Correlations between Viscosity and Composition for Aerosol Proxies via Analytical Chemistry and Spectroscopy

  • Ahmad Alluhaybi

Student thesis: Phd

Abstract

Physical properties such as surface tension and viscosity are known to affect the rates of multiphase processes in atmospheric aerosols. New methods of correlating the composition and viscosity of proxy were evaluated. Correlations between IR spectral features and viscosity were studied using solutions of organic acids, saccharides and the products of α-pinene ozonolysis. In aqueous solutions of oxalic, malonic, succinic and glutaric acids, the viscosity increased with chain length. IR spectra showed the presence of broad features in the H-bonded stretching spectral region (3700 – 2500 cm–1) that appear to be correlated with viscosity, although no direct numerical correlation could be obtained because of extensive overlap with solvent absorptions. A more detailed investigation of oxalic acid as a function of concentration in a range of solutions using water and protic and aprotic organic solvents showed a clearer numerical correlation in a different part of the spectrum. In addition to a broad, structured H-bonded motif, changes were identified in the carbonyl region that can be assigned to structural changes in oxalic acid molecules in solution that affect the activity of the IR bands. Detailed analysis suggests that the H-bonding interactions in aqueous solution are strong enough to pull the oxalic acid molecules into the cis-isomer. It is likely that these strong interactions are the cause of increased viscosity. This effect was not observed in organic solvents, although weaker H-bonded motifs were seen for polar protic solvents such as methanol and ethanol and were absent in acetonitrile. IR spectra of a range of saccharides in aqueous solution also show spectral changes that appear to be correlated with viscosity. For glucose, a linear, numerical correlation was established between features assigned to disruption of the water structure and viscosity. However, this was not possible for more complex saccharides with larger numbers of OH groups and intramolecular H-bonding, with multiple absorption bands from solvent, intra- and intermolecular H-bonding overlapping the key spectral regions. Based on literature reporting that the viscosity of aerosols containing α-pinene oxidation products varies with composition, a system for solution-phase ozonolysis was developed and used to ozonise α-pinene in ethyl acetate. Substantial changes in viscosity were measured as the degree of ozonolysis increased. Features in the IR spectra were shown to track quantitatively the loss of α-pinene and the formation of carbonyl functional groups, and the intensity of the latter were shown to be correlated with viscosity. Compositional analysis by GC/MS revealed that a number of species were formed, including α-pinene oxide, α-campholenal, pinonaldehyde and pinonic acid. However, the viscosity of test solutions made using pinonic acid was much lower than that of ozonised α-pinene, confirming that these lower molecular weights, primary ozonolysis products are not directly responsible for increased viscosity. More detailed compositional analysis of samples during ozonolysis using ESI/MS, APCI/MS and MALDI revealed the presence of small quantities of dimers and higher oligomers not identified in GC/MS methods. The observed linear correlation between the total carbonyl content measured in the IR spectral data and the viscosity suggests that viscosity changes during α-pinene ozonolysis are largely due to increasing numbers of carbonyl-containing, higher molecular weight products that are most likely formed by reactions involving some of the primary carbonyl products.
Date of Award1 Aug 2020
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorCarl Percival (Supervisor) & Andrew Horn (Supervisor)

Cite this

'