Freezing effects of oil-in-water emulsions studied by sum-frequency scattering spectroscopy

Temperature-dependent sum-frequency scattering spectroscopy is used to study the properties of hexadecane and dodecane oil droplets in water. The sum-frequency scattering spectra contain vibrational bands that correspond to the symmetric and antisymmetric CH stretching vibrations of the methylene (CH2) and methyl (CH3) groups of the alkane molecules. The relative amplitudes of the vibrational bands provide information on the surface structure and the shape of the oil droplets. We study the sum-frequency scattering spectra over a temperature range of -48 to 24 degrees C, including the freezing transitions of the water matrix and the oil droplets. Hexadecane oil droplets freeze at a higher temperature than the surrounding water, whereas dodecane oil droplets freeze at a lower temperature than the surrounding water. This allows us to independently study the freezing effect of oil and water on the surface structure of the oil droplets. In both cases, freezing leads to a change in the polarization dependencies that are valid in the case of the spherical-symmetric shapes that the oil droplets assume when both water and oil are liquid. We find that the freezing of water leads to a strong distortion of the liquid dodecane surface but has little effect on the surface of already solidified hexadecane. For completely frozen emulsions a further decrease in temperature is observed to lead to a further distortion of the surface of the solid oil particles, which might be caused by increasing hardness of the ice matrix encapsulating the particles. Published by AIP Publishing.

Metal Clusters on Surfaces

Michael Hugentobler

The morphology, stability and reactivity of nanometre-sized particles (Au, Pt) is investigated on three different supports. Gold nanoparticles with a diameter comprised between 4 and 6nm are stabilized in nanosized pits of well defined depth in highly oriented pyrolytic graphite (HOPG). These pits are produced by creation of artificial defects, followed by etching under a controlled oxygen atmosphere. At low Au coverage, clusters are found on the edges of the hexagonal pits maximizing the contact to dangling bonds on graphite multisteps. Larger coverage results in Au beads of well defined shape and with a constant bead density per unit length. Most remarkable is the stability of these nanostructures under ambient conditions. Temperatures as high as 650 K do not alter the morphology of the gold clusters. Above 700 K, the gold clusters catalyze the etching of graphite layers when heated under atmospheric conditions. The depth of the etching channel is defined by the depth of the pit and the channel width can be controlled by the cluster size. Hydrogen, oxygen and water are dissociated on the Au clusters at low temperatures (> 400 K). CO and CO2 is produced above 700 K which confirms the etching of the multilayer graphene sheets. The so-called water-gas shift reaction has been observed where oxygen and water dissociate on the Au clusters at low temperatures. Thermal desorption spectroscopy (TDS) measurements on the Pt=YSZ system have been performed and the desorption peaks of CO and O2 have been evidenced. Carbon monoxide desorbs from the surface at a temperature of 535 K and oxygen at 705 K, in good agreement with values reported in literature. Catalytic measurements have confirmed the known strong catalytic activity of Pt. The CO oxidation takes place at temperatures between 450 K and 700 K. The adsorption and desorption temperature of the two educts have been confirmed during the catalysis measurements. Size-selected Aun clusters (n = 5, 7) are deposited from the gas phase at room temperature with well-controlled kinetic energy on rutile TiO2 . Two different surface reconstructions have been used as support: TiO2(110) – (1 × 1) and TiO2(110) – (1 × 2). Systematic studies of morphology, stability and adsorption sites have been performed using scanning tunnelling microscopy (STM). The mean size of the clusters is maintained during deposition at a kinetic energy of 7.1eV per atom, indicating that there is no fragmentation. Au clusters are not stable neither on the TiO2(110) – (1 × 1) nor on the TiO2(110) – (1 × 2) reconstruction during annealing of the substrate to 800 K. A progressive transition into a pronounced 3-d formation is observed. The size distribution of the particles is small and the growth mode (Ostwald ripening) independent of the surface reconstruction. The higher corrugation of the TiO2(110) – (1 × 2) reconstruction results in a pronounced stabilization of the clusters on terraces, in contrast to the TiO2(110) – (1 × 1) reconstruction where 90% of the clusters are found on step edges at elevated temperatures. The catalytic activity of the Au clusters sets in during the 2d - 3d transition. These measurements have to be confirmed and a full understanding of the process should be found.

EPFL2010

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

Metal Clusters on Surfaces

Michael Hugentobler

EPFL2010

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

Hilton Barbosa De Aguiar

EPFL2011