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X-Ray Spectroscopy of Liquids and Interfaces

  • Author(s): Lam, Royce Ka-Jun
  • Advisor(s): Saykally, Richard J
  • et al.

The molecular structure of liquids and solutions and the behavior of solvated species, both in solution and at interfaces, play a vital role in a wide range of systems and phenomena. While much has been done to characterize these systems, significant advances are still needed to extend existing experimental methodologies to those which were previously inaccessible. For example, while the introduction of liquid microjets into soft X-ray spectroscopy enabled the study of liquids by this element-selective technique, studies of highly volatile liquids were impractical due to the high vapor background perturbing the liquid signal. The development of a new detection scheme, presented herein, enabled the studies of such systems. In this dissertation, I describe the development of new liquid microjet techniques, including a detailed study elucidating the mechanism of electrokinetic energy conversion, studies exploring the liquid structure of prototypical volatile liquids and binary mixtures, and hydration and interfacial fractionation in the aqueous carbonate system using soft X-ray absorption (XAS) and photoemission spectroscopy (XPS), as well as the development of a new interface-specific soft X-ray spectroscopy, soft X-ray second harmonic generation (SHG).

In Chapter 2, I detail the use of liquid microjet electrokinetics for energy conversion, exploring thermal energy as a pressure source, the mechanism of charge separation in the liquid microjets, and techniques for tuning the microjet properties through targeted use of surface coatings via silanization.

In Chapter 3, I investigate high vapor pressure liquids using liquid microjet XAS, focusing on the structure of liquid alkanes (n-nonane and n-decane). This study required the development of a new detection scheme which significantly reduced the vapor contribution to the measured signal. The detection scheme employed a straightforward modification of our standard liquid microjet assembly and leveraged a detection methodology similar to that of the electrokinetic energy conversion experiments, wherein a signal can be detected both upstream and downstream of the point of charge separation.

In Chapter 4, water-alcohol mixtures are characterized by XAS, providing new insight into the hydrogen bonding environments, with methanol- and ethanol-water exhibiting significant differences to that of isopropanol-water. The measured spectra evidence a significant enhancement of hydrogen bonding originating from the methanol and ethanol hydroxyl groups upon the addition of water. These additional hydrogen bonding interactions would strengthen the liquid-liquid interactions, resulting in additional ordering in the liquid structures and leading to a reduction in entropy and a negative enthalpy of mixing, consistent with existing thermodynamic data. In contrast, the spectra of the isopropanol-water mixtures exhibit an increase in the number of broken alcohol hydrogen bonds for mixtures containing up to 0.5 water mole fraction, an observation consistent with existing enthalpy of mixing data, suggesting that the measured negative excess entropy is a result of clustering or micro-immiscibility.

In Chapter 5, the aqueous carbonate system is investigated, characterizing the hydration environment by XAS and the interfacial fractionation of the species by XPS. Studying the aqueous carbonate system necessitated the development of a new fast-flow liquid microjet mixing system, which facilitated generation of short-lived species immediately prior to interaction with the X-ray probe. This advance permitted the continuous generation of aqueous carbonic acid through the protonation of bicarbonate in solution. The decomposition of the generated acid further enabled the characterization of dissolved CO2 by XAS. Utilizing XPS, which exploits attenuation length of the emitted photoelectrons to achieve depth profiling, the relative fractionation of carbonate, bicarbonate, and carbonic acid at the liquid/vapor interface, finding that both carbonate and carbonic acid are present in higher concentrations than bicarbonate in the interfacial region – a surprising result as the concentration of the doubly charged, strongly hydrated carbonate anion was found to be enhanced relative to the singly charged, less strongly hydrated bicarbonate and in apparent contrast to current models for ion adsorption to hydrophobic interfaces.

In Chapter 6, the development of a new surface and interface sensitive soft X-ray spectroscopy, soft X-ray SHG, is detailed. The experimental results and accompanying first principles theoretical analysis highlight the effect of resonant enhancement and show the technique to be interfacially sensitive in a centrosymmetric sample, with second harmonic intensity arising primarily from the first atomic layer at the open surface. This technique and the associated theoretical framework demonstrate the ability to selectively probe interfaces, including those that are buried, with elemental specificity, thus providing a new tool for a range of scientific applications. Additionally, the transmission of the soft X-ray pulses from the FERMI free electron laser through carbon films of varying thickness was examined to quantify other nonlinear effects of high intensity pulses above and below the carbon K-edge.

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