Water Structure at the Hydrophobic Nanodroplet Surface Revealedby Vibrational Sum Frequency Scattering Using Isotopic Dilution br

The water structure at the hydrophobic/waterinterface is key toward understanding hydrophobicity at themolecular level. Herein, we characterize the hydrogen-bondingnetwork of interfacial water next to sub-micron-sized hydrophobicoil droplets dispersed in water using isotopic dilution vibrationalsum frequency scattering (SFS) spectroscopy. The relativeintensity of different modes, the frequency shift of the uncoupledO-D spectrum, and a low-frequency shoulder (2395 cm-1) revealthat water forms an overall stronger hydrogen-bonding networknext to hydrophobic droplets compared to bulk water and the air/water interface. Half of the spectral width of the oil droplet SFSspectrum is determined by inter- and intramolecular coupling of water molecules. Isotopic dilution also confirms the presence of abroad distribution (ca. 2640-2745 cm-1) of non-water-hydrogen-bonded O-D modes that are red-shifted and broadenedcompared to similar species at the air/water interface. This band corroborates the presence of charge transfer between water and oil

Charge transfer across C-H center dot center dot center dot O hydrogen bonds stabilizes oil droplets in water

Sergey Kulik, Saranya Pullanchery Sankara Narayanan, Benjamin Rehl, Sylvie Roke

The hydrophobic-water interface plays a key role in biological interactions. However, both the hydrophobic-water interfacial molecular structure and the origin of the negative zeta potential of hydrophobic droplets in water are heavily contested. We report polarimetric vibrational sum-frequency scattering of the O-D and C-H stretch modes of 200-nanometer hexadecane oil droplets dispersed in water. An unusually broad spectral distribution (2550 to 2750 per centimeter) of interfacial water molecules that were not hydrogen bonded to other water molecules was observed, as well as a blue shift in the vibrational frequency of the interfacial hexadecane C-H stretch modes. Oil and water frequency shifts correlated with the negative electrostatic charge. Molecular dynamics simulations demonstrated that the unexpected strong charge-transfer interactions arose from interfacial C-H center dot center dot center dot O hydrogen bonds.

AMER ASSOC ADVANCEMENT SCIENCE2021

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

Nikolay Smolentsev

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.

EPFL2017

Resolving the charging paradox of neat oil droplets in water

Sergey Kulik

The thesis covers the series of questions regarding the molecular structure and charge of oil nanodroplets in water. This work proposes a mechanism responsible for the stabilization of oil nanodroplets in water, which does not involve OH- adsorption. First, it describes a numerical model for absorptive processes in sum frequency scattering (SFS) experiments on nanoobjects in absorptive media. The developed approach made it possible to analyze the O-D stretch region of SFS spectra for oil droplets in D2O. With this analytical tool, the isotopic dilution of water revealed the uncoupled O-D stretch spectrum with characteristics of a stronger hydrogen bonding network compared to the bulk. Additionally, these SFS experiments revealed a broad distribution of non-hydrogen bonded modes that were red shifted and broadened compared to similar species found previously at the air/water interface. Furthermore, a polarimetric analysis of SFS spectra demonstrated that charge transfer at the oil/water interface may be responsible for stabilization of bare oil droplets in water. Finally, complimentary second harmonic scattering measurements provided further evidence that OH- adsorption can not be responsible for negative charge at the droplet's surface and an alternative mechanism was proposed, which involved charge transfer and the high conductivity of OH- ions.

EPFL2022

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

Nikolay Smolentsev

EPFL2017