer of Ionic Species at Polarized Liquid|Liquid Interfaces as Studied by Potential Modulated Fluorescence Spectroscopy

Fundamental expressions for analyzing potential modulated fluorescence (PMF) responses were derived within the framework of a phenomenological model for adsorption and transfer of ionic species across polarized liquid|liquid interfaces. For small periodic perturbations of the Galvani potential difference, PMF signals can be linearized and the contribution of each process can be uncoupled in the frequency domain. The PMF response for kinetically controlled adsorption is expressed as a semicircle in the complex plane in which the characteristic frequency of maximum imaginary component is proportional to the adsorption and desorption rate constants. Considering that the potential dependence of adsorption exhibits opposite sign whether the process take place from the aqueous or organic phase, the corresponding PMF responses appear in different quadrants of the complex plane. The present model delivers useful diagnostic criteria for analyzing the nature of the various processes contributing to the periodic fluorescence response. The adsorption dynamics of meso-tetrakis(N-methyl-4-pyridyl)porphyrinato zinc(II) at the water|1,2-dichloroethane interface were evaluated from the frequency dependent PMF responses. Studies performed at various Galvani potential differences clearly confirm that the adsorption can take place at two distinctive planes located at the aqueous and organic sides of the interface. Basic aspects in connection to the nature of the adsorption planes are briefly discussed.