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

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.

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

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

Lipid membrane characterization with second harmonic scattering

Cornelis Ulrich Lütgebaucks

Membranes, composed of a variety of lipids and other biomolecules, mediate signaling processes between cells and their aqueous environment. To fulfill this function, membranes can vary their composition leaflet-specific and thus alter their surface properties. To fully understand the impact of these processes on the molecular level, it is necessary to develop tools that can access the molecular properties of free-floating model membranes label-free. These tools are ideally surface-specific. In this thesis, we apply the nonlinear optical techniques second harmonic scattering (SHS) and vibrational sum-frequency scattering (SFS) together with electrokinetic measurements to label-free characterize the interfacial properties, hydration structure and surface potentials of liposomes in aqueous solutions. First, we generalize the nonlinear optical theory to describe the second-order surface response from interfaces with aqueous solutions independent of the ionic strength for reflection, transmission and scattering geometries. We demonstrate that interference effects from oriented water molecules in the bulk aqueous solution alter the probing depth and the expected second-order response at low ionic strengths. Then, we apply this theory to demonstrate that SHS patterns of liposomes and oil droplets contain all necessary information to extract the absolute surface potential of the respective particles without assuming a model for the interfacial structure. By analyzing scattering patterns that capture the orientational distribution of water around the particles, we find surface potentials of -38 mV for bare oil-droplets and -11 mV for zwitterionic liposomes in water. For anionic liposomes the surface potential varies between -150 mV and -23 mV in solutions containing different amounts of NaCl ranging from

\sim

0 mM to 10 mM. These values are remarkably different for solutions to the Gouy-Chapman model considering a fixed surface charge density. Next, we characterize the hydration and lipid asymmetries in binary mixed membranes using SHS and SFS. The liposomes exhibit hydration asymmetry between the inner and outer leaflet. The lipid number density between the inner and outer leaflet is the same, although geometrical packing arguments would suggest a different density. However, an asymmetric lipid distribution between the leaflets can be induced by fine tuning specific intermolecular interactions between the lipids. This is shown with dipalmitoylphosphoserine and dioleoylphosphocholine mixtures creating a membrane structure that allows intermolecular H-bonding between the phosphate and amine groups of the lipids. Finally, we quantify the surface properties of membranes composed of lipids containing phosphoserine and phosphocholine headgroups. Surprisingly, we find a very high degree of counterion condensation on anionic membranes in pure water: only 1 % of all lipids are ionized. This indicates a tightly packed layer of ions around the membrane that needs to be considered when modelling the interfacial structure around membranes.

EPFL2017