Water-Mediated Interactions In Aqueous Salt Solutions
Open Access
- Author:
- Drexler, Chad
- Graduate Program:
- Chemistry
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- December 10, 2020
- Committee Members:
- Paul S Cremer, Dissertation Advisor/Co-Advisor
Paul S Cremer, Committee Chair/Co-Chair
Lasse Jensen, Committee Member
Christine Dolan Keating, Committee Member
Ralph H Colby, Outside Member
Philip C Bevilacqua, Program Head/Chair - Keywords:
- Electrolytes
Local Electric Fields
Ion Hydration
Ion Pairing
Proton Transfer
Hofmeister - Abstract:
- Previous studies of the liquid phase have revealed intermolecular interactions between ions and surfaces in aqueous solution. Such work points to the importance of water structure in the first hydration shell around ions and their specific binding sites. An interaction often exists between two ions, ions and dipoles or ions and hydrophobic groups. The resulting ‘ion-receptor’ complexes can form either direct contacts or water-mediated structures. Contact pairs exist when both the ion and the binding site dehydrate and directly interact. Water-mediate pairs exist between species that share hydration water. While contact pairs have been shown previously to be important for numerous processes, water-mediated binding events have received less attention. Herein, ion-ion and ion-dipole interactions are studied in aqueous solutions. These investigations revealed consequences of water-mediated pairing in two separate phenomena. In a first set of studies, the impact of water-mediated ion pairs on the conductivity of salt solutions was investigated. Hydroxide has been shown previously to undergo transport by a phenomenon known as the Grotthuss mechanism. This results in fast diffusion of OH- compared to simple ions like Na+ or Cl- due to proton transfer between water and OH-. Here, water-mediated ion pairs modulated the diffusion constant of alkali hydroxide salts. In a second set of studies, Onsager’s reaction field theory was tested in salt solutions. Water-mediated interactions between polar organic molecules and metal halide salts were of central importance to this study. Focus was given to molecules containing either carbonyl or nitrile bonds. The C=O oxygen atom formed water-separated pairs with well-hydrated metal cations like Mg2+, while the C≡N nitrogen did so with poorly-hydrated ones like Cs+. Moreover, I- formed these pairs with the positive terminus of either dipole better than Cl-. Such interactions resulted in the establishment of a local electric field. These findings revealed that Onsager’s model breaks down in electrolytes due to water-mediated ion-dipole interactions.