Partner und Internationale Organisationen
(Englisch)
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Chemistry Department, Universityof Liverpool, (UK),University of Limerik, The National Technological Park 7 (IRL), Departamento de Quimica, Faculdade de Ciencias do Porto (P), Departamento de Termodinamica (E), University College, Department of Chemistry, London (UK), Laboratoire d'électrochimie, EPFL (CH)
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Abstract
(Englisch)
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The overall objective of the European network ODRELLI was to investigate the fundamental correlation between molecular organisation and reactivity at polarised liquid|liquid interfaces. Throughout the project, we have demonstrated that these boundary systems provide an ideal framework to model life sustaining processes such as ion transport across biological membranes and photosynthesis. Furthermore, this fundamental knowledge allows envisaging and developing new applications in the areas of amperometric ion detectors, ion selective electrodes, pharmacokinetics, photocatalysis and solar energy conversion. The main contributions of the electrochemistry group of the EPFL included:
1.- Understanding of the interactions leading to specific adsorption of molecules at liquid|liquid junctions,
2.- Dynamic studies of electron and ion transfer reactions,
3.- Characterisation of the lipophilicity of ionisable drugs,
4.- Modelling artificial photosynthetic processes
5.- Development of new amperometric detectors for ion chromatography
6.- New approaches to two-phase photocatalytic processes and photoelectrochemical solar cells.
One of the key advantages of electrochemical measurements at polarisable interfaces between two immiscible electrolyte solutions (ITIES) is the fine control over the Galvani potential difference. This potential difference determined the partition of ionic species between the two electrolyte solutions. The transfer of charge species across the interface manifests itself as a current response under potentiostatic conditions. Consequently, the Gibbs energy of ion transfer, which is directly related to the partition coefficient, can be conveniently obtained from cyclic voltammetry. This basic information allows designing ionic partition diagrams, in which the lipophilicity of ionisable species can be represented by potential - pH relationships. This representation is rather valuable for modelling the transport of drugs across biological membranes.
The Gibbs energy of transfer is determined by the difference in the chemical potential of ions between the two liquid phases. Alkali metal ions are rather hydrophilic; therefore their Gibbs energy of transfer from water to an organic electrolyte is rather positive. Interfacial complexation by suitable ligands allows decreasing the Gibbs energy of transfer for a specific ion. Our group has intensively studied assisted ion transfer process, exploiting these phenomena in selective detection of hydrophilic ions. Electrochemical and spectroscopic techniques have been employed for accessing the mechanism of complexation, stoichiometry of the complex and the dynamics of charge transfer process. Novel amperometric detectors for ion chromatography, featuring excellent selectivity for alkali metal ions, have been developed from these studies.
Charge transfer kinetics across ITIES as a function of the Galvani potential difference provide not only information on the reactivity but also on the structure of the liquid|liquid boundary system. Dynamic electrochemical measurements such as chronoamperometry and ac-impedance provide a rather sensitive mean to study the rate of charge transfer process. However, parallel process such as double layer charging or transfer of supporting electrolyte can obscure the electrochemical response of the ion of interest. During this project we have developed dynamic spectroscopic techniques that allows direct monitoring interfacial changes in the concentration of chromophores at ITIES. Phenomenological transfer rate constant were calculated from chrono-absorptometry as well as potential modulated reflectance spectroscopy for a variety of processes including ion transfer, assisted ion transfer and heterogeneous electron transfer. These results provide valuable insights into the potential dependence of the charge transfer rate constant, interfacial distribution of ionic species and mechanistic aspects in connection to ion transfer processes.
The inhomogeneous solvation properties at ITIES are also responsible for the specific adsorption of ionic species exhibiting groups with different hydrophilicity. Classical examples include surfactant molecules which can self-assemble at the liquid|liquid boundary with the hydrophobic groups pointing towards the organic phase while the polar heads face towards the aqueous phase. In this project, we have studied in detail the adsorption properties of non-surfactant ionic species, which coverage can be affected by the Galvani potential difference. Two approaches were employed in order to access the spectroscopic signature of the molecules adsorbed at the interface, surface second harmonic generation and potential modulated fluorescence spectroscopy. The former is a non-linear optical technique that allows estimating the second order susceptibility of the interfacial region. This parameter can be related to the hyperpolarisability of the adsorbed species, revealing information such as surface coverage and molecular orientation. Potential modulated fluorescence spectroscopy relies on periodic changes in the coverage of adsorbed species induced by the ac-potential. This approach not only provides insights into the potential dependence of the coverage, but also describes the dynamics of the adsorption processes. The combination of these two techniques has proved rather successful in the characterisation of adsorption process at the molecular level and opens up unique possibilities for studying local potential distributions as well as the structure and properties of water in contact with low dielectric media.
The interest in the fundamental aspects of specific adsorption at ITIES is also connected to the fascinating photoreactivity exhibited by surface-active porphyrin species. These species are able to undergo heterogeneous redox process with species in the organic phase upon photoexcitation. The photoinduced electron transfer manifests itself as photocurrent responses, which can be employed to characterise the reactivity of the system and as a mean to convert light energy into electricity. Our group has pioneered this field by describing the dynamic aspects of the photoelectrochemical responses, the properties of the photoactive species at the molecular level, characteristic output power of liquid|liquid photovoltaic junction and the efficiency of model photosynthetic reactions. The implications of these studies are potentially important for new developments in molecular solar cells and two-phase photocatalysis.
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