Investigating Ion Interactions with Monolayer/Water Interfaces Using Vibrational Sum Frequency Generation Spectroscopy
Restricted (Penn State Only)
- Author:
- Gonzalez Velez, Nicole
- Graduate Program:
- Chemistry
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- August 12, 2024
- Committee Members:
- Paul Cremer, Chair & Dissertation Advisor
Christine Keating, Major Field Member
John Asbury, Major Field Member
Ralph Colby, Outside Unit & Field Member
Kenneth Knappenberger, Program Head/Chair - Keywords:
- Sum Frequency Generation Spectroscopy
Monolayer
Air/Water Interfaces
Ion Pairing
Ion-Specific Effect
Langmuir Trough
Alkali Ions
Grahame Model - Abstract:
- Vibrational Sum Frequency Generation (SFG) spectroscopy is a second-order nonlinear optical technique used to probe the vibrational resonance of oriented water molecules. This method was employed to investigate the interfacial water structure at the air/water interface in the presence of both charged and uncharged Langmuir monolayers and varying salt concentrations in the subphase. Understanding the water structure and interfacial potential is crucial for elucidating phenomena such as protein folding, proton exchange membrane behavior, ion and metabolite transport across membranes, and the development of advanced catalysts. At the air/water interface adjacent to a charged monolayer, water molecules exhibit distinct orientations based on the charge of the membrane. Specifically, water molecules either orient with their hydrogen bonds pointing upward towards the interface (H-up) or downward towards the bulk solution (H-down), depending on whether the monolayer is positively or negatively charged. Three distinct populations of water are identified: (1) water molecules chemically bound to the surfactant headgroups in the Stern layer, (2) water molecules in the diffuse layer oriented by the electric field generated by the surface charge, and (3) the bulk water layer, which is unaffected by the electric field and does not contribute to the SFG signal. The orientation of water molecules correlates with the interfacial potential, which is related to surface charge density through the Grahame equation. We utilized the Gouy-Chapman theory and the Grahame equation to differentiate between water in the Stern and diffuse layers. In this study, we introduce a new methodology that allows us to maintain a constant surface potential by varying the surface charge density and salt concentration or to maintain a constant surface charge density by adjusting the surface potential through changes in salt concentration. This approach allows for the precise separation of water molecules oriented directly by monolayer headgroups from water molecules ordered by the electric field and for investigating potential saturation effects. SFG spectroscopy was used to examine the saturation of the OH stretch region at the air/water interface as a function of surface charge density. Experiments were conducted with negatively and positively charged monolayers, demonstrating that the OH water structure increased continuously with the surface potential. These experiments included variations in salt concentration, surface charge density, and surface potential. The surface charge density was modulated by incorporating a neutral surfactant as a filler in the monolayer. All measurements were performed using the SSP polarization combination in a Langmuir trough at constant pressure. Ultimately, we observed that water molecules orient with their hydrogen atoms pointing downwards toward the bulk in the presence of alcohol Langmuir monolayers, as indicated by the negative signal in the Maximum Entropy Method (MEM) spectra. This finding prompted an investigation into the origins of this signal and its variation with changes in subphase composition, pH, alkyl chain length, and surface pressure. Our findings demonstrate the critical role of surface pressure in determining water molecule orientation. Specifically, in the gel phase of Langmuir monolayers, water molecules consistently oriented with H-down, whereas in the gas phase, the orientation shifted to H-up at the surface. These orientation changes were consistent across different subphase species and salt concentrations. Understanding water molecule orientation at interfaces, particularly as influenced by monolayer phase transitions, is essential for elucidating biochemical processes and advancing the development of novel materials and technologies.