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UV Second-Harmonic Studies of Concentrated Aqueous Electrolyte Interfaces


The nature of liquid-vapor interfaces is a rapidly developing field of research, aimed at ascertaining the properties and structure of this unique microscopic environment. The mechanism of aqueous electrolyte partitioning and chemistry in the aqueous-vapor interface region is explored herein, using the surface selective technique of second-harmonic generation to probe these systems via strong electronic resonances in the ultraviolet.

In Chapter 1, the current descriptions of the neat water- and electrolyte solution-vapor interfaces are reviewed. Mechanistic explanations for anion adsorption to such interfaces are highlighted as an object of research. Previous applications of the surface second-harmonic technique to aqueous electrolyte systems are also described, and the results of these studies are built upon herein.

In Chapter 2 the principles of applying second harmonic generation as a spectroscopic probe of liquid-vapor interfaces are discussed, followed by a consideration of the adapted Langmuir model used to interpret such studies. This model is then developed to include both cations and anions as a consequence of the requirement of electroneutrality of the solution-vapor interface. The resulting expressions are then identified as diagnostics for mechanisms of ionic adsorption to the solution interface. A discussion of the technical requirements and developments necessary to apply femtosecond ultraviolet second-harmonic spectroscopy to these systems in a reliable and reproducible manner is also presented in Chapter 2.

The models developed in Chapter 2 are used to interpret the interfacial adsorption processes of aqueous sodium nitrite and sodium nitrate solutions in Chapter 3. Nitrite surface activity is found to exhibit second-order bulk concentration dependence which is then interpreted to indicate the adsorption of nitrate and sodium into the interface region as ion-pairs. These ion-pairs are found to adsorb with a standard Gibbs free energy of -37±1 kJ*mol-1. The related sodium nitrate electrolyte is not found to be strongly surface active.

The temperature dependence of the surface activity of aqueous potassium thiocyanate is explored in Chapter 4, and in terms of an adapted Langmuir adsorption model is found to be an exothermic process (-5.9±0.2 kJ*mol-1) with a weakly unfavorable entropic component (-8±1 J*mol-1). The process is also found to exhibit first-order dependence on bulk electrolyte concentration, indicating that the cation and anion translations are not highly correlated. These results are discussed in the framework of current theories of anionic interface adsorption mechanisms and alternative explanations are considered.

The UV photochemical products of aqueous potassium and sodium thiocyanate are also observed to obscure the second-harmonic response of thiocyanate in the interface, and elemental sulfur is proposed to be excluded from the bulk solution into the interface in this system. In Chapter 5 the effect of oxidation on the interface of aqueous sodium iodide solutions is also investigated, determining that much like thiocyanate the second-harmonic signal is obscured by the products that are generated. The potential for exploring rate-law behavior at aqueous electrolyte interfaces by this method is also established. Lastly, the ultraviolet second-harmonic spectrum of sodium iodide at molar and millimolar concentrations is found to be qualitatively different to previously reported spectra. Two postulates are made to explain this variance, one being an experimental configuration difference, the other being due to the oxidation products found in these systems.

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