Molecular Level Interfacial Studies of Nanoemulsions: Surface Structure, Specific Ion Effects and Stability

This thesis is a study on the molecular structure of the interface of nanometer-sized oil droplets dispersed in water, a system known as oil-in-water nanoemulsion. The motivation for this research is the large significance of the interfacial nanostructures both for in industry and biology. Here we apply the nonlinear optical techniques of sum frequency scattering and second harmonic scattering to study the stabilization mechanism of nanoemulsions, as well as specific ion effects at the nanoscale. We begin with the study of the interfacial structure of oil nanodroplets stabilized with a positively charged, a negatively charged and a neutral surfactant, along with the stability of each system. We show that the surface density of charged surfactants on nanodroplets is an order of magnitude lower than on planar interfaces, due to repulsive interactions between like charges on opposing sides on the droplets surface through the oil phase, allowed by the small droplet size. Moreover, we find no experimental correlation between stability and surfactant surface density. Instead droplet stability is found to depend on the relative cooperativity between charge-charge, charge-dipole and hydrogen bonding interactions. Then, we study the effect of the inversion of the oil and the water phases on the droplet stability for nanometer-sized and micrometer-sized droplets. We employ an oil-soluble neutral surfactant, a water-soluble anionic surfactant, and the combination of the two. We find that, while microdroplets and water-in-oil nanodroplets follow the widely accepted empirical rules, and are stabilized only with a surfactant soluble in the continuous phase, nanometer-sized oil-in-water emulsions can be stabilized with an oil-soluble surfactant. Moreover, the structure of the surfactant is different when it approaches the interface from the dispersed or the continuous phase. Next, we study the interaction of four anions (SCN-, NO3-, Cl- and SO4-2) with different molecular structure with the nanointerface of droplets stabilized with a surfactant with a positively charged headgroup (trimethylammonium) for different anionic concentrations. Our results reveal a unique adsorption pattern for each anion that changes with concentration, possibly involving reorientation of the anions and adsorption to different patches of the interface. Interestingly, not only the weakly-hydrated SCN- and NO3 , but also the well-hydrated SO¬4-2 approaches the nanointerface, inducing strong interfacial water ordering. Last, we further study the interaction of SCN- with the positively charged nanointerface employing ab-initio molecular dynamics simulations. We find ion-pairing of SCN- with the interfacial trimethylammonium groups at concentrations as low as 5 millimolar. Moreover, a variety of ion species emerge at different ionic strengths, with differently oriented SCN- groups adsorbed on hydrophilic and/or hydrophobic parts of the surface. This diverse and heterogeneous chemical environment is surprisingly different from the behaviour at extended liquid planar interfaces, where ion pairing is typically detected at molar concentrations.

Molecular ordering effects in bulk and at lipid/water nanoscopic interfaces: A story of water, light, and order

Nathan Dupertuis

Water is ubiquitous on Earth, playing a critical role for a plethora of structures, processes and chemical reactions in nature. Its unique physicochemical properties originate mainly from its hydrogen bond network. Despite its recognized importance, many aspects of the structure of water as a liquid remain elusive. It is especially difficult to understand the complex nature of fluctuations and ordering in pure water, or to get a complete picture of aqueous interfaces at the molecular level. In this thesis, our aim is to improve our understanding of the underlying interactions between water and ions, macromolecules, and interfaces, especially the long range interactions. To this aim, we use two nonlinear optical techniques, second harmonic scattering (SHS) and vibrational sum frequency scattering (SFS), on liquid water as well as on model lipid membranes mimicking the cell membranes.We investigate with SHS how the orientational ordering of water molecules is affected by temperature changes and nuclear quantum effects. Scattering measurements of pure water reveal that the intermolecular correlations undergo a symmetry transformation with increasing temperature. On the other hand, with increasing temperature, aqueous electrolyte solutions exhibit a diminished influence of the combined electrostatic field on the water- water correlations. This trend is predicted qualitatively by a Debye-Hückel model, but not quantitatively due to its non-inclusion of hydrogen bonding and nuclear quantum effects. These insights facilitate the elaboration of future nuclear quantum mechanical models of water.The same interactions are further studied by a comparison of pure liquid water and pure liquid carbon tetrachloride. SHS measurements and simulations reveal that transient nanoscale voids exist in water and carbon tetrachloride. The coherent SHS emissions observed in water are generated by charge density fluctuations around the cavities. These measurements strongly hint towards non-uniformity in the structure of water, on femtosecond timescale and nanometric range.Next, we turn towards the structure of water around model lipid membranes. We explore first the three-dimensional confinement of liquid water inside liposomes of various size. By analyzing SHS patterns of zwitterionic and anionic liposomes, we observe that the effect of confinement is visible on a range of a hundred nanometers, larger compared to the previously reported ranges on the order of a nanometer or about ten nanometers. The water orientational ordering at the interface as well as the surface potential of the liposomes are retrieved with a nonlinear light scattering modeling, showing that the confinement effect is mainly attributable to the hydrogen bond network. The effect is in particular 8 times larger for light water than for heavy water.Finally, we investigate how the water structure is modified around lipid monolayers and bilayers. The hydration of lipid membranes is asymmetric with respect to charge. SHS and vibrational SFS reveal that the charge hydration asymmetry is drastically different depending on the nature of the lipid layer. It depends on a delicate balance between electrostatic and hydrogen bonding interactions. The charge hydration asymmetry is further modified by the presence of charges on the lipid headgroups on both sides of the bilayers.

EPFL2021

From Hydrophobic to Hydrophilic: The Structure and Density of the Hexadecane Droplet/Alkanol/Water Interface

Yixing Chen, Kailash Chandra Jena, Sylvie Roke

The molecular structure of the hexadecane droplet/alkanol/water interface has been investigated using vibrational sum frequency scattering and second harmonic scattering. This combination of methods allows us to investigate the interfacial alkyl chain conformation of the oil and several alkanols, ranging from 1-pentanol to 1-dodecanol, the orientational distribution of the methyl groups, the surface density of the alkanols, as well as the orientational alignment of water. For the hexadecane/1-dodecanol/water interface, dodecanol alkyl chains form a fluid layer with a wide distribution of tilt angles of the terminal CH3 groups. Indistinguishable spectra are recorded for the alkanol alkyl chains of 1-pentanol, 1-hexanol, 1-octanol, and 1-dodecanol, and alkanols with chain length longer than 6 C atoms all form films with similar densities. In contrast, the alkyl chains of the oil phase are relatively more distorted with respect to the pure oil/water interface for alkanols with shorter chain lengths. The projected surface area of a saturated film of hexanol is 29 +/- 5 angstrom(2), which requires a free energy of adsorption of Delta G = -26.3 +/- 0.7 kJ/mol. In addition, with increasing alkanol density the interfacial water structure loses its initial orientational alignment, which matches with the added number of interfacial 1-hexanol molecules. The found structures differ significantly from those reported on the alkanol/water and alkanol/air interface and charged surfactant/oil/water interface.

American Chemical Society2015

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.