Product analysis of reactions of a model system for organics on atmospheric particles: Ozonolysis of Self-Assembled Monolayers on Silicon Surfaces
EMSL Project ID
25605
Abstract
Airborne particles have significant effects on climate, visibility, human health and atmospheric reactions. Organics associated with airborne particles are thought to be oxidized to polar, hygroscopic species with enhanced cloud-nucleating properties. This lab has studied the ozone (O3) oxidation of unsaturated self-assembled monolayers (SAMs) as proxies for organics on airborne dust particles. In a NSF-sponsored project, it has been observed that the gas phase O3 oxidation of the SAMs leads to the formation of large hydrophobic particles/aggregates on the coated surface that do not increase the uptake of water as previously assumed. The molecular identity composing the large aggregates and the mechanisms of their formation has not been resolved. It is proposed that the atmospheric formation of hydrophobic aggregates is generally controlled by the availability of water rather than acid. For instance, sulfuric acid complexes water at lower humidity, and the lack of additional free water may lead to enhanced oligomerization. However, since water is readily available in the atmosphere, it may also quench particle formation. Associated with the findings of these studies, comparison between simulation and experimental results shows a gap between predicted and measured ozone residence times. This is speculated to be associated with the growth in a rough oxide layer formed from multiple SAM deposition/strip cycles on these substrates. Past work with Dr. A. Scott Lea and Dr. Daniel J. Gaspar in experiments performed at PNNL, has shown that these large particles were clearly organic in nature. The Auger electron spectroscopy (AES) experiments carried out by Dr. A. Scott Lea showed that the reactions leave some carbon-containing material on the substrate surrounding the large aggregate particles. This was supported by Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) data collected by Dr. Daniel J. Gaspar that was subjected to principal component analysis which showed that some residual carbon remained on the substrate surrounding particles after oxidation of C8=. As a result, during the ozone reaction, alkyl monolayer chains must be cleaved somewhere along the chain, leaving organic material on the surface surrounding the particles. Clearly, the mechanism of ozonolysis of alkene SAMs is markedly different than for gas or liquid phase reactions.Recently, additional studies on the oxidation product aggregates with Mr. Paul L. Gassman using a FTIR microscope showed 2 different types of particles according to their transmission IR signatures. The purpose of this proposal is to continue the studies on these unusual particles formed from ozonolysis of SAMs but as a function of relative humidity, and for mixtures of SAMS. We are also interested in the effect on the substrate surface when the oxide layer is removed using ammonium fluoride, re-oxidized, and subsequently reacted with SAM precursors.
SAMs on hydroxylated surfaces such as glass, quartz and oxidized silicon surfaces will be prepared, at UCI, with 2 different SAM precursor molecules, and on Si surfaces treated with ammonium fluoride, and these SAMs will be oxidized with ozone in the presence and absence of water. EMSL user equipment, Auger electron spectroscopy (AES)/scanning Auger microprobe and the FTIR microscope, will be used to examine the chemical composition of these particles. Concomitant with measurements by EMSL scientists will be real-time ATR-FTIR analysis of SAMs coated surfaces as a function of O3 concentration, under dry conditions, and as a function of relative humidity conducted at UCI. Surface roughness and topography, before and after O3 reaction, will be evaluated using atomic force microscopy (AFM). The combination of these experimental techniques is critical to assess the potential contribution of the polymer/aggregate formation in the atmosphere.
Project Details
Project type
Large-Scale EMSL Research
Start Date
2007-06-01
End Date
2010-09-30
Status
Closed
Released Data Link
Team
Principal Investigator
Related Publications
T. M. McIntire, O. S. Ryder, P. L. Gassman, Z. Zhu, S. Ghosal, and B. J. Finlayson-Pitts, ?Why Ozonolysis May Not Increase the Hydrophilicity of Particles.? Atmospheric Environment, 44(7), 939-944 (2010). DOI: 10.1016/j.atmosenv.2009.11.009