Electrocapillarity and quartz crystal microbalance studies of the interface between two immiscible electrolyte solutions
Dragan, Simona A.
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In this thesis, the use of the quartz crystal microbalance (QCMB) as a technique for the study of the polarized interface between two immiscible electrolyte solutions is presented. The possibility of using the information obtained from QCMB measurements as a starting point in developing a method for determining the interfacial tension of some liquid/liquid systems is explored. The interface between aqueous solutions of lithium chloride and nitrobenzene solutions of tetrabutylammonium tetraphenylborate (TBATPB) was studied. This interface behaves as an ideally- polarized interface in a certain potential range. Three different types of systems were investigated: (1) equal concentration of LiCl and TBATPB in the aqueous and nitrobenzene phases, respectively; (2) constant concentration of LiCl (0.1 mol/L) and variable concentration of TBATPB in the nitrobenzene phase; and, (3) variable concentration of LiCl in the aqueous phase and constant concentration of TBATPB (0.1 mol/L). A quartz crystal was immersed across the water/nitrobenzene interface and changes in its resonant frequency were recorded while the interface was polarized. Simultaneously, cyclic voltammograms were recorded. The oscillating frequency of the crystal with the applied potential follows a parabolic-like profile. It is assumed that the changes in the oscillating frequency of the crystal are due to: (1) formation of precipitate caused by the transfer of electrolyte ions from the phase where they are soluble into the other phase; and (2) change of the shape of the meniscus that the two liquids form with the crystal surface dining the voltammetric sweep. For all systems investigated, the interfacial capacitance at different polarizations of the interface was determined from impedance spectroscopy measurements. Electrocapillary curves were obtained by double integration of the capacitance-potential curves with respect to potential. The potential of zero charge shifts toward more positive values with the concentration of TBATPB in systems (1) and (2). This shift suggests that the tetraphenylborate anion specifically adsorbs at the water/nitrobenzene interface. A quantitative measure of the specific adsorption occurring at the interface is given by the values of the Esin-Markov coefficients. The correlation of the QCMB measurements with electrocapillary and interfacial capacitance vs. applied potential curves is attempted. Additionally, a support study which investigates if the quartz crystal immersed across the water/nitrobenzene polarized interface can be used without insulating material on its surface is presented. It was found that in the potential window in which only charging current corresponding to the double layer region is generated, the crystal can be used without insulating material on its surface.