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Proton Transfers at the Air-Water Interface


Mishra, Himanshu (2013) Proton Transfers at the Air-Water Interface. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/A9HR-PN89.


Proton transfer reactions at the interface of water with hydrophobic media, such as air or lipids, are ubiquitous on our planet. These reactions orchestrate a host of vital phenomena in the environment including, for example, acidification of clouds, enzymatic catalysis, chemistries of aerosol and atmospheric gases, and bioenergetic transduction. Despite their importance, however, quantitative details underlying these interactions have remained unclear. Deeper insight into these interfacial reactions is also required in addressing challenges in green chemistry, improved water quality, self-assembly of materials, the next generation of micro-nanofluidics, adhesives, coatings, catalysts, and electrodes. This thesis describes experimental and theoretical investigation of proton transfer reactions at the air-water interface as a function of hydration gradients, electrochemical potential, and electrostatics. Since emerging insights hold at the lipid-water interface as well, this work is also expected to aid understanding of complex biological phenomena associated with proton migration across membranes.

Based on our current understanding, it is known that the physicochemical properties of the gas-phase water are drastically different from those of bulk water. For example, the gas-phase hydronium ion, H3O+(g), can protonate most (non-alkane) organic species, whereas H3O+(aq) can neutralize only relatively strong bases. Thus, to be able to understand and engineer water-hydrophobe interfaces, it is imperative to investigate this fluctuating region of molecular thickness wherein the ‘function’ of chemical species transitions from one phase to another via steep gradients in hydration, dielectric constant, and density. Aqueous interfaces are difficult to approach by current experimental techniques because designing experiments to specifically sample interfacial layers (< 1 nm thick) is an arduous task. While recent advances in surface-specific spectroscopies have provided valuable information regarding the structure of aqueous interfaces, but structure alone is inadequate to decipher the function. By similar analogy, theoretical predictions based on classical molecular dynamics have remained limited in their scope.

Recently, we have adapted an analytical electrospray ionization mass spectrometer (ESIMS) for probing reactions at the gas-liquid interface in real time. This technique is direct, surface-specific,and provides unambiguous mass-to-charge ratios of interfacial species. With this innovation, we have been able to investigate the following:

1. How do anions mediate proton transfers at the air-water interface?

2. What is the basis for the negative surface potential at the air-water interface?

3. What is the mechanism for catalysis ‘on-water’?

In addition to our experiments with the ESIMS, we applied quantum mechanics and molecular dynamics to simulate our experiments toward gaining insight at the molecular scale. Our results unambiguously demonstrated the role of electrostatic-reorganization of interfacial water during proton transfer events. With our experimental and theoretical results on the ‘superacidity’ of the surface of mildly acidic water, we also explored implications on atmospheric chemistry and green chemistry. Our most recent results explained the basis for the negative charge of the air-water interface and showed that the water-hydrophobe interface could serve as a site for enhanced autodissociation of water compared to the condensed phase.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Proton transfer, air-water interface, catalysis, electrospray ionization mass spectrometer, acid-base neutralization, pKa switching, acidity, basicity, isoprene, long-range specific ion effects, autodissociation
Degree Grantor:California Institute of Technology
Division:Engineering and Applied Science
Major Option:Materials Science
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Goddard, William A., III (advisor)
  • Hoffmann, Michael R. (co-advisor)
Thesis Committee:
  • Goddard, William A., III (chair)
  • Hoffmann, Michael R.
  • Johnson, William Lewis
  • Bhattacharya, Kaushik
Defense Date:15 April 2013
Non-Caltech Author Email:hmishra1983 (AT)
Record Number:CaltechTHESIS:05092013-220921048
Persistent URL:
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Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:7693
Deposited By: Himanshu Mishra
Deposited On:17 May 2013 22:41
Last Modified:04 Oct 2019 00:00

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