Molecular-level characterization of curved water interfaces with sum frequency and second harmonic scattering

Aqueous interfaces are omnipresent in nature. Some of them are visible: droplets in clouds, aerosols, and surfaces of oceans, lakes, and rivers. Many more surfaces are hidden: water in contact with cell membranes, proteins, and the surface of droplets in emulsions. The hydrophobic interaction plays a key role in many biological and industrial processes; hence understanding of water structure at the hydrophobic interfaces is needed. To elucidate molecular-level water structure at these interfaces and to learn about the molecular-level mechanisms behind hydrophobicity we use sum frequency and second harmonic scattering spectroscopy. We apply these techniques to probe the molecular structure (orientational order and strength of the hydrogen bonds) at the interfaces of droplet and liposomes. First, we demonstrate a way to use the focusing properties of a liquid jet for nonlinear scattering experiments. As the jet surface is continuously refreshed, reduced heating effects and cleanliness are advantages of this technique. Then, investigating water structure in water droplets in oil, we find that hydrogen bonds of water at the surface of the droplet resemble that of planar oil/water interface made of the same chemicals at 50°C lower temperature. Using water droplets dispersed in CCl4 we show that second harmonic scattering data has a high sensitivity to the ionic strength of the solution. We estimate the concentration of free charge carriers (reverse micelles and hydrated ions) and discuss changes in the surface water ordering and the surface potential, which reflect the formation of reversed micelles at the droplet interface. Finally, we study water and lipid transmembrane asymmetry using liposomes composed of neutral and charged lipids, which are a good model system of a phospholipid bilayer of a cell membrane. We calculate the distribution of the phosphate tilt angle and quantify lipid transmembrane asymmetry. Furthermore, we discuss a mechanism responsible for the observed lipid asymmetry that involves intermolecular hydrogen bonding between phosphate and amine groups of adjacent lipids.