Spectroscopic studies of molecular interaction at the liquid–liquid interface

Oil-in-Water Emulsions Probed Using Fluorescence Multivariate-Curve-Resolution Spectroscopy

Hydrophobic surfaces in contact with aqueous media are omnipresent in nature. A plethora of key biological and physiological processes occur at the interface of immiscible fluids. Besides its fundamental importance, probing such interfaces is rather challenging, especially when one medium is bathed in the other. Herein, we demonstrate a fluorescence-based method that probes the oil-water interface and interfacial processes through surface dielectric perturbations. The fluorescence response of Nile Red is measured in hexadecane in water nanoemulsions. Three major spectral components appear: two from the bulk liquid media (hexadecane and water) and a distinct band at around 640 nm due to the interfacial component. Such spectra are deconvoluted using the multivariate-curve-resolution algorithm, and interface-correlated fluorescence spectra are attained. The influence of anionic sodium dodecylbenzenesulfonate (SDBS) and cationic cetyltrimethylammonium bromide (CTAB) surfactants on the oil-water interface is elucidated with concentration-dependent measurements. A charge-dependent spectral shift is observed. The interface correlated band at 641 nm for bare hexadecane nanoemulsions red shifts in the presence of anionic surfactants, indicating an apparent dielectric increase. In contrast, the same band gradually blue shifts with increasing cationic surfactant concentration, indicating an apparent interface dielectric decrease. Such a method can be utilized to probe alterations at interfaces beyond the oil/water interface.

Liquid Heterostructures: Generation of Liquid-Liquid Interfaces in Free-Flowing Liquid Sheets

Chemical reactions and biological processes are frequently governed by the structure and dynamics of the interface between two liquid phases, but these interfaces are often difficult to study due to the relative abundance of the bulk liquids. Here, we demonstrate a method for generating multilayer thin film stacks of liquids, which we call liquid heterostructures. These free-flowing layered liquid sheets are produced with a microfluidic nozzle that impinges two converging jets of one liquid onto opposite sides of a third jet of another liquid. The resulting sheet consists of two layers of the first liquid enveloping an inner layer of the second liquid. Infrared microscopy, white light reflectivity, and imaging ellipsometry measurements demonstrate that the buried liquid layer has a tunable thickness and displays well-defined liquid-liquid interfaces and that this inner layer can be only tens of nanometers thick. The demonstrated multilayer liquid sheets minimize the amount of bulk liquid relative to their buried interfaces, which makes them ideal targets for spectroscopy and scattering experiments.

Potential-Induced Aggregation of Anionic Porphyrins at Liquid|Liquid Interfaces

The adsorption and self-aggregation of anionic porphyrins were studied at the polarized water|1,2-dichloroethane (DCE) interface by polarization-modulation total internal reflection fluorescence (PM-TIRF) spectroscopy. 5,10,15,20-Tetrakis(4-sulfonatophenyl)porphyrin diacid (H₄TPPS^2-) and protoporphyrin IX (H₂PP^2-) exhibited high surface activities at the interface. The selective excitation of interfacial species in PM-TIRF measurements elucidated the potential-induced aggregation mechanism of the porphyrins. The J-aggregates of H₄TPPS^2- were reversibly formed only at the water|DCE interface by applying appropriate potentials even when the porphyrins exist as monomers in the aqueous and organic solutions. In the H₂PP^2- system, the slow aggregation process was found in the negative potential region. The spectral characteristics and the signal phase of PM-TIRF indicated that the H₂PP^2- monomers were adsorbed with relatively standing orientation and that the long axis of the J-aggregates was nearly in plane of the interface. H₂PP^2- was also investigated at the biomimetic phospholipid-adsorbed water|DCE interface. The competitive adsorption of neutral glycerophospholipids effectively inhibited the potential-dependent adsorption and interfacial aggregation processes of H₂PP^2-. The results demonstrated that the aggregation state of the charged species can reversibly be controlled at liquid|liquid interfaces as a function of externally applied potential.

Fundamentals and applications of self-assembled plasmonic nanoparticles at interfaces

This tutorial review covers applications and fundamentals of self-assembled plasmonic nanoparticles at liquid|liquid and liquid|air interfaces.

Total internal reflection spectroscopy for studying soft matter

Total internal reflection (TIR) spectroscopy is a widely used technique to study soft matter at interfaces. This tutorial review aims to provide researchers with an overview of the principles, experimental design and applications of TIR spectroscopy to enable them to understand how this class of techniques might be used in their research. It also highlights limitations and pitfalls of TIR techniques, which will assist readers in critically analysing the literature. Techniques covered include attenuated total reflection infrared spectroscopy (ATR-IR), TIR fluorescence, TIR Raman scattering and cavity-enhanced techniques. Other related techniques are briefly described.