Aqueous interfaces are ubiquitous in the forms of, such as, cell membranes in body, aerosols in the atmosphere, and emulsions in food and pharmacy industries. The processes and interactions occurring at these interfaces are governed by interfacial molecular physical and chemical properties. A molecular-level understanding of water is crucial for unraveling aqueous interfaces. In this thesis, we aim to investigate the molecular structure of water at various aqueous interfaces: water outside oil-in-water droplets emulsions, water inside water-in-oil droplet emulsions, and water around liposome membranes, using a combination of different techniques including dynamic light scattering, nonlinear light scattering, molecular dynamics (MD) simulations, and atomic force microscopy.

We first investigate interfacial inversion, interference, and infrared (IR) absorption effects on two inversed droplet emulsions prepared using the same three chemicals: hexadecane oil, water, and a neutral Span80 surfactant. Theoretical analysis and simulation reveal the IR absorption effect of adsorptive media on sum frequency scattering (SFS) signal and how this effect can be corrected. The comparison of vibrational C-H stretches spectra from Span80 covered water-in-oil and oil-in-water emulsions reveals the interfacial-inversion-induced conformational difference of interfacial Span80 molecules. Using the isotope dilution method, we analyze the interfacial interference of both droplet emulsions and disentangle the interference from IR absorption effects through an absorption correction procedure.

We next focus on finite volume effects of Span80-covered water-in-oil emulsions using electrophoretic mobility and vibrational SFS measurements. Theoretical calculations reveal a nearly vanishing electrostatic field throughout the whole water droplets in oil. Notable, there is substantial difference in the mobility, the alkyl chain conformation of interfacial Span80, and the orientational ordering of interfacial water molecules between two inversed droplet systems. We further utilize the isotopic dilution of water to disentangle the local H-bonding from vibrational coupling effects, which shows that water inside water droplets forms a more heterogeneous H-bonding network than that outside oil droplets. We next investigate how electrolytes of different species and ionic strengths affect the interfacial water responses from Span80-covered water-in-oil emulsions. The SFS results indicate that as the ionic strength increases, all electrolytes reduce the water signal progressively in a similar trend even though with subtle differences.

Finally, we examine how the interfacial structure of water next to biological membranes is determined by its interactions with simple chiral prebiotic molecules, such as lipids and sugars. MD simulations show that a chiral water arrangement can form within a methyl-β-cyclodextrin (mβCD) dimer. We observed from second harmonic scattering responses that interfacial water next to 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-L-serine (POPS) liposomes becomes chiral and forms thin microns long suprastructures after incubating with mβCD. SFS further reveals that water inside these extended chiral suprastructures is highly ordered and spectroscopically ice-like. The formation of such extended chiral water-lipid-mβCD complexes is likely driven by intermolecular H-bonding.