Interfaces play a crucial role in phenomena such as wetting, adsorption, adhesion, friction, heterogenous ice-nucleation, and biocompatibility. The interfacial molecules exhibit unique behavior due to missing interactions at the surface (or differing interactions across an interface). Designing molecules for targeted applications demands a thorough understanding of the connection between molecular-level interfacial interactions and macroscopic observables, which is currently limited due to the difficulties in accessing the buried solid/liquid and solid/solid interfaces in situ. In this dissertation, we employ interface-sensitive infrared-visible sum frequency generation (SFG) spectroscopy to probe the interfacial structure of simple liquids (or liquid mixtures) as well as complex proteins next to a sapphire substrate and discuss the ramifications for macroscopic phenomena such as adsorption, solidification, and adhesion. SFG, being a second order non-linear optical technique, provides insights into the interfacial structure, orientation, and concentration of molecules.
First, the competitive adsorption to sapphire from three binary liquid mixtures, acetone-chloroform, tetrahydrofuran (THF)-benzene, and N,N-dimethylformamide (DMF)-benzene, has been investigated using SFG. The preferential adsorption of one component over another forms the basis for a variety of applications such as separation or purification using membranes or column chromatography, as well as biological implant acceptance or rejection. The relative interfacial concentrations of the two components from binary mixtures are determined by analyzing the shape of the sapphire hydroxyl peak. By fitting the adsorption isotherm with the thermodynamic Everett model, the differences in interfacial energies (Δγ) of the two components with the sapphire substrate are determined. These are then compared with the Δγ values calculated using the Dupré-Fowkes approach. The calculated Δγ values are consistent with the experimental values for acetone-chloroform and THF-benzene mixtures. However, the DMF-benzene mixture displays unexpected behavior, highlighting complications in the broader use of the Dupré-Fowkes-Everett equation for calculating interfacial concentrations.
The second study investigates the influence of headgroup-substrate interactions on the thermal stability of self-assembled monolayers (SAMs) for the three different amphiphiles (octadecanol, octadecylamine, and octadecanoic acid) by probing the sapphire/melt interface using SFG. Each of the three amphiphiles forms an ordered SAM on the sapphire substrate at temperatures above its bulk melting temperature (Tm) on account of acid-base interactions between the polar headgroup (OH, COOH, or NH2) and the surface hydroxyls of sapphire. Each SAM undergoes an order-disorder transition (ODT) at a temperature much higher than the bulk Tm. The three amphiphiles exhibit a different increase in surface ODT temperature relative to bulk Tm. However, the variation in surface ODT temperatures for the three amphiphiles does not correlate with the interaction strength calculated by analyzing the shifted sapphire surface hydroxyl peak, highlighting the need to take into consideration additional parameters such as headgroup size, monolayer packing, and entropy change during ODT.
The third study involves the examination of interfacial water structure next to graphene (supported on a sapphire substrate) to gain insights into graphene-water interfacial interactions. A molecular-scale understanding of the graphene-water interface is critical for optimizing the performance of graphene in various applications including energy storage, sensing, desalination, and catalysis. SFG measurements reveal an enhanced ordering of interfacial water molecules next to graphene coated sapphire relative to bare sapphire underscoring strong interactions between graphene and water. In addition, graphene displays better ice-nucleating properties relative to bare sapphire due to strong layering of water molecules revealed by in situ freezing experiments. The ordered water structure and proton-disordered ice structure indicate similarities between graphene and hydrophobic (or negatively charged) surfaces despite graphene's low water contact angle (51±5°).
The last study deals with using the knowledge gleaned from the above studies to reveal the secrets behind the success of spider aggregate glue in humid environments by in situ investigation of the aggregate glue/sapphire interface using a combination of SFG and infrared spectroscopy. The SFG results demonstrate that the glycoproteins (present in aggregate glue) act as primary binding agents at the interface. As humidity increases, reversible changes occur in the interfacial secondary structure of glycoproteins. Although the infrared spectroscopy measurements show a consistent increase in liquid-like water inside the bulk with increasing humidity, no liquid-like water is detected at the interface, highlighting the role played by hygroscopic low molecular mass compounds in sequestering interfacial water. Using hygroscopic compounds to sequester interfacial water provides a novel design concept for developing water-resistant synthetic adhesives.