UC Berkeley

Understanding the structure and composition of the air-water interfaces is of utmost important to many areas of scientific study. Aqueous interfaces are ubiquitous in nature and as such, are intimately involved in chemical processes ranging from those affecting atmospheric aerosols to those governing cellular structuring. The current state of the atmosphere in relation to climate change is of utmost importance, and surface sensitive spectroscopies are only now beginning to shed light on the chemical mechanisms involved. An understanding of how ions adsorb to the water surface will be invaluable in predicting and combatting the environmental chemistry which is central to climate change. Despite the importance of the air-water interface, it remains incompletely understood – particularly in the case of those solutions which contain ions and/or biomolecules – due to the experimental challenges involved in probing it. However, recent experimental and theoretical results have established the existence of certain anions that are preferentially adsorbed to the air-water interface. In this dissertation, I discuss my development of a broadband deep-UV sum frequency generation (DUV-SFG) spectroscopy experiment which was used to explicitly measure the adsorption of ions at the air-water interface. I also use a Raman thermometry experiment that, while it is not a surface specific probe, is a surface sensitive probe and so interfacial information can be drawn from those experiments.

In Chapter 2 I describe broadband DUV-SFG studies of the aqueous iodide ion wherein the first full interfacial charge-transfer-to-solvent (CTTS) spectrum of an ion was measured. Interestingly, the spectrum is redshifted by ~8 nm relative to the bulk spectrum as well as significant line narrowing – 17% for the J = 1/2 peak and 66% for the J = 3/2 peak. These results may indicate very interesting dynamics at the air-water interface as well as reporting on the hydration structure of those ions at the water surface.

A recent publication indicated that sodium nitrite adsorbs strongly to the interface as a contact ion pair, wherein the Gibbs free energy of adsorption to the air-water interface was found to be -17.8 kJ/mol. In Chapter 3 I describe broadband DUV-SFG studies on the aqueous sodium nitrite system in an attempt to confirm or deny the contact ion pairing hypothesis. It was found that despite elaborate attempts to purify the salt prior to studying it in solution, there was always a significant degree of hydrocarbon contamination. This contamination likely resulted in the previous study’s misinterpretation of large SHG intensity as nitrite adsorption to the air-water interface. Interestingly, the broadband SFG experiment was found to be a good method for confirming sample purity by examining the nonresonant signal collected.

Clearly the development of nonlinear optical spectroscopies has greatly advanced the field of interfacial science, but there is still much to be learned. In Chapter 4 I describe what I feel are the most important future steps toward furthering our understanding of interfaces. I have outlined several experimental techniques that need to be developed or refined, as well as indicated the next steps for theoreticians. This chapter does not encompass all of what can or should be done, but it is a good starting point.

Nonlinear optical spectroscopies are incredibly powerful due to their interfacial specificity, but they are also quite difficult to utilize because they are naturally a low signal method and are very susceptible to contamination. In Chapter 5 I describe the use of Raman thermometry to study the evaporation rate of water from aqueous HCl droplets. By definition, evaporation is a process that occurs at the interface, and so despite Raman spectroscopy not being a surface specific probe, this experimental technique allows us to draw conclusions about interfacial properties. The kinetics and energetics of cloud droplet and aerosol formation in the atmosphere are highly governed by the evaporation and condensation rates of water, yet the magnitude and mechanism of evaporation remains incompletely characterized. Of particular import (and controversy) is the nature of interfacial water pH and its effect on the evaporation rate and environmental reactivity. Raman thermometry measurements of freely evaporating micro-droplets were used to determine evaporation coefficients () for two different hydrochloric acid solutions, both which result in a significant deviation from γwater. With a 95% confidence level, it is found that the evaporation coefficient for 1.0 M HCl is 0.24 ± 0.04 – a ~60% decrease relative to pure water, and for 0.1 M HCl is 0.91 ± 0.08 – a ~45% increase relative to pure water. These results suggest a large perturbation in the surface structure induced by hydronium at the water surface.