Abstract

Deep UV Second Harmonic Generation Studies of Thiocyanate at Hydrophobe/Water Interfaces

By

Debra Lynn McCaffrey

Doctor of Philosophy in Chemistry

University of California, Berkeley

Professor Richard J. Saykally, Chair

Our understanding of the air/water interface and ion adsorption to this interface has developed rapidly in the last decade. While tremendous progress has been made in this area, it is essential for the field to branch out into additional interfaces. This dissertation describes the work I’ve done to extend the field to additional hydrophobic interfaces.

Chapter 1 gives historical context for the work done on the air/water interface. Macroscopic measurements, such as surface tension, suggested that ions should be depleted from the air/water interface. Microscopic measurements, such as molecular dynamics simulations and second harmonic generation spectroscopy, show that some ions are enhanced at the air/water interface and that the degree of enhancement follows the Hofmeister series. The mechanism that drives adsorption is a delicate balance between factors such as solvent repartitioning, electrostatics, capillary waves, and configurational entropy.

Chapter 2 presents nonlinear optical spectroscopy theory and outlines the experimental apparatus. It also details the sample preparation and the data analysis procedure. The processed data are fit to a Langmuir model, derived therein.

Chapter 3 details studies on hydrocarbon/water interfaces. Hydrocarbons provide a condensed phase analogue to the air/water interface. While the data are preliminary and need to be refined, the general trend is that thin layers of hydrocarbons have little effect on the free energy of adsorption of thiocyanate.

Chapter 4 details studies on the graphene/water interface. While graphene has a high charge carrier mobility, it seems that graphene behaves like a hydrophobe in regards to thiocyanate adsorption. Experimental studies showed that the free energy of adsorption to the graphene/water interface is similar to that of the air/water interface. Molecular dynamics simulations reveal that the adsorption mechanism is drastically different, however.

Chapter 5 evolved out of a concern to include a surface potential term in the Langmuir model. This created nested models that were compared with several model comparison metrics. The better model seems to depend on the dataset. The method of calculating errors for the fit parameters was also examined, but further work still needs to be done on finding the most accurate method.

Chapter 6 presents some broad conclusions and directions for future study. It seems that thin, uncharged monolayers inserted into an interface have little effect on the free energy of adsorption, although the mechanism can change. This chapter also proposes a new study to help elucidate the effect of surface charge. Surfactants can mix with alkanes to form an ordered monolayer on water. Varying the concentration of the surfactant can vary the surface charge.