Semester

Fall

Date of Graduation

2022

Document Type

Dissertation

Degree Type

PhD

College

Eberly College of Arts and Sciences

Department

Chemistry

Committee Chair

Fabien Goulay

Committee Co-Chair

Blake Mertz

Committee Member

Blake Mertz

Committee Member

Justin Legleiter

Committee Member

Stephen Valentine

Committee Member

Dady Dadyburjor

Abstract

The surface-bulk partitioning of small organic molecules in aqueous droplets was investigated using molecular dynamics. The air-particle interface was modeled using a 80-Å cubic water box containing series of organic molecules and surrounded by gaseous OH radicals. The properties of the organic solutes within the interface and the water-bulk were examined at a molecular-level using density profiles and radial pair distribution functions. Molecules containing only polar functional groups such as urea and glucose are found predominantly in the water bulk, forming an exclusion layer near the water surface. Substitution of a single polar group by an alkyl group in sugars and amides leads to the migration of the molecule toward the interface. Within the first 2 nm from the water surface, surface-active solutes lose their rotational freedom and adopt a preferred orientation with the alkyl group pointing toward the surface. The different packing within the interface leads to different solvation shell structures and enhanced interaction between the organic molecules and absorbed OH radicals. The simulations provide quantitative information about the dimension, composition, and organization of the air-water interface as well as about the non-reactive interaction of the OH radicals with the organic solutes. It reveals that the enhanced reactivity of surface-active molecules is due to increased concentrations, preferred orientation, and decreased solvation near the air-water surface. The results are important to explain how heterogeneous oxidation mechanisms and kinetics within interfaces may differ from those of the bulk. An atmospheric pressure flow-tube reactor coupled with offline GC-MS analysis techniques was used to determine the kinetics of the OH-initiated oxidation of equimolar aqueous organic aerosol. Saccharides (glucose and MGP) and amides (propionamide, urea, and acetamide), were chosen as model molecules because of their partitioning properties, availability in the atmosphere, and the important role they play in atmospheric chemistry. The decay rates of the solutes (saccharides and amides) were determined by measuring the loss in signal of solutes in the particle phase as a function of OH exposure (time-integrated total concentration of OH radical). The reactivity of MGP towards OH radicals was shown to be higher in the presence of urea (a surface in-active molecule) than when in the presence of propionamide (a surface in-active molecule). The decreases in MGP reactivity was shown by the change in rate coefficients from 1.2(±0.1) x 10-11 cm-3 s-1 in the urea-MGP particles to 4.7(±0.3) x 10-12 cm-3 s-1 in the propionamide-MGP particles. These findings highlight the importance of surface interactions over bulk interactions in determining the reaction rate of reactive species in aqueous aerosols. The change in particle size with change in chemical composition was also demonstrated.

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