Christopher T. Konek

Interaction of Nitrate, Barium, Strontium and Cadmium Ions with Fused Quartz/Water Interfaces Studied by Second Harmonic Generation

Inorganic anions and cations are ubiquitous in environmental chemistry. Here, we use second harmonic generation to track the interaction of the environmentally important metal cations barium, strontium, and cadmium and the nitrate anion with fused quartz/water interfaces at pH 7. Using a dynamic flow system, we assess the extent of reversibility in the binding process and report the absolute number density of adsorbed cations, their charge densities, and their free energies of adsorption. We also present resonantly enhanced second harmonic generation experiments that show that nitrate is surface active and report the free energies and binding constants for the adsorption process. The second harmonic generation spectrum of surface-bound nitrate shows a new adsorption band that cuts further into the solar spectrum than nitrate in the aqueous or solid state. The results that we obtain for all four inorganic ions and the implications for tropospheric and aquatic chemistry as well as geochemistry are discussed in the context of fundamental science as well as pollutant transport models.

Jammed Acid−Base Reactions at Interfaces

Using nonlinear optics, we show that acid-base chemistry at aqueous/solid interfaces tracks bulk pH changes at low salt concentrations. In the presence of 10 to 100 mM salt concentrations, however, the interfacial acid-base chemistry remains jammed for hours, until it finally occurs within minutes at a rate that follows the kinetic salt effect. For various alkali halide salts, the delay times increase with increasing anion polarizability and extent of cation hydration and lead to massive hysteresis in interfacial acid-base titrations. The resulting implications for pH cycling in these systems are that interfacial systems can spatially and temporally lag bulk acid-base chemistry when the Debye length approaches 1 nm.

Chapter 4 Tracking the Interaction of Transition Metal Ions with Environmental Interfaces using Second Harmonic Generation

Abstract Surfaces and interfaces are ubiquitous in the environment. Nonlinear optical (NLO) techniques, such as second harmonic and sum frequency generation (SHG and SFG, respectively), allow for highly sensitive surface measurements under environmentally relevant temperature and solute concentration conditions. In this work, we present three experimental studies that demonstrate how NLO methods can be used to investigate the interaction of manganese and chromate ions with environmental interfaces. The environmental implications of this work are discussed in the context of understanding heterogeneous processes in geomedia.