Md Mosfeq Uddin

Topic

Studying the Electrical Double Layer at Charged Aqueous Interfaces Using Variable-Angle Surface Spectroscopy

Department of Chemistry

Date & location

• Monday, September 14, 2026
• 11:30 A.M.
• Elliott Building, Room 230

Examining Committee

Supervisory Committee

• Dr. Dennis Hore, Department of Chemistry, University of Victoria (Supervisor)
• Dr. Peter Loock, Department of Chemistry, UVic (Member)
• Dr. Tao Lu, Department of Electrical and Computer Engineering, UVic (Outside Member)

External Examiner

• Dr. Eric Borguet, Department of Chemistry, Temple University

Chair of Oral Examination

• Prof. Malcolm Gaston, School of Public Administration, UVic

Abstract

The structure and orientation of water molecules at charged solid–liquid interfaces fundamentally influence interfacial chemistry and molecular behavior. In aqueous environments, hydration at surfaces and around ions differs markedly from bulk water due to specific chemical interactions and electric fields. Vibrational sum-frequency generation (SFG) spectroscopy provides a powerful, interface-specific probe of interfacial water under ambient, in situ conditions. In this work, we develop a variable-angle SFG approach to isolate signals from distinct regions of the electrical double layer (EDL). 

We first develop a variable-angle SFG approach that exploits changes in the nonlinear coherence length to separate the responses of water in the bonded layer and the diffuse layer. Applying this method to the charged silica–water interface over a range of NaCl concentrations shows that water molecules in the bonded and diffuse layers share the same net orientation, while water in the bonded layer exhibits a slight preference for donating hydrogen bonds to surface silanol groups. This preference decreases with increasing ionic strength.

Next, we establish an intensity calibration procedure using z-cut α-quartz as an absolute reference. This enables quantitative determination of both the magnitude and phase of the bonded-layer nonlinear susceptibility, together with the bonded layer potential that separates the bonded and diffuse water responses. Application of this method to the silica–water interface shows that the bonded layer potential exceeds the magnitude of the corresponding ζ -potential by approximately 30% at neutral pH and low ionic strength. 

Finally, we extend this calibrated variable-angle SFG method to charged polymer–water interfaces. This approach is especially useful for thin films because measurements at high ionic strength provide a practical way to account for local field effects. We find that poly(dimethylsiloxane) (PDMS) and polystyrene show different bonded-layer water responses and bonded-layer potentials, demonstrating that polymer surface chemistry strongly affects interfacial water structure and ion interactions.

Overall, the variable-angle, intensity-calibrated SFG method in this thesis provide a quantitative way to study water structure and bonded layer potentials in different regions of the electrical double layer. These methods can be applied to many charged aqueous interfaces and improve our understanding of interfacial hydration, ion interactions, and processes that are important in electrochemistry, biomaterials, and surface science.