Self-Assembly at Water Nanodroplet Interfaces Quantified with Nonlinear Light Scattering

The interfaces of water micro- and nanodroplets drive environmental, medical, catalytic, biological, and chemical biphasic processes. The interfacial droplet structure and electrostatics greatly determine the reactivity and efficiency of these processes. Droplet interfacial properties are elusive and generally inferred from bulk measurements and are therefore anything but exact. Here, we quantify the interfacial ordering of water and the electrostatic surface potential of nanoscale water droplets in an apolar liquid using angle-resolved polarimetric second-harmonic scattering. We also present a method to determine the amount of free charges in the hydrophobic phase, reaching a sensitivity that is 3 orders of magnitude better than conductivity measurements. Investigating the structural and surface electrostatic changes induced by AOT surfactant adsorption, we find that both the hydrogen bonding as well as the electrostatics strongly depend on the surfactant concentration. Above the critical micelle concentration, the interface mediates micelle self-assembly.

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

Evangelia Zdrali

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.

EPFL2019

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

Vibrational Sum-Frequency Scattering Studies of Oil-in-Water Emulsions

Hilton Barbosa De Aguiar

Oil-in-water emulsions consist of oil droplets dispersed in an aqueous phase. Interfacial properties of the droplets determine the stability of emulsions. Even though this statement is widely accepted, a molecular level picture of the droplet interface is yet to be determined. In this thesis, we use Vibrational Sum-Frequency Scattering (SFS), an interface specific technique, to probe the interface of the droplets with chemical specificity. We start with a detailed description of how SFS measurements can be performed in water and what the effects of penetration depth, pulse energy and particle density are. This is followed by a comparison between non phase-matched SF scattering and phase-matched SF Generation from planar interfaces. We then show studies on hexadecane oil droplets stabilized by the anionic surfactant sodium dodecylsulfate (SDS) dipersed in D2O. We measure the interfacial activity of SDS by probing the response of symmetric SO3 stretch of the headgroup. Surprisingly, the results show that the interfacial density of SDS is very low, namely one order of magnitude lower than what would be expected at the corresponding planar interface. As a consequence of such a low interfacial density, the interfacial tension barely changes from the value of the pristine oil-water interface. Moreover, we use SFS to retrieve molecular order of the alkyl chains from the CH stretch responses of either SDS or oil using a selective deuteration scheme. The spectral analysis shows that the SDS alkyl chains are highly disordered at the droplets interface. In contrast, the alkyl chains of the oil molecules at the interface are more ordered than SDS. Interference experiments show that the oil molecules are, in contrast to what is seen on most planar interface, oriented predominantly parallel with respect to the interface. We also probe the alkyl chain order of SDS adsorbed at interfaces of dispersions of polymer particles. Regardless of the chemical structure of the dispersed phase the SDS molecules are always very disordered. We can therefore conclude that SDS does not show any specific interaction with the droplets or particles. In the last part of this thesis, we use SFS to probe electrostatic properties of the droplets interfaces. Adsorption of charged surfactants at the droplet interface creates a strong electrostatic field that is screened by water and counter ions in the solution. The electrostatic field, together with the incoming laser fields, excite the OD vibrational stretch of the water molecules in the vicinity of the interface with a strength proportional to the surface potential. The surface charge densities are probed by the response of the symmetric SO3 stretch of the SDS headgroup. Therefore, we are able to independently measure the surface potential and the surface charge density. We show that it is essential to separate the changes of surface potential due to charging of the interface from changes due to screening effects. The results are in agreement with the Gouy-Chapman model.

EPFL2011