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Chemistry of Indoor-Relevant Carboxylic Acids on Titanium Dioxide Surfaces


Compounds identified in indoor air are emitted from various sources like cooking, cleaning products, and human metabolism. Since a majority of homes experience dampness, these gaseous compounds can partition into the aqueous phase. Chemical interactions between these compounds and surfaces play an important role in altering the composition of indoor air, yet there is still a lack of knowledge on the mechanisms behind indoor surface chemistry. TiO2 is a major component of paints and self-cleaning surfaces and, therefore, of interest in indoor air chemistry. The adsorption of two indoor-relevant carboxylic acids, lactic acid and pyruvic acid, onto TiO2 particle surfaces is investigated in the aqueous phase at acidic and indoor-relevant pH values. Specifically, by using attenuated total reflection-FTIR spectroscopy coupled with quartz crystal microbalance with dissipation, this thesis focuses on the surface chemistry of lactic and pyruvic acid on TiO2, including adsorption kinetics, surface coverage, and speciation as a function of pH. The results conclude that lactic and pyruvic acid deprotonate upon adsorption and bind to TiO2 via bidentate bridging. Adsorption and desorption rate constants do not differ significantly when pH changes. Surface coverage is dependent on solution pH and decreases with increasing pH. Solution and surface speciation for lactic acid remain constant as a function of pH, but solution and surface speciation differ for pyruvic acid at higher pH values, suggesting that surface adsorption induces a change in species equilibrium. An overall molecular picture emerges from these studies of the adsorption of these important organic acids on TiO2 surfaces.

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