INTERFACIAL PROPERTIES OF IONIC LIQUIDS:ELECTRIC PROPERTIES OF THIN FILMS AND INTERACTION WITH MODEL MEMBRANES AND LIVING CELLS
Tesi di Dottorato
Data di Pubblicazione:
2014
Citazione:
INTERFACIAL PROPERTIES OF IONIC LIQUIDS:ELECTRIC PROPERTIES OF THIN FILMS AND INTERACTION WITH MODEL MEMBRANES AND LIVING CELLS / M. Galluzzi ; tutor: A. Podestà ; coordinatore: M. Bersanelli. DIPARTIMENTO DI FISICA, 2014 Jan 24. 26. ciclo, Anno Accademico 2013. [10.13130/galluzzi-massimiliano_phd2014-01-24].
Abstract:
Room-Temperature Ionic Liquids (ILs) have attracted considerable interest in recent years. This interest is motivated by the physico-chemical properties of these systems, tunable modifying the chemical structure of ions. Generally, ILs show chemical and thermal stability, i.e. they do not easily decompose or react. Furthermore, these compounds remain liquid over an extended range of temperatures, in which they show also a remarkably low volatility. The low vapor pressure of ILs, promote them as good solvents for the growing field of the ”Green Chemistry”, in substitution of the volatile organic compounds. The fact that ILs are composed solely by ions, and can have a quite wide electrochemical window, make them very interesting as electrolytes. For these purposes, this PhD thesis is devoted to the investigation of ILs in contact with solid interfaces, primary targets of interaction. To deepen the analysis of electric properties at the solid interface, thin layers of ILs deposited on conductive substrates were investigated by means of AFM. The ”Green” character of these compounds was investigated studying their interaction with biomembrane models and external membranes of living cells, by means of AFM and electrochemical methods. Because of their ionic nature, ILs can be used as electrolytes in several devices aimed at conversion and storage of energy, such as electrochemical supercapacitors, Graetzel solar cells and batteries. In these devices a key role is played by the interface between the surface of the electrodes and the electrolyte; in particular, structural-morphological and electrical properties of the first few nanometers of IL interacting with the solid electrode surface are expected to have the strongest impact on device performance. AFM morphological analysis of small quantity of [C 4 MIM] [NTf 2 ] IL, deposited on various insulating surfaces revealed a population of nanodroplets and new structures. Remarkably, the solid surfaces induce the organization of the ionic liquid into regular, lamellar solid-like nanostructures presenting a high degree of vertical structural order and high mechanical resistance to normal compressive stresses. Nanomechanical investigation reveals that the structures resist to normal compressive loads up to 1.5 MPa; beyond that limit, indentation, in discrete steps, occurs. Furthermore, lamellar [C 4 MIM] [NTf 2 ] islands are not affected when scanned by a biased AFM tip under the influence of an electric field as intense as 10 8 V/m, while the liquid nano-and micro-droplets are easily swept away. These results confirm the solid-like character of the ordered lamellar nanostructures observed when thin films of [C 4 MIM] [NTf 2 ] are deposited on solid surfaces, and suggest that these films may possess an insulating, dielectric behavior, at odd with the case of the bulk ionic liquids. Nanoscale impedance measurements (capacitance vs. distance) and electrostatic force spectroscopy (electric force vs. distance) between a conductive AFM tip and the IL structures confirmed that values of the dielectric constant (ε r = 3-5) are significantly smaller than those measured in the bulk liquid (ε r = 9-15). These results support the picture of solid-like ordered domains where the ion mobility is significantly inhibited with respect to the bulk liquid phase. These findings also highlight the potentialities of scanning probe techniques for the quantitative investigation of the interfacial electrical properties of thin ionic liquid films, suggesting that ILs at electrified solid surfaces may possess unexpected electrical and structural properties, thus influencing the behavior of photo-electrochemical devices. The ”green” character of ionic liquids (ILs) is dependent on their negligible vapor pres
Tipologia IRIS:
Tesi di dottorato
Keywords:
ionic liquids ; intefacial properties ; DOPC ; nanomechanic ; atomic force microscopy
Elenco autori:
M. Galluzzi
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