Reports: UNI649505-UNI6: Second Harmonic Generation (SHG) Spectroscopic Studies of Polycyclic Aromatic Hydrocarbons (PAH's) at Liquid Water Surfaces

Elizabeth A. Raymond, PhD , Western Washington University

My research group has been studying the adsorption of environmentally relevant polycyclic aromatic hydrocarbons (PAH’s) to the air/water and oil/water interfaces, using surface tension, UV-Vis, fluorescence, and second harmonic generation (SHG).  PAH molecules are naturally found in crude oil, are readily formed as products in incomplete combustion, and are of great concern to human (and other organism) health, due to their toxic and often carcinogenic properties.  The ultimate goals of these studies are to measure the time scale of adsorption and interfacial concentrations of these molecules using surface tension as well as to investigate differences between their bulk and interfacial molecular environments (as characterized by molecular orientations and spectroscopic parameters) by comparing the results of bulk UV-Vis and fluorescence spectroscopy and the surface-specific second harmonic generation (SHG) spectroscopy.

            In the past year, we have focused on a family of simple PAH’s, anthracene, dibenzofuran (DBF), and dibenzothiophene (DBF).  These molecules were chosen for their simple (but related) structures and for their solubilities in both aqueous and organic solution.  Solutions of all three molecules in hexane have been characterized using UV-Vis and fluorescence spectroscopy, yielding spectra which agree with published work, and which show vibronic structure.  The vibronic structure observed in bulk organic solution, which is influenced by the molecules’ electronic and vibrational environments, is then compared with the structure observed in the surface selective SHG spectra.  Past work on the project focused on obtaining reproducible, consistent surface tension measurements at both the air/water and oil (hexane)/water interfaces using the Wilhelmy plate technique.  These measurements provided the time scale to achieve equilibrium surface concentrations (typically less than twenty minutes), and a quantitative measure of the PAH surface concentrations.  

            In the past year, we have focused on obtaining SHG spectra of these three molecules at the hexane/water interface, as spectra at the air/water interface have been difficult to obtain.  A total internal reflection (TIR) geometry (which enhances the total amount of SHG collected) was used, with the fundamental and SHG beams passing through the water phase to minimize adsorption by the bulk solution PAH molecules.  All three PAH molecules have been studied over a wide range of bulk concentrations, ranging from 100 nM to 100’s of mM.  Comparison of the surface specific SHG spectra with bulk absorption and fluorescence spectra has led to the following conclusions about the surface populations and surface molecular orientations. 


Although anthracene is a centrosymmetric molecule (which is symmetry forbidden from producing SHG), SHG spectra were obtained at the hexane/water interface.  The origin of this signal has two possible sources; an asymmetry induced in the molecule arising from its orientation and solvation at the surface, or quadrupolar contributions to the SHG signal from the anthracene molecules, which imply that not all of the SHG signal is coming from the interfacial region.  The square root of the SHG intensity (proportional to the number of molecules) is not consistent with the surface tension results, indicating molecular re-orientations are occurring as the surface layer of anthracene packs. 


            Both the surface tension and the SHG spectral intensity indicate that the DBF has the sparsest surface coverage, even at maximum surface coverage.  From the spectra and surface tension, there appears to be little or no molecular re-orientation which occurs as the surface layer packs.  This seemingly fixed orientation likely arises from the strong potential DBF has for hydrogen bonding with the aqueous phase, providing a fixed orientation, rather than one that changes as the molecules pack.  Molecular dynamics simulations would help elucidate the specific molecular orientations present at this interface.


            Initial spectra of DBT at the hexane/water interface show little to no molecular re-orientation at low bulk concentrations of DBT, but at moderate to high concentrations (25 -100 mM) the square root of the intensity no longer tracks with the number of DBT molecules at the surface.  Again, this is indicative of significant molecular re-orientation.  A possible explanation for this change in molecular orientations is that the interaction of DBT’s sulfur with the aqueous phase may provide an aligning force at low surface coverages, but as the DBT molecules pack at higher surface coverages, the DBT-DBT interactions dominate.  Work continues on this system, refining both the SHG and surface tension measurements to determine if this result is reproducible and/or outside the experimental error.  This is another system that would greatly benefit from molecular dynamics simulations. 

            Future work on this project includes further data collection on the interfaces mentioned above; to reduce error bars and fill in additional concentrations in both the surface tension and SHG results.  In addition, we are expanding the number of PAH molecules being studied to include fluorene (the all carbon analog of DBT and DBF) and several other PAH’s which have been functionalized to have higher solubility in water for study at the air/water interface.  Molecular dynamics simulations that produce meaningful results are beyond the time constraints and abilities of the current research group, so I am exploring potential collaboration opportunities to provide this important component. 

            The results of this work were presented by undergraduate students Everett Jackson and Jennifer McCarville at the Puget Sound ACS Undergraduate Research Symposium and Western Washington University Scholars Week poster sessions, and in a talk at the fall national ACS meeting in Denver by E. Raymond.

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