Physical adsorption at solid-liquid interface is an effective way to dynamically alter the surface properties \textit{in-situ} and is prevalent in biology, geoscience, biotechnology, catalysis, food processing, agriculture, textiles, coatings, adhesives and lubricants, and cosmetics. Over the last century an extensive framework of the thermodynamics and kinetics of adsorption as well as the intermolecular forces that govern the physical interactions between the substrate, solute, and solvent has been established, helping elucidate the structure of adsorbates. However, there lacks an understanding of the connection between the local conformation of the adsorbed molecules and its influence on the macroscopic interfacial phenomena. In this research we have employed vibrational spectroscopy techniques namely the interface-selective non-linear sum frequency generation (SFG) and linear attenuated total reflectance Fourier transform infrared (ATR-FTIR) in conjugation with contact mechanics to develop the molecular level insights into macroscopic phenomena of wetting, adhesion, and sliding friction. We study the interfacial structure of small molecules (surfactants and solvents) as well as macromolecules adsorbed on model surfaces of sapphire (aluminum oxide) and silicone dioxide.
Using SFG, we discovered a strongly coordinated ice-like water layer confined between two charged surfaces, under hydration pressures, formed by the adsorption of a cationic surfactant (cetyl trimethyl ammonium bromide, CTAB) on dissimilar hydrophobic surfaces (phenylethyl trichlorosilane, PETS self-assembled monolayer, and polydimethylsiloxane elastomer, PDMS). This strongly coordinated water structure forms past the surfactant concentration needed for a monolayer surface coverage and is observed to reduce the sliding friction. Correlating interface-selective spectroscopy with hydration forces, and their macroscopic manifestation on adhesion and friction requires us to reconsider how we understand water under confinement and its significance in more complicated interfacial processes prevalent in biology, chemistry, and engineering.
To investigate the adsorption of a basic polymer (PMMA) on acidic surfaces in carbon tetrachloride (neutral), Chloroform (acidic), and acetone (basic) solvents, we used SFG and ATR-FTIR to build on to the established concept of acid-base interactions that explains limited adsorption from basic and acidic solutions compared to neutral solution due to competitive interactions. We show that besides the differences in adsorbed amount, chains adsorbed from an acidic solvent adopt a flat conformation with a much smaller ratio of segments of loops and tails to trains compared to those adsorbed from a neutral solvent. Surface interaction parameters alone cannot predict the differences in conformation of chains adsorbed from acidic or neutral solvents. Such differences in the static and dynamic conformations have consequences in understanding the exchange kinetics, colloidal stabilization, chromatographic separations, adhesion and friction, and stabilization of nanocomposites.
The adsorption of polyelectrolytes driven by Coulombic interactions becomes complicated when mixed with oppositely charged surfactant which modulates the net charge and solubility of polyelectrolytes in water resulting in a characteristic bulk phase diagram. We have investigated the adsorption from pre-mixed solutions of a cationic polysaccharide (PQ10) and the anionic surfactant sodium dodecyl sulfate (SDS), on an amphoteric alumina surface using quartz crystal microbalance with dissipation (QCMD). By tuning the surface charge of the amphoteric alumina, we confirmed the importance of electrostatic interactions on the adsorption on a hydrophilic charged surface, a suggestion that was made earlier solely based on the measurements on negatively charged surfaces. We also directly correlate the bulk phase transitions at the interface adsorption by observing a maximum extent of adsorption on both negatively and positively charged surfaces from a solution corresponding to the maximum turbidity. Using the Voight based viscoelastic model, QCMD also provided information on the viscosity, shear modulus, and thickness of the adsorbed polymeric complex, findings of which were corroborated by underwater atomic force microscopy (AFM) measurements.
Shifting gears from adsorption to understanding the polymer thin films surface structure in different environments, we used SFG to observe the surface restructuring of an amphiphilic glassy homopolymer poly(alpha-hydroxy-n-butyl acrylate, PHNB) from hydrophobic to aqueous environment. We observed relative changes in ordering of hydrophobic and hydroxyl pendant groups upon going from air to water at different temperatures showing their impact on wetting of the polymer surface observed by means of dynamic contact angles measurement.